(Note: The only change from the previous edition dated January 1, 2008 is that a Sand Filter with specific design criteria was added as an LID BMP in Table 6.1 on page 47.)

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Section 1. Purpose .......................................................................................................................... 1
Section 2. Definitions..................................................................................................................... 2
Section 3. Performance Criteria ..................................................................................................... 7
Section 4. Low Impact Development Site Planning ...................................................................... 8
4.1
Introduction ..................................................................................................................... 8
4.2
LID Goals........................................................................................................................ 8
4.3
Key Considerations for LID Site Planning ..................................................................... 9
4.4
Minimizing Impacts to the Curve Number (CN) .......................................................... 10
4.4.1
Introduction ........................................................................................................... 10
4.4.2
Land Cover Type .................................................................................................. 10
4.4.3
Reduction and Disconnection of Impervious Areas ............................................. 14
4.5
Providing Retention Storage to Maintain Existing CN................................................. 15
4.5.1
Residential Retention Storage ............................................................................... 15
4.5.2
Commercial/Industrial Retention Storage............................................................. 16
4.6
Methods for Additional Detention Storage ................................................................... 16
4.7
Maintaining Existing Time of Concentration (Tc) ....................................................... 17
Section 5. LID Hydrologic Analysis ............................................................................................ 19
5.1
Introduction ................................................................................................................... 19
5.2
Hydrologic Comparison Between Conventional and LID Approaches ........................ 19
5.3
Key LID Hydrologic Definitions .................................................................................. 22
5.4
Hydrologic Evaluation .................................................................................................. 23
5.4.1
LID Runoff Curve Number (CN) ......................................................................... 23
5.4.2
Maintaining the Pre-development Time of Concentration (Tc) ........................... 27
5.4.3
Maintaining the Pre-development Curve Number and Runoff Volume ............... 29
5.4.4
Potential Requirement for Additional Detention Storage ..................................... 30
5.5
Process and Computational Procedure for LID Hydrologic Analysis .......................... 31
5.5.1
Introduction ........................................................................................................... 31
5.5.2
Data Collection ..................................................................................................... 31
5.5.3
Determining the LID Runoff Curve Number (CN) .............................................. 31
5.5.4
Development of the Time of Concentration (Tc) ................................................. 34
5.5.5
LID Storm Water Management Requirements ..................................................... 34
5.5.6
Determination of Design Storm Event .................................................................. 44
Section 6. LID Site Design Best Management Practices (BMPs) ............................................... 46
6.1
Introduction ................................................................................................................... 46
6.2
LID BMPs and Their Functions .................................................................................... 46
6.2.1
“Helpful Hints” for the Proper Design and Installation of BMPs ........................ 48
6.3
Runoff Reduction BMPs ............................................................................................... 49
6.4
Retention BMPs ............................................................................................................ 50
6.5
Detention BMPs ............................................................................................................ 55
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6.6
Pollution Prevention Practices ...................................................................................... 57
Section 7. Erosion and Sediment Control Considerations for LID .............................................. 59
7.1
Introduction ................................................................................................................... 59
7.2
Erosion and Sediment Control Principles ..................................................................... 59
7.3
Principle One: Planning ............................................................................................... 59
7.4
Principle Two: Scheduling of Operations .................................................................... 61
7.5
Principle Three: Soil Erosion Control Practices .......................................................... 61
7.6
Principle Four: Sediment Control Practices ................................................................. 63
7.7
Principles Five and Six: Maintenance and Inspection ................................................. 64
Section 8. Construction and Maintenance ................................................................................... 65
8.1
Introduction ................................................................................................................... 65
8.2 Permitting and Processing........................................................................................... 65
8.3
Erosion and Sediment Control Principles for Bioretention Applications ..................... 65
8.4
Construction Technique and Sequencing for Bioretention ........................................... 66
8.4.1
Site Preparation and Planning ............................................................................... 66
8.4.2
Design Considerations .......................................................................................... 67
8.4.3
Minimize Lot Grading/Clearing ........................................................................... 68
8.4.4
Install Sediment Control and Diversion Devices .................................................. 68
8.4.5
Excavation Preparation ......................................................................................... 70
8.4.6
Underdrain Specification ...................................................................................... 70
8.4.7
Observation/Cleanout Standpipe .......................................................................... 71
8.4.8
Gravel Bed ............................................................................................................ 71
8.4.9
Filter Fabric ........................................................................................................... 72
8.4.10
Soil Preparation and Installation ....................................................................... 73
8.4.11
Plant Preparation and Planting Methodology ................................................... 76
8.4.12
Installation of Mulching Materials.................................................................... 78
8.5
Maintenance and Operation .......................................................................................... 79
8.5.1
Maintenance Access.............................................................................................. 79
8.5.2
Maintenance Requirements for all BMPs ............................................................. 79
8.5.3
Special Maintenance Requirements for Bioretention Facilities............................ 80
8.5.4
Warrantees ............................................................................................................ 82
8.6 Typical Sequence of Construction for Bioretention ...................................................... 82
8.6.1
Inspectors Checklist for Bioretention ................................................................... 82
Section 9. Plan Submittal/Review................................................................................................ 85
9.1
Introduction ................................................................................................................... 85
9.2
Site Evaluation Tool ..................................................................................................... 85
9.3
Plan Submittal Requirements ........................................................................................ 86
9.4
Bonding ......................................................................................................................... 89
9.4.1
Posting Performance Bonds .................................................................................. 89
9.4.2
Bond Reduction / Release ..................................................................................... 90
9.4.3
Maintenance Bonds ............................................................................................... 90
9.4.4
Bonds for Public Entities ...................................................................................... 90
Section 10. Inspection and Enforcement ..................................................................................... 91
10.1
Introduction ................................................................................................................... 91
10.2
Authority ....................................................................................................................... 91
10.3
Inspection Responsibilities ........................................................................................... 91
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10.4
Inspection Requirements During Construction ............................................................. 91
10.4.1
Drywell or Infiltration Trench .......................................................................... 92
10.4.2
Bioretention BMPs............................................................................................ 92
10.5
Maintenance Inspections ............................................................................................... 93
10.5.1
Introduction ....................................................................................................... 93
10.5.2
Responsibilities and Procedures ....................................................................... 93
10.6
Approvals and Reports .................................................................................................. 93
10.6.1
Field Inspection Report ..................................................................................... 94
10.7
Administration and Enforcement .................................................................................. 94
10.7.1
Right to Appeal ................................................................................................. 94
10.7.2
Penalties ............................................................................................................ 94
List of Tables:
Table 5.1. LID Techniques and Hydrologic Design and Analysis Components .......................... 24
Table 5.2. Comparison of Conventional and LID Land Covers ................................................... 25
Table 5.3. LID Planning Techniques to Reduce the Post-Development LID CN ....................... 25
Table 5.4. LID Techniques to Maintain the Pre-development Time of Concentration ................ 28
Table 5.5. Representative LID Curve Numbers............................................................................ 31
Table 5.6. Representative Percentages of Site Required for Volume and Peak Control .............. 41
Table 6.1. Approved LID BMPs for Use in Huntersville ............................................................. 47
Table 6.2. Conventional Storm Water BMPs ............................................................................... 47
Table 6.3. Grassed Swale Design Considerations ........................................................................ 56
Table 8.1. Specifications for the Soil Mixture in Bioretention Facilities ..................................... 74
Forebay ......................................................................................................................................... 77
List of Figures:
Figure 4.1. Schematic of Residential Single-Family Low Impact Lot Layout ............................. 11
Figure 4.2. Grass Swale and Infiltration Trench ........................................................................... 13
Figure 4.3. Providing Bioretention Within Landscaped Buffer Areas ......................................... 17
Figure 5.1. LID Analysis Procedure ............................................................................................. 20
Figure 5.2. Hydrologic Response of Conventional BMPs ............................................................ 21
Figure 5.3. Comparison of the Hydrologic Response of Conventional and LID BMPs .............. 22
Figure 5.4. Comparison of Land Covers Between Conventional and LID CNs........................... 24
Figure 5.5. Effect of LID CN on the Post-Development Hydrograph Without BMPs ................. 26
Figure 5.6. LID Hydrograph that has a Reduced CN and Maintains Tc Without BMPs ............. 28
Figure 5.7. Retention Storage Required to Maintain Peak Development Runoff Rate ................ 29
Figure 5.8. Effect of Additional Detention Storage on LID Retention Practices ......................... 30
Figure 5.9. Procedure to Determine Storage Volume Required for BMPs to Maintain Pre-
Development Volume and Peak Flow Rate. ................................................................................. 36
Figure 5.10. Comparison of Retention of Storage Volumes Required to Maintain Peak Runoff
Rate Using Retention and Detention ............................................................................................. 38
Figure 5.11. Storage Volume Required to Maintain Peak Runoff Rate ....................................... 39
Figure 5.12. Comparison of Storage Volumes for Various Tcs ................................................... 39
Figure 6.1. Typical Swale Design ................................................................................................. 49
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Figure 6.2. Typical Bioretention BMP ......................................................................................... 52
Figure 6.3. Typical Dry Well ........................................................................................................ 53
Figure 6.4. Typical Rain Barrel .................................................................................................... 54
Figure 6.5. Typical Rock Trench Level Spreader ......................................................................... 57
Figure 6.6. Concrete Level Spreader ............................................................................................ 58
Figure 6.7. Slotted Drain Level Spreader During Dry Conditions and a Rain Event ................... 58
Figure 8.1. Rain Garden Detail ..................................................................................................... 69
Figure 8.2. Installation of Underdrain on Top of Filter Fabric in Trench Bottom ....................... 71
Figure 8.3. Placing Gravel Over the Underdrain .......................................................................... 72
Figure 8.4. Placing Filter Fabric Over the Gravel Covering the Underdrain ............................... 73
Figure 8.5. Installing Amended Soil Mix Over Filter Fabric ....................................................... 75
Figure 8.6. Silt Fence Around the Perimeter to Protect From Sedimentation .............................. 76
Figure 8.7. Hal Marshall Rain Garden Planting Design ............................................................... 77
Figure 8.8. Planted and Mulched Rain Garden ............................................................................. 79
Figure 8.9. Sequence of Construction for Bioretention BMPs ..................................................... 84
List of Examples:
Example 5.1. Detailed CN Calculation ......................................................................................... 33
Example 5.2: Determine Site Area Required to Maintain Volume (CN) Using Chart ................. 37
Example 5.3: Calculation of Volume for Water Quality Control ................................................. 37
Example 5.4: Calculation of Additional Storage Above Volume Required to Maintain CN and
Maintain Pre-development Peak Runoff Rate Using Hybrid Approach ....................................... 42
Example 5.5: Calculation of Storage Volume Required to Maintain the Peak Runoff Rate Using the Hybrid Approach of Retention and Detention ........................................................................ 44
Example 7.1: Calculation of Slope Steepness and Slope Length ................................................. 60
List of Appendices:
Appendix A: Huntersville Post-Construction Ordinance ............................................................. 97
Appendix B: Storage Volume Required to Maintain the Pre-Development Runoff Volume Using
Retention Storage ........................................................................................................................ 139
Appendix C: Storage Volume Required to Maintain the Pre-Development Peak Runoff Rate
Using 100% Retention Storage - Type 2 24-Hour Storage Chart Series .................................... 147
Appendix D: Storage Volume Required to Maintain the Pre-Development Peak Runoff Rate
Using 100% Detention Storage - Type 2 24-Hour Storage Chart Series .................................... 155
Appendix E: Design Examples and Cost Analyses for Use of LID ........................................... 163
Appendix F: Rain Garden Plant List ........................................................................................... 197
Appendix G: BMP Operation and Maintenance Agreement (Declaration of Covenants).......... 202
Appendix H: BMP Maintenance Plans ....................................................................................... 212
Appendix I: BMP Maintenance and Inspection Check Lists ...................................................... 224
Appendix J: Soil Verification Form ............................................................................................ 242
For more information, contact the Mecklenburg County Water Quality Program at 704-
336-5500. Additional information is also available on the following website: http://stormwater.charmeck.org
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
The purpose of this document is to provide the information necessary for compliance with the
Huntersville Post-Construction Ordinance adopted by the Huntersville Town Board on February
17, 2003 and incorporated into the Huntersville Zoning Ordinance. Appendix A contains a copy of this Ordinance. The goal of the Ordinance and this manual is to establish storm water management requirements and controls to prevent surface water quality degradation to the extent practicable in the streams and lakes within the Town Limits and Extraterritorial Jurisdiction of
Huntersville and to protect and safeguard the general health, safety, and welfare of its residents.
Low Impact Development (LID) techniques combined with conventional storm water retention and detention structures are the primary mechanisms discussed in this document for meeting this goal and complying with the Ordinance. The goal of LID is to develop site design techniques, strategies, BMPs, and criteria to store, infiltrate, evaporate, retain, and detain runoff on the site to replicate pre-development runoff characteristics and mimic the natural and unique hydrology of the site thereby preventing an increase in pollutant loads above pre-development conditions. The selection of these strategies and techniques for compliance with Water Quality Performance
Criteria is discretionary and shall be detailed in the Concept Plan Application submitted during the Preliminary Plan review process. Detailed designs are provided with the submittal of the
Storm Water Management Permit Application following approval of the Concept Plan. This manual provides the information necessary to select and design these strategies and techniques.
The following website provides additional guidance and information for compliance with
Huntersville’s Post-Construction Ordinance: http://stormwater.charmeck.org (select “Post-
Construction Programs” and “Mecklenburg County, Towns of……”). A copy of the
Administrative Manual developed for the ordinance is available on this website along with all necessary forms, a process guide, Charlotte-Mecklenburg BMP Design Manual and other a variety of other important information. This information is available for use by Mecklenburg
County staff at the following LAN site: G:\WQ_Xfer\WQ\Post-Construction Ordinance.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Best Management Practices (BMPs): A structural or non-structural management-based practice used singularly or in combination to reduce non-point source input to receiving waters in order to achieve water quality protection goals.
• Non-structural BMPs - Non-engineering methods to control the amount of non-point source pollution. These may include land-use controls and vegetated buffers.
• Structural BMPs - Engineered structures that are designed to reduce the delivery of pollutants from their source or to divert contaminants away from the waterbody.
Bioretention: The use of vegetation in retention areas designed to allow infiltration of runoff into the ground and transpiration by plants as well as evaporation. The plants provide additional pollutant removal and filtering functions while infiltration allows the temperature of the runoff to be cooled. Also referred to as a Biofilter or Rain Garden.
Cistern: A receptacle for holding water or other liquid (i.e. tank for catching and storing rainwater).
Conveyance: The process of moving water from one place to another.
Curbs: Concrete barriers on the edges of streets used to direct storm water runoff to an inlet or storm drain and to protect lawns and sidewalks from vehicles.
Curve Number: An index that represents the amount of runoff from the combined hydrologic effect of soil, land use, agricultural land treatment class, hydrologic condition, and antecedent soil moisture. Curve numbers have a range of 0 to 100.
Design Storm: A rainfall event of specific depth or intensity (i.e. 3.12 inches or 4.80 inches/hour) and return frequency (e.g., 1- year storm) that is used to calculate runoff volume and peak discharge rate.
Detain: To store and slowly release storm water runoff following precipitation by means of a surface depression or tank and an outlet structure. Detention structures are commonly used for pollutant removal, water storage, and peak flow reduction.
Developed Area: That portion of the site that has been cleared, graded or built-upon.
Dry Well: Small excavated trenches filled with stone to control and infiltrate runoff, usually from rooftops.
Evaporation: The change by which any substance is converted from a liquid state and carried off in vapor.
Evapotranspiration: The combination of evaporation and transpiration of water into the atmosphere from living plants and soil.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Filter Strip: Grass strips along roads or parking lots that remove pollutants from runoff as it passes through, allowing infiltration and velocity reduction.
Floodplain: An area adjacent to a stream or river where water overflows its banks during high flow events.
Ground Water: The supply of fresh water found beneath the earth's surface (usually in aquifers) that provides base flow to streams and rivers and is often used for supplying wells and springs.
The inflow to a ground water reservoir is called ground water recharge.
Gutter: The edge of a street (below the curb) designed to drain water runoff from streets, driveways, parking lots, etc. into catch basins and storm drains.
Hydrograph: A graphic plot of changes in the flow of water or in the elevation of water level plotted against time.
Hydrologic Cycle (Water Cycle): The cycle of water movement from the atmosphere to the earth and back to the atmosphere through various processes.
Hydrologic Soil Group (HSG): See Soil Group definition.
Hydrology: The science dealing with the properties, distribution and circulation of water.
Impervious Surface: A surface that cannot be penetrated by water such as pavement or rock and prevents infiltration, thus generating runoff.
Integrated Management Practices (IMP): A LID practice or combination of practices that are the most effective and practicable (including technology, economic, and institutional considerations) means of controlling the pre-development site hydrology.
Level Spreader: An outlet designed to convert concentrated runoff to sheet flow and disperse it uniformly across a slope to prevent erosion.
Low Impact Development (LID): The integration of site ecology and environmental goals and requirements into all phases of urban planning and design from the individual residential, commercial and industrial lot level to the entire watershed.
Mecklenburg County Land Use & Environmental Services Agency (LUESA): The department or division of Mecklenburg County government (regardless of the title given to it by Mecklenburg
County) which has responsibility for storm water and water quality matters, acting as the agent of the Town of Huntersville for various purposes in connection with the enforcement of this regulation.
National Pollution Discharge Elimination System (NPDES) Permit: A permit issued pursuant to the federal Clean Water Act for the purpose of controlling discharges of pollutants to surface
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012 waters and protecting water quality. In North Carolina, NPDES Permits are issued by the N.C.
Department of Environment and Natural Resources.
Nonpoint Source (NPS) Pollution: Forms of pollution caused by sediment, nutrients, organic and toxic substances originating from land use activities, which are carried to lakes and streams by surface runoff. Nonpoint source pollution occurs when the rate of materials entering these waterbodies exceeds natural levels.
Permeability: The property of a soil to transmit water under a gradient. It is measured by the quantity of water passing through a unit cross section, in a unit time, under a hydraulic gradient.
Pollutant Load: A calculated quantity that is the result of a flow rate and pollutant concentration applied over a given amount of time.
Rain Barrels: Barrels designed to collect and store rooftop runoff.
Receiving Waters - A river, ocean, stream, or other watercourse into which runoff from precipitation is discharged.
Recharge: The addition of water into the ground water via the surface of the ground.
Retain: To capture and hold storm water runoff following precipitation by means of a surface depression allowing the water to infiltrate into the soil, evaporate and possibly transporate thus reducing the hydrologic and pollution impacts downstream. Retention structures are commonly used for pollutant removal, water storage, and peak flow reduction.
Riprap: A facing layer (protective cover) of stones placed to prevent erosion or the sloughing off of a structure or embankment of a waterbody.
Runoff: Water from a precipitation event that flows across the ground surface. Run-off carries nonpoint source pollutants to receiving streams.
Sediment: The layer of soil, sand and minerals at the bottom of surface water, such as streams, lakes, and rivers that may absorb contaminants.
Sedimentation: The removal, transport, and deposition of detached soil particles by flowing water or wind.
Siltation: The deposition of finely divided soil and rock particles upon the bottom of a waterbody.
Site Evaluation Tool (SET): A spreadsheet-based model that assesses and compares predevelopment and post-development runoff, infiltration, and pollutant loading rates, which provides a methodology to aid in better site design and evaluation of BMP effectiveness.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Site Fingerprinting: A development approach that places land disturbing activities away from environmentally sensitive areas (wetlands, steep slopes, etc.), future open spaces, tree save areas, future restoration areas, and temporary and permanent vegetative forest buffer zones. Ground disturbance is confined to areas where structures, roads, and rights-of-ways will exist after construction is completed.
Soil Group: Hydrologic Soil Groups (HSG) are used to estimate runoff from precipitation. Soils not protected by vegetation are placed in one of four (4) groups on the basis of the intake of water (infiltration) after the soils have been wetted and have received precipitation from long duration storms. Group A soils have a high infiltration rate and usually include course sand and gravel. Group B soils have a moderate infiltration rate and include soils with moderately fine to coarse texture. Group C soils have a slow infiltration rate and include soils that have a moderately fine to fine texture. Group D soils have a very slow infiltration rate and include fine textured clays (Soil Conservation Service, 1986).
Soil Moisture: Water diffused in the soil. It is found in the upper part of the zone of aeration where water is discharged by transpiration from plants or by soil evaporation.
Storm Water: Water that runs across the surface of the ground during and after precipitation events.
Swale: An open depression or wide, shallow ditch that intermittently contains or conveys runoff.
Can be used as a BMP to detain and filter runoff.
Time of Concentration (Tc): The time required for runoff to travel from the hydraulically most distant point (in time) in the watershed, to the point of interest in the watershed.
Total Phosphorus (TP): A nutrient that is essential to the growth of organisms but when it occurs in high enough concentrations it can negatively impact water quality conditions. Total phosphorus includes both dissolved and suspended forms of reactive phosphorus, acid hydrolyzable phosphorus and organic phosphorus as measured by Standard Method 4500-P.
Total Suspended Solids (TSS): Total suspended matter in water which includes particles collected on a filter with a pore size of 2 microns as measured by Standard Method 2540-D, which is commonly expressed as a concentration in terms of milligrams per liter (mg/l) or parts per million (ppm).
Travel Time (Tt): The time it takes water to travel from one location to another in a watershed.
It is a component of the time of concentration (Tc).
Treatment Train: A series of BMPs or natural features, each designated to treat runoff that are implemented together to maximize pollutant removal effectiveness.
Water Table: The upper surface of a zone of saturation or ground water.
Watershed: The area of land that contributes surface runoff to a waterbody.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Wet Pond: A BMP designed to detain urban runoff and always contain water.
Wetlands: An area (including swamp, marsh, bog, prairie pothole, or similar area) that is typically inundated or saturated by surface or groundwater at a frequency and duration sufficient to support the growth and regeneration of vegetation requiring an abundant water source.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Land development activities shall be performed in such a manner as to minimize the degradation of water quality conditions through compliance with the Performance Criteria listed below.
Tables 6.1 and 6.2 in Section 6 of this manual contain a description of approved BMPs for meeting each of these Criteria. a) All storm water treatment systems used to meet these Performance Criteria shall be designed to achieve average annual 85% Total Suspended Solids (TSS) removal for the developed area of a site. Areas designated as open space that are not developed do not require storm water treatment. All sites must employ LID practices to control and treat runoff from the first 1inch of rainfall. A combination of LID and conventional BMPs is allowed in the form of a treatment train design provided the conventional BMPs meet the optimal design threshold and the treatment train meets a minimum pollutant removal efficiency of 85% total suspended solids and 70% total phosphorus in accordance with the specifications contained in Section 4 of the Charlotte-Mecklenburg BMP Design Manual. b) LID practices or a combination of LID and conventional storm water management practices shall be used to control and treat the increase in storm water runoff volume associated with post-construction conditions as compared with pre-construction (existing) conditions for the
2-year frequency, 24-hour duration storm event (3.12 inches) in the Rural and Transitional
Zoning Districts. For all other Zoning Districts, LID practices or a combination of LID and conventional storm water management practices shall be used to control and treat the increase in storm water runoff volume associated with post-construction conditions as compared with pre-construction (existing) conditions for the 1-year frequency, 24-hour duration storm event (2.58 inches). This may be achieved by hydrologic abstraction, recycling and/or reuse, or other accepted management practice as described in Section 6 of this manual. c) Where any storm water BMP employs the use of a temporary water quality storage pool as a part of its treatment system, the drawdown time shall be a minimum of 48 hours and a maximum of 120 hours. d) Peak storm water runoff rates shall be controlled for all development above 12% imperviousness. The peak storm water runoff release rates leaving the site during postconstruction conditions shall be equal to or less than the pre-development peak storm water runoff release rates for the 2-year frequency, 24-hour duration storm event and 10-year frequency, 24-hour duration storm event. The emergency overflow and outlet works for any pond or wetland constructed as a storm water BMP shall be capable of safely passing a discharge with a minimum recurrence frequency of 50 years. For detention basins, the temporary storage capacity shall be restored within 72 hours. Requirements of the Dam
Safety Act shall be met when applicable. e) No one BMP shall receive runoff from an area greater than five (5) acres. However, the total drainage area from BMPs used in series (i.e., integrated) can exceed this five (5) acre maximum.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
4.1 Introduction
The Water Quality Performance Criteria discussed in Section 3 shall be achieved using LID site planning and techniques (BMPs) or a combination of LID and conventional storm water structures. The selection of these strategies and techniques is discretionary and shall be detailed in a Concept Plan submitted during the plan review process (see Section 9, Plan
Submittal/Review). This Section focuses on the LID planning strategies and techniques that can be used in the development of the Concept Plan for achieving the Performance Criteria. The goal of LID site planning is to allow for the maximum reasonable utilization of the property while maintaining the pre-development hydrologic regime (volume, peak runoff rate for a given frequency). The LID approach combines a hydrologically functional site design with pollution prevention measures (BMPs) to reduce development impacts on hydrology and water quality.
The goal is to maintain the pre-development storm water runoff volume, peak runoff rates, and to mimic pre-development runoff conditions. Storm water is managed in small, source control landscape features rather than in large, end-of-pipe pond structures located at the downstream extent of drainage areas. However, ponds may be required in addition to LID BMPs to create a
“treatment train” affect designed to meet the Performance Criteria described in Section 3.
Through LID, hydrologic functions such as infiltration, peak and volume of discharges, and ground water recharge can be maintained with the use of reduced impervious surfaces, functional grading, open channel sections, disconnection and utilization of runoff, and the use of bioretention/filtration landscape areas.
4.2 LID Goals
The goal of LID is to develop site design techniques, strategies, BMPs, and criteria to store, infiltrate, evaporate, transpire, retain, and detain runoff on the site to replicate pre-development runoff characteristics and mimic the natural and unique hydrology of the site. Since every aspect of site development affects the hydrologic response of the site, LID runoff control techniques also can address every aspect of site development. There is a wide array of impact reduction and site design techniques that allow the site designer to create storm water control mechanisms that function in a similar manner to natural control mechanisms. The net result will be to mimic the watershed’s natural hydrologic functions or water balance between runoff, infiltration, storage, ground water recharge, and evapotranspiration. With the LID approach, receiving waters experience little change in the volume, frequency, or quality of runoff or in the base flows fed by ground water.
The goals of LID are discussed and demonstrated throughout this manual. The list below highlights and reinforces some of the main goals and principles of LID:
• Provide enhanced technology for environmental protection of receiving waters.
• Provide economic incentives to provide environmentally sensitive development.
• Develop the full potential of environmentally sensitive site planning and design.
• Encourage public education and participation in environmental protection.
• Reduce maintenance costs of the storm water infrastructure.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
• Utilize concepts, technologies, and objectives for storm water management such as micromanagement and multi-functional landscape features (rain gardens, swales, and conservation areas) to mimic or replicate hydrologic functions, and maintain the ecological/biological integrity of receiving streams.
LID planning and techniques will reduce impervious areas, reduce the need for conventional pipe and pond technology, and reduce or limit clearing and grading. LID techniques integrate storm water controls throughout the site in small, discrete units. BMPs are distributed across each site, reducing the need for a centralized BMP.
4.3 Key Considerations for LID Site Planning
Examples of the LID site planning techniques include, but are not limited to:
• Maintaining natural drainage ways and patterns and directing runoff to depression areas.
• Preserving as many trees as possible, especially those located on soils with the highest permeability rates.
• Reducing the percentage of impervious area.
• Locating BMPs in soils with the highest permeability rates.
• Disconnecting impervious areas.
• Limiting clearing and grading in areas containing soils with the highest permeability rates.
• Locate impervious areas on less permeable soils.
• Maintaining existing natural topography and terrain. Avoid disturbance of, and construction in, steep slope areas (>15%).
• Limiting clear-cutting and mass-grading through “site fingerprinting” techniques.
Selectively clear wooded lots to preserve the tree canopy, understory vegetation, and natural vegetative buffers. See Article 7 of the Huntersville Zoning Ordinance.
• Flattening slopes only within existing cleared and graded areas, where feasible, to facilitate on-lot storage and infiltration.
• Revegetating areas that have been cleared and graded.
• Dispersing storm water flow to the natural drainage ways rather than concentrating it in swales, pipes, or channels.
The appropriate combination of these techniques to maintain the curve number (CN) and time of concentration (Tc) will result in a design that maintains the pre-development runoff volume, peak rate, and frequency. The remainder of this Section offers guidance on how these and other
LID site planning techniques can be used to achieve the LID hydrology and Performance Criteria
(Section 3). The Section is divided into the four hydrologic components that are discussed in the hydrologic analysis:
• Minimizing development impacts on the existing CN (Section 4.4).
• Providing retention storage to maintain the CN (Section 4.5).
• Providing additional detention storage to maintain the CN (Section 4.6).
• Maintaining the Tc (Section 4.7).
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
4.4 Minimizing Impacts to the Curve Number (CN)
4.4.1 Introduction
The CN is used to determine the runoff volume from both the pre- and post-development condition. The aggregate CNs assigned to each area are used to arrive at a composite, or weighted, CN for the site watershed. As the CN from the pre-development condition to the postdevelopment condition increases, storage is required to mimic the pre-development CN. Site development factors most responsible for the determination of the CN fall into four categories:
• Land cover type.
• Percentage and connectivity of impervious areas.
• Hydrologic conditions (good, fair, poor).
• Soil infiltration rate (HSG).
The evaluation and management of land cover type, and percentage and connectivity of impervious areas are different for LID and discussed in detail below.
4.4.2 Land Cover Type
The most important factor for the change in CN from the pre- to post-development condition is land cover. The land cover type is the distribution of the physical components of the site. It is typically a broad definition, such as residential ¼ acre lots, but can be defined in detail, such as trees in good, fair, or poor condition. For example, the land use might be low-density residential on an HSG B, which carries an average CN of 65 (see TR55 Table 2-2a, Soil Conservation
Service, 1986); however, if examined more closely, each lot might incorporate LID site planning and retention practices that would effectively reduce the CN. Figure 4.1 illustrates a low-density residential lot that includes tree preservation, minimized impervious areas, and low-impact BMP retention and detention practices. As shown in Figure 4.1, the land cover of a site is directly affected by the extent of the limits of clearing and grading. To minimize these hydrologic impacts, LID site planning strategically reduces clearing and grading, preserves more trees wherever possible, and minimizes the percent and connectivity of impervious areas within a given drainage area. The designer should keep in mind that the change in the CN from pre- to post-development will directly affect the need for compensatory storm water storage volume; hence, reducing this change can potentially reduce the storage volume requirement, thereby increasing the reasonable use of the property.
Determining the existing land cover requires delineating the surface coverage of existing land cover features and sensitive areas such as woodlands, permeable soils, streams, floodplains, critical areas, wetlands, and steep slopes. Delineation of these areas is important for understanding existing site conditions and site hydrology. It should be noted that this process is exactly the same as the conventional site planning process. The difference is the increased amount of detail and effort required to calculate the CN.
Reduction in Land Cover Changes.
Reduction in land cover changes is the first step in maintaining existing CN. There are several ways to reduce land cover changes including:
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
• Reduce the size of cleared area (i.e., preserve as much woodland as possible) and increase reforestation areas.
• Locate cleared/graded areas outside permeable soils and vegetated areas.
• Design roads, sidewalks, and parking areas to minimize land cover impacts.
•
Reduce or disconnect site imperviousness.
10 Feet @ 5%
10 Feet
@ 5%
10 Feet
@ 5%
Figure 4.1. Schematic of Residential Single-Family Low Impact Lot Layout
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Reduce Limits of Clearing and Grading.
The limits of clearing and grading refer to the site area to which development is directed. This development area will include all impervious areas such as roads, sidewalks, rooftops, and graded lawn areas and open drainage BMPs. To have minimal hydrologic impacts on existing site land cover (i.e., to reduce the percent change in the CN), the area of development should be located where the impact on the pre-development CN is less sensitive (e.g., developing barren C and D type soils will have a lesser hydrologic impact than developing forested A and B type soils). At a minimum, current requirements place the areas of development outside stream and lake buffer areas. Increasing these buffer areas, thereby reducing the limits of clearing and grading, will lead to a reduction in the percent change in the CN.
Site Fingerprinting.
Site fingerprinting (minimal disturbance techniques) can be used to further reduce the limits of clearing and grading, thereby minimizing the percent change in the CN. Site fingerprinting includes restricting ground disturbance by identifying the smallest possible area and clearly delineating it on the site. Land cover impacts can be minimized through minimal disturbance techniques that include the following:
• Minimizing the size of construction easements, materials storage areas, and siting stockpiles within the development envelope.
• Careful siting of lots and home layout, clearing and grading to avoid steep slopes, removing existing trees and excessive grading.
• Minimizing imperviousness by reducing paved surfaces on private areas.
• Homes on crawl spaces or basements that conform to natural grades without creating a flattened pad area for slab construction; thus saving clearing and grading costs.
• Flagging the smallest site disturbance area possible to minimize soil compaction on the site.
Install orange construction fencing and tree protection areas where necessary to protect root structure along the limit of encroachment during the construction phase.
• Disconnecting all impervious areas to increase the Tc and flow paths.
• Re-vegetating cleared and graded areas to provide an effective way to decrease the postdevelopment runoff CN. Sometimes it is impractical or impossible to develop a lot or group of lots using the minimal disturbance techniques. Re-vegetation can be used to connect bioretention facilities to natural drainage ways or to increase the size of riparian buffer areas.
Re-vegetated areas also increase the permeability of the soil. In addition, these BMPs can add aesthetic value to a site.
• Maintaining existing topography and drainage patterns to encourage dispersed flow.
Locate Cleared and Graded Areas Outside Permeable Soils and Vegetated Areas.
Locating roadways, houses, and graded lawn areas within the following areas should be minimized as much as possible:
Within pervious soils. Sensitive soils for LID sites include soils with good infiltration characteristics. Site planning that locates impervious surfaces or directs compaction within areas of highly pervious soils creates the greatest possible change in infiltration between predevelopment and post-development conditions. Areas that contain well-drained soils should be preserved. Preserving permeable soils should be promoted in all unpaved areas throughout the site, when considering optional ways to reduce the difference between the pre- and post-
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012 development CNs. Maintaining permeable soils also helps maintain recharge areas for ground water and base stream flows.
Within existing vegetated areas.
Protection of woodland areas can help to reduce impacts on existing land cover and associated CN. Expansion of protected vegetated areas adds to the benefits of reducing CN changes. Saving existing trees on a development site is a cost-effective and quality-enhancing practice. If the trees are appropriate for preservation and adequate protective measures are taken during construction, the preservation of existing trees on a site has many rewards. When these protected vegetated areas are combined with riparian buffers, they can provide added benefits of reduced storm water velocities, increased storm water infiltration, filtration of pollutants, protection of existing wildlife habitat, and stream bank and bed stability.
Use Alternative Roadway Designs.
Roadways, sidewalks, driveways, and parking areas are the greatest contributors to increasing the CN due to associated imperviousness and land clearing for cutting and filling. The primary considerations of road design are safety and balanced earthwork for the site. For LID sites, minimizing the effective imperviousness contributed by road and parking area pavement is also an important site planning and design consideration. In a LID layout, the roadway is designed to minimize hydrologic impacts by using minimal grading and clearing techniques and open drainage sections. LID roadway designs emphasize the need to keep paved areas to an absolute minimum. Reducing both pavement width and road length can help achieve this goal. The following features can be incorporated into the roadway to minimize impacts:
Narrow road sections. Small sections can be used to minimize site imperviousness and clearing and grading impacts.
Grassed swales and infiltration trenches.
In Huntersville’s Rural Zoning District, grassed swales and infiltration trenches can be used in place of curb and gutter in some circumstances (see
Figure 4.2).
Figure 4.2. Grass Swale and Infiltration Trench
Curvilinear road layouts. Local and collector streets with curves and changes in alignment allow the flexibility to fit the road into the existing site topography. Following existing contours will minimize grading and make earthwork operations easier. It should be noted that curvilinear road layouts must meet current AASHTO design criteria.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Location of roads on ridges. Strategically locate the roadway entrance to minimize disturbance by positioning the entrance on the high point of the ingress, following mild contour slopes to the greatest degree possible. As an added benefit, following mild contour slopes when laying out roads allows reduction in cut and fill requirements and associated grading costs. It is also more aesthetically pleasing than rigid rectangular blocks.
Reduced application of sidewalks to one side of primary roads. Decrease site imperviousness by reducing sidewalk widths or by using alternate materials such as pervious pavers. The reduction of sidewalks to one of side of the street is allowed only in Huntersville’s Rural Zone.
4.4.3 Reduction and Disconnection of Impervious Areas
By reducing and disconnecting site impervious areas, the amount of direct runoff can be significantly decreased. This requires careful planning. There must be adequate circulation so that pedestrians and vehicles do not use grassed areas and cause erosion. In turn, reduced widths and lengths result in reduction of associated infrastructure such as drainage pipes, etc.
Minimize rooftop imperviousness. Rooftops contribute to site imperviousness, and the number of lots per acre or lot coverage generally determines the site’s rooftop impervious area. House type, shape, and size can affect rooftop imperviousness. As an example, more rooftop coverage is generally required for ranch-type homes that spread out square footage over one level. With this in mind, vertical construction is favored over horizontal layouts to achieve less impact.
Disconnect impervious areas.
Reductions in post-development CNs can be gained by redirecting and dissipating concentrated flows from impervious areas onto vegetated surfaces. Strategies for accomplishing this include:
• disconnecting roof drains and directing flows to vegetated areas;
• directing flows from small swales to stabilized vegetated areas;
• breaking up flow directions from large paved surfaces; and
• encouraging sheet flow through vegetated areas.
Carefully locate impervious areas. To the extent possible, place impervious areas so that they drain to natural drainage features, vegetated buffers, natural resource areas, or permeable zones/soils. Additional LID site planning techniques aimed at reducing site imperviousness include:
• Using shared driveways in Water Supply Watershed Critical Areas. (Using shared driveways in other areas is recommended but may require a subdivision waiver. Shared driveways also need the approval of the Town of Huntersville.)
• Limiting residential driveway width to 9 feet (for both single and shared driveways).
• Minimizing building setback to reduce driveway lengths.
• Efficient parking lot layout. Avoid single loaded aisles, angled parking, and excessive spaces.
• Using private roads where possible to allow flexibility in reducing road widths (see Figure
4.2).
• Planning efficient lot layout to minimize the road length, single loaded streets and double frontage lots.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
4.5 Providing Retention Storage to Maintain Existing CN
Once all of the appropriate strategies have been applied to the site to take advantage of infiltration and storage opportunities in the watershed, there still may be a need for additional retention storage to maintain the CN. The objective is to provide this additional storage as source control BMPs. These are small BMPs distributed strategically throughout the site and close to the origin of storm water runoff. When the need arises, additional detention storage basins may be required to maintain the pre-development peak runoff rate.
4.5.1 Residential Retention Storage
For residential LID sites, lot layouts must be planned to distribute retention storage volume as much as possible throughout the site. At the site planning stage, it is important to allocate enough area to provide for needed storm water retention storage. This involves developing a preliminary layout where LID techniques can be incorporated. At a minimum, this should include designating (1) road layout with potential LID geometric modifications, (2) sidewalk areas, (3) building footprint, driveway and accessory structures, and (4) potential BMP areas. When laying out the site, the designer should always keep in mind the four important factors that affect the change in pre- to post-development storm water runoff volume and peak rate including: land cover type, percentage and connectivity of impervious areas, hydrologic conditions, and soil type
(HSG). The site should be planned to maximize lot yield while minimizing impacts on the hydrologic regime. Residential retention storage can be incorporated onto individual lots or common areas. Due to concerns with the future maintenance of these BMPs, locating them in common areas dispersed throughout the site is recommended.
Information and calculations provided in Section 5 can be used to determine the required retention storage area. Zoning requirements exist regarding maximum lot coverage in residential zones for various lot sizes in accordance with the Huntersville Zoning Ordinance. Lot coverage for residential zones relates to maximum impervious surface allowed per lot and can be used to determine available green space for on-lot retention storage area. Keep in mind the need for allowing “reasonable use” of a property and restrictions on locating storage areas within the building restriction line. Estimated requirements for retention storage areas (evenly dispersed throughout the development) can be compared with estimated available green space. In most cases, adequate space will be available to provide retention storage in zones. However, situations may arise for ½-acre lots and smaller where the magnitude of change in predevelopment and post-development curve numbers results in storage requirements that cannot be fully accommodated while allowing reasonable use of lawn and/or open space area. Refer to
Section 4.6
for more detailed information on this situation (i.e., incorporating more detention storage). This is important to realize when identifying the limits of clearing and grading on each lot. It may be necessary to minimize the clearing and grading to reduce changes in the CN to accommodate storage retention requirements. Terraces and enlarged drainage swales can also be used to provide additional detention storage as necessary. When locating on-lot retention storage areas on residential LID sites, follow these recommended guidelines:
• Locate swales and bioretention BMPs (rain gardens) where they can provide a green space connection between existing wooded or natural areas.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
• Bioretention practices must be located outside a public road right-of-way to avoid conflict with underground utilities.
• Infiltration or enhanced swales may be used in the public right-of-way.
• Locate bioretention and infiltration in areas containing permeable soils.
• Keep all LID storm water management BMPs outside all sensitive areas and respective buffers.
• Insure that the contributing drainage area to the site is stabilized prior to bioretention installation.
4.5.2 Commercial/Industrial Retention Storage
Planning for retention storage volume for commercial and industrial LID sites is focused on two areas: perimeter buffer areas and green areas located within parking lots. On-site retention storage can be provided as interior bioretention, preferably located within the required landscape islands, or as cistern or rainbarrel facilities. In the event that the available green space within the parking area is insufficient to provide for required storage volume (as computed in Section 5), additional space can be obtained by providing bioretention within the landscaped buffer area located on the perimeter of the commercial or industrial site (see Figure
4.3). For example, retention storage volume could be located within required commercial and industrial site setback areas or landscape buffers. Existing minimum green space requirements plus the size of perimeter buffers and parking space requirements will dictate the feasibility of providing all required storage within surface swales, terraces, or bioretention facilities (refer to the Huntersville Zoning ordinance and the landscape details for specific requirements).
4.6 Methods for Additional Detention Storage
In some cases, it may be necessary to provide for additional detention storage to augment retention in maintaining the pre-development storm water runoff peak discharge rate. The following LID practices can be used for detention within residential or commercial/ industrial sites:
• swales with check dams;
• restricted drainage pipe and inlet entrances;
• wider swales and terraces;
• rain barrels;
• parking lot storage;
• rooftop storage; and
• diversion structures.
Additional planning considerations for swales include:
• Open drainage conveyance (swales or natural drainage ways) should be used to the extent allowed by existing ordinances. Terraces also can be designed for and used as detention.
Drainage along primary roads must be contained in piped storm drains.
• Use 4:1 slopes for roadway swales out of the public right-of-way. These slopes, once graded, are to be stabilized with fiber mats or netting and then planted with perennials or wild flowers or dense ground cover or woody shrubs. The use of 4:1 slopes will minimize disturbance and preserve more existing trees. The slope of the graded swale to the building pad must be 1 percent minimum.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
•
Locate on-lot swale facilities where they can provide green space connection between existing wooded or natural areas.
Figure 4.3. Providing Bioretention Within Landscaped Buffer Areas
4.7 Maintaining Existing Time of Concentration (Tc)
The time of concentration (Tc), in conjunction with the CN, determines the peak discharge rate for a storm event. From theoretical considerations, site and infrastructure components that affect time of concentration and travel time include:
• travel distance (flow path);
• slope of the ground surface and/or water surface;
• ground surface roughness; and
• channel shape and pattern.
These concepts are applied to LID by using techniques to control the Tc by modifying the following aspects of flow and conveyance within the development:
• maximize sheet flow;
• modify/lengthen flow path;
• site and lot slopes;
• open swale BMP; and
• site and lot vegetation.
Overland Sheet Flow. The site should be graded to maximize the overland sheet flow distance and to minimize disturbance of woodland along the Tc flow path. This practice will increase travel times thereby increasing the time of concentration and ultimately the peak discharge rate
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012 will be decreased. To provide sufficient contact time and allow for settlement of suspended solids, flow velocity in areas graded to natural drainage ways should not exceed 1 fps to the extent practicable. Install a stable, level spreader (timber tie, edging, etc.) along the upland edge of the natural drainage way buffer, or create a flat grassy area about 30 feet wide on the upland side of the buffer where runoff can spread out. This grassy area can be incorporated into the
Upland Zone of the S.W.I.M. buffer; however, level spreaders must be located outside the buffer.
Flow Path.
Increase flow path or travel distance of surface runoff to increase infiltration and travel time. One of the challenges of LID is to provide as much overland or sheet flow as possible to increase the time it takes for rooftop and driveway runoff to reach open swale drainage features or in some cases piped storm drains. Typically, the designer can perform one or both of two potential techniques for accomplishing this task. First, rooftop and driveway runoff can be permanently infiltrated or stored within infiltration trenches, dry wells, or cisterns strategically located to capture the runoff prior to it reaching the lawn. Second, strategic lot grading can increase both the surface roughness and the travel length of the runoff.
Site and lot slopes.
Flatten lot slopes to approach a minimum 1 percent to increase infiltration and travel time .
The building pad area is a 10-foot perimeter around the building with a positive drainage in accordance with the building code, or a minimum of five percent (5%) slope. Lot areas outside the building pad perimeter should contain a positive slope of at least 1 percent. As described in the Charlotte-Mecklenburg Storm Water Design Manual, the designer is responsible for ensuring that the slope of the lot does not cause flooding during the 100-year storm event.
Soil compaction of original soils (not fill) in the lot area should be avoided to maximize the infiltration capacity of the soil. These infiltration areas can receive runoff from impervious surfaces such as rooftops and driveways to decrease travel times for these areas.
Open Swales. Wherever possible, LID aims to use open swales in lieu of more conventional storm drain structures .
To alleviate flooding problems and reduce the need for conventional storm drain systems wherever possible vegetated or grassed open drainage BMPs should be provided as the primary means of conveying surface runoff between lots and along roadways.
Lots should be graded so as to minimize the quantity and velocity of surface runoff within the open drainage BMPs. On-line infiltration BMPs and terraces can be used to reduce the quantity and travel time of the surface runoff as the need arises. At no time shall 5 cfs be exceeded.
Site and Lot Vegetation.
Re-vegetate and/or plant graded areas to promote natural retention and increase travel time. Re-vegetating graded areas, planting, or better yet, preserving existing vegetation can reduce the peak discharge rate by creating added surface roughness as well as providing for additional retention and reducing the surface water runoff volume. Designers can connect vegetated buffer areas with existing vegetation or forest to gain retention/detention credit for runoff volume and peak rate reduction and to avoid “paved area” as the Tc flow path for the
“shallow concentrated flow” part of the Tc calculation. The benefit of such practices will be to minimize the need for on-lot bioretention facilities.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
5.1 Introduction
The purpose of this Section is to provide LID hydrologic analysis and computational procedures for use in determining LID storm water management requirements. Design concepts are illustrated by the use of runoff hydrographs that represent responses to both conventional and
LID approaches. LID site planning and BMPs are defined and categorized into components of
LID objectives. Computational procedures for determining BMP requirements are demonstrated through design examples located in Appendix E. A strategy for using these techniques is provided in Section 4. The process for developing the LID hydrology is illustrated in Figure 5.1.
This figure lists the sequential steps and the sections in the manual where the methods to calculate or determine the specific requirements are provided.
5.2 Hydrologic Comparison Between Conventional and LID Approaches
Conventional storm water conveyance systems are designed to collect, convey, and discharge runoff as efficiently as possible. Conventional storm water management BMPs are typically sited at the most downstream point of the entire site (end-of-pipe control). The storm water management requirement is usually to maintain the peak runoff rates at pre-development levels for a particular design storm event. Figure 5.2 illustrates the hydrologic response of the runoff hydrograph to conventional BMPs.
• Hydrograph 1 represents the response to a given storm of a site that is in a pre-development condition (e.g., woods, meadow). The hydrograph is defined by a gradual rise and fall of the peak discharge and volume.
• Hydrograph 2 represents the response of a post-development condition with no storm water management BMPs. This hydrograph definition reflects a shorter time of concentration (Tc) and higher runoff curve number (CN) than that of the pre-development condition, a rapid decrease in the time to reach the peak runoff rate, a significant increase in the peak runoff discharge rate and volume, and increased duration of the discharge volume.
• Hydrograph 3 represents a post-development condition with conventional storm water BMPs, such as a detention pond. Although the peak runoff rate is the same, the hydrograph exhibits significant increases in the runoff volume and duration of runoff from the pre-development condition.
In comparison with conventional storm water management, the objective of LID hydrologic design is to retain the post-development excess runoff volume in discrete units throughout the site to emulate the pre-development hydrologic regime. Management of both runoff volume and peak runoff rate is included in the design. The approach is to manage runoff at the source rather than at the end of pipe. Preserving the hydrologic regime of the pre-development condition requires both structural and nonstructural techniques to compensate for the hydrologic alterations of development. Typically alterations to the hydrologic regime as a result of development include, but are not limited to, the following:
• Increased runoff volume and velocity;
• Increased flow frequency, duration, and peak runoff rate;
•
Reduced infiltration (ground water recharge);
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
LID Hydrologic Analysis Process
Implement Additional
LID Tc Techniques and
Recalculate Tc NO
Legend
∀
Q Storage Volume Needed for
Water Quality Control
∀
R Storage Volume to Maintain
CN Using Retention Chart A
∀
R100 Storage Volume to Maintain
Peak Using 100% Retention
Chart B
∀
D100 Storage Volume to Maintain
Peak Using 100% Detention
Chart C
H
H’
Storage Volume for Hybrid
Design
Storage Volume for Hybrid
Design with Limited
Retention
Start
Data Collected
Calculate Existing Tc
(Section 5.5.2)
(Section 5.5.2)
(Section 5.5.4)
(Section 5.5.4)
Calculate Existing CN
Prepare Preliminary Layout
Calculate Proposed CN Using LID
(Section 5.5.3)
(Section 5.5.3)
Concepts
(Section 5.5.4)
Calculate Proposed Tc (Section 5.5.4)
Proposed Tc> Existing Tc?
YES
Determine Design Storm Event
Calculate Volume Required to Maintain
Existing CN Using Chart Series in
Appendix B for Each Design Storm
∀
R
(Section 5.5.6)
(Section 5.5.6)
(Section 5.5.5
Step 1)
(Section 5.5.5
Step 1)
(Section 5.5.5
Step 2)
Calculate the Storage Volume Area
Required for Quality Control
∀
Q
(Section 5.5.5
Step 2)
(Section 5.5.5
Step 2)
Select Higher Values of
∀
Q or
∀
R for
Storage Volume
Required for BMP
(Section 5.5.5
Step 3)
Hybrid Approach
Calculate Additional
Volume to Maintain Both
Predevelopment Peak and
Volume H Using
∀
R,
∀
D100 ,
∀
R100
Series in Appendix C for Each Design Storm
YES
(Section 5.5.5
Step 6)
Predevelopment Peak Discharge Using Chart
Is
∀
R <
∀
R
100
NO
∀
R
100
(Section 5.5.5
Step 6)
(Section 5.5.5
Step 4)
NO
(Section 5.5.5
Step 5)
Can Site Conditions
Accommodate 100% of
BMPs for
∀
R or
∀
Q
Use Chart Series in
Appendix D to Calculate
YES
Select Appropriate BMPs
LID Final Storm Water Design
Figure 5.1. LID Analysis Procedure
20
(Section 5.5.5
Step 3)
Determine
Storage Volume
Area That Is
Acceptable for
Retention and
Recalculate
Storage Volume
(Section 5.5.5
Step 7)
∀
D
100
,
∀
∀
R
R,
100
(Section 5.5.5 Step 7)

Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Figure 5.2. Hydrologic Response of Conventional BMPs
• Modification of the flow pattern;
• Faster time to peak, due to shorter Tc through storm drain features; and
• Loss of storage.
In LID, the design approach is to leave as many undisturbed areas as practicable to reduce runoff volume and runoff rates by maximizing infiltration capacity. Storm water management BMPs are then integrated throughout the site to compensate for the hydrologic alterations of development. The approach of maintaining areas of high infiltration and low runoff potential in combination with small, source control storm water management BMPs creates a “hydrologically functional landscape.” This functional landscape not only helps maintain the pre-development hydrologic regime but also enhances the aesthetic and habitat value of the site. Figure 5.3 illustrates a comparison of LID and conventional BMPs:
• For hydrograph 1, refer to Figure 5.2 for description.
• For hydrograph 3, refer to Figure 5.2 for description.
• Hydrograph 4 represents the response of a post-development condition that incorporates LID storm water management. LID uses undisturbed areas and smaller retention storage areas distributed throughout the site (on-lot or in common areas) to reduce runoff volume. The peak runoff rate and volume remain the same as the pre-development condition through the use of common area or on-lot retention and/or detention. The frequency and duration of the runoff are also much closer to the existing condition than those typical of conventional
BMPs.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012 practices to control runoff volume and, if these retention practices are not sufficient to control the peak runoff rate, to then use additional detention practices to control the peak runoff rate. Detention is temporary storage that releases excess runoff at a controlled rate.
The use of a combination of retention and detention to control the peak runoff rate is defined as the hybrid approach.
• Flow Frequency/Duration Control: Since LID is designed to emulate the pre-development hydrologic regime through both volume and peak runoff rate controls, the flow frequency and duration for the post-development conditions will be almost identical to those for the predevelopment conditions (see Figure 5.3). Thus, the impacts on the sediment and erosion and stream habitat potential at downstream reaches can then be minimized.
• Water Quality Control: LID is designed to provide water quality treatment of runoff from the first 1 inch of rainfall using retention practices. Storm water treatment shall be designed to achieve average annual 85% Total Suspended Solids (TSS) removal and must apply to the volume of post-construction runoff. Drawdown time for this treated volume of runoff shall be a minimum of 2 days. The storage required for water quality control is compared to the storage required to control the increased runoff volume. The greater of the two volumes is the required retention storage. LID also provides pollution prevention by modifying human activities to reduce the introduction of pollutants into the environment. LID practices also aid in cooling runoff from developed sites thus lessening thermal peaks in receiving streams.
The low-impact analysis and design approach focuses on the following hydrologic analysis and design components:
• CN: Minimizing change in the post-development CN by reducing impervious areas and preserving more trees and meadows to reduce the storage requirements to maintain the predevelopment runoff volume.
• Tc: Maintaining the pre-development Tc by minimizing the increase of the peak runoff rate after development by lengthening and flattening flow paths and reducing the length of the piped runoff conveyance systems.
• Retention: Providing retention storage for volume and peak control, as well as water quality control, to maintain the same storage volume as the pre-development condition.
• Detention: Providing additional detention storage, if required, to maintain the same peak runoff rate and/or prevent flooding and erosion downstream.
Table 5.1 provides a summary of LID techniques that affect these components.
5.4 Hydrologic Evaluation
The purpose of the hydrologic evaluation is to determine storm water management requirements for LID sites. The evaluation method is used to determine the amount of retention and/or detention to control the runoff volume and peak runoff rate. Appropriate detention and/or retention techniques are then selected to meet these requirements.
5.4.1 LID Runoff Curve Number (CN)
Calculation of the LID CN is based on a detailed evaluation of the existing and proposed land cover so that an accurate representation of the potential for runoff can be obtained. This calculation requires the designer to investigate key parameters associated with a LID:
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Table 5.1. LID Techniques and Hydrologic Design and Analysis Components
LID Technique
Low-Impact Hydrologic
Design and Analysis
Components
Lower Post-Development
       
CN
Increase Tc
Retention
Detention
             
 
 




  
• Land cover type;
• Percentage of and connectivity of impervious areas;
• Hydrologic soils group (HSG); and
• Hydrologic condition (average moisture or runoff condition).
Comparing conventional and LID CN calculations, the conventional CNs are based on the land cover assumptions whereas the LID CN is based on a detailed evaluation of the land cover and parameters listed above. For example, as illustrated in Figure 5.4, customizing the CN for a LID site allows the developer/engineer to take advantage of and get credit for such LID site planning practices as the following:
• Narrower driveways and roads (minimizing impervious areas);
• Maximized tree preservation or re-forestation (tree planting);
• Site fingerprinting (minimal disturbance);
• Open drainage swales, sheet flow, maintain natural drainage patterns;
• Preservation of soils with high infiltration rates to reduce CN; and
• Location of BMPs on high infiltration soils.
Figure 5.4. Comparison of Land Covers Between Conventional and LID CNs
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Table 5.2 illustrates a comparison of LID CN land covers with those of a conventional CN for a typical 1-acre lot. Figure 5.4 illustrates a comparison of conventional CN land covers with a LID customized CN for a 1-acre lot.
Table 5.2. Comparison of Conventional and LID Land Covers
Conventional Land Covers
20% impervious
80% grass
(TR-55 Assumptions) LID Land Covers
15% imperviousness
25% woods
60% grass
Table 5.3 provides a list of LID site planning practices and their relationship to the components of the LID CN. Key LID techniques that will reduce the post development CN, and corresponding runoff volumes, are as follows:
• Preservation of Permeable Soils: This approach includes site planning techniques such as minimizing disturbance of soils, particularly vegetated areas with high infiltration rates.
Additional planning should limit the placement of infrastructure and impervious areas such as houses, roads, and buildings on more permeable soils. These areas of permeable soils should be reserved for infiltration practices. Care must be taken when determining the suitability of soils for proposed construction practices. Adequate geotechnical information
(in addition to County Soils Maps) is required for planning practices.
Table 5.3. LID Planning Techniques to Reduce the Post-Development LID CN
Suggested Options
Affecting Curve
Number
Land Cover Type
Percent of
Imperviousness
  
  

 
Hydrologic Soils
 
Group
Hydrologic Condition
Disconnectivity of
 
Impervious Area

  
Storage & Infiltration
 
• Preservation of Existing Natural Vegetation: Woods and other vegetated areas provide many opportunities for storage and infiltration of runoff. By maintaining the surface coverage to
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012 the greatest extent possible, the amount of compensatory storage for BMPs is minimized.
Naturally vegetated areas also can be used to provide surface roughness, thereby increasing the Tc. In addition, plant life functions to filter out and uptake pollutants, particularly nitrogen, phosphorus and heavy metals.
• Minimization of Site Imperviousness: Reducing the amount of imperviousness on the site will have a significant impact on the amount of compensatory BMP storage required since there is almost a one-to-one corresponding relationship between rainfall and runoff for impervious areas.
• Disconnection of Site Imperviousness: Impervious areas are considered disconnected if they do not connect to a storm drain structure or other impervious areas through direct or shallow
Figure 5.5. Effect of LID CN on the Post-Development Hydrograph Without BMPs
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
• Hydrograph 5 represents the resulting post-development hydrograph using the LID CN only.
There is a reduction in both the post-development peak rate and volume. Section 5.5.3 describes the process and computational procedure for determining the LID runoff CN.
5.4.2 Maintaining the Pre-development Time of Concentration (Tc)
The LID hydrologic evaluation requires that the post-development time of concentration (Tc) be greater than or equal to the pre-development Tc. The travel time (Tt) throughout individual lots or areas should be approximately the same so that the Tc is representative of the drainage. To maintain the Tc, LID uses the following site planning techniques:
• Maintaining pre-development flow path length by dispersing and redirecting flows, generally, through open swales and natural drainage patterns.
• Increasing surface roughness (e.g., preserving woodlands, using vegetated swales).
• Detaining flows (e.g., open swales, check dams, rain gardens).
• Minimizing disturbance (minimizing compaction and changes to existing vegetation).
• Flattening grades in impacted areas.
• Disconnecting and dispersing runoff from impervious areas (e.g., eliminating curb/gutter and redirecting downspouts).
• Connecting pervious and vegetated areas (e.g., reforestation, tree planting).
To maintain the pre-development Tc, an iterative process that analyzes different combinations of the above appropriate techniques may be required. These site planning techniques are incorporated into the hydrologic analysis computations for post-development Tc to demonstrate an increase in post-development Tc above conventional techniques and a corresponding reduction in peak discharge rates. Figure 5.6 illustrates the hydrologic response to maintaining equal pre-development and post-development Tc.
• For hydrograph 1, refer to Figure 5.2.
• For hydrograph 5, refer to Figure 5.5.
• Hydrograph 6 represents the effects of the LID techniques to maintain the Tc. This effectively shifts the postpeak runoff time to that of the pre-development condition and lowers the peak runoff rate.
The greatest gains for increasing the Tc in a small watershed can be accomplished by increasing the Manning’s roughness “n” for the initial surface flow at the top of the watershed and increasing the flow path length for the most hydraulically distant point in the drainage area.
After the transition to shallow concentrated flow, additional gains in Tc can be accomplished by:
• Decreasing the slope;
• Increasing the flow length; and
• Directing flow over pervious areas.
In LID sites, the amount of flow in closed channels (pipes) should be minimized to the greatest extent possible. Swales and open channels should be designed with the following features:
• Increase surface roughness to retard velocity.
• Use a network of wider and flatter swales and channels to avoid fast-moving flow (maximum
5cfs for swales during a 10-year, 24-hour storm event).
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Figure 5.6. LID Hydrograph that has a Reduced CN and Maintains Tc Without BMPs
• Increase channel flow path.
• Maximize swale and channel width to increase storage capacity.
• Reduce channel gradients to decrease velocity .
• The swale and channel should flow over pervious soils wherever possible to increase infiltration and reduce runoff to maximize infiltration capacity.
Table 5.4 identifies LID techniques and volume objectives to maintain pre-development Tc.
Table 5.4. LID Techniques to Maintain the Pre-development Time of Concentration
Low Impact Development Technique
LID Objective
Minimize disturbance
Flatten grades
  
 
Reduce height of slopes
Increase flow path (divert and redirect)
  
Increase roughness “n”
  
    
  
  
  
    
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
5.4.3 Maintaining the Pre-development Curve Number and Runoff Volume
Once the post-development Tc is maintained at the pre-development conditions and the impact of CN is minimized, any additional reductions in runoff volume must be accomplished through distributed on-site storm water management techniques. The goal is to select the appropriate combination of management techniques (see Section 6) that emulate the hydrologic functions of the pre-development condition to mimic the existing CN and runoff volume. LID uses retention to accomplish this goal. Placing these facilities strategically located in common areas or on individual lots will provide volume controls at the source.
Retention storage allows for a reduction in the post-development volume and the peak runoff rate. The increased storage and infiltration capacity of retention BMPs allows the predevelopment volume to be maintained. Retention BMPs to maintain the pre-development CN include, but are not limited to the following:
• Infiltration trenches;
• Retention ponds;
• Rain barrels;
• Bioretention (Rain Garden);
• Irrigation ponds; and
• Rooftop storage.
As the retention storage volume of the LID BMPs is increased, there is a corresponding decrease in the peak runoff rate in addition to runoff volume reduction. If a sufficient amount of runoff is stored, the peak runoff rate may be reduced to a level at or below the pre-development runoff rate. This concept is illustrated in Figure 5.7. This storage may be all that is necessary to control the peak runoff rate when there is a small change in CN. However, when there is a large change in CN, it may be less practical to achieve flow control using volume control only.
Figure 5.7. Retention Storage Required to Maintain Peak Development Runoff Rate
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
• Hydrograph 7 represents the BMP inflow hydrograph for the post-development condition for a site using LID. Because of BMP retention storage, runoff is not released until the maximum retention storage volume is exceeded. Line A represents the limit of retention storage.
•
Hydrograph 8 is the outflow hydrograph from the LID retention BMP. The release begins at the limit of retention storage, represented by line A. The storage maintains the predevelopment volume and controls the peak runoff rate. For this situation, the falling limb of the hydrograph represents a condition where the inflow (hydrograph 7) equals the outflow
(hydrograph 8).
5.4.4 Potential Requirement for Additional Detention Storage
Even though the post-development Tc and CN are maintained at the pre-development level, in some cases additional detention storage is needed to maintain the pre-development peak runoff rate due to the spatial distribution of the retention storage provided. The amount of storage that maintains the pre-development runoff volume might not be sufficient to also maintain the predevelopment peak runoff rate. Therefore, additional common areas or on-lot storage is required in detention storage. LID storm water management techniques for providing detention storage include, but are not limited to the following:
• Swales with check dams, restricted drainage pipe, and inlet entrances;
• Wider swales;
• Rain barrels;
• Rooftop storage; and
• Diversion structures.
The effect of this additional detention storage is illustrated in Figure 5.8.
Figure 5.8. Effect of Additional Detention Storage on LID Retention Practices
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
• For hydrograph 1, refer to Figure 5.2.
• Hydrograph 9 represents the response of a post-development condition that incorporates LID retention practices. The amount of retention storage provided is not large enough to maintain the pre-development peak runoff discharge rate. Additional detention storage is required.
• Hydrograph 10 illustrates the effect of providing additional detention storage (hybrid design, see page 43) to reduce the post-development peak discharge rate to pre-development conditions.
5.5 Process and Computational Procedure for LID Hydrologic Analysis
5.5.1 Introduction
The hydrologic analysis of LID is a sequential decision making process that can be illustrated by the flow chart shown in Figure 5.1. Several iterations may occur within each step until the appropriate approach to reduce storm water impacts is determined. The procedures for each step are given in the following Section. Design charts have been developed to determine the amount of storage required to maintain the existing volume and peak runoff rates to satisfy storm water management requirements (see Appendices B, C, and D).
5.5.2 Data Collection
The basic information used to develop the LID site plan and used to determine the CN and Tc for the pre- and post-development condition is the same as conventional site plan and storm water management approaches.
5.5.3 Determining the LID Runoff Curve Number (CN)
The determination of the LID CN requires a detailed evaluation of each land cover within the development site. This will allow the designer to take full advantage of the storage and infiltration characteristics of LID site planning to maintain the CN. This approach encourages the conservation of more woodlands and the reduction of impervious area to minimize the need of BMPs. The steps for determining the LID CN are as follows:
Step 1: Determine percentage of each land use/cover.
Because LID design emphasizes minimal site disturbance (tree preservation and site fingerprinting), it is possible to retain much of the pre-development land cover and CN.
Therefore, it is appropriate to analyze the site as discrete units to determine the CN. Table 5.5 lists representative land cover CNs used to calculate the composite “custom” LID CN.
Table 5.5. Representative LID Curve Numbers
Land Use/Cover Curve Number for Hydrologic Soils Groups
1
Impervious Area
Grass (good condition, >75%)
A
98
39
Woods (fair condition) 36
1
Table 2.2, TR-55 (Soil Conservation Service, 1986).
B
98
61
60
C
98
74
73
D
98
80
79
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Step 2: Calculate composite custom CN.
The initial composite CN is calculated using a weighted approach based on individual land covers without considering disconnectivity of the site imperviousness. This is done using
Equation 5.1. This weighted approach is illustrated in Example 5.1.
Equation 5.1.
CN c
=
CN
1
A
1
+
A
1
CN
2
+
A
2
...
A
2
...
+
+
CN
A j j
A j
Where:
CNc = composite curve number;
A j
= area of each land cover; and
CN j
= curve number for each land cover
Overlays of Soil Conservation Service (HSG) boundaries onto homogeneous land cover areas are used to develop the LID CN. What is unique about the LID custom made CN technique is the way this overlaid information is analyzed as small discrete units that represent the hydrologic condition, rather than a conventional TR-55 approach that is based on a representative national average. This is appropriate because of the emphasis on minimal disturbance and retaining site areas that have potential for high storage and infiltration. This approach provides an incentive to save more trees and maximize the use of more permeable soils for recharge. Careful planning can result in significant reductions in post-development runoff volume and BMP costs.
Step 3: Calculate LID CN based on the connectivity of site impervious area. When the impervious areas are less than 30 percent of the site, the percentage of the unconnected impervious areas within the watershed influences the calculation of the CN (Figure 2-4, Soil
Conservation Service, 1986). Disconnected impervious areas are impervious areas that do not connect to a drainage feature or impervious surface through direct or shallow concentrated flow.
For example, roof drains from houses could be directed onto lawn areas where sheet flow occurs, instead of to a swale or driveway where shallow concentrated flow occurs. By increasing the ratio of disconnected impervious areas to pervious areas on the site, the CN and resultant runoff volume can be reduced. Equation 5.2 is used to calculate the CN for sites with less than 30% impervious area.
Equation 5.2.
CN c
=
CN p
+


P imp
100


×
( 98
−
CN p
)
×
( 1
−
0 .
5 R ) where:
R = ratio of unconnected impervious area to total impervious area;
CN c
= composite CN;
CN p
= composite pervious CN; and
P imp
= percent of impervious site area
Example 5.1 uses steps 1 through 3 to compare the calculation of the curve number using conventional and LID techniques using the percentages of land cover for a typical 1-acre residential lot from Figure 5.4.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Example 5.1. Detailed CN Calculation
Given:
One-acre residential lot
Conventional CN: 68 (from TR-55 Table 2.2a) Runoff curve numbers for urban areas
(Soil Conservation Service, 1986) Table 2.2a assumes HSG B, 20% imperviousness with a CN of 98 and 80% open space in good condition.
Custom Made LID CN: CN for individual land covers based on Table 2.2a. Assume 25% of the site will be used for reforestation/landscaping (see Figure 5.4) HSG B.
Procedure:
Step 1: Determine percentage of each land cover occurring on site and the CN associated with each land cover.
Land Use
Impervious (Directly Connected)
Impervious (Unconnected)
Open Space (Good Condition, Graded)
Woods (Fair Condition)
HSG
(1)
B
B
B
B
CN
(2)
98
98
61
55
% of
Site
(3)
5
10
60
25
Land
Coverage (ft
2
)
(4)
2,178
4,356
26,136
10,890
Step 2: Calculate composite custom CN (using Equation 5.1).
CN c
=
98
×
4,356
+
98
×
2,178
+
61
×
26,136
+
55
×
10,890
43,560
CN c
=
65
Step 3: Calculate LID CN based on the connectivity of the site imperviousness (using
Equation 5.2).
×
,
+ ×
CN
= p
,
CN p
=
59 2
R
=
10
15
R
=
0 67
CN c
CN c
CN c
=
CN p
+ 



P imp
100




×
(
98
−
CN p
)
×
=
59 .
2
+



15
100
=
63.1
(use 63)



×
(
98
−
59 .
2
) (
1
−
(
1
−
0 .
5
×
R
)
0 .
5
×
0 .
67
)
LID custom CN of 63 is less than conventional CN of 68 (pre-development CN is 55).
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
5.5.4 Development of the Time of Concentration (Tc)
The pre- and post-development calculation of the Tc for LID is exactly the same as that described in the TR-55 (Soil Conservation Service, 1986) and NEH-4 (Soil Conservation
Service, 1985) manuals. Tc can be maintained through techniques discussed in 5.4.2.
5.5.5 LID Storm Water Management Requirements
Once the CN and Tc are determined for the pre- and post-development conditions, the storm water management storage volume requirements can be calculated. The LID objective is to maintain both the pre-development volume, pre-development peak runoff rate, for each storm frequency. The required storage volume is calculated using the design charts in Appendices B, C, and D. The remaining LID hydrologic analysis techniques are based on the premise that the post-development Tc is the same as the pre-development condition. If the post-development Tc does not equal the pre-development Tc, additional LID site design techniques must be implemented to maintain the Tc. Three series of design charts are needed to determine the storage volume required to control the increase in runoff volume and peak runoff rate using retention and detention practices. The required storages shown in the design charts are presented as a depth in hundredths of an inch (over the development site). Recommended design depths for BMPs are provided in Section 6. Equation 5.3 is used to determine the volume required for
BMPs. However, a 6-inch depth is recommended as the maximum depth for bioretention basins used in LID.
Equation 5.3.
Volume for BMP
=
( depth from chart , inches developmen t size , acres
)
100
The amount, or depth, of runoff lost by infiltration or by the process of evapotranspiration is not included in the design charts. Reducing surface area requirements through the consideration of these factors can be determined by using Equation 5.4.
Equation 5.4.
Volume of Site Area for BMP
=
( initial volume of site
100 for BMP
) (
100
− x
) where: x = % of the storage volume infiltrated and/or reduced by evaporation or transpiration. x% should be minimal (less than 10% is considered).
Storm water management is accomplished by selecting the appropriate BMP, or combination of
BMPs, to satisfy the calculations for surface area and volume requirements. The design charts to be used to evaluate these requirements are:
• Appendix B: Storage Volume Required to Maintain the Pre-development Runoff Volume
Using Retention Storage.
• Appendix C: Storage Volume Required to Maintain the Pre-development Peak Runoff Rate
Using 100% Retention.
• Appendix D: Storage Volume Required to Maintain the Pre-development Peak Runoff Rate
Using 100% Detention.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
These charts are based on the following general conditions:
• The land uses for the development are relatively homogeneous.
• The storm water management measures are to be distributed evenly across the development, to the greatest extent possible.
• The design storm is based on 1-inch increments. Use linear interpolation for determining intermediate values.
The procedure to determine the BMP requirements is outlined in Figure 5.9 and described in the following Sections.
Step 1: Determine storage volume required to maintain pre-development volume or CN using retention storage.
The post-development runoff volume generated as a result of the postdevelopment custom made CN is compared to the pre-development runoff volume to determine the storage volume required for volume control. Use Chart Series in Appendix B: Storage
Volume Required to Maintain the Pre-development Runoff Volume Using Retention Storage.
The procedure for calculating the depth required for maintaining runoff volume is provided in
Example 5.2. The practical and reasonable use of the site must be considered. The BMPs must not restrict the use of the site. The storage volume, expressed as hundredths of an inch, is for volume control only; additional storage may be required for water quality control. The procedure to account for the first 1-inch of rainfall, which is the current Performance Criteria, is found in
Step 2.
Step 2: Determine storage volume required for Performance Criteria.
The runoff volume, expressed as hundredths of an inch, is then compared to the volume required for water quality control. The volume requirement for storm water management quality control is based on the requirement to treat the first 1-inch of rainfall. This volume can be translated to a percent of the site area by assuming a storage depth of 6 inches. The procedure for calculating the site area required for quality control is provided in Example 5.3. The greater number is used as the required storage volume to maintain the CN. From the results of Example 5.3, 0.0011 inches of storage is required for water quality using retention; from Example 5.2, 0.19 inches of storage is required to maintain the runoff volume using retention. Since the storage required to maintain the volume is larger, in this case 0.19 inches over the site should be reserved for retention BMPs using LID measures.
Step 3: Determine storage volume required to maintain peak storm water runoff rate using 100 percent retention.
The amount of storage required to maintain the pre-development peak runoff rate is based on the chart in Appendix C. This chart is based on the relationship between storage volume, ∀ s
∀ r
, and discharge,
Q o
Q i
, to maintain the pre-development peak runoff rate.
Where:
∀ s
= volume of storage to maintain t he predevelop ment peak runoff rate using
100% retention;
∀ r
= post development peak runoff volume;
Q o
= peak outflow discharge rate; and
Q i
= peak inflow discharge rate.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Step 1
Determine storage volume required to maintain runoff volume or CN.
Use Chart Series in Appendix B: Storage Volume Required to Maintain the Pre- development Runoff Volume Using Retention Storage (Example 5.2).
Step 2:
Determine storage volume for water quality volume requirements.
Determine storage volume required for quality control BMPs. Use larger of volumes to maintain CN (Step 1, Example 5.2) or water quality volume
(Example 5.3).
Step 3:
Determine storage volume required to maintain predevelopment peak runoff rate using 100% retention . Use Chart Series in Appendix C: Storage Volume Required to
Maintain the Predevelopment Peak Runoff Rate Using 100% Retention.
Step 4:
Determine whether additional detention storage is required to maintain predevelopment peak runoff rate . Compare the results of Steps 1 and 2 to the results of
Step 3. If the storage volume in Steps 1 and 2 is determined to be greater than that in
Step 3, the storage volume required to maintain the predevelopment CN also controls the peak runoff rate. No additional detention storage is needed. If the storage volume in
Step 1 is less than that in Step 3, additional detention storage is required to maintain the peak runoff rate (Example 5.4).
Step 5 (use if additional detention storage is required):
Determine storage volume required to maintain predevelopment peak runoff rate using 100% detention . Use Chart Series in Appendix D: Storage Volume Required to
Maintain the Predevelopment Peak Runoff Rate Using 100% Detention. This is used in conjunction with the Chart Series in Appendices B and C to determine the hybrid volume in Step 6.
Step 6 (use if additional detention storage is required):
Hybrid approach . Use results from Chart Series in Appendices B, C and D to determine storage volume to maintain both the predevelopment peak runoff rate and runoff volume. Refer to Equations 5.5 and 5.6 as found in Example 5.4.
Step 7 (use if additional detention storage is required):
Determine appropriate storage volume available for retention practices . If the storage volume available for retention practices is less than the storage determined in
Step 3, recalculate the amount of BMP area required to maintain the peak runoff rate while attenuating some volume using the procedure in Example 5.6 using Equations 5.7 and 5.8.
Figure 5.9. Procedure to Determine Storage Volume Required for BMPs to Maintain Pre-Development
Volume and Peak Flow Rate.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Example 5.2: Determine Site Area Required to Maintain Volume (CN) Using Chart
Series in Appendix B: Storage Volume Required to Maintain the Pre-development
Runoff Volume Using Retention Storage
Given:
Site Area is 18 acres
Existing CN is 60
Proposed CN is 65
Design storm is 3.12 inches (2-year, 24-Hour Event)
Design depth of BMP is 6 inches
Solution: Use Chart Series in Appendix B: Storage Volume Required to Maintain
Runoff Volume or CN.
0.19” of storage over the site is required to maintain the runoff volume.
Therefore: 0.57 acres (18 acres x 0.19 / 6 ) of BMPs distributed evenly throughout the site are required to maintain the runoff volume, or CN.
Additional Considerations:
Account for depths other than 6 inches (using Equation 5.3):
Site BMP Area = 0.57 acres if 6” depth is used
Depth of BMPs = 4”
Size of BMP Area = 0.57 x 6”/4”
Size of BMP Area = 0.855 acres
Account for infiltration and/or evapotranspiration (using Equation 5.4)
If 10% of the storage volume is infiltrated and/or reduced by evaporation and transpiration.
BMP Area = (storage volume) x (100 – x)
/100
BMP Area = 0.57 x (100-10)/100
Area of BMP = 0.51 acres
Example 5.3: Calculation of Volume for Water Quality Control
Given: Water Quality Rainfall = 1.0 inch
Propose CN = 65
Solution:
Runoff Depth , Q
=
(
P
P
−
+
0 .
2 S
)
2
0 .
8 S
Q = 0.0011 inches
Where P
=
1 .
0 inch , and S
=
(
1000 / CN
)
−
10
0.19 inches (from Example 5.2) is more than 0.0011 inches for the Water Quality
Control, therefore use the volume of storage for Runoff Volume Control to also meet
Water Quality requirement.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Qo
Qi
(
(
Peak
Peak
Outfall
Inflow
Discharge
Discharge
Rate
Rate
)
)
Figure 5.10. Comparison of Retention of Storage Volumes Required to Maintain Peak Runoff
Rate Using Retention and Detention
The relationship for retention storage to control the peak runoff rate is similar to the relationship for detention storage. Figure 5.10 is an illustration of the comparison of the storage volume/ discharge relationship for retention and detention. Curve A is the relationship of storage volume to discharge to maintain the pre-development peak runoff rate using the detention relationship from Figure 6.1 of the TR-55 Manual (SCS, 1986) for a Type II 24-hour storm event. Curve B is the ratio of storage volume to discharge to maintain the pre-development peak runoff rate using
100 percent retention. Note that the volume required to maintain the peak runoff rate using detention is less than the requirement for retention. This is graphically demonstrated in Figure
5.11.
• For hydrograph 2, refer to Figure 5.1 for description.
• For hydrograph 10, refer to Figure 5.8 for description.
∀
1 is the storage volume required to maintain the pre-development peak ratio using 100% detention storage. The combination of
∀
1 and
∀
2 is the storage volume required to maintain the pre-development peak discharge rate using 100% retention storage. The following calculations apply to Chart Series in Appendix C:
• The Tc for the post-development condition is equal to the Tc for the pre-development condition. This equality can be achieved by techniques such as maintaining sheet flow lengths, increasing surface roughness, decreasing the amount and size of storm drain pipes, and decreasing open channel slopes. Section 5.4.2 provides more details on these techniques.
• The depth of storage for the retention structure is 6 inches. For other depths, see Example
5.2.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Figure 5.11. Storage Volume Required to Maintain Peak Runoff Rate
If the Tc is equal for the pre-development and post-development conditions, the peak runoff rate is independent of Tc for retention and detention practices. The difference in volume required to maintain the pre-development peak runoff rate is practically the same if the Tc’s for the predevelopment and post-development conditions are the same. These concepts are illustrated in
Figure 5.12. In Figure 5.12 note that the difference in the required BMP area between a Tc of
0.5 and a Tc of 2.0 is minimal if the pre-development and post-development Tc’s are maintained.
Figure 5.12. Comparison of Storage Volumes for Various Tcs
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Step 4: Determine whether additional detention storage is required to maintain the predevelopment peak runoff rate. The storage volume required to maintain the pre-development volume using retention, as calculated in Step 1, might or might not be adequate to maintain both the pre-development volume and peak runoff rate. As the CNs diverge, the storage requirement to maintain the volume is much greater than the storage volume required to maintain the peak runoff rate. As the CNs converge, however, the storage required to maintain the peak runoff rate is greater than that required to maintain the volume. Additional detention storage will be required if the storage volume required to maintain the runoff volume (determined in Step 1) is less than the storage volume required to maintain the pre-development peak runoff rate using 100 percent retention (determined in Step 3). The combination of retention and detention practices is defined as a hybrid BMP. The procedure for determining the storage volume required for the hybrid approach is described in Step 5. Table 5.6 illustrates the percentage of site area required for volume and peak control for representative curve numbers. For example, using a 5-inch
Type II 24-hour storm event, with a pre-development CN of 60, the following relationships exist:
• For a post-development CN of 65, 5.9 percent of the site area (column 4) is required for retention practices to maintain the pre-development volume. To maintain the predevelopment peak runoff rate (column 5), 9.5 percent of the site is required. Therefore, additional detention storage or a hybrid approach (calculated in column 7) is required.
• For a post-development CN of 90, 42.9 percent of the site area (column 4) is required for retention practices to maintain the pre-development volume. To maintain the predevelopment peak runoff rate (column 5) 37.2 percent of the site is required. Therefore, the storage required to maintain the runoff volume is also adequate to maintain the peak runoff rate. However, 42.9 percent of the site for BMPs is not a practical and reasonable use of the site. Refer to Step 7, hybrid approach, for a more reasonable combination of retention and detention storage.
Step 5: Determine storage required to maintain pre-development peak runoff rate using 100 percent detention. (This step is required if additional detention storage is needed.) Charts contained in Appendix D: Storage Volume Required to Maintain the Pre-development Peak
Runoff Rate Using 100% Detention are used to determine the amount of storage to maintain the peak runoff rate only. This information is needed to determine the amount of detention storage required for hybrid design, or where site limitations prevent the use of retention storage to maintain runoff volume. This includes sites that have severely limited soils for infiltration or retention practices. The procedure to determine the site area is the same as that of Step 3. Using chart contained in Appendix D, the following assumptions apply:
• The Tc for the post-development condition is equal to the Tc for the pre-development condition.
• The depth of storage for the detention structure is 6 inches. For other depths, see Example
5.2.
• The storage volume, expressed as a depth in hundredths of an inch, is for peak flow control.
These charts are based on the relationship and calculations from Figure 5.1 (Approximate
Detention Basin Routing for Rainfall Types I, IA, II and III) in TR-55 (SCS, 1986).
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Table 5.6. Representative Percentages of Site Required for Volume and Peak Control
Type of
24-Hour
Storm
Event
(1)
Runoff Curve No.
Existing
(2)
% of Area Needed for BMP Hybrid
Design
(Eq. 5.6)
(7)
3”
5”
7”
50
60
70
75
50
60
70
75
50
60
70
75
90
80
85
90
90
75
80
85
60
65
70
80
65
70
75
75
80
85
90
65
70
75
90
80
85
90
55
60
65
70
80
80
85
90
55
70
75
90
75
80
85
90
Proposed
(3)
55
60
65
70
80
65
53.7
8.9
17.9
27.2
36.7
9.1
18.4
27.9
15.6
23.9
32.5
50.5
8.3
16.9
25.8
5.9
12.3
19.1
42.9
6.9
14.3
22.2
30.7
7.4
15.3
23.8
7.6
Volume Control
Using 100%
Retention
Appendix B
(4)
1.7
4.0
6.9
10.4
19.3
2.9
4.8
10.5
17.1
4.8
10.1
16.0
22.4
36.7
6.3
10.5
27.5
4.1
8.9
14.6
21.2
49.7
20.4
26.8
33.4
42.3
22.1
28.6
35.3
18.6
25.0
31.4
44.5
16.6
23.2
29.9
9.5
14.6
19.8
37.2
13.2
18.9
24.5
30.5
15.0
20.6
26.7
12.3
Peak Control
Using 100%
Retention
Appendix C
(5)
1.6
3.4
6.2
9.3
18.0
3.9
7.5
11.8
16.6
6.9
11.1
15.6
20.6
32.8
6.7
10.0
24.9
5.9
9.7
13.9
18.7
30.7
10.9
14.7
18.9
23.0
11.5
15.6
19.8
10.7
15.1
19.6
30.0
9.0
13.2
17.3
5.3
8.4
12.0
25.3
7.2
10.7
14.3
18.2
8.1
11.6
15.2
6.8
Peak Control
Using 100%
Detention
Appendix D
(6)
0.9
2.4
4.5
7.3
15.8
2.3
4.2
7.0
10.2
4.0
6.9
10.4
14.5
23.9
4.4
7.1
18.7
3.4
5.8
8.8
12.6
100
55
75
87
94
53
73
85
88
97
100
100
61
80
90
71
88
97
100
63
82
93
100
73
91
100
77
93
100
100
100
60
81
92
71
Percent of
Volume
Retention for Hybrid
Design
(Eq. 5.5)
(8)
100
100
100
100
100
80
96
100
100
77
94
100
100
8.3
13.9
19.6
42.9
10.9
17.4
23.8
30.7
6.6
11.4
17.1
6.3
10.9
16.0
22.4
36.7
12.3
18.9
25.7
10.7
1.7
4.0
6.9
10.4
19.3
3.6
6.6
10.5
27.5
5.3
9.5
14.6
21.2
53.7
16.1
23.8
31.5
39.2
17.1
25.1
32.9
17.7
24.7
32.5
50.5
13.6
21.2
28.7
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Step 6: Use hybrid facility design (required for additional detention storage).
When the storage volume for peak control exceeds that for volume control as determined in Step 3, a hybrid approach must be used. For example, a dry swale (infiltration and retention) may incorporate additional detention storage. Equation 5.5 is used to determine the ratio of retention to total storage. Equation 5.6 is then used to determine the additional storage volume, above the storage volume required for runoff volume control, needed to maintain the pre-development peak runoff rate.
Equation 5.5. x
=
(
∀
R 100
50
− ∀
D 100
)
×
(
−∀
D 100
+ ∀ 2
D 100
+
4
×
(
∀
R 100
− ∀
D 100
)
× ∀
R ) where
∀
R = Storage volume required to maintain pre-development runoff volume (see Chart Series in Appendix B)
∀
R100
= Storage volume required to maintain pre-development peak runoff rate using 100% retention (see Chart Series in Appendix C)
∀
D100 = Storage volume required to maintain pre-development peak runoff rate using 100% detention (see Chart Series in Appendix D) x = Area ratio of retention storage to total storage
Equation 5.6.
H =
∀
R x (100 ÷ x ) where:
H = Hybrid storage volume
Equations 5.5 and 5.6 are based on the following assumptions:
• x% of the total storage volume is the retention storage required to maintain the predevelopment CN calculated from the Chart Series in Appendix B: Storage Volume Required to Maintain Pre-development Volume using Retention Storage.
• There is a linear relationship between the volume of storage required to maintain the peak pre-development runoff rate using 100% retention and 100% detention (Chart Series in
Appendices C and D) .
The procedure for calculating hybrid facilities size is shown in Example 5.4.
Example 5.4: Calculation of Additional Storage Above Volume Required to
Maintain CN and Maintain Pre-development Peak Runoff Rate Using Hybrid
Approach
Given:
• 3.12 -inch Storm Event (2-year, 24-hour event)
• Existing CN = 60
• Proposed CN = 65
• Storage volume required to maintain volume (CN) using retention storage =
0.19 inches (from Chart Series in Appendix B)
• Storage volume required to maintain peak runoff rate using 100% retention =
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
0.275 inches (from Chart Series in Appendix C)
• Storage of site required to maintain peak runoff rate using 100% detention =
0.14 inches (from Chart Series in Appendix D)
Step 1: Solve for x (ratio of retention to total storage) using Equation 5.5: x
=
(
0 .
275
50
−
0 .
14
)
× −
(
0 .
14
+
0 .
14
2 +
4
(
0 .
275
−
0 .
14
)
×
0 .
19
) x = 77.62
Therefore: 0.19 inches of storage needed for runoff volume control is 77.62% of the total volume needed to maintain both the pre-development volume & peak runoff rates.
Step 2: Solve for the volume to maintain both the peak runoff rate and volume using
Equation 5.6.
H = 0.19 x 100 / 67
H= 0.24 inches
Therefore, the difference between 0.19 inches and 0.24 inches is the additional volume needed to maintain peak discharge using 100% retention.
Step 7: Determine hybrid amount of BMP site area required to maintain peak runoff rate with partial volume attenuation using hybrid design (required when retention area is limited).
Site conditions, such as high percentage of site needed for retention storage, poor soil infiltration rates, or physical constraints, can limit the area of the site that can be used for retention practices.
For poor soil infiltration rates, bioretention is still an acceptable alternative, but an underdrain must be installed. In this case, the bioretention basin is considered detention storage. When this occurs, the amount available for retention BMPs is less than that required to maintain the volume, or CN. A variation of the hybrid approach is used to maintain the peak runoff rate while attenuating as much of the increased runoff volume as possible. First, the appropriate storage volume available for volume control
( )
is determined by analyzing the site constraints.
Equation 5.7 is used to determine the ratio of retention to total storage. Equation 5.8 is then used to determine the total site BMP area in which the storage volume available for retention practices
(
∀
R’) is substituted for the storage volume required to maintain the runoff volume.
Equation 5.7
X
′
=
(
∀
R 100
50
− ∀
D 100
×
(
−∀
D 100
+ ∀ 2
D 100
+
4
×
(
∀
R 100
− ∀
D 100
)
× ∀
R
′
)
) where:
∀
R = storage volume acceptable for retention BMPs. The total storage with limited retention storage is:
Equation 5.8
H ′ = ∀ R ′ x (100 ÷ X ′ )
H’= Hybrid area, with a limited storage volume available for retention BMPs.
Example 5.5 illustrates this approach.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Example 5.5: Calculation of Storage Volume Required to Maintain the Peak Runoff
Rate Using the Hybrid Approach of Retention and Detention
Given:
• 3.12-inch storm event (2-year, 24-hour event)
• Existing CN = 60
• Proposed CN = 65
• Storage volume required to maintain volume (CN) = 0.19” (from Appendix B)
• Storage volume required to maintain peak runoff rate using 100% retention = 0.275”
(from Appendix C)
• Storage volume required to maintain peak runoff rate using 100% detention = 0.14”
(from Appendix D)
• Only half of the required site area is suitable for retention practices, remainder must incorporate detention.
• (R’=0.19 x 0.50 = 0.095)
Step 1: Determine appropriate amount of overall BMP volume suitable for retention practices. Half of area is appropriate (given above). Use Equation 5.7: x x
'
=
(.
275
50
−
46 .
77 %
.
14 )
×
(
−
.
14
+
.
14
2 +
4
×
(.
275
−
.
14 )
×
0 .
095
)
'
=
Therefore, 0.095 inches of storage available for runoff volume control on the site is 46.77% of total volume needed for maintaining the pre-development peak runoff rate.
Step 2: Solve for the total storage required to maintain the peak runoff rate using Equation
5.8. Solve for H’
Η′ =
0 .
095
100
46 .
77
Η′ =
0 .
20 inches
Therefore, 0.20” of storage is required to maintain predevelopment peak runoff rate but not runoff volume. Of the 0.20” of storage, 0.095” is required for retention volume.
5.5.6 Determination of Design Storm Event
Conventional storm water management runoff quantity control is generally based on not exceeding the pre-development peak runoff rate for the 2-year and 10-year 24-hour Type II storm events. The amount of rainfall used to determine the runoff for the site is derived from
Technical Paper 40 (Department of Commerce, 1963). For Mecklenburg County, these rainfall amounts are 3.12 and 4.80 inches, respectively. The 2-year storm event was selected to protect receiving channels from sedimentation and erosion. The 10-year event was selected for adequate flow conveyance considerations. In situations where there is potential for flooding of structures,
44

Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012 the 100-year event is used. For LID the design storm event shall be the 2-year, 24-hour storm.
Post-developed peak and volume shall not exceed the pre-developed peak and volume.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
6.1 Introduction
In Section 5, the required runoff storage volume, expressed in hundredths of an inch, and the type of best management practice (BMP) required for its control (i.e., retention, detention, or hybrid) were determined through a systematic hydrologically-based process. Section 5 explored site planning considerations for maintaining the curve number (CN) and time of concentration
(Tc), which can be used to achieve the objectives of Section 5. This Section builds upon the planning strategies presented in Sections 4 and 5 by providing considerations key to the proper design of the BMPs. The Section begins with a definition and description of the functions provided by various types of LID BMPs that are available to the designer and concludes by outlining some consideration for their design.
6.2 LID BMPs and Their Functions
LID BMPs are sited and designed to provide the following:
• Site hydrologic source controls for Tc, CN, and ground water recharge.
• Small common area or on-lot BMPs that are more easily maintained.
• An added aesthetic value.
• Multiple use of landscaped areas. In some cases, the on-lot or commercial hydrologic control also can satisfy the Town of Huntersville’s requirements for green or vegetated buffer space.
LID site design techniques and BMPs can be organized into four major categories, as follows:
• Pollution prevention practices that contribute to the overall improvement of water quality.
• Runoff prevention measures designed to minimize impacts that would otherwise change the pre-development curve number (CN) and time of concentration (Tc).
• Retention facilities that store runoff and have no positive outlet, or release, for the design storm event. Excess runoff is infiltrated, exfiltrated, or evaporated.
• Detention facilities that temporarily store a portion of the increased runoff volume and release it through a measured outlet.
LID BMPs shall be applied in the following order of preference:
Step One: Incorporate runoff reduction BMPs into the design to minimize changes in the site hydrology, which result from changes in the CN and Tc .
Step Two: Meet site storm water quality treatment objectives through the use of retention/detention practices.
Step Three: Use retention BMPs, to the greatest extent possible, for storm water management volume and peak flow rate control, using the procedures from Section 5.
Step Four: Use detention and hybrid BMPs if required.
Step Five: Use pollution prevention through source reduction and elimination.
Tables 6.1 and 6.2 provide a list of BMPs approved for use to satisfy the Huntersville Ordinance as well as the applicable zones for use of the BMPs in Huntersville (Urban, Transitional, Rural), the section of the Performance Criteria (see Section 3) they are best suited to satisfy and whether they serve a water quality or volume control function (WQ or VC). These lists are not all
46

Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012 inclusive. New BMPs will be reviewed and adapted on a case-by-case basis by the Mecklenburg
County Water Quality Program as they are made available. In addition, BMPs placed in series in the form of a treatment train or modifications to BMP designs can be developed to combine both retention and detention storage. Placing BMPs in treatment trains will provide for the maximum on-lot storm water runoff control (i.e., maximum mitigation of site development impacts on CN and Tc). This type of design BMP is known as a “hybrid.” Hybrid BMPs can be used to increase the post-development Tc. Table 6.1 gives each LID BMP type and its primary function.
The design source for all the BMPs in Tables 6.1 and 6.2 is the Charlotte-Mecklenburg BMP
Design Manual developed by Charlotte-Mecklenburg Storm Water Services.
Table 6.1. Approved LID BMPs for Use in Huntersville
BMP
Applicable
Zoning Districts
U, T, R
(1)
Applicable
Performance
Criteria (2)
3(a)
Designs &
Specifications
(3)
Function (4)
(WQ, VC, PC)
Strategic Clearing & Grading
Reduce Impervious Surfaces
Bioretention (Rain Garden)
Infiltration Trench
Enhanced Grass Swale
Filter Strip/Wooded Buffer Strip
Dry Well, Cistern & Rain barrel
Curb & Gutter Elimination
Rooftop Storage
<6% BUA for Entire Site (5)
Sand Filter (6)
U, T, R
U, T, R
U, T, R
U, T, R
U, T, R
U, T, R
R
U, T, R
U, T, R
U, T, R
3(a)
3(a), 3(b)
3(a), 3(b)
3(a)
3(a)
3(b)
3(b)
3(b)
All
3(a), 3(b)
Chapter 4.1
Chapter 4.6
Chapter 4.4
Chapter 4.7
Chapter 4.8
WQ, VC, PC
WQ, VC, PC
WQ, VC, PC
WQ, VC, PC
WQ, VC
WQ, PC
WQ, VC, PC
WQ, PC
VC, PC
WQ, VC, PC
WQ, VC, PC
(1) Applicable Zoning Districts: These are the Zoning Districts where the BMP can be used including:
T = Transitional; R = Rural; and U = All other zones EXCEPT transitional and rural.
(2) Applicable Performance Criteria: These are the Performance Criteria Section numbers (see Section 3) that the
BMP can be used to satisfy.
(3) Designs & Specifications: All BMP designs and specifications are contained in the Charlotte-Mecklenburg BMP
Design Manual in the specific chapter indicated in the above table.
(4) Functions: These are the dominate functions that the BMPs perform including: WQ = Water Quality; VC =
Volume Control, PC = Peak Control.
(5) Less than six (6) percent built upon area (BUA) for the entire development site along with grass channels will satisfy the requirements of the Ordinance.
(6) Sand Filter: To be considered an LID BMP, the sand filter must be above ground with a native soil bottom that has been scarified and not compacted. A double-ringed infiltrometer test of the bottom must show infiltration capabilities. In addition, the under drain must be designed to create a one-foot internal water storage layer above the bottom. Sand filters not meeting these criteria will be considered Conventional Storm Water BMPs.
Table 6.2. Conventional Storm Water BMPs
Applicable Zoning
BMP
Wet Pond
Extended Dry Pond
Districts (1)
U, T, R
T, R
Storm Water Wetland T, R
Applicable Performance
Criteria (2)
3(b), 3(d)
3(b), 3(d)
3(b)
Designs &
Specifications (3)
Chapter 4.2
Chapter 4.9
Chapter 4.3
Function (4)
(WQ, VC, PC)
WQ, VC, PC
VC, PC
WQ, VC, PC
(1) Applicable Zoning Districts: These are the Zoning Districts where the BMP can be used including:
T = Transitional; R = Rural; and U = All other zones EXCEPT transitional and rural.
(2) Applicable Performance Criteria: These are the Performance Criteria Section numbers (see Section 3) that the
BMP can be used to satisfy.
(3)
Designs & Specifications: All BMP designs and specifications are contained in the Charlotte-Mecklenburg BMP
Design Manual in the specific chapter indicated in the above table.
(4)
Functions: These are the dominate functions that the BMPs perform including WQ = Water Quality; VC =
Volume Control, PC = Peak Control.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
The functions listed in Table 6.1 are subdivided into two major categories as follows:
• Water Quality: A combination of LID and conventional BMPs (as listed in Tables 6.1 and
6.2) is allowed in the form of a treatment train or hybrid design provided the conventional
BMPs meet the optimal design threshold and the treatment train meets a minimum pollutant removal efficiency of 85% total suspended solids and 70% total phosphorus in accordance with the specifications contained in Section 4 of the Charlotte-Mecklenburg BMP Design
Manual. BMPs must be designed to control and treat runoff from the first inch of rainfall.
• Volume Control: The LID BMPs listed in Table 6.1 or a combination of LID and conventional storm water management practices as listed in table 6.2 can be used to satisfy the volume and peak runoff rate control (See Performance Criteria described in Sections 3(b) and 3(d) of this manual.) Section 3(c) requires that whenever any storm water BMP employs the use of a temporary water quality storage pool as a part of its treatment system, the drawdown time must be a minimum of 48 hours and a maximum of 120 hours. Section 3(e) requires that no one BMP can receive runoff from an area greater than five (5) acres.
However, the total drainage area from BMPs in series (i.e., integrated) can exceed this five
(5) acre maximum.
Some approaches are used to convey storm water while minimizing or eliminating erosion potential. For example, a swale can be designed to detain, retain, and control runoff velocities simply by altering its geometry and slope and adding an infiltration trench beneath the swale and a weir control structure at the inlet of the culvert beneath the driveway (see Figure 6.1). The implication of this is that velocities are reduced and particulate matter is infiltrated or filtered from the runoff prior to its delivery to a particular BMP or downstream receiving water body.
Some conveyance techniques also can be designed specifically for runoff water quality treatment if that objective is considered during the concept plan design.
6.2.1 “Helpful Hints” for the Proper Design and Installation of BMPs
The following is a list of “Helpful Hints” that are oftentimes overlooked but which are very important for ensuring the proper design and installation of BMPs:
1.
All plans and details for BMPs must be drawn to scale for clarity and to aid in construction.
Drawings not to scale often lead to construction problems and field errors, or simply can not be built. Standard NCDOT details do not require a specific scale.
2.
Do not assume the final elevations based on contour maps as their accuracy is only 1/2 the contour interval. Use in-field elevations to set final grades for BMPs and flared end sections.
3.
All bioretention plans must contain sufficient contours (1 foot increment) and spot elevations to accurately construct in the field.
4.
Make sure you have shown roof leaders that may directly connect to bioretention BMPs.
These must discharge as sheet flow into the bioretention areas and not as a point source.
Care must be taken to show the end invert of these pipes as they will control the final elevation of the mulch layer.
5.
Clearly show all elevations on BMP plans, cross sections and details that impact the design of the BMP. Show all inverts, top of mulch, top of berm, spillway etc. elevations to insure that proper construction can be accomplished.
6.
The required surface area for the BMP must be clearly shown on the plan in square feet and graphically depicted for clarity.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
7.
The biggest threat to the functionality of a bioretention area is sediment, which will clog the soil media and prevent infiltration. Take great care to prevent sediment from entering the bioretention area by surrounding it with silt fence and ensuring that the land draining to the area has been stabilized. Additional information regarding erosion and sediment control is provided in Section 7.
8.
It is extremely important to properly dissipate the flow entering a bioretention area through the use of level spreaders as discussed in Section 6.5.
Figure 6.1. Typical Swale Design
6.3 Runoff Reduction BMPs
Runoff reduction BMPs are site development practices that are implemented to reduce changes in storm water runoff volume and peak flow rate. This is accomplished by minimizing changes to pre-development land cover, reducing impervious surfaces, and preserving trees, especially those found on pervious soils. Runoff reduction BMPs are the first line of defense to reduce changes in existing land cover and to disconnect “effective” impervious surfaces. These runoff reduction BMPs may be the least expensive way to minimize CN impacts and maintain existing
Tc. By including these techniques, the need for retention and detention BMPs to mitigate development hydrologic impacts is reduced. A brief description of these techniques/practices is provided below.
49

Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Strategic clearing and grading practices are the most effective method of reducing storm water quantity and erosion impacts on downstream receiving waters and aquatic habitat. Minimizing clearing and grading within forested and densely vegetated areas is the most efficient method of reducing changes to the CN. This is due to the natural dispersion and infiltration function of forests and densely vegetated areas. It is possible to increase the post-development Tc by flattening the slopes, increasing the flow path, maximizing sheet flow, and increasing surface roughness on cleared areas.
Vegetated buffers are strips of vegetation, either natural or planted, around sensitive areas, such as water bodies, wetlands, or highly erodible soils. In addition to protecting sensitive areas, vegetated strips help to reduce storm water runoff impacts by trapping sediment and sedimentbound pollutants, encouraging infiltration, and slowing and dispersing storm water flows over a wide area.
Proper landscaping is one method of mitigating the hydrologic impacts of clearing and grading.
In some cases, a large portion of the site will have to be cleared and graded, resulting in the loss of much woody vegetation and debris. A carefully designed landscaping plan can be used to reestablish some of the vegetative functions lost during this process. Heavily revegetated areas can improve sediment removal, infiltration, and community aesthetics.
Curb elimination or flat curbs addresses both quantity as well as water quality functions. When curbs are removed flattened or depressed, site imperviousness is disconnected by allowing storm water runoff, normally conveyed along the gutter and discharged directly into a storm drain BMP or receiving water body, to be dispersed to vegetated buffer areas or roadside swales. This process helps to minimize CN impacts, increase or maintain the TC, and filter pollutants leaving a given site.
Changes in CN due to clearing and grading and the addition of impervious surfaces result in changes in storm water runoff volume and peak flow rate. Both the pre- and post-development
CNs must be determined to evaluate the magnitude of change and potential requirement for retention and detention storage volume. By reducing the change in CN, the requirement for onsite BMP storage area is reduced. Calculation of the post-development CN should be based on prototypical lot layouts that represent the LID design techniques that will be incorporated into the development.
6.4 Retention BMPs
LID BMPs are used to satisfy the water quality and storage volume requirements (Sections 3(a) and 3(b)) calculated in Section 5. They are the preferred method because they maintain the predevelopment runoff volume. Therefore, they are the most effective method for managing the hydrologic regime. In the hydrologic analysis presented in Section 5, storage volume for retention is first calculated to maintain pre-development runoff volume and peak flow rate.
These BMPs are sized to retain 100% of the excess post-development volume; however, under some circumstances additional detention storage will be required. Ideally, the design goal is to locate retention BMPs at the source on level ground within individual lots of the development.
LID retention BMPs include standard infiltration-type facilities. They typically are designed and located to provide retention controls for very small drainage areas serving up to 2 to 5 acres.
Retention BMPs that are applicable to LID include multifunctional landscape areas, bioretention facilities, dry wells, and roof runoff controls such as rain barrels and cisterns.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
In most cases, LID on-lot or common open space retention storage will be feasible for storm water management in zones which specify ½-acre lots and larger. However, situations may arise where the magnitude of change in pre-development and post-development CNs results in on-lot storage requirements that cannot be accommodated within available lawn or open space area.
Therefore, in these cases, the use of runoff reduction BMPs is important. In the case of commercial and industrial zoned properties, the minimum internal landscaping requirements plus the size of perimeter buffers and perimeter parking landscape will dictate the feasibility of providing all required storage within surface swales or bioretention facilities. (Refer to the Town of Huntersville’s Zoning Ordinance and landscape manual for specific requirements). The following provides a brief description of retention BMPs.
• Bioretention: Bioretention BMPs (also called Rain Gardens) are applicable as retention facilities for treatment of up to 5 acres. Typically they are designed to mimic forested BMPs that naturally control hydrology through infiltration and evapotranspiration. They are especially suited to residential and commercial areas where additional landscaping can provide aesthetic benefits. Figure 6.2 shows a typical plan and detail for a bioretention cell.
Design Considerations: In addition to design criteria currently listed in various State and local manuals, the following design considerations should be followed for LID sites.
Bioretention areas should include an upstream pretreatment system. The type of pretreatment used depends on the type of land use in the upstream service area. It is recommended that at a minimum bioretention areas incorporate vegetated filter strips as pretreatment devices. In commercial areas where space is limited, parking area sweeping is recommended as a pretreatment practice. It is further recommended that bioretention areas be placed “off-line.” The first flush should be diverted to the BMP and those flows in excess of the first inch of rainfall should be bypassed. Make sure you have shown roof leaders that directly connect to bioretention areas. These must discharge as sheet flow into the bioretention areas and not as a point source. Care must be taken to show the end invert of these pipes as they will control the final elevation of the mulch layer.
• Dry Wells: Dry wells are small excavated trenches backfilled with stone. Dry wells function as infiltration BMPs used to control runoff from building rooftops. Figure 6.3 shows a typical detail for a dry well.
Design Considerations: Dry wells should infiltrate the design storm within three (3) days from the beginning of the storm. Dry wells should be located close to runoff sources.
However, they must be located a sufficient distance away on the down slope side of the structure and from building foundations to prevent seepage into basements. The designer must make sure to site these facilities away from slopes >20%, particularly when the slope consists of fill materials over native ground. The designer must also ensure that the overflow is directed to the downslope bioretention BMPs, swales, or other management areas in a nonerosive fashion. Conveyance areas should be well vegetated and have slopes of <5%, or appropriately sized riprap should be used.
• Cisterns: Storm water runoff cisterns are roof water management devices that provide retention storage volume in underground storage tanks. On-lot storage and later reuse of storm water also provides an opportunity for water conservation and the possibility of reducing water utility bills.
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PARKING LOT SHEET FLOW
OPTIONAL
LEVEL
SPREADER
Cleanout
OVERFLOW “CATCH
BASIN” SET GRATE 6”
ABOVE GRADE
SHEET FLOW
OPTIONAL LEVEL SPREADER
FILTER
FABRIC
MEASURE DEPTH OF SOIL MIX FROM
TOP OF STONE OVER UNDERDRAIN
TYPICAL SECTION
1’ PONDING
3” OF MULCH
2’ TO 4’ OF
CONDITIONED
SOIL
(DEPENDING ON
DESIGN)
FILTER FABRIC ON
BOTTOM (NOT SIDES)
6” UNDERDRAIN
IN 12” - # 57 STONE
4’ MIN.
Figure 6.2. Typical Bioretention BMP
FILTER FABRIC TOTALLY WRAPPED AROUND
STONE COVERING UNDERDRAIN
6” UNDERDRAIN
PROFILE
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Figure 6.3. Typical Dry Well
Design Considerations: Cisterns are applicable to residential, commercial, and industrial
LID sites. Due to the size of rooftops and the amount of imperviousness of the drainage area, increased runoff volume and peak discharge rates for commercial or industrial sites may require larger capacity cisterns. Individual cisterns can be located beneath each downspout, or storage volume can be provided in one large, common cistern. Cisterns should be located where readily accessible in case maintenance or replacement becomes necessary.
• Rain Barrels: Rain barrels are low-cost, effective, and easily maintainable retention devices applicable to both residential and commercial/industrial LID sites. Rain barrels operate by retaining a predetermined volume of rooftop runoff (i.e., they provide permanent storage for a design volume); an overflow pipe provides some detention beyond the retention capacity of the rain barrel. Figure 6.4 shows a typical rain barrel. Rain barrels also can be used to store runoff for later reuse in lawn and garden watering. However, water from rain barrels is not to be used for potable use.
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Scale: Not to Scale
Figure 6.4. Typical Rain Barrel
Design Considerations: Rain water from any type of roofing material may be directed to rain barrels. To be aesthetically acceptable, rain barrels can be incorporated into the lot’s landscaping plans, or patio or decking designs. Rain barrels placed at each corner of the front side of the house should be landscaped for visual screening. Rain barrels are not allowed within a public right-of-way nor within the required setback, required side yard, or within five (5) feet of an established rear yard as regulated by the Huntersville Zoning
Ordinance. Since all rain barrels will be located on private property, they must be included in any private storm water maintenance agreement. Gutters and downspouts are used to convey water from rooftops to rain barrels. Filtration screens should be used on gutters to prevent clogging of debris. Rain barrels should be designed so that complete draining of the
BMP is possible. Rain barrels should also be equipped with a drain spigot that has garden hose threading, suitable for connection to a drip irrigation BMP. An overflow adapter could be used to connect two or more barrels to divert surplus water away from foundations. An overflow outlet must be provided to bypass runoff from large storm events. Rain barrels must be designed with removable, child-resistant covers and mosquito screening on water entry holes. The size of the rain barrel is a function of the rooftop surface area that drains to the barrel, as well as the inches of rainfall required for retention storage. For example, one
42-gallon barrel provides 0.5 inch of runoff storage for a rooftop area of approximately 133 square feet.
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6.5 Detention BMPs
LID detention BMPs are used to satisfy the detention storage volume requirement calculated in
Section 5. As noted in Section 5, the storage required to maintain the pre-development volume using retention might not be adequate to maintain both the pre-development volume and peak flow rate. Therefore, additional detention storage is required in the form of LID detention
BMPs. For LID sites, detention BMPs are preferably used in combination with retention BMPs.
The ratio of retention storage to detention storage is calculated in Section 5.5.5, Step 6.
Detention structures store the volume and then release the discharge through a control structure at the pre-development rate. Some water quality benefit may be realized as pollutants settle out during detention. The SCS Rational Method is used to determine the discharge in order to size the outlet. LID detention BMPs include filter strips, grassed swales, level spreaders, and rooftop storage. (Refer to Table 6.1 for a complete listing.) The following are brief descriptions of some LID detention BMPs:
• Filter Strips: Filter strips are typically bands of close-growing vegetation, usually grass, planted between pollutant source areas and a downstream receiving waterbody. They also can be used as outlet or pretreatment devices for other storm water control practices. For
LID sites, a filter strip should be viewed as only one component in a storm water management BMP. Filter strips remove pollutants and help mitigate concentrated peak flows.
Design Considerations: LID filter strips should be planted in combination with existing natural vegetation to meet the needed filter width. Usually the minimum width for grassed filter areas is 15 feet, while that of wooded areas is 35 feet. Depending on specific site conditions, filter areas may be as wide as 150 feet. The width is based on a required detention. Filter strips function best when they are level in the direction of storm water flow toward the receiving water. This orientation creates proper sheetflow through the strip, increasing infiltration and filtering of sediments and other organic solids. To prevent erosion or channel formation, a level spreader should be situated along the top edge of the strip.
• Grassed Swales: Grassed swales are earthen channels covered with a dense growth of a hardy grass, such as tall fescue. Grassed swales are typically located at the outlets of road culverts, as conveyance between homes, and as highway medians. Swales provide detention storage. They can also provide a water quality benefit if designed for pollutant removal.
Design Considerations: Maximum swale depth is to be 2.9 feet, with a maximum head of flow over a private driveway of 0.50 feet. Culvert structures should have a weir located no closer than 1 foot from the inlet to control the peak flow rate. LID swales may be designed with a curvilinear width to maximize storage volume for detention. For LID sites, side slopes outside a right-of-way may be 2:1. Table 6.3 provides design considerations for grassed swales.
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Design Storm
Channel Capacity
Soils
Channel Shape
Bottom Width
Side Slopes
Table 6.3. Grassed Swale Design Considerations
10-year storm event
Swale must be sized to convey the peak discharge of the 10-year storm event.
The permeability (infiltration rate) of the soils will determine whether a dry or wet swale can be used. It is recommended that soils used for dry swales have infiltration rates of 0.27 – 0.50 inches per hour.
Trapezoidal or parabolic shape recommended.
2 foot minimum, 6 foot maximum.
3:1 or flatter for mowable slopes in lawn areas.
Channel Longitudinal Slop 1.0% minimum, 6.0% maximum.
Flow Depth 4.0 inches for water quality treatment (maximum).
Manning’s n Value 0.15 for water quality treatment (depth < 4 inches) 0.15 – 0.03 for depths between 4 inches and 12 inches, 0.03 minimum for depth 12
Flow Velocity
Length of Channel inches.
1.0 fps for water quality treatment – 5.0 fps for 2-year and 10-year storms.
Length necessary for 10 minute residence time.
Maintenance Routine landscape maintenance required.
• Level Spreaders: A level spreader typically is an outlet designed to convert concentrated runoff to sheet flow and disperse it uniformly across a slope to prevent erosion. One type of level spreader is a shallow trench filled with crushed stone. The lower edge of the level spreader must be exactly level if the spreader is to work properly. Figure 6.5 shows a typical rock-filled trench level spreader detail. Timber ties or landscape edging can also be used.
Figures 6.6 and 6.7 show the concrete and slotted drain level spreaders.
Design Considerations: Sheet flow, or overland flow, is the movement of runoff in a thin
(usually less than 1 inch in depth) layer over a wide surface, which begins when water ponded on the surface of the land becomes deep enough to overcome surface retention forces. Level spreaders can be used to convey sheet flow runoff from lawn areas within graded areas to bioretention facilities and transition areas. They can also be used to deliver runoff from parking lots and other impervious areas to infiltration areas. The receiving area of the outlet must be uniformly sloped and not susceptible to erosion. Particular care must be taken to construct the outlet lip completely level in a stable, undisturbed soil to avoid formation of rilling and channeling. Erosion-resistant matting might be necessary across the outlet lip, depending on expected flows. Alternative designs to minimize erosion potential include hardened structures (landscape edging, timber ties), stiff grass edges, slotted drain
(see Figure 6.7) or trench drain level spreaders, and segmenting of discharge flows into a number of smaller, adjacent spreaders. Sheet flow should be used over well-vegetated areas, particularly lawns to achieve additional opportunity for retention and increase the Tc.
• Detention Ponds: In some unique circumstances, LID structures will need to be augmented with conventional storm water structures such as dry detention ponds to achieve the Water
Quality Target described in Section 4 (see the Charlotte-Mecklenburg Storm Water Design
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Manual for design specifications). Both dry and wet ponds provide detention storage; however, only wet ponds are effective for water quality purposes.
Figure 6.5. Typical Rock Trench Level Spreader
6.6 Pollution Prevention Practices
Pollution prevention, through public education, implementation of site maintenance and management plans, and industrial process changes, is an integral part of low-impact site development. By emphasizing public education and outreach programs, the site developer can prevent the entrance of pollutants into storm water runoff, thereby reducing the burden on retention and detention BMPs and enhancing their pollutant removal effectiveness. The nature of pollution prevention techniques will complement existing structural controls and will help promote citizen stewardship and local participation in environmental restoration and enhancement efforts.
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Figure 6.6. Concrete Level Spreader
Figure 6.7. Slotted Drain Level Spreader During Dry Conditions and a Rain Event
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7.1 Introduction
Erosion and sediment control and storm water management are interrelated. In conventional practice, the design of erosion and sediment controls tend to follow the design of storm water management control BMPs. It is not uncommon to find all of the site area directed to a large sediment control pond, located at the low point of the site. After the site is stabilized, the sediment pond is cleaned out and converted to a storm water management pond. Mass grading operations often disturb more area than is necessary and negative water quality impacts can result from increased soil erosion.
In the application of LID technology the designer must be careful and conscious to carry the LID concepts through to the erosion and sediment control elements of resource protection. If this aspect is overlooked erosion and sediment control problems will be encountered. The application of LID concepts and the associated emphasis on minimizing the areas disturbed, as well as breaking up drainage areas into small manageable subcatchment areas, is consistent with the basic principles of erosion and sediment control outlined below.
7.2 Erosion and Sediment Control Principles
The following six (6) basic commonsense principles govern the development and implementation of a sound erosion and sediment control plan for any land development activity.
1.
Planning. Plan the operation to fit the existing site features including; topography, soils,
2.
drainage ways, and natural vegetation.
Scheduling of Operations.
Schedule grading and earthmoving operations to expose the smallest practical area of land for the shortest possible time.
3.
4.
Soil Erosion Control.
Apply soil erosion control practices as a first line of defense against offsite damage.
Sediment Control.
Apply sediment control practices as a second line of defense against
5.
6.
offsite damage.
Maintenance.
Implement a thorough maintenance program before, during and after development is completed.
Inspection.
All erosion and sediment control structures must be inspected at least once a week and within 24 hours (weekends and holidays included) after any storm event >.5 inches of rain per 24 hour period (see Mecklenburg County Soil Erosion and Sedimentation
Ordinance).
The following sections describe key elements of the above principles in detail.
7.3 Principle One: Planning
The first principle of erosion and sediment control is to plan the development to fit the site features, including; topography, soils, drainage ways, and natural vegetation. It should be observed that this principle is very similar to the planning guidelines provided for LID in Section
4 of this Design Manual. Listed below are key considerations of the planning element.
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Topography. The primary considerations are slope steepness and slope length (see Example
7.1). Due to the runoff effects, longer and steeper slopes provide for the greatest erosion potential. The percent of slope can be determined from site topography. Areas of similar slope can be grouped together to produce a slope area map, which identifies areas of similar steepness.
Slope steepness can be grouped into three or more general ranges of erosion potential as listed below:
0 - 7 % - Low erosion hazard (buildable, no constraints)
7 - 15 % - Moderate erosion hazard (moderate building constraints, selective clearing)
15 % or over - High erosion hazard (do not disturb, preserve)
Within these slope ranges, the greater slope length results in a greater the erosion hazard.
Therefore, in determining potential critical areas the site planner should be aware of excessively long slopes. As a general rule, the erosion hazard will become critical if slope lengths exceed the following values:
0 - 7 % -
7 - 15 % -
15 % or over -
300 feet
150 feet
75 feet
Example 7.1: Calculation of Slope Steepness and Slope Length
In the sample cross section through a development site at right, the elevation change is 130 feet (780 – 650) and the slope length is
1200 feet. The slope steepness is calculated as follows:
( 780
−
650 )
=
0 .
025 * 100
=
10 .
8 %
1200
This example results in a moderate erosion potential. However, when the slope length is taken into account, the erosion hazard is considered critical because slope steepness values from 7 – 15 % with slope lengths in excess of 150 feet indicate a critical erosion hazard.
780 ft msl
1200 feet
650 ft msl
Drainage ways. Natural drainage patterns existing on the site should be identified in order to plan around these critical areas where water will concentrate. Where it is possible natural drainage ways should be used to convey runoff over and off the site to avoid the expense and problems of constructing an artificial drainage BMP. These natural drainage ways should be protected with vegetative buffers whenever possible or required.
Man made ditches, diversions, and waterways will erode if they are not properly stabilized. Care should also be taken to be sure that increased runoff from the site will not erode or flood the
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012 existing natural drainage feature.
Soils. Major soil considerations from an erosion and sediment control standpoint include; erodibility, permeability, depth to water table and bedrock, and soils with special hazards including shrink/swell potential or slippage tendencies. The Mecklenburg County Soil Survey provides data on these properties for the soils found in the county.
Erodibility is a term that describes the vulnerability of a soil to erosion. Soil erodibility is influenced by the average particle size and gradation (texture), percentage of organic matter, and soil structure. The most erodible soils generally contain high proportions of silt and very fine sand. The presence of clay or organic matter tends to decrease soil erodibility. Clays are sticky and tend to bind soil particles together, which along with organic matter helps to maintain stable soil structure (aggregates). By combining the soils information with the topography, drainage, and vegetation on the site, the planner can determine the critically erodible and sensitive areas that should be avoided if possible during construction.
Natural Vegetation .
Ground cover is the most important factor in terms of preventing erosion.
Any existing vegetation that can be saved will help prevent erosion. Vegetative cover shields the soil surface from raindrop impact and the root mass holds soil particles in place. Vegetation can
"filter" sediment from runoff. Thus grass "buffer strips" can be used to remove sediment from surface runoff. Vegetation also slows the velocity of runoff and helps maintain the infiltration capacity of a soil. Trees and unique vegetation protect the soil as well as beautifying the site after construction. Where existing vegetation cannot be saved, the planner should consider staging construction, temporary seeding, or temporary mulching.
7.4 Principle Two: Scheduling of Operations
The second erosion and sediment control principle is to expose the smallest practical area of land for the shortest possible time. The reason for this principle is that 1 acre of exposed land will yield less sediment than 2 acres of exposed land, and an area exposed for 3 months will yield less sediment than an area exposed for 6 months.
The clearing, grubbing and scalping (mass clearing or grading) of excessively large areas of land at one time promotes erosion and sedimentation problems. As previously described in Section 4 entitled Planning for LID, these initial earth disturbing activities should be kept to a bare minimum. On the areas where disturbance takes place the site designer should consider staging construction, temporary seeding and /or temporary mulching as a technique to reduce erosion.
Staging construction involves stabilizing one part of the site before disturbing another. In this way the entire site is not disturbed at once and the duration of soil exposure is minimized.
Temporary seeding and mulching involves seeding or mulching areas that would otherwise lie open for long periods of time. The time of exposure is limited and therefore the erosion hazard is reduced.
7.5 Principle Three: Soil Erosion Control Practices
The third important principle is to apply soil erosion control practices as a first line of defense
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012 against offsite damage. This principle relates to using practices that control erosion on a disturbed area to prevent excessive sediment from being produced. Control does not begin with the perimeter sediment trap or basin, rather it begins at the source of the sediment and extends down to the control structure.
Soil particles become sediment when they are detached and moved from their initial resting place. This process, which is called erosion, is accomplished for the most part by the impact of falling raindrops and the energy exerted by moving water and wind. A reduction in the rate of soil erosion (Soil Loss) is achieved by controlling the vulnerability of the soil to erosion processes or the vulnerability of the soil to erosion processes, or the capability of moving water to detach soil particles. In humid regions, such as Mecklenburg County, this is accomplished through the use of “soil stabilization” and “runoff control practices”.
Soil stabilization practices include a variety of vegetative, chemical, and structural measures used to shield the soil from the impact of raindrops, or to bind the soil in place, thus preventing it from being detached by surface runoff or wind erosion. Approved soil stabilization practices in the North Carolina Erosion and Sediment Control Design Manual under Section 6.10, Surface
Stabilization, and includes the following practices:
• Temporary Seeding
• Permanent Seeding
• Sodding
• Trees, Shrubs, Vines, and Ground Covers
• Mulching
• Riprap
Additional guidance can be found in the Charlotte Mecklenburg Land Development Standards
Manual, Section 30.17.
The use of mulch to achieve temporary stabilization before permanent vegetation is established is gaining increased attention and recognition. Ongoing research efforts are confirming the fact that mulching is a very effective method of reducing runoff as well as removing pollutants from runoff.
Runoff control practices, in contrast, include a number of measures designed to reduce the amount of runoff that is generated on a construction site, prevent offsite runoff from entering the disturbed area, or slow the runoff moving through and exiting the disturbed area. Drainage system outlet velocities shall not exceed 5 feet per second (fps). Approved runoff control practices are provided in the following Sections of the North Carolina Erosion and Sediment
Control Planning and Design Manual:
Section 6.2 – Runoff Control Measures, Provides standards for the following practices: 1) temporary diversions; 2) permanent diversions; 3) diversion dikes (perimeter protection); and 4) right-of-way diversions (water bars).
Section 6.3 – Runoff Conveyance Measures, provides standards for the following practices: 1) grass lined channels; 2) riprap channels; and 3) temporary slope drains.
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Section 6.4 – Outlet Protection, provides standards for the following practices: 1) level spreader; and 2) outlet stabilization structure.
Additional guidance is available in the Charlotte-Mecklenburg Land Development Standards
Manual.
7.6 Principle Four: Sediment Control Practices
The fourth principle is to apply sediment control practices as a second line of defense against offsite damage. Even with the best erosion control plan, some sediment will be generated and controlling it is the objective of this principle. Whereas soil erosion control practices are designed to prevent soil particles from being detached, sediment control involves using practices that prevent the detached particles from leaving the disturbed area and getting to receiving waterways. This is accomplished by reducing the ability of surface runoff to transport sediment and by containing the sediment onsite.
Sediment control practices are designed to slow the flow of water by spreading, ponding, or filtering. By so doing, the ability of the water to transport sediment is reduced, and sediment settles out of suspension. Commonly used control practices include: 1) the preservation or installation of vegetated buffer areas downslope of the disturbed area to slow and filter the runoff, 2) the construction of small depressions or dikes to catch sediment (particularly coarsetextured material) as close to its point of origin as possible; and 3) the construction of sediment traps or basins at the perimeter of the disturbed area to capture additional sediment from the runoff.
The amount of sediment removed from the runoff is mostly dependent upon (1) the speed at which the water flows through the filter, trap or basin; (2) the length of time the water is detained; and (3) the size, shape and weight of the sediment particles.
Sediment control practices which have been approved for use in North Carolina are presented in the North Carolina Erosion and Sediment Control Planning and Design
Manual under Section 6.60 - Sediment Traps and Barriers, which provides information on the following control devices:
• Temporary Sediment Trap
• Sediment Basin
• Sediment Fence
• Rock Dam
Section 6.5 – Inlet Protection provides standards for the following practices; 1) Excavated Drop
Inlet Protection, 2) Fabric Drop Inlet Protection, 3) Block and Gravel Inlet Protection, and 4) Sod
Drop Inlet Protection.
Currently, the most frequently used approach to sediment control is simply to direct all surface runoff into a large sediment basin, which is later cleaned out and converted to a storm water management pond. While this approach is arguably the simplest and lowest cost method to
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012 control sediment, it often fails to address the other principles described above and thus may not represent the best way to prevent and control sediment.
One of the underlying concepts of LID technology involves breaking up the drainage areas of a given site into very small catchment areas and to provide opportunities to increase the time of concentration and thus reduce peak discharges. Accordingly this approach will benefit sediment control efforts by diffusing surface flow into many directions and providing more flexibility in the use of a variety of sediment control practices.
This approach will provide more opportunity to use silt fences and small traps to control small catchment areas generally in the range of one to three acres in size. It will also allow more opportunity to integrate the use of vegetative buffers in sediment control. When bioretention practices are planned for storm water management, they can first be used as a small temporary trap, by excavating the top two feet of soil. After the site is stabilized, the trap and accumulated silt can be removed and the bioretention cell can be completed. See Section 8.2.1 of this manual for additional information.
7.7 Principles Five and Six: Maintenance and Inspection
The final important control principle is to implement a thorough maintenance and follow up operation. This principle is vital to the success of an erosion and sediment control program. A site cannot be controlled effectively without thorough, periodic checks of all erosion and sediment control practices. When inspections reveal problems, modifications, repairs, cleaning or other maintenance operations must be performed expeditiously.
Particular attention must be paid to water-handling structures such as; 1) diversions, 2) sediment traps, 3) grade control structures, 4) sediment basins, and 5) areas being revegetated. Breaches in the structures or areas being revegetated must be repaired before the next rainfall.
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8.1 Introduction
The effectiveness of an LID system is a function of the design and the construction techniques employed. Of these two parameters, construction is far more critical at achieving quality results. Poor construction techniques will cause the best designed
BMP to fail prematurely, usually from sedimentation and/or clogging. To prevent this from occurring, adequate and proper inspection is required. This Section covers the basic concepts associated with LID construction and inspection. Refer to the Charlotte-
Mecklenburg BMP Design Manual for specific bioretention design criteria.
8.2 Permitting and Processing
Typically, the installation of bioretention is a component of the grading and storm water management permit associated with the development or individual lot. Conceptual storm water management approval and construction permits have already been obtained by this point and are not covered here. For specific permitting information, contact Mecklenburg County Land Use and Environmental Services Agency (LUESA).
8.3 Erosion and Sediment Control Principles for Bioretention Applications
During the construction phase, sedimentation and erosion problems can be greatest due to exposed earth, clearing and grubbing operations, and equipment soil compaction. For this reason, erosion and sediment controls are required to contain sediment onsite. For conventional storm water management design, this meant that the designer simply had to place a sediment control pond at the lowest point of the property under development. The sediment basin would then be used for storm water management control after construction was completed. Sites that incorporate bioretention for storm water control require closer attention to detail because drainage areas are reduced and massive site grading to one low point is discouraged. As a result, grading and sediment control practices are typically applied on a lot-by-lot basis to minimize the opportunity for soil transport. The following principles are identified and briefly explained for the user of this manual.
Principle 1: Planning and phasing. PRIOR to construction and even design, proper planning for sediment control is needed for each lot. Bioretention is a source BMP that requires placement within the lot area or common open space area. Therefore, when laying out the development, the designer must analyze the topography, existing tree cover to be preserved, the building location and associated setbacks, slope steepness and length, drainage ways, and soil types.
Principle 2: Schedule of Operations. Expose the smallest area of land for the shortest possible time. All sediment control devices must be in place prior to the start of the main construction.
At the end of each workday, inspect the devices to be sure of their adequacy and safeguard any trenches or excavations. Provide temporary stabilization for disturbed areas as quickly as possible or as directed by the inspector. Areas that have been disturbed and are not actively being worked, as well as areas that are on final grade, must be stabilized within 14 days.
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Principle 3: r Soil Erosion Control. This is the first line of defense against contamination of the bioretention area. This would include the installation of on-lot silt fences, diversion swales, stabilization and runoff control techniques. Make sure that silt fencing is properly keyed into the ground to prevent undermining. See the Charlotte Mecklenburg Land Development Standards for guidance.
Principle 4: Sediment Control. Even with the best erosion control techniques, sediment transport will occur. For this reason, on-lot sediment traps and/or super silt fence control practices are recommended.
Principle 5 : Inspection and Maintenance. Erosion and sediment control practices must be inspected and maintained on a routine schedule. Accumulated sediment must be removed on a periodic basis, and inspected for excessive accumulation after every major storm. Particular attention should be paid to the stabilization of disturbed areas and integrity of the sediment control devices. All sediment traps must have room for additional sediment loading capacity.
Proper disposal of removed sediment is imperative to reduce the probability of downstream contamination.
Sedimentation Rates for Bioretention. Although no specific sedimentation studies have been done on bioretention to determine the rate of accumulated sediment, estimated vertical settling rates may be derived from Stokes Law. Sedimentation rates will vary significantly and are a function of the following factors:
• Soil Particle size distribution and load of the influent (affected by land use activities)
• Retention time in the ponded area (24 hour drawdown time)
• Physical features of the bioretention facility surface and resulting flowpath length
• Surface area
• Water temperature affecting fluid viscosity
• Wind resuspension (minimal effect)
8.4 Construction Technique and Sequencing for Bioretention
8.4.1 Site Preparation and Planning
Most importantly, the erosion and sediment control principles discussed in Section 7 must be followed to insure sediment will not affect the BMP. In the planning and lot layout phase, the potential bioretention locations are identified. Bioretention facilities should be located within the development envelope, minimizing the need to clear areas unnecessarily. Bioretention areas may make use of existing wooded areas, with minimal clearing required. The BMP must be located on commonly owned property such as property owned by the Homeowners’ Association. BMPs are not to be located on private property due to concerns regarding proper maintenance. Any deviation from this policy must be approved by the Storm Water Administrator.
Two methods of sediment control are typically applied to bioretention facilities as follows.
Method One
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The first method (most typical) is to avoid disturbing the proposed bioretention area after the initial rough grading and temporary stabilization has been performed. During the construction phase, all drainage must be directed away from the BMP location to avoid excessive sedimentation. Flow can be directed away from the bioretention BMP by utilizing silt fencing materials and temporary diversion swales that direct flows to small on-lot silt traps.
Method Two
The second method of erosion and sediment control design allows the area proposed for the bioretention BMP to be used for the installation of a sediment control structure. If a sediment control structure is to become a bioretention BMP, the following conditions must be met:
• The proposed invert of the bioretention BMP must be greater than 1 foot below the invert for the sediment control structure.
• All remnant sediment must be removed.
• If geotechnical tests show that the in-situ soils meet or exceed the soil medium guidelines for infiltration rates (see section 8.4.10), no underdrain will be required, although it is still highly recommended.
• The in-situ soils and ponded sediment materials shall be removed and the remaining surface scarified to increase the likelihood of adequate infiltration potential.
8.4.2 Design Considerations
Bioretention facilities must be designed and sized to treat runoff from the first inch of rainfall and should bypass larger flows to prevent disruption of plants, mulch and soil media. It is difficult to maintain sheet flow conditions for these larger flows, which oftentimes can lead to clogging of outlet structures or increased maintenance of the bioretention facility. It is important to insure that inflow velocity is dissipated and dispersed such that sheet flow (even spread of inflow) is created. Sheet flow allows for low water velocity to be evenly distributed into the facility thus causing little to no disruption of plants, mulch or soil media. This improves the overall functionality of the bioretention facility, decreases maintenance and prolongs the life of the device. Sheet flow can be achieved in a variety of ways including:
1. Plunge pool/sediment forebay (shallow) to reduce inflow velocities.
• Ensure 3:1 side slopes and design for the appropriate entry volume and velocity.
• Discharge rim of pool or forebay should be level for even distribution into the device (use uniform stone or structural weir).
2.
Slotted or perforated pipe (level or even discharge contoured swale, etc.).
• Use pipe with reverse flow. Water channeled into the pipe is discharged via the lateral openings in the pipe.
3.
Level spreader (flush curb, structural weir, lateral grass swale)
4.
Indirect discharge from overland flow, parking lot, various sheet flow. It is extremely important to avoid all direct discharges to the bioretention facility from flumes, swales, pipes, downspouts, etc. unless one of the above methods or other similar level spreading device is utilized. A forebay is an effective device for dispersing inflow velocity. A forebay will also act to settle out sediment from the inflow which will protect the bioretention area from clogging. However, if a forebay is installed it will need to be inspected at least monthly and accumulated sediment removed as necessary. Depending on the type of forebay liner material (rip rap/concrete) be sure to construct a level outlet to prevent the re-concentration
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012 of flow and maintain sheet flow. If using rip rap, a smaller stone may be warranted to fill outlet voids between larger stones to reduce multiple point discharges.
Another important design consideration for a bioretention facility is to provide 100% ground cover around the perimeter of the facility. Bare ground contributes directly to sediment runoff which clogs the bioretention facility thus preventing it from functioning properly. Two instant ground cover solutions are sodding and mulching. Seeding and matting may also be used with the addition of silt fence around the entire perimeter of the bioretention facility to prevent sediment entry during seed germination. The application of ground cover around the perimeter of the bioretention facility should occur after the excavation of the basin and prior to the installation of the underdrain or soil media. Access points should be included to allow for the completion of construction activities.
Tall grasses can also be an effective tool for controlling velocity and sediment. These grasses can be located along the edge of the bioretention facility at the point of inflow to slow velocities and filter particles.
When utilizing a direct entry from curbing via a curb cut, rip rap is generally installed at the inlet of the bioretention facility for velocity control. For effective use, the rip rap should be “keyed” below the invert of the curb line to allow for positive flow into the facility without the potential for bypass to occur due to buildup of sediment along the curbline.
When entry slopes of 3:1 or less cannot be achieved due to elevation constraints, a drop structure should be considered to avoid potential slope failures. In addition, sheet flow should be re-introduced at the outlet using the previous examples in numbers 1, 2, 3, or 4 above.
Soil compaction tests are required on any berms >5 feet in height from the natural grade. Soil compaction must be at 95% proctor and certified by a licensed soil engineer.
8.4.3 Minimize Lot Grading/Clearing
Bioretention facilities should be located within the development envelope, minimizing the need to clear areas unnecessarily. Bioretention areas may make use of existing wooded areas, without grading the wooded area to install the facility. Grading of any catchment area draining to the facility should be done sparingly and stabilized immediately (within 14 days).
8.4.4 Install Sediment Control and Diversion Devices
Install the necessary sediment control devices to protect the facility from contamination by sediment by following the approved sediment and erosion control plan. Essentially, the placement of silt fence material around the perimeter should be sufficient to prevent flow from entering the area during construction. If the inspector approves the installation of bioretention facilities prior to the final asphalt coat, flow diversion devices must be installed to prevent storm water flow into the BMPs. This is essential because this flow will likely contain fine soil particles that will clog the media in the bioretention facility thus rendering it ineffective.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Cleanout consisting of rigid non-perforated PVC pipe, 6 inches in diameter with cap
3:1 Slope
12”of Stone
4” minimum
3:1 Slope
NOTES:
1.
Excavate the rain garden to the design dimensions.
2.
Place non-woven filter fabric (Mirafi 140 or equal filter fabric) across the bottom of the excavation in locations for underdrains.
3.
Install underdrain system on top of the filter fabric in the bottom of the excavation. Double wall HDPE corrugated plastic pipe is recommended due to its rigidity and long life expectancy. Other pipe materials may be substituted at the designer’s discretion and with the inspector’s approval.
4.
Install cleanout standpipes and outlets to the underdrain. The cleanout standpipes must be located at the upper end of the structure and be capped above the water level elevation. The cleanouts should consist of rigid non-perforated PVC pipe, 6 inches in diameter.
5.
Place #57 wash stone (gravel) over the underdrain to a depth of 12 inches and width of 4 feet.
6.
Place filter fabric over the gravel bed to fully enclose it on the top, bottom and sides.
7.
Backfill with a soil mixture containing 80% white mortar sand and 20% organic material
(compost, topsoil) to a depth of 2 to 4 feet (depending on design) measured from the top of the stone over the underdrain. Soil must not be installed until all the contributing drainage area has been stabilized and approved by the inspector. Soil mixtures must be placed in lifts of 12 inches. No heavy equipment is allowed in the basin area.
8.
Install vegetation as per approved plans.
9.
Place mulch on top of the soil mixture to a depth of 3 inches.
10.
Stabilize the slopes draining to the rain garden with grass sod to a minimum width of 4 feet and surround the structure with silt fence until drainage area is fully vegetated to prevent sediment from contaminating the soil mixture.
11.
An energy dispersion device is required for concentrated flow entering the structure.
12. Refer to Section 4.1 of the Charlotte-Mecklenburg BMP Design Manual for specific design criteria for rain gardens/bioretention systems.
Figure 8.1. Rain Garden Detail
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
8.4.5 Excavation Preparation
Excavate the facility to the design dimensions (see figure 8.1). Excavated materials must be placed away from the facility sides to avoid contamination and possible side wall instability.
Large tree roots (>2 inches in diameter) must be trimmed flush with the trench bottom in order to prevent fabric puncturing or tearing during subsequent installation procedures. The sidewalls of the trench must be roughened where sheared and sealed by heavy equipment.
8.4.6 Underdrain Specification
Underdrains are required on all bioretention BMPs. The underdrain system (pipe capacity and orifice capacity) must be designed assuming that 50 percent of the capacity is lost due to clogging. The following information provides guidance for underdrain installation. Figure 8.2 shows the installation of a typical underdrain.
Location.
Underdrains are typically located at the invert of the bioretention BMP to capture and remove any filtered water that does not infiltrate into the surrounding soils. The underdrain is placed at the bottom of the excavated trench under a 12-inch gravel layer, 4 feet in width
(minimum) with at least 4 inches of gravel above the top of the pipe. If multiple underdrain pipes are used, they must be spaced at a maximum of 10 feet on center. Soil over the gravel layer must be at least 2 feet in depth and up to 4 feet if trees are planted. Placement of 2 to 3 inches of gravel bedding is recommended beneath the discharge points. Underdrains must
“daylight” or connect to an existing drainage system and achieve a positive flow condition
(minimum of 2.5 feet per second flow velocity for full flow conditions or 0.5% slope). Prior to covering the underdrain system, the inspector must observe the underdrain itself, the connections and the gravel bedding. Suitable discharge points include: -8
• Grass swale areas, flush cut with side slope areas.
• Storm drain pipe conveyance system.
Underdrain Material Types . Underdrain systems may be composed of a variety of materials.
Double wall HDPE corrugated plastic pipe is recommended due to its rigidness and long life expectancy. Other pipe materials may be substituted at the designer’s prerogative and with the inspector’s approval such as 6-inch PVC pipe. All underdrain pipes must be a minimum of 6 inches in diameter.
Connections . Pipe joints and storm drain structure connections must be adequately sealed to avoid leaks. Pipe sections must be coupled using suitable connection rings and flanges. Field connections to storm drain structures and pipes must be sealed with polymer grout material that is capable of adhering to surfaces. The underdrain pipe must be capped until site completion.
Perforations . Perforated PVC pipe sections are available from local hardware and building supply stores. The perforation locations are not critical for proper operation, as long as the total opening area exceeds the expected flow capacity of the underdrain itself. Commonly marketed perforated PVC pipe has ¼ to ½ inch perforations, 6 inches from center to center, along two or three longitudinal rows. Whether or not the perforations are placed at the invert of the pipe or elsewhere, depends upon the design of the facility. Typically, the perforations are placed closest to the invert of the pipe to achieve maximum potential for draining the facility. The perforations
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012 can be placed near the top of the pipe if an anaerobic zone is intended. Water below the perforated portion of the underdrain will have a tendency to accumulate during periods of saturation. Otherwise, water will have a tendency to infiltrate into the surrounding in-situ soils.
Figure 8.2. Installation of Underdrain on Top of Filter Fabric in Trench Bottom
8.4.7 Observation/Cleanout Standpipe
An observation/cleanout standpipe must be installed to the underdrain in every bioretention
BMP. The standpipe will serve two primary functions: 1) it will indicate how quickly the bioretention BMP dewaters following a storm; and 2) it provides a maintenance port. The cleanout standpipe must be located at the upper end of the structure and be capped above the water level elevation. It must consist of a rigid, non-perforated PVC pipe, 6 inches in diameter.
A cleanout must be installed at both ends of the system and at all bends. If the bioretention BMP exceeds 100 feet in length; additional cleanouts must be installed in series every 50 feet. The top of the cleanout must be capped with a screw, or flange type cover to discourage vandalism and tampering. Locking is not necessary.
8.4.8 Gravel Bed
Gravel bed materials are used to protect an underdrain pipe to reduce clogging potential.
Placement of the gravel over the underdrain must be done with care. Avoid dropping the gravel from a backhoe at elevated heights. Spill gravel gently over the underdrain and spread manually.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
The gravel stone size shall be no greater than ½ to1½ inches in diameter (blue stone, double washed, #57 stone). The depth of the gravel must not exceed 12 inches. Figure 8.3 shows the placement of the gravel over the underdrain.
Figure 8.3. Placing Gravel Over the Underdrain
8.4.9 Filter Fabric
Filter fabric must be placed over the gravel bed to: 1) prevent the transport of silt or sediment into the gravel bed that would cause serious clogging problems, and 2) control the direction of flow through the gravel. Figure 8.4 shows filter fabric placement over the gravel covering the underdrain. Filter fabric installations must meet the following specifications:
• Must meet a minimum permeability rate of 75 gal/min/ft
2
and shall not impede the infiltration rate of the soil medium (Example: 140 Mirafi nonwoven).
• Non-woven fabric is preferred over the woven variety. Filter fabric installation requires at least 1-foot overlap at the ends and staking in-place during construction at the turned up surfaces. After soil is placed over the filter fabric, excess fabric may be cut away. Single layers of fabric should be used. Overlapping must be minimized. Filter fabric must completely surround the gravel over the underdrain, including bottom and sides. Filter fabric is not necessary along the sides or the entire bottom of the trench.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Figure 8.4. Placing Filter Fabric Over the Gravel Covering the Underdrain
8.4.10 Soil Preparation and Installation
Soil specifications for proposed bioretention sites are required and must be submitted as part of the Preliminary Plans. Soil preparation can be accomplished by thoroughly mixing soil components, amendments and additives, as needed utilizing a backhoe or front-end loader.
Table 8.1 contains the specifications for the soil mixture to be used in a bioretention facility.
Test methods are specified for all parameters except sand and clay content but tests are required only for total phosphorus and particle size for the soil media mix. However, the inspector may at their discretion test suspected soil media for any of the parameters indicated in Table 8.1 and disapprove it for use if results do not meet the acceptable values. A Soil Verification Form
(Appendix J) must be filled out and notarized to verify that total phosphorus and particle size tests have been performed and that results are within the acceptable values indicated in Table
8.1. This form must be provided to the County inspector prior to the placement of the soil in the bioretention facility. Soil preparation can be performed off-site and transported to the facility location when ready for installation. In these situations, the Soil Verification Form must be approved by the inspector prior to bringing the soil on-site. In all situations, the County inspector must inspect the soil on-site prior to installation.
Soil must not be installed until all of the contributing drainage area has been stabilized and approved by the inspector. Provisions for sediment control must be installed and maintained as specified in the sediment and erosion control plans. Soil should not be delivered to the site until
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012 excavation has taken place and the fabric and underdrain system has been installed. Scarification of in-situ soil surfaces by manually raking to aerate and reduce soil compaction is recommended.
Figure 8.5 shows initial placement of amended soil mix.
Table 8.1. Specifications for the Soil Mixture in Bioretention Facilities
Parameter Acceptable Values
Testing
Required (1)
Test Method
Sand Content (ASTM
C-144 recommended)
Organic Material
(compost, sandy loam and loamy sand)
80%
20%
No
No
None
TMECC 05.07-A
Clay Content Less than 6% No None
Phosphorus Index
(total Phosphorus) (2)
10 to 30
(12 to 36 ppm on a dry basis)
Yes
Mehlich 3 Extraction,
Mehlich 2 Extraction
(Mehlich 1 Extraction is acceptable but result must be multiplied by 1.7 for comparison) pH 5.5 to 7.0 No TMECC 04.11-A
Permeability 1 to 4 in/hr No
ASTM D2434
(compacted to 20%)
Particle Size Analysis
Sieve 2 inch (50 mm)
Sieve No. 4 (4.75 mm)
Sieve No. 8 (2.36 mm)
Sieve No. 10 (2.0 mm)
Sieve No. 16 (1.18 mm)
Sieve No. 30 (600 um)
Sieve No. 50 (300 um)
Sieve No. 100 (150 um)
Acceptable Minimum
% Passing by Weight
Lower
100
Upper
100
98
95
86
70
40
10
2
100
100
100
100
75
35
15
Yes ASTM D422
Sieve No. 200 (75 um) 0 5
(1) Even though testing is not required for all parameters, the inspector reserves the right to test suspect material and disapprove it for use if results show that parameters do not meet the acceptable values.
(2) Phosphorus Index test methods are based on information received from Dr. Myers at the N.C.
Department of Agriculture.
Installation of soils must be done in a manner that will ensure adequate filtration. After the invert area of the proposed facility has been scarified, soil should be applied in lifts of 12 inches. This will minimize compaction and application time. Lifts may be applied using a small track machine with low ground pressure such as a minihoe. The lifts may be lightly watered to encourage natural compaction. Removing overfill is easier than adding soil when attempting to
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012 bring the facility to the correct elevation. Avoid over compaction by allowing time for natural compaction and settlement. No additional manual compaction of soil is necessary. Rake soil material as needed to level out. It is recommendation that consolidation be allowed to occur followed by the application of additional soil material to bring the facility to the design elevation.
Depending upon the soil material, up to 20% natural compaction may occur. In order to speed up the natural compaction process, presoaking the placed soil may be performed. Significant settlement can occur after the first presoak, and additional settlement may occur subsequent to the initial wetting. If time and construction scheduling permits, it is preferable to allow natural settlement to occur with the help of rain events to presoak the soil media. The surface of the facility does not have to be uniform. A slight variation due to settling or mulch application is acceptable as long as the possible ponding does not exceed 12 inches. For areas where excessive settlement occurs, apply sand to fill low areas and check to insure the entire surface is level before planting and mulching the area.
Figure 8.5. Installing Amended Soil Mix Over Filter Fabric
In-situ (or in-place) soil used for bioretention must also be prepared. Scarification of the soil surfaces by manually raking to aerate and reduce soil compaction is recommended. When in-situ soils are being proposed for use, soils investigation/geotechnical reports are required. A copy of the geotechnical report shall be supplied to the inspector at the pre-construction meeting. The report shall include the boring location at the bioretention facility and include USDA soil classification, boring log with penetration depths at least 2’ below the proposed facility invert, depth to groundwater or impervious layer (if present), and infiltration rate of the in-situ soil. The bioretention facility shall not be placed in service until all of the contributing drainage area has been stabilized and approved by the inspector. Provisions for sediment control shall be in-place as specified within the sediment and erosion control plans.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Delivery of materials such as soil medium, plants, gravel, filter fabric, and underdrains will need to be coordinated to avoid stockpiling and contamination problems. Soil materials should not be delivered until the bioretention facility has been excavated or graded to the design elevations and filter fabrics and underdrain systems are in place. A primary reason for bioretention BMP failure is the clogging of the soil mix with fine clays from soil erosion during site installation. To avoid this problem, silt fence should be installed around the entire perimeter of the basin until ground cover is fully established (see Figure 8.6). An alternative would be to use sod to stabilize the soil around the perimeter of the basin. Planting materials should not be delivered until after the soil medium has had time to settle and trimmed to the proper grade elevation. Weather and seasonal conditions will also affect planting requirements.
Figure 8.6. Silt Fence Around the Perimeter to Protect From Sedimentation
8.4.11 Plant Preparation and Planting Methodology
When ordering plants to be installed in a bioretention BMP, adequate preparation of the bedding soils must occur prior to delivery. Timing in relation to season and readiness of the BMP is very important. The recommended ordering times for plants are early spring or fall, depending upon the species selected. Often times, plant materials need to be stockpiled while the BMP is being prepared. Keeping root balls wet during this period, and providing a shaded storage location will improve the plants survivability.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
The initial density of the planting arrangement will be thick. This is to ensure that adequate vegetative cover will quickly take hold. Once the plants continue to grow and spread out, some plants may be removed or divided by the property owner and transplanted elsewhere. Refer to
Appendix F for specific species requirements. Selected plants must be able to survive in the
80% sand mixture.
A minimum of three (3) species of trees and three (3) species of shrubs should be selected to insure diversity. In addition to reducing the potential for monoculture mortality concerns, a diversity of trees and shrubs with differing rates of transpiration may ensure a more constant rate of evapotranspiration and nutrient and pollutant uptake throughout the growing season. To add interest to the rain garden, a variety of plant material should be selected to provide blooming and color at different times of the year. Figure 8.7 illustrates the planting design used at a rain garden installed at the Mecklenburg County Hal Marshall Building, 700 N. Tryon Street in
Charlotte in November 2003. The rain garden is located at the eastern side of the parking lot located at 11 th
and College Streets and is available for inspection.
19
5
15
1
5
20
5
22
11
5
13
21
2
16
20
4
2
21
23
4
7 6
7
10
23
17
8
9
10
12
8 3
8
3
9
21
14
18 17
Forebay
17
18
19
20
13
14
15
16
21
22
23
#
Trees
1
2
3
Shrubs
4
5
Scientific Name
Acer rubrum
Myrica cerifera
Magnolia virginiana
6
7
8
9
Clethra alnifolia
Cornus serecia
Ilex verticillata – male
Ilex verticillata
Ilex vomitoria ‘pendula’
Itea virginica
Herbaceous Plants
10 Andropogon glomeratus
11
12
Aster nova-angliea
Aster oblongifolius
Aster tataricus
Chasmanthium latifolium
Helianthus angustifolia
Hibiscus coccineus
Iris virginicus
Oenothera fruticosa
Panicum virgatum
Rudbeckia fulgida var. fulgida
Schizachyrium scoparium
Silphium perfoliatum
Solidago rugosa
Common Name
Red Maple
Wax Myrtle
Sweet Bay Magnolia
Hummingbird Clethra
Red Twig Dogwood
Deciduous Holly
Winter Red Deciduous Holly
Weeping Yaupon Holly
Henry’s Garnet Sweetspire
Bushy Bluestem
New England Aster
October Skies
River Oats
Swamp Sunflower
Red Hibiscus
Blue Flag Iris
Sundrops
Heavy Metal Switchgrass
Balck-eyed Susan
Little Bluestem
Cup Plant
Fireworks Rough-leafed Goldenrod
Figure 8.7. Hal Marshall Rain Garden Planting Design
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Shipping of the plant materials is typically the responsibility of the nursery or landscaping contractor. It is preferable to have the plants shipped directly to the facility site ready for planting.
All plant materials shall be tagged for easy identification with the American Standard for
Nursery Stock. Tags shall be checked by the inspector for compliance with the landscaping planting list shown on the design plans or Storm Water Management Permit Application plans.
Approved plants for use in bioretention BMPs are provided in Appendix F of this manual.
Variations of plant type, quantity or quality, requires Mecklenburg County L.U.E.S.A. approval and may necessitate a plan revision submittal.
The number of tree and shrub plantings may vary, especially in areas where aesthetics and visibility are vital to the site development, and should be determined on an individual site basis.
On average, 2,000 trees, shrubs, and herbaceous plants should be planted per acre. Bioretention gardens that are less than 1000 square feet do not require trees and should have 70% shrubs and
30% herbaceous plants. Bioretention gardens that are 1,000 square feet or greater should have
10% trees, 70% shrubs, and 20% herbaceous plants. Grasses that are a minimum of 3 feet in height at installation may account for 20% of the shrubs that are used.
At installation, trees should be 2½ inches in caliper, and shrubs 3 to 4 feet in height or 18 to 24 inches in spread. Ground cover may be as seed, or preferably, plugs. The relatively mature size requirements for trees and shrubs are important to ensure that the installation of plants is readily contributing to the bioretention process (i.e. evapotranspiration, pollutant uptake). Initially fertilizer such as 10-10-10 may be tilled into the surface prior to applying mulch or plants in order to “jump start” plant growth. Fertilizer should be used very sparingly and only during installation.
8.4.12 Installation of Mulching Materials
Mulch should be placed on the surface ponding area of the BMP. The mulch material should be fresh, double ground and screened hardwood to help retain soil moisture and maximize nutrient uptake. This type of mulch material also helps resist flotation when BMP is fully ponded. Select your mulch carefully. Mulch cannot contain soil or fine organics, which have a tendency to create a barrier to infiltration thus the importance of making sure the mulch is “screened” to remove these fines. Do not use pine straw mulch.
The layer of mulch should not exceed 3 inches in depth. Greater depths keep plant roots from making good contact with the soil. Mulch materials should not be mounded around the base of tree trunks as this practice encourages pests and diseases. The mulch layer should be placed after the plants and groundcover have been installed. Protect and lift groundcover vegetation to place mulch material underneath and between plantings. The mulch layer surface should approximate the final elevation as shown on the design plans. Figure 8.8 shows the rain garden constructed at the Hal Marshall Building parking lot after installation of hardwood mulch.
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
Figure 8.8. Planted and Mulched Rain Garden
8.5 Maintenance and Operation
8.5.1 Maintenance Access
Adequate access must be provided into all BMP areas for inspection, maintenance, and landscaping upkeep. A 20-foot wide permanent maintenance access easement from a public right-of-way must be provided for all BMPs. The cleared access area within this easement must have a minimum stabilized width of 12 feet, maximum longitudinal grade of 15 percent, and maximum cross slope of 5 percent. In addition, a 10-foot wide permanent maintenance access easement must be provided around the perimeter of all BMPs to allow for adequate maintenance and repair.
8.5.2 Maintenance Requirements for all BMPs
BMPs that are constructed on privately-owned land and that are not within a public easement shall be maintained by a Homeowners Association or the owner of the subject property. BMPs that are constructed on public land within public rights-of-way, and/or within public easements
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012 shall be maintained by the public body with ownership/jurisdiction of the subject property. The following requirements shall be met for all BMPs that have been constructed on privately-owned property and not within a public easement.
1.
Maintenance Covenants. Prior to plan approval and issuance of a storm water management permit, the applicant or owner of the BMP shall establish a formal Maintenance Covenant approved by the Mecklenburg County Land Use and Environmental Services Agency and recorded in the Office of the Register of Deeds in which the owner acknowledges the duty of the owner and all subsequent owners of the property to maintain the BMP in accordance with the terms of the Covenant. A maintenance plan and schedule shall be included as part of the covenant as well as a mechanism for funding maintenance and repairs. This Maintenance
Covenant shall also specify the Homeowners Association or other party responsible for maintenance of the BMP. A Homeowners Association or similar legal entity has the power to compel contributions from residents of a development to cover their proportionate shares of the costs associated with BMP maintenance. Appendix G contains an example of a typical
Maintenance Covenant. Appendix H contains an example Maintenance Plan and inspection checklists.
2.
Requirements for the Maintenance Covenants. BMPs shall be inspected at a minimum of annually by a qualified professional as described in the Maintenance Covenant. The purpose of this inspection is to identify maintenance and repair needs to ensure the long-term functionality of the BMP. Any identified maintenance and/or repair needs shall be addressed in a timely manner. The inspection and maintenance requirement may be increased as deemed necessary by the Mecklenburg County Water Quality Program (MCWQP) to ensure proper functioning of the BMP.
3.
Records of Installation and Maintenance Activities. Parties responsible for the inspection, operation, and maintenance of a BMP shall maintain records of the installation of all the maintenance and repairs and shall retain the records for the life of the BMP. After inspections and maintenance have been performed, a copy of the inspection checklist (see
Appendix H) shall be forwarded by the owner to MCWQP within two weeks of the inspection. MCWQP staff will file submitted inspection forms and enter the inspection information into a BMP maintenance data base to track inspections and maintenance performed. All inspection checklists are to be mailed to 700 North Tryon Street, Charlotte,
NC 28202, Attention: Water Quality Program.
Watering and maintenance responsibilities during different phases of a project shall generally be defined as follows, unless contractual obligations require otherwise:
• Construction Phase: Developer/Builder
• Project Acceptance: Builder
• Property Ownership Transfer: Builder/Property Owner
• Warrantee Phase: Property Owner
• Operation Phase: Property Owner/Homeowners Association
8.5.3 Special Maintenance Requirements for Bioretention Facilities
In traditional, intensively cropped landscapes, soil fertility (and especially the level of available nitrogen) is considered the limiting factor to plant growth. By design, bioretention facilities are located in areas where nutrients (especially nitrogen) are significantly elevated above natural
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012 levels. Therefore, it is unlikely that soil fertility will be the limiting factor in plant growth, and thus fertilization would be unnecessary. Excess fertilization, (besides compromising the BMP’s pollutant reduction effectiveness) leads to weak plant growth, promotes disease and pest outbreaks, and inhibits soil life. If soil fertility is in doubt, a simple soil test can resolve the question. Persons responsible for maintenance may consult with their local nursery or contact the
Mecklenburg County Cooperative Extension Office to determine fertility needs. If fertilization should become necessary, an organic fertilizer will provide nutrients as needed without disrupting soil life.
Like any garden area that includes grasses or woody plant materials, harvesting and pruning of excess growth will need to be done occasionally. Trimmed materials may be recycled back in with replenished mulch material. Mulch should be inspected after every significant precipitation event and replaced as necessary. In most cases, shredded mulch should only need replacement once per year.
Prince George’s County, Maryland 4-13
Typically, watering of the BMP will not be necessary once plants have become established, except during drought conditions. Plant species for bioretention have been selected based on their hardiness and ability to survive extreme conditions. However, watering will be needed during the plant establishment stage. As with any landscaping feature, the designer should consider affects on moisture condition and the ability of the owner to apply watering as needed.
Facilities susceptible to drying conditions include:
• Landscape parking lot islands
• Median areas
• Windy, exposed areas
Weeding of the facilities is not absolutely necessary for the proper functioning of the bioretention BMP. However, unwanted plants can be invasive, consuming the intended planting and destroying the aesthetic appeal. Therefore, weeding is encouraged to control growth of unwanted plants, especially where facilities are placed in prominent settings.
Seasonal Care
Spring:
• Prune deciduous trees and shrubs before leaves appear (usually early to mid-March).
• Prune flowering trees and shrubs after blossoming (usually early June).
• Divide ornamental grasses and perennials as soon as the soil becomes soft.
Summer:
• During extended drought, water deeply in the morning every seven to ten days.
• Check trees and shrubs for signs of disease or insect pests. Plant diseases usually can be easily treated when detected early.
• Weed regularly, preferably by hand.
Fall:
• Cut perennials back to the ground after the first frost and remove annuals.
• Plant new trees and shrubs as long as the soil temperature remains above 32 degrees.
• Mulch trees and shrubs to help conditions the soil for spring and to protect roots.
Winter:
• Cut back ornamental grasses and remove clippings.
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Troubleshooting Problems
• Look for signs that plants are too wet including wilting, yellowing, ringed spots on leaves, and a soft or rotting base.
• If erosion is occurring at drainage paths, stabilize the erosion.
• If plants are dying, plants more tolerant of drier/wetter conditions may be necessary.
• If water is not dissipating within a couple of days, the BMP is not functioning properly.
Water will pond longer in winter and early spring.
• Do not walk or mow in ponding areas.
8.5.4 Warrantees
The landscaping work and materials shall be guaranteed by the developer/builder for one year from the date of the final approval. After one year the property owner will assume responsibility for all landscaping work and materials and shall replace plants if they die.
8.6 Typical Sequence of Construction for Bioretention
The sequence of construction for bioretention areas is closely tied to the grading plans for the development. Because bioretention is a source control BMP, drainage area catchments are kept relatively small and manageable during the construction phase for control of sediment. Basic sediment control practices are employed for each lot.
A typical sequence of construction with a typical construction schedule is provided in Figure 8.9 of this document. The sequence of construction will vary for every project but the designer may utilize this sequence of construction as a general guide. Variations to the sequence must be noted and conveyed to the County inspector. The sequence of construction shall be placed on the plans.
8.6.1 Inspectors Checklist for Bioretention
The following checklist has been developed for use when evaluating a bioretention facility during different phases:
Bioretention Inspection Checklist
1. Pre-construction Meeting

Two copies of approved Storm Water Management Plan with Latitude & Longitude of
BMPs

Submit one copy to Mecklenburg County Water Quality Program for BMP mapping

Disseminate inspection requirements; what needs inspection
 Ticket and tag requirements & a copy of the geotechnical report (if available)
2. Excavation of Bioretention Area

Soil Permeability

Suitable sub-grade materials

Presence of moisture or water
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 Dimensions and placement of excavation conforms with plans

Sediment and erosion control devices in place
3. Installation Phase (Only after all contributing drainage area has been stabilized)

Proper placement of filter fabric (non-woven) below locations of underdrains

Proper placement of underdrains and cleanouts (size, schedule, location)

Proper Placement of gravel layer on top of underdrains

Proper placement of filter fabric around gravel layer

Backfill soil conforms with specifications and placed per details and specifications

Proper grade establishment
 Silt fence installed around perimeter of BMP to keep sediment out of fill material

Proper placement of plant materials (type, size, quantity, tags)

Correct placement of mulch cover

Install sod filter strip around perimeter
4. Final Inspection and As-Built

Changes in grading, facility depth, size, soil medium, plant materials, etc., shall require an As-built Plan whether private or public to reflect the changes.

Maintenance Agreement/Covenant for bioretention facilities located on private property

All landscaping installed/landscape warrantee documentation received
 Bioretention configuration, size and depth are in accordance with approved plans

Landscaping certification documentation for bioretention facility(ies)

Drainage area conforms to approved plan

Drainage area completely stabilized
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1.
2.
Install sediment control devices as shown on the plans.
Construction time : ½ Day
Grade site to elevations shown on plan. If applicable, construct curb openings, and/or remove and replace existing concrete as specified on the plan. Curb openings shall be blocked or other measures taken to prohibit drainage from entering construction area. At the end of each workday, all excavations shall be protected by construction safety fencing or temporary backfill as needed.
Construction time : 1 Day
3. Stabilize grading within Limit of Disturbance except for Bioretention Area. Bioretention areas may be utilized as sediment traps if the proposed invert of the bioretention facility is 1 foot lower than the sediment trap and if approved on the plans.
Construction time : ½ Day
4. Excavate bioretention area to proposed invert depth and scarify the existing soil surfaces, taking care not to compact the in-situ materials. Install filter fabric on bottom of excavation below locations for underdrains. It is not necessary to cover the entire bottom of the excavation with filter fabric.
Construction time : ½ Day
5. Install underdrain system, cleanout pipe and outlet device.
Construction time : ½ Day
6.
7.
Inspection required of underdrain and amended soil mix.
Cover underdrain with washed stone (#57). Surround stone with filter fabric. Install amended soil mix as specified in the plans and detailed in the specifications.
Construction time : ½ Day
8. Presoak the planting soil prior to planting vegetation to allow for settlement. This can be done by water truck or allowing water to enter the pit from a rain event.
Construction time : ¼ Day
9. Excavate or fill to achieve proper design grade, leaving space for the upper layer of mulch and/or topsoil that will bring the surface to final grade and ready for planting. Create sheet flow into bioretention area to avoid concentrated flow and erosion.
Construction time : ¼ Day
10. Plant vegetation specified in the planting plan for Bioretention Area and sod perimeter.
Construction time : ½ Day
11. Mulch and install erosion protection at entrance points. Remove sediment control practices or entrance blocks with inspector authorization.
Construction time : ½ Day
12. Final inspection required.
Total Estimated Construction Time = 5.0 Days
Figure 8.9. Sequence of Construction for Bioretention BMPs
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9.1 Introduction
Plan submittal requirements will remain the same for all commercial and residential projects.
Storm Water Management Permit Application plans demonstrating compliance with Section 3,
Performance Criteria, are required for all projects, unless exempted under Applicability of the
Huntersville Post-Construction Ordinance.
9.2 Site Evaluation Tool
The Site Evaluation Tool (SET) is a water quality model that assesses pre-development runoff and pollutant loading rates and provides a methodology for implementing LID best management practices (BMPs) into a development for achieving the established Performance Criteria (Section
3). The SET is intended to be an integral part of the development of the Concept Plan and SET output must be submitted along with the Concept Plan Application. The SET requires the following input describing pre- and post-construction conditions:
1. The size of the project.
2. For residential development, the number of homes within the project to be served by septic systems.
3. For commercial development using on-site wastewater disposal BMPs, the estimated waste volume in gallons/year.
4. The fraction of the project area that is distributed within each of the hydrologic soil groups
A, B, C, or D.
5. The land areas (in square feet) shown on the detailed site plan for the proposed development as occupied by the following pervious areas: a. Forest/Wetland b. Meadow (open space maintained in a natural condition) c. Lawn
6. The areas (in square feet) of the project that will be covered by the following impervious areas: a. Rooftops (all buildings) b. Driveways and/or Parking Lots (including gravel surfaces) c. Roads d. Sidewalks e. Other Impervious Areas (e.g. tennis courts, patios)
7. The surface areas (in square feet) of all storm water management facilities or BMPs planned for the project including structural features (ponds, wetlands) and design features such as swales, channels and infiltration galleries).
8. The same division of land uses, impervious areas, and storm water management facilities as items 5-7 for the condition of the site prior to development. For most sites the existing land use will be a combination of forest/wetland and meadow.
9. A division of the project area into distinct drainage areas that are served by specific storm water management facilities and/or BMPs.
10. Additional inputs are required for some BMPs; ponds and wetlands require entry of detention storage, and stream buffers require average buffer width and the proportion of the drainage
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012 area within 150 feet + the buffer width of the stream. Also, pollutant removal efficiencies and hydraulic properties must be specified on the site plan that are not included in the menu of
BMP choices.
11. The average slope and the longest flow length for each drainage area.
A copy of the SET and detailed documentation can be obtained from the following sources:
• Via USMail: Mecklenburg County Water Quality Program,
700 North Tryon Street
Charlotte, NC 28202
Attn: David Kroening
• Via Telephone: 704-336-5500
• Via the Internet: http://stormwater.charmeck.org (select Developers, Contractors, Engineers
& Regulators; select Low Impact Development (LID); select Huntersville
LID; select Mecklenburg County Site Evaluation Tool)
9.3 Plan Submittal Requirements
Plan submittal procedures will remain as described in the Town of Huntersville Zoning
Ordinance and the Mecklenburg County Land Use and Environmental Services Agency
Plan (LUESA) Submittal Guidelines, with the following additions:
1.
Concept Plan: A Concept Plan meeting is required with Town staff and the Mecklenburg
County Water Quality Program (700 N. Tryon Street, Charlotte, NC 28202) prior to a formal plan submittal. The plan shall be on a topographical map showing original contours at intervals of not less than two feet and existing tree lines. It should show in sketch form the proposed layout of streets, lots, and other features in relation to existing conditions as well as how the layout minimizes the impact to steep slopes, natural drainage ways and wooded areas. This Low Impact Design shall include the following:
• The boundary/property lines of the property being developed as well as the location of property lines that intersect the property being developed;
• Water courses on the land to be subdivided or developed;
• Impervious area calculations;
• The location, names, and rights-of-way of any existing streets on or within 300 feet of the land to be subdivided or developed;
• The location of all property lines which intersect the boundaries of the property being subdivided or developed;
• Limits of all wooded areas (locate all trees 6-inches in diameter or larger for special or conditional uses);
• Soils type (HSG) and limits;
• Contour map at two (2) foot intervals extending 100 feet beyond the property boundary;
• Slope Analysis showing ( 0% - 10%, 10% - 15%, 15% - 25%, >25% );
• Natural drainage ways (woodland swales, concentrated flows), ponds;
• Wetlands limits (copy of appropriate Federal and State permits/verification to be submitted with preliminary plans);
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• Rough finished grades, the location of proposed streets, lots, parks or other open spaces, building lines, street cross-sections, number and type of buildings, and the location of any building restriction flood lines;
• Zoning information for the proposed project site and adjacent properties;
• Proposed front, rear, and side yard dimensions for each building type along each street type;
• The location and width of required S.W.I.M. buffers;
• SET Calculations;
• Proposed LID BMPs and locations along with hydrologic analysis;
• The location and width of any S.W.I.M. stream buffers;
• The location of general buffers or screens required for the project area, as a whole;
• The scale of the plan, which shall not be smaller than 100 feet to the inch (All plans and details for BMPs must be drawn to scale for clarity and to aid in construction. Drawings not to scale often lead to construction problems and field errors, or simply can not be built. Standard NCDOT details do not require a specific scale.)
• North point; date; and
• A small scale vicinity map.
2.
Preliminary Plan: Preliminary Plans shall include the above listed information in addition to the Preliminary Plan submittal requirements outlined in the Huntersville Zoning Ordinance and the Mecklenburg County Land Use and Environmental Services Agency Plan Submittal
Guidelines.
3.
Storm Water Management Permit: A Storm Water Management Permit must be obtained by submitting an application demonstrating compliance with the Performance Criteria (see
Section 3) and containing the following information:
• BMP summary table, which lists all BMPs on the site and corresponding NAD 83 (feet) coordinates;
• Calculations and design drawings with detailed BMP plans at 1 inch = 2 feet with 1-foot contours and spot elevations for each BMP and overall site hydrology calculations (see
Section 5) illustrating compliance with Section 3, Performance Criteria. All special details shall be drawn to a specified scale for clarity.
• Planting plan/schedule for each BMP illustrating plant location, species, and quantities; and
• A Maintenance Covenant and Plan for all BMPs. This plan shall include the responsible party for each BMP and a schedule of routine maintenance activities to ensure proper performance. Prior to the approval of the Preliminary Plan and Storm Water Permit, an approved signed copy of the Maintenance Covenant and Plan that has been stamped by the Register of Deeds Office must be submitted to LUESA.
Upon approval of the Storm Water Management Permit, a record plat is required on all BMP structures and where improvements are not complete, a means of financial security must be posted as required by the Huntersville Post-Construction Ordinance. Upon completion of BMP structures and prior to the release of the financial security, a Maintenance Bond shall be required for a period of two years.
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4.
Plat Submittal: In addition to typical Final Plat requirements in the Huntersville Zoning
Ordinance and the Mecklenburg County Land Use and Environmental Services Agency Plan
Submittal Guidelines, the following additional requirements shall apply:
• All BMPs shall be named and recorded on the final plat and applicable deeds, with their corresponding NAD 83 (feet) coordinates. The following naming convention should be used for all BMPs: “Project or subdivision name – BMP Type – Number.” For example, “Birkdale Phase V – Bioretention Garden – 1”.
• Any vegetation, tree save areas, open space or site conditions that contribute to the project’s compliance with Section 3, Performance Criteria, shall be called out, protected, and recorded through the plat and applicable deeds.
• Show all storm drainage easements leading to the BMP.
• Show a 12-foot wide stable maintenance access route. The access route must be contained within a 20-foot wide maintenance access easement from the BMP facility to a public right-of-way.
• Dimensions of BMP easement area with the corresponding BMP identification name/number should be illustrated on the plat.
• Show storm drainage easements around BMP or leading to BMP including access easement.
• Show the maximum impervious for watershed protection areas.
• The following statements must be including on the plat:
“The purpose of the BMP is to treat/reduce the pollutants associated with storm water runoff in order to minimize negative effects to downstream receiving waters. The easement around the BMP is to allow storm water conveyance and system maintenance.
Any buildings and/or obstructions which impede storm water flow or maintenance are prohibited.”
“This plat contains water quality features that must be maintained in accordance with the recorded Maintenance Covenant as specified in Section 8.17.13(a) of the Huntersville
Zoning Ordinance. Removal of plants or disturbance of the BMP structure or otherwise affecting the overall functionality of the BMP for reasons other than maintenance is prohibited.”
Prior to approval of the plat, an approved, signed copy of the Maintenance Covenant and
Plan that has been stamped by the Register of Deeds Office must be submitted to
LUESA.
5.
As-Built Plans: As-Built BMP plans are required for Wet Ponds, Storm Water Wetlands and
Dry Detention Ponds. As-builts are not typically required for other BMPs unless a significant change is made during construction. If this occurs, the inspector may require that an as-built be submitted. When required, as-built plans must be approved prior to issuing an occupancy permit or releasing a performance bond.
6.
Occupancy Permit: Prior to the issuance of an Occupancy Permit for any building within a permitted development served by a BMP, the applicant or owner of the BMP shall establish a formal Maintenance Covenant, approved by the Mecklenburg County Land Use and
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Environmental Services Agency, and recorded in the Office of the Register of Deeds in which the owner acknowledges the duty of the owner and all subsequent owners of the property to maintain the BMP in accordance with the terms of the Covenant. This
Maintenance Covenant is required for all BMPs constructed on privately-owned property and not within a public easement. Appendix G contains an example of a typical Maintenance
Covenant. See Section 8.3.2 of this manual for additional information on maintenance responsibilities.
9.4 Bonding
In extenuating circumstances when BMP construction cannot reasonably be completed before the building is ready for occupancy, a performance bond shall be required. The bond shall be for a time period deemed appropriate by the inspector, not exceeding one year from the time of full occupancy or three years for phased development. Once the bond and bond agreement have been accepted by the County, the Water Quality Hold on the building permit can be released.
9.4.1 Posting Performance Bonds
A bond must be posted for the BMPs on each plat. The purpose of the bond is to ensure that in the absence of required completed work, a means of financial security is posted to ensure the completion of work. Bonds provide the County with financial resources to complete required public improvements in the event of developer/builder default. Bonds will be considered in default and will be called by Mecklenburg County if BMPs are not constructed within 3 years from the construction bond date or if the drainage area is >75% built out. A bond estimate should include complete construction, materials and installation costs for each BMP. Estimates will only be accepted by the design engineer or by a contractor in the form of a Certified
Contractor Bid in detailed line item format. Estimates without adequate detail will not be accepted. The bond amount will be 120% of the certified bid + $10,000. If Mecklenburg
County estimates the bond amount, the following formula will be used: (Surface Area of
Structure x $17.00 / square foot + $15,000). The bond amount will also include contingencies and legal fees and are subject to change. Bonding for the BMP may be posted in a common bond with other required subdivision or zoning improvements or may be posted separately.
The developer may choose from the following options regarding bonds:
• Surety Bond – Issued through a surety company (similar to an insurance policy).
• Letter of Credit – Issued through a credited bank with Mecklenburg County or a full service bank within Mecklenburg County.
• Cash Bond – A certified check to Mecklenburg County or a Company Check to Mecklenburg
County.
Blank bond forms are available on the following website: http://stormwater.charmeck.org
(select
“Post-Construction Programs,” select “Mecklenburg County, Towns….,” select “Forms,” select
“BMP Installation Bond Application Forms”).
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9.4.2 Bond Reduction / Release
A one-time bond reduction is permitted on performance bonds when at least 50% of the construction has been completed and upon approval from the inspector. Bonds are administered at the LUESA North Office and reduction requests should be made to this office.
9.4.3 Maintenance Bonds
BMP Maintenance or Warranty Bonds are required for all structural BMPs. Blank bond forms are available on the following website: http://stormwater.charmeck.org
(select “Post-
Construction Programs,” select “Mecklenburg County, Towns….,” select “Forms,” select “BMP
Maintenance Bond Application Forms”). The purpose of these bonds is to ensure hat funds are secured to maintain BMPs if the owner should fail to do so in which case Mecklenburg County would cash the bond to obtain the money to perform the necessary maintenance. The value of the bond shall be: 30% of BMP construction or $5.10 Square Foot of surface area plus a $10,000 base bond amount. The bonds shall be posted by the owner for a period of not less than two (2) years from the final approval of the BMP by the Storm Water Administrator. The owner is responsible for all repairs and/or material replacement required to ensure the operation of the
BMP in accordance with approved plans and specifications. After the two (2) year warranty period, the owner may request that the Storm Water Administrator release the bond. Before the bond is released, the Water Quality inspector shall conduct a field evaluation of the BMP to ensure proper operation and maintenance. If this field evaluation reveals that the BMP is not being maintained, the inspector may choose to extend the maintenance bond for an additional year. If it is confirmed that the BMP is being properly maintained, the inspector shall complete a
Bond Release Form. This form shall be provided to the bond reviewer and the bond shall be released.
9.4.4 Bonds for Public Entities
Public entities are not required to obtain sureties for either the installation or maintenance of water quality BMPs provided a letter signed by the manager or director of the project provides suitable assurances that the necessary improvements will be installed and maintained in accordance with ordinance requirements. Public entities shall include but not be limited to
Charlotte-Mecklenburg Schools, City of Charlotte, Charlotte-Mecklenburg Utilities, Charlotte-
Mecklenburg Storm Water Services, Mecklenburg County and all the Towns as well as the State and Federal government. The letter of assurance shall be addressed to the Storm Water
Administrator and upon his approval the surety requirement will be waived and all holds/ approvals for the project released.
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10.1 Introduction
This Section establishes inspection and enforcement guidelines to be followed for this manual and corresponding Ordinance. The guidelines are to be applied uniformly for inspection of private and public water quality measures.
10.2 Authority
The provisions of this Ordinance shall be enforced by the Mecklenburg County Land Use and
Environmental Services Agency (LUESA). LUESA staff shall be authorized by the
Huntersville Town Manager to perform the following functions:
1.
Conduct inspections and file reports as necessary during construction of water quality BMPs to ensure compliance with the approved plans and permits.
2.
Furnish the permitting agent or owner of the property the results of the inspection in a timely manner (See Section 10.6).
3.
Issue a Field Inspection Report (FIR) to the permitting agent or owner when any portion of the work does not comply with the approved plans and/or permits (see Section 10.6.1).
4.
Issue a Notice of Violation in accordance with these guidelines as the result of unsatisfactory work or progress, failure to comply with approved plans and permits, and any noncompliance of the requirements of this Ordinance.
5.
Issue a Stop Work Order or Revocation of Permit as the result of unsafe conditions, working without a permit, unsatisfactory work or progress, or other non-compliance.
6.
Issue Civil Citations(s), Civil Actions(s), or Criminal Action(s) listed in Section 10.7 due to unsafe conditions, non-compliance with a Stop Work Order, unsatisfactory work or progress, or other non-compliance (see appropriate Section of the Huntersville Zoning Ordinance).
7.
Perform a final inspection upon the completion of water quality structures to determine if the completed work is constructed in accordance with the approved Storm Water Management
Plan and associated plans/documents.
10.3 Inspection Responsibilities
LUESA staff shall be responsible for performing all inspection activities described in this
Section under authorization of the Huntersville Town Manager.
10.4 Inspection Requirements During Construction
Inspections will be conducted at the request of the owner/developer and at specified stages of construction for each water quality BMP and/or technique described below. Final inspections/approvals are required for all BMPs, but additional inspections/ approvals are required during the construction phase of Drywell/Infiltration Trenches and Bioretention BMPs, as specified in Section 10.4.1 and 10.4.2. The inspectors will also make unscheduled inspections to ensure compliance with the requirements of this manual and corresponding Ordinance as they deem necessary. The inspector will be authorized to accept minor field changes proposed by the owner/contractor. Inspections shall be requested at least 24 hours in advance. When requesting
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10.4.1 Drywell or Infiltration Trench
The following inspections are required for construction of a Drywell or Infiltration Trench. The inspector may require additional inspections.
1.
Excavation Inspection - An inspection must be conducted after excavation and immediately prior to the final topsoil installation. During this inspection, the inspector shall verify the following: a.
Trench dimensions comply with approved plans. b.
Adequate sediment control protection has been installed. c.
Approved filter fabric has been cut and installed to cover the trench beneath the underdrain(s). A six-inch minimum overlap is required between strips of cloth.
Extruding tree roots or other obstacles must be removed from the trench base to prevent the fabric from tearing. d.
Observation well or inlet has be installed as specified on plans prior to stone placement.
Perforations shall not extend beyond gravel trench. e.
Installation of 1 1/2 inch to 3 inch washed stone. Care must be used when dumping the stone to ensure the filter cloth does not tear. f.
Installation of the inlet pipe for Drywells. g.
Placement of topsoil on-site (but not installed).
2.
Final Inspection - During this inspection, the inspector will verify the following: a.
Gravel surface must be completely covered with cloth and backfilled with topsoil for
Drywells and Infiltration Trenches. For Dry wells. surface inlets must be installed as per approved plans and the observation well must be capped.
Upon stabilization of the area, if the above items have been completed satisfactorily, a final inspection report will be issued and approved by the inspector. The final inspection is required before requesting a Final Certificate of Occupancy.
10.4.2 Bioretention BMPs
The following inspections are required for construction of a bioretention BMP. The inspector may require additional inspections.
1. Excavation and Underdrain Inspection - An inspection shall be conducted after excavation and underdrain installation and immediately prior to the final stone and topsoil installation.
During this inspection, the inspector shall verify the following: a. Trench dimensions comply with approved plans. b. Adequate sediment control protection has been installed. c. Stone, underdrain and filter fabric have been installed as per the approved plan. d.
Proper grade has been establishment. e.
Placement of topsoil on-site (but not installed).
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2. Final Inspection - During this inspection, the inspector shall verify the following: a.
Proper topsoil installation. b.
Proper plant installation (native plants, size) and plant health as per the approved plan. c.
Maintenance agreement/covenant complete. d.
Outlet properly installed. e.
Proper placement of ground cover/mulch. f.
Drainage area completely stabilized. g.
Drainage area conforms to approved plan.
Upon stabilization of the area, if the above items have been completed satisfactorily, a final inspection report will be issued and approved by the inspector. The final inspection is required before requesting a Final Certificate of Occupancy.
10.5 Maintenance Inspections
10.5.1 Introduction
Maintenance inspections must be performed at a minimum of once a year to ensure that water quality BMPs remain functional and properly maintained. These inspections must be conducted by a qualified registered North Carolina professional engineer or landscape architect performing services only in their area of competence. Written inspections reports must be provided to the
Mecklenburg County Water Quality Program at 700 North Tryon Street, Charlotte, N.C. 28202.
Inspection reports will be maintained on file at this office.
10.5.2 Responsibilities and Procedures
The owner of the property will be contacted prior to the maintenance inspection. If the owner cannot be contacted, the inspection will be performed and a report will be sent to the owner.
1.
A Warning Citation and/or Notice of Violation will be issued for any maintenance and/or repairs required.
2.
The owner shall begin to make corrections to the violations within 10 days of the date of the
Warning Citation and/or Notice of Violation. After 10 days, Civil Citations may be issued as governed in Section 10.7.2.3.
3.
If an imminent hazard exists, the inspector may post the Water Quality BMP unsafe as per the North Carolina State Building Code and require immediate repair.
4.
Upon the owner’s failure to comply with the Warning Citation, Notice of Violation, Civil
Citation(s), the inspector may issue a Civil or Criminal Action against the property owner and/or contractor.
10.6 Approvals and Reports
Approvals and inspection reports shall be maintained by the inspector to insure proper notification of construction approvals and failure to comply with the approved plans to the owner, contractor or developer.
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10.6.1 Field Inspection Report
The purpose of the Field Inspection Report (FIR) is to notify the owner and or contractor/developer of construction deficiencies noted by the inspector, and to direct repairs and corrections.
FIRs will be issued when the permittee or agent is directed to make changes to their work to satisfy this manual and corresponding Ordinance, approved storm water design plans, or specifications. The notice shall set forth the nature of the corrections required and the time allotted to make the necessary corrections. FIRs shall be issued when the following has occurred:
1.
Failure to comply with the design plan. Incorrect measurements, using improper materials or failing to follow proper procedures can prompt the issuance of a FIR. FIRs shall be issued in writing except when a verbal notice would result in immediate compliance as the work is being completed. Verbal notices shall be noted in the project file.
2.
Failure to provide certification for water quality structures. The inspector shall issue a FIR to the owner/developer requesting certifications and/or as-builts. A compliance date and a mailing address for sending needed information shall be supplied.
10.7 Administration and Enforcement
If an application for a Building Permit, Grading Permit, Storm Water Permit, Zoning
Land Use Permit or Certificate of Occupancy is denied because of non-compliance with this
Ordinance, the inspector shall provide notification of the denial and of the reasons therefore.
If the owner fails to comply with the FIR during construction, the inspector shall provide notification of the denial and of the reasons therefore.
10.7.1 Right to Appeal
If a request for a permit is disapproved or if a ruling of the Zoning Administrator is questioned, any aggrieved party may appeal such ruling to the Huntersville Board of Adjustment as provided in Section 10.3 of the Huntersville Zoning Ordinance. An appeal or variance to the Board of
Adjustment, lawfully and completely filed within 30 days of the date of the decision , shall stay enforcement action and penalties until a hearing has been held and a decision rendered by the
Board of Adjustment.
10.7.2 Penalties
In case any water quality BMP is installed, constructed, reconstructed, altered, repaired, converted or maintained in violation of these regulations, an action for injunction, mandamus, or other appropriate action or proceeding to prevent such violation may be instituted by the Zoning
Administrator or other authority designated by the Board of Commissioners as enforcement agent(s) for this ordinance. Penalties and remedies are stated in Section 10.2.3 of the
Huntersville Zoning Ordinance and as listed below:
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1.
Notice of Violation.
The inspector will issue a Notice of Violation to the owner upon noncompliance of this Ordinance. In most cases a FIR will be used for the first offense.
Subsequent noncompliance with these regulations or failure to complete the items on the FIR within the specified period of time will result in Notice of Violation.
2.
Stop Work Order Issuance and Revocation of Permits. Whenever a water quality BMP or part thereof is being installed, constructed, renovated, altered, or repaired in violation of any provisions of this ordinance, the inspector may order the specific part of the work that is in violation, or would be when the work is completed, to be immediately stopped. The Stop
Work Order shall be in writing, directed to the person doing the work and/or owner, and shall state the specific work to be stopped, the specific reasons for cessation and the action(s) necessary to lawfully resume work.
The Zoning Administrator may revoke any permit (e.g. building, grading, storm water, zoning use, certificate of occupancy) by written notification to the permit holder and/or owner when violations of this manual or corresponding Ordinance have occurred. Permits may be revoked when false statements or misrepresentations were made in securing the permit, work is being or has been done in substantial departure from the approved application or plan, there has been a failure to comply with the requirements of this manual or corresponding Ordinance, or a permit has been mistakenly issued in violation of this manual or corresponding Ordinance.
Failure to comply with the proper construction sequence as outlined in this Ordinance may cause a Notice of Violations/ Stop Work Orders to be issued in such cases as described below: a.
Failure to notify the Department before beginning any work to implement the water quality BMP (including not requesting a pre-construction meeting):
Any work that has been placed without a required inspection approval shall be certified in writing by a registered professional engineer before the next phase of construction begins. The inspector reserves the right to require investigative materials testing on all un-inspected facilities or devices at the sole expense of the permittee/owner. Any deficiencies that need to be corrected for work already started shall be listed and given a compliance date. The permittee shall be notified to call for future inspections as required, as well as any additional inspections required by the inspector. b.
Failure to have work inspected and approved before continuing work:
It is required that inspection point(s) not approved be certified in writing by a registered professional engineer. The inspector reserves the right to require investigative or materials testing on all un-inspected facilities or devices at the sole expense of the permittee/owner. c.
Failure to call for a final inspection:
The inspector shall list all deficiencies that need to be corrected, give a compliance date, and request a letter of certification and/or as-builts (if required) be submitted. The owner or contractor/developer shall request a re-inspection after completing the corrections so another final inspection can be made. d.
Failure to provide certification for completed water quality BMPs:
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Town of Huntersville Water Quality Design Manual………………………………...July 6, 2012
If an engineer’s certification and/or as-built is not received by the compliance date as required by a previously issued FIR, a Notice of Violation to the owner and/or developer shall be sent requesting certification and/or as-builts.
3.
Civil Penalty.
Pursuant to NC General Statute 160A-175, the regulations and standards of the manual or corresponding Ordinance may be enforced through the issuance of civil penalties by the Zoning Administrator for the following situations: a.
When a FIR, Notice of Violation and/or Stop Work Order has not been complied with or there has not been substantial progress in complying with the Notice of Violation and/or Stop Work Order. b.
On abandoned sites where no work has been on going, and continued non-compliance with a Notice of Violation may result in the issuance of repeated citations. c.
When a Stop Work Order is in effect and work continues in defiance of the order. d.
When repeated recurring violations of the same section of this manual or corresponding Ordinance. Each day that a violation remains uncorrected constitutes a separate violation of applicable code or ordinance.
Subsequent citations for the same violation may be issued by the Zoning Administrator if the offender does not pay the citation (except as otherwise provided in a warning situation) after it has been issued unless the offender has sought an Appeal to the
Decision of the Zoning Administrator through the Board of Adjustment. Once the tenday warning period has expired, each day which the violation continues shall subject the violator to additional citations to be issued by the Zoning Administrator.
The following penalties are hereby established: a.
Warning Citation and/or Notice of Violation - Correct Violation within 10 days. b.
First Citation - $50. c.
Second Citation for Same Offense - up to $200. d.
Third and Subsequent Citations for Same Offense - up to $500, then $500 per day.
A citation may be issued every day the violation continues after the 10 day warning period. Hazardous violations or violations that cause risk to the public’s health or welfare may be fined immediately. If the offender fails to pay the civil penalties within
10 days after having been cited, the Town of Huntersville may recover the penalties in a civil action in the nature of debt.
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8.17
Water Quality
.1 Purpose.
The purpose of this regulation is to establish storm water management requirements and controls to prevent surface water quality degradation to the extent practicable in the streams and lakes within the Town Limits and Extraterritorial Jurisdiction of Huntersville and to protect and safeguard the general health, safety, and welfare of Huntersville’s residents. This regulation seeks to meet this purpose by fulfilling the following objectives: a) Minimize increases in storm water runoff from development or redevelopment in order to reduce flooding, siltation and streambank erosion, and maintain the integrity of stream channels; b) Minimize increases in non-point source pollution caused by storm water runoff from development or redevelopment that would otherwise degrade local water quality; c) Minimize the total volume of surface water runoff that flows from any specific site during and following development in order to replicate pre-development hydrology to the maximum extent practicable; d) Reduce storm water runoff rates and volumes, soil erosion and non-point source pollution, to the extent practicable, through storm water management controls
(BMPs) and to ensure that these management controls are properly maintained and pose no threat to public health or safety; and e) Meet the requirements of the National Pollution Discharge Elimination System
(NPDES) Storm Water Permit and other requirements as established by the Clean
Water Act.
This regulation and the Huntersville Water Quality Design Manual require the use of
Low Impact Development (LID) practices, which more closely replicate a site’s predevelopment characteristics compared to conventional storm water management techniques.
.2 Applicability.
This regulation shall apply to all of the land located within the Town Limits and
Extraterritorial Jurisdiction of Huntersville. The effective date of this regulation is June
30, 2007. This regulation governs the development and use of all land and structures.
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.3
.4
.5
No building, structure, or land shall be used, occupied or altered, and no building, structure, or part thereof shall be erected, constructed, reconstructed, moved, enlarged, or structurally altered, unless in conformity with all the provisions of this regulation and all other applicable regulations, except as otherwise provided by this regulation.
Exceptions to Applicability. a) All properties shall be subject to this regulation except those properties which, as of the effective date of June 30, 2007, fit into one of the following categories:
(1) Have been issued a Certificate of Building Code Compliance;
(2) Have a valid building permit; or
(3) Are included on a valid preliminary subdivision plan and/or a valid sketch plan.
(4) Any complete conditional rezoning application and subdivision sketch plan submitted by May 1, 2007. b) Redevelopment of non-single family homes that disturbs less than 20,000 square feet, does not decrease existing storm water controls, and renovation and/or construction costs do not exceed 100% of the tax value of the property is not subject to the provisions of this regulation. c) New development that includes the creation of less than or equal to 6% built upon area is not subject to the provisions of this regulation. d) Residential development activity that disturbs less than one acre of land and is not part of a larger common plan of development or sale, including new development, redevelopment or expansions, is not subject to the provisions of this regulation. e) Non-residential development activity that disturbs less than ½ acre of land and is not part of a larger common plan of development or sale, including new development, redevelopment or expansions, is not subject to the provisions of this regulation.
No Development or Redevelopment Until Compliance and Permit.
No development or redevelopment shall occur except in compliance with the provisions of this ordinance or unless exempted. No development for which a permit is required pursuant to this ordinance shall occur except in compliance with the provisions, conditions, and limitations of the permit.
Map.
The provisions of this ordinance shall apply within the areas designated on the map titled “Post-Construction Ordinance Map of the Town of Huntersville, North
Carolina” (hereafter referred to as the “Post-Construction Ordinance Map”), which is adopted simultaneously herewith. The Post-Construction Ordinance Map and all
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.6 explanatory matter contained thereon accompanies and is hereby made a part of this ordinance. The Post-Construction Ordinance Map shall be kept on file by the Storm
Water Administrator or designee (hereinafter referred to as the “Storm Water
Administrator”) and shall be updated to take into account changes in the land area covered by this ordinance and the geographic location of all structural BMPs permitted under this ordinance. In the event of a dispute, the applicability of this ordinance to a particular area of land or BMP shall be determined by appeal through the Storm Water
Administrator
Definitions.
For the purposes of this Ordinance, the following words and phrases shall be defined as specified below:
Administrative Manual. A manual developed by the Storm Water Administrator and distributed to the public to provide information for the effective administration of this ordinance, including but not limited to application requirements, submission schedule, fee schedule, maintenance agreements, criteria for mitigation approval, criteria for recordation of documents, inspection report forms, requirements for submittal of bonds, a copy of this ordinance, and where to obtain the Design Manual.
Best Management Practices (BMPs). A structural or nonstructural management based practice used singularly or in combination to reduce non-point source input to receiving waters in order to achieve water quality protection goals.
• Non-structural BMPs - Non-engineering methods to control the amount of nonpoint source pollution. These may include land-use controls and vegetated
• buffers.
Structural BMPs - Engineered structures that are designed to reduce the delivery of pollutants from their source or to divert contaminants away from a waterbody.
Built-Upon Area (BUA). That portion of a development project that is covered by impervious or partially impervious surface including, but not limited to, buildings; pavement and gravel areas such as roads, parking lots, and paths; and recreation facilities such as tennis courts. “Built-upon area” does not include a wooden slatted deck or the water area of a swimming pool.
Design Manual. The storm water design manual shall be approved for use in the Town of
Huntersville by the North Carolina Department of Environment and Natural Resources and shall be at least as stringent as the storm water design manual approved for use in
Phase II jurisdictions by the Department for the proper implementation of the requirements of the federal Phase II storm water program. All references herein to the
Design Manual are to the latest published edition or revision.
Detain. To store and slowly release storm water runoff following precipitation by means of a surface depression or tank and an outlet structure. Detention structures are commonly used for pollutant removal, water storage, and peak flow reduction.
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Hydrologic Abstractions. Physical processes of interception of rainfall or overland storm water flow by vegetation, evaporation from land surfaces and upper soil layers, transpiration by plants, infiltration of water into soil surfaces, and storage of water in surface depressions.
Low Impact Development (LID). The integration of site ecology and environmental goals and requirements into all phases of urban planning and design from the individual residential lot level to the entire watershed.
Mecklenburg County Land Use and Environmental Services Agency. The department or division of Mecklenburg County government (regardless of the title given to it by
Mecklenburg County) which has responsibility for storm water and water quality matters, acting as the agent of the Town of Huntersville for various purposes in connection with the enforcement of this regulation.
National Pollution Discharge Elimination System (NPDES) Permit. A permit issued pursuant to the federal Clean Water Act for the purpose of controlling discharges of pollutants to surface waters and protecting water quality. In North Carolina, NPDES
Permits are issued by the N.C. Department of Environment and Natural Resources.
Non-Point Source (NPS) Pollution. Forms of pollution caused by sediment, nutrients, organic and toxic substances originating from land use activities and carried to lakes and streams by surface runoff.
Retain. To capture and hold storm water runoff following precipitation by means of surface depression allowing the water to infiltrate into the soil, thus reducing the hydrologic and pollution impacts downstream. Retention structures are commonly used for pollutant removal, water storage, and peak flow reduction.
Site Evaluation Tool (SET). A spreadsheet-based model that assesses and compares predevelopment and post-development runoff, infiltration, and pollutant loading rates, which provides a methodology to aid in better site design and evaluation of BMP effectiveness.
Storm Water Administrator. The Mecklenburg County Water Quality Program Manager that has been designated by the Town of Huntersville Board of Commissioners to administer and enforce this ordinance.
Storm Water Management Permit. A permit required for all development and redevelopment unless exempt pursuant to this ordinance, which demonstrates compliance with this ordinance.
Total Suspended Solids (TSS). Total suspended matter in water, which is commonly expressed as a concentration in terms of milligrams per liter (mg/l) or parts per million
(ppm).
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.7 Interpretation. a) Meaning and Intent
All provisions, terms, phrases, and expressions contained in this ordinance shall be construed according to the general and specific purposes set forth in Section
8.17.1, Purpose. If a different or more specific meaning is given for a term defined elsewhere in the Town of Huntersville ordinances, the meaning and application of the term in this ordinance shall control for purposes of application of this ordinance. b) Text Controls in Event of Conflict
In the event of a conflict or inconsistency between the text of this ordinance and any heading, caption, figure, illustration, table, or map, the text shall control. c) Authority for Interpretation
The Storm Water Administrator has authority to interpret this ordinance. Any person may request an interpretation by submitting a written request to the Storm
Water Administrator who shall respond in writing within 30 days. The Storm
Water Administrator shall keep on file a record of all written interpretations of this ordinance. d) References to Statutes, Regulations, and Documents e) Computation of Time
The time in which an act is to be done shall be computed by excluding the first day and including the last day. If a deadline or required date of action falls on a
Saturday, Sunday, or holiday observed by the Town of Huntersville, the deadline or required date of action shall be the next day that is not a Saturday, Sunday or holiday observed by the Town of Huntersville. References to days are calendar days unless otherwise stated. f)
Whenever reference is made to a resolution, ordinance, statute, regulation, manual
(including the Design and Administrative Manuals), or document, it shall be construed as a reference to the most recent edition of such that has been finalized and published with due provision for notice and comment, unless otherwise specifically stated.
Delegation of Authority
Any act authorized by this ordinance to be carried out by the Storm Water
Administrator of the Town of Huntersville may be carried out by his or her designee.
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g) Usage
(1) Mandatory and Discretionary Terms
The words “shall,” “must,” and “will” are mandatory in nature, establishing an obligation or duty to comply with the particular provision.
The words “may” and “should” are permissive in nature.
(2) Conjunctions
Unless the context clearly indicates the contrary, conjunctions shall be interpreted as follows: The word “and” indicates that all connected items, conditions, provisions or events apply. The word “or” indicates that one or more of the connected items, conditions, provisions or events apply.
(2) Tense, Plurals, and Gender
Words used in the present tense include the future tense. Words used in the singular number include the plural number and the plural number includes the singular number, unless the context of the particular usage clearly indicates otherwise. Words used in the masculine gender include the feminine gender, and vice versa.
.8 h) Measurement and Computation
Lot area refers to the amount of horizontal land area contained inside the lot lines of a lot or site.
Huntersville Water Quality Design Manual.
Subject to the approval of the Storm Water Administrator, the Mecklenburg County Land
Use and Environmental Services Agency shall furnish, maintain and update a Water
Quality Design Manual for the Town of Huntersville containing additional policy, criteria and information that shall be followed for the proper implementation of the requirements of this Ordinance. This Design Manual shall be at least as stringent as the storm water design manual approved for use in Phase II jurisdictions by the N.C. Department of
Environment and Natural Resources for the proper implementation of the requirements of the federal Phase II storm water program. All references herein to the Design Manual are to the latest published edition or revision. BMPs that are designed and constructed in accordance with the criteria contained in this Manual shall be accepted as meeting the minimum Performance Criteria.
If the specifications or guidelines of the Design Manual are more restrictive or apply a higher standard than other laws or regulations, that fact shall not prevent application of the specifications or guidelines in the Design Manual.
Standards, specifications, guidelines, policies, criteria, or other information in the Design
Manual in affect at the time of acceptance of a complete application shall control and
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shall be utilized in reviewing the application and in implementing this ordinance with regard to the application. The Design Manual may be updated and expanded from time to time, based on advancements in technology and engineering, improved knowledge of local conditions, or local monitoring or maintenance experience. Prior to amending or updating the Design Manual, proposed changes shall be generally publicized and made available for review, and an opportunity for comment by interested persons shall be provided.
Relationship to Other Laws, Regulations and Private Agreements. .9
(a) Conflict of Laws
This ordinance is not intended to modify or repeal any other ordinance, rule, regulation or other provision of law. The requirements of this ordinance are in addition to the requirements of any other ordinance, rule, regulation or other provision of law, and where any provision of this ordinance imposes restrictions different from those imposed by any other ordinance, rule, regulation or other provision of law, whichever provision is more restrictive or imposes higher protective standards for human or environmental health, safety, and welfare, shall control.
(b) Private Agreements
This ordinance is not intended to revoke or repeal any easement, covenant, or other private agreement. However, where the regulations of this ordinance are more restrictive or impose higher standards or requirements than such easement, covenant, or other private agreement, then the requirements of this ordinance shall govern. Nothing in this ordinance shall modify or repeal any private covenant or deed restriction, but such covenant or restriction shall not legitimize any failure to comply with this ordinance. In no case shall the Town of Huntersville be obligated to enforce the provisions of any easements, covenants, or agreements between private parties.
.10 Severability.
If the provisions of any section, subsection, paragraph, subdivision or clause of this ordinance shall be adjudged invalid by a court of competent jurisdiction, such judgment shall not affect or invalidate the remainder of any section, subsection, paragraph, subdivision or clause of this ordinance.
.11 Effective Date and Transitional Provisions.
(a) Effective Date
This ordinance shall take effect on June 30, 2007.
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(b) Violations Continue
Any violation of the provisions of this ordinance existing as of the effective date of this ordinance shall continue to be a violation under this ordinance and be subject to penalties and enforcement unless the use, development, construction, or other activity complies with the provisions of this ordinance.
.12 Performance Criteria.
Land development activities shall be performed in such a manner as to minimize the degradation of water quality conditions through compliance with the Performance
Criteria listed below. Section 6 of the Huntersville Water Quality Design Manual contains a description of approved BMPs for meeting each of these Criteria. a) All storm water treatment systems used to meet these Performance Criteria shall be designed to achieve average annual 85% Total Suspended Solids (TSS) removal for the developed area of the site. Areas designated as open space that are not developed do not require storm water treatment. All sites must employ LID practices to control and treat runoff from the first one inch of rainfall. b) LID practices or a combination of LID and conventional storm water management practices shall be used to control and treat the increase in storm water runoff volume associated with post-construction conditions as compared with preconstruction (existing) conditions for the 2-year frequency, 24-hour duration storm event in the Rural and Transitional Zoning Districts. For all other Zoning
Districts, LID practices or a combination of LID and conventional storm water management practices shall be used to control and treat the increase in storm water runoff volume associated with post-construction conditions as compared with pre-construction (existing) conditions for the 1-year frequency, 24-hour duration storm event. This may be achieved by hydrologic abstraction, recycling and/or reuse, or other accepted management practice as described in Section 6 of the Huntersville Water Quality Design Manual. c) Where any storm water BMP employs the use of a temporary water quality storage pool as a part of its treatment system, the drawdown time shall be a minimum of 48 hours and a maximum of 120 hours. d) Peak storm water runoff rates shall be controlled for all development above 12% imperviousness. The peak storm water runoff release rates leaving the site during post-construction conditions shall be equal to or less than the pre-development peak storm water runoff release rates for the 2-year frequency, 24-hour duration storm event and 10-year frequency, 24-hour duration storm event. The emergency overflow and outlet works for any pond or wetland constructed as a storm water BMP shall be capable of safely passing a discharge with a minimum recurrence frequency of 50 years. For detention basins, the temporary storage capacity shall be restored within 72 hours. Requirements of the Dam Safety Act shall be met when applicable.
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e) No one BMP shall receive runoff from an area greater than five (5) acres.
However, the total drainage area from BMPs used in series (i.e., integrated) can exceed this five (5) acre maximum. f) Storm water runoff from the development shall be transported from the development by vegetated conveyance to the maximum extent practicable.
.13 Low Impact Development (LID).
The Performance Criteria shall be achieved using LID site planning and techniques or a combination of LID and conventional storm water management practices. The goal of
LID is to develop site design techniques, strategies, and BMPs to store, infiltrate, evaporate, retain, and detain runoff on the site to more closely replicate pre-development runoff characteristics and to better mimic the natural and unique hydrology of the site thereby limiting the increase in pollutant loads caused by development. The selection of these strategies and techniques for compliance with the Performance Criteria is discretionary and shall be detailed in a storm water management permit application submitted during the preliminary plan review process. Specific requirements regarding the design, installation and maintenance of LID structures and a discussion of LID site planning is contained in the Huntersville Water Quality Design Manual.
.14 Storm Water Administrator. a) Designation. The Mecklenburg County Water Quality Program Manager has been designated as the Storm Water Administrator by the Town of Huntersville for the purpose of administering and enforcing this ordinance. Any act authorized by this ordinance to be carried out by the Storm Water Administrator of the Town of Huntersville may be carried out by his or her designee. b) Powers and Duties. In addition to the powers and duties that may be conferred by other provisions of other laws, the Storm Water Administrator shall have the following powers and duties under this ordinance:
(1) To review and approve or disapprove applications submitted pursuant to this ordinance.
(2) To make determinations and render interpretations of this ordinance.
(3) To establish application requirements and schedules for submittal and review of applications and appeals.
(4) To enforce this ordinance in accordance with its enforcement provisions.
(5) To maintain records, maps, and official materials as relate to the adoption, amendment, enforcement, or administration of this ordinance.
(6) To provide expertise and technical assistance upon request to the
Huntersville Town Board.
(7) To designate appropriate other person(s) who shall carry out the powers and duties of the Storm Water Administrator.
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(8) To provide information and recommendations relative to variances and information as requested by the Town of Huntersville in response to appeals.
(9) To take any other action necessary to administer the provisions of this ordinance.
.15 REVIEW PROCEDURES. a) Permit Required; Must Apply for Permit. A storm water management permit is required for all development and redevelopment unless exempt pursuant to this ordinance. A permit may only be issued subsequent to a properly submitted, reviewed and approved permit application, pursuant to this Section. The content and form of the permit shall be established by the Storm Water Administrator.
(1) Concept Plans. A Concept Plan shall be submitted to the Storm Water
Administrator for review prior to the submittal of the Storm Water
Management Permit Application. This Concept Plan shall include the information necessary to evaluate the proposed development site for compliance with Performance Criteria as detailed in the Huntersville
Water Quality Design Manual.
(2) Site Evaluation Tool. A storm water management permit application shall include Site Evaluation Tool (SET) output along with sketch plans for each proposed development site. Section 9 of the Huntersville Water
Quality Design Manual contains detailed information concerning the submission requirements for SET. b) Effect of Permit. A storm water management permit shall govern the design, installation, and construction of storm water management and control practices on the site, including structural BMPs and elements of site design for storm water management other than structural BMPs. The permit is intended to provide a mechanism for the review, approval, and inspection of the approach to be used for the management and control of storm water for the development or redevelopment site consistent with the requirements of this ordinance.
Compliance after project construction is assured by the maintenance provision of this ordinance. c) Authority to File Applications. All applications required pursuant to this ordinance shall be submitted to the Storm Water Administrator by the land owner or the land owner’s duly authorized agent or anyone having interest in the property by reason of a written contract with the owner. d) Establishment of Application Requirements, Schedule, and Fees.
(1) Application Contents and Form. The Storm Water Administrator shall establish requirements for the content and form of all applications and
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shall amend and update those requirements from time to time. At a minimum, the storm water management permit application shall describe in detail how post-construction storm water runoff will be controlled and managed, the design of all storm water facilities and practices, and how the proposed project will meet the requirements of this ordinance. The permit application shall contain SET outputs, supporting computations, drawings, soil analyses, calculations for each BMP and overall site hydrology calculations as well as other information sufficient to describe the manner, location, and type of measures for managing storm water from the development in compliance with this ordinance. In addition, the permit application shall specify those parties responsible for long term maintenance of all BMPs.
(2) Submission Schedule. The Storm Water Administrator shall establish a submission schedule for applications. The schedule shall establish deadlines by which complete applications must be submitted for the purpose of ensuring that there is adequate time to review applications, and that the various stages in the review process are accommodated.
(3) Permit Review Fees. The Town of Huntersville shall establish permit review fees as well as policies regarding refund of any fees upon withdrawal of an application, and may amend and update the fees and policies from time to time.
(4) Administrative Manual. For applications required under this ordinance, the Storm Water Administrator shall compile into an Administrative
Manual the application requirements, submittal checklist, submission schedule, fee schedule, maintenance agreements, a copy of this ordinance, and where to obtain the Design Manual, as well as other information and materials necessary for the effective administration of this ordinance. This
Administrative Manual shall be made available to the public. e) Submittal of Complete Application. Applications shall be submitted to the Storm
Water Administrator pursuant to the application submittal schedule in the form established by the Storm Water Administrator, along with the appropriate fee established pursuant to this Section. An application shall be considered as timely submitted only when it contains all elements of a complete application pursuant to this ordinance, along with the appropriate fee. If the Storm Water Administrator f) finds that an application is incomplete, the applicant shall be notified of the deficient elements and shall be provided with an opportunity to submit a complete application. However, the submittal of an incomplete application shall not suffice to meet a deadline contained in the submission schedule established above.
Review. Within 30 working days after a complete application is submitted, the
Storm Water Administrator shall review the application and determine whether the application complies with the standards of this ordinance.
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(1) Approval. If the Storm Water Administrator finds that the application complies with the standards of this ordinance, the Storm Water
Administrator shall approve the application and issue a storm water management permit to the applicant. The Storm Water Administrator may impose conditions of approval as needed to ensure compliance with this ordinance. The conditions shall be included in the permit as part of the approval.
(2) Fails to Comply. If the Storm Water Administrator finds that the application fails to comply with the standards of this ordinance, the Storm
Water Administrator shall notify the applicant and shall indicate how the application fails to comply. The applicant shall have an opportunity to submit a revised application.
(3) Revision and Subsequent Review. A complete revised application shall be reviewed by the Storm Water Administrator within 15 working days after its re-submittal and shall be approved, approved with conditions or disapproved. If a revised application is not re-submitted within sixty (60) calendar days from the date the applicant was notified, the application shall be considered withdrawn, and a new submittal for the same or substantially the same project shall be required along with the appropriate fee.
.16 Applications for Approval.
(a) Concept Plan and Consultation Meeting
Before a storm water management permit application is submitted, the Storm
Water Administrator or land owner or the land owner’s duly authorized agent or anyone having interest in the property by reason of a written contract with the owner may request consultation(s) on a concept plan for the post-construction storm water management system to be utilized in the proposed development project. This consultation meeting(s) should take place at the time of the preliminary plan of the subdivision or other early step in the development process.
The purpose of this meeting(s) is to discuss the post-construction storm water management measures necessary for the proposed project, as well as to discuss and assess constraints, opportunities and potential approaches to storm water management designs before formal site design engineering is commenced. Local watershed plans and other relevant resource protection plans may be consulted in the discussion of the concept plan.
At the time of concept plan submittal, the following information should be included in the concept plan, which should be submitted in advance of the meeting as specified in the Administrative Manual:
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(1) Existing Conditions / Proposed Site Plans. Existing conditions and proposed site layout sketch plans, which illustrate at a minimum: existing and proposed topography; perennial and intermittent streams; mapping of predominant soils from soil surveys; boundaries of existing predominant vegetation and proposed limits of clearing and grading; proposed
Undisturbed Open Space area; and location of existing and proposed roads, buildings, parking areas and other impervious surfaces.
(2) Natural Resources Inventory. Written or graphic inventory of the natural resources at the site and surrounding area as it exists prior to the commencement of the project. This description should include a discussion of soil conditions, forest cover, geologic features, topography, wetlands, and native vegetative areas on the site, as well as the location and boundaries of other natural feature protection and conservation areas such as lakes, ponds, floodplains, stream buffers and other setbacks (e.g., drinking water well setbacks, septic system setbacks, etc.). Particular attention should be paid to environmentally-sensitive features that provide particular opportunities or constraints for development .
(3) Storm Water Management System Concept Plan. A written or graphic concept plan of the proposed post-construction storm water management system including: preliminary selection and location of proposed structural storm water controls; low impact design elements; location of existing and proposed conveyance systems such as grass channels, swales, and storm drains; flow paths; location of proposed Undisturbed Open Space areas; location of all floodplain/floodway limits; relationship of site to upstream and downstream properties and drainages; and preliminary location of proposed stream channel modifications, such as bridge or culvert crossings. b) Storm Water Management Permit Application
The storm water management permit application shall detail how postconstruction storm water runoff will be controlled and managed and how the proposed project will meet the requirements of this ordinance. All such plans submitted with the application shall be prepared by a registered North Carolina professional engineer or landscape architect. The engineer or landscape architect shall perform services only in their area of competence, and shall verify that the design of all storm water management facilities and practices meets the submittal requirements for complete applications, that the designs and plans are sufficient to comply with applicable standards and policies found in the Design Manual , and that the designs and plans ensure compliance with this ordinance.
The submittal shall include all of the information required in the submittal checklist established by the Storm Water Administrator. Incomplete submittals shall be treated pursuant to Section 8.17.11(e).
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(c) As-Built Plans and Final Approval
The applicant shall certify that the completed project is in accordance with the approved plans and designs, and shall submit actual “as- built” plans for all storm water management facilities or practices after final construction is completed.
Failure to provide approved as-built plans within the time frame specified by the
Storm Water Administrator may result in assessment of penalties. At the discretion of the Storm Water Administrator, performance securities or bonds may be required for storm water management facilities or practices until as-built plans are approved.
As-built plans shall show the final design specifications for all storm water management facilities and practices and the field location, size, depth, and planted vegetation of all measures, controls, and devices, as installed. The designer of the storm water management measures and plans shall certify, under seal, that the asbuilt storm water measures, controls, and devices are in compliance with the approved plans and designs and with the requirements of this ordinance. The exact boundary of all storm water management measures shall be shown on final plats prepared by a registered surveyor. Further, final plats shall contain the following statement: "This lot contains a water quality feature that must be maintained in accordance with the recorded Maintenance Covenant as specified in
Section 8.17.17(a) of the Huntersville Zoning Ordinance
Final as-built plans and a final inspection and approval by the Storm Water
Administrator are required before a project is determined to be in compliance with this ordinance and before performance securities shall be released. At the discretion of the Storm Water Administrator, certificates of occupancy may be withheld pending receipt of as-built plans and the completion of a final inspection and approval of a project.
Upon submittal of as-built plans, the location of storm drainage pipes, inlets and outlets as well as the location of all BMPs must be delivered to the Storm Water
Administrator in the digital format specified in the Administrative Manual.
.17 Approvals. a) Effect of Approval
Approval authorizes the applicant to go forward with only the specific plans and activity authorized in the permit. The approval shall not be construed to exempt the applicant from obtaining other applicable approvals from local, state, and federal authorities.
(1) Time Limit/Expiration
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A Storm Water Management Permit and accompanying plan approved under the provisions of this ordinance shall remain valid for a period of three years from the date of approval. If no work on the site in furtherance of the plan has commenced within the three-year period, the permit and plan approval will become null and void and a new application will be required to develop the site. If work on the site in furtherance of the plan has commenced that involves any utility installations or street improvements except grading, the permit and plan shall remain valid and in force and the project may be completed in accordance with the approved plan.
.18
Incentives. a) Purpose. The purpose of this Section is to set forth incentives to offset restrictions that LID may have on the development of certain sites. b) Setbacks. In order to accommodate water quality BMPs, required setbacks, side yards and rear yards in the Rural and Transitional Zoning Districts may be reduced up to 25%. The reductions may not compromise public safety such as the site distance triangles as defined by this Zoning Ordinance. c) Sidewalks. To reduce impervious cover and promote LID, sidewalks on one side of the street may be waived in the Rural Zoning District. d) Required Plant Reduction and Substitution. In order to accommodate water quality BMPs, the number of planted trees may be reduced in buffer yards by
10%, 50% of the required trees may be 1.5 inches in caliper, and all shrubs may be 24 inches in height. e) Encroachments. Water quality BMPs may encroach into a required buffer yard as long as the encroachment does not disturb existing vegetation. Minor understory may be disturbed in order to accommodate water quality structures. Trees and shrubs shall be placed to maximize screening where the encroachment takes place. If the encroachment runs parallel to the buffer, the width of the buffer shall be increased by the amount of the encroachment.
.19
Appeals and Variances.
An appeal to reverse or modify the order, decision, determination, or interpretation of the
Storm Water Administrator or any other designated administrative officer shall comply with the procedures and standards of Section 11.3 of these regulations.
.20 Posting of Financial Security.
Approval of a storm water permit application shall be subject to the owner filing a surety bond or letter of credit or making other financial arrangements in favor of Mecklenburg
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County as agent for the Town of Huntersville acceptable to the Mecklenburg County
Land Use and Environmental Services Agency guaranteeing the installation and maintenance of required BMPs until the issuance of certificates of occupancy for seventy-five percent (75%) of all construction which might reasonably be anticipated to be built within the area which drains into the BMP, allowing credit for improvements completed prior to the submission of the final plat. At such time that this level of occupancy is achieved, written notice thereof shall be submitted by the owner to the
Mecklenburg County Land Use and Environmental Services Agency. The owner shall verify the adequacy of the Maintenance Covenant for the BMPs including the necessary financing to support the proposed maintenance practices. The owner shall also provide professional engineer certification that the BMP is designed and constructed in accordance with approved plans and specifications. The Mecklenburg County Land Use and Environmental Services Agency will inspect the structural BMP and verify the effectiveness of the Maintenance Covenant and, if both are found to be satisfactory, the
Mecklenburg County Land Use and Environmental Services Agency will notify the owner in writing within 30 days of the date of notice regarding approval of the BMP.
Following the issuance of this written approval, the owner can request the release of the surety bond, letter of credit or other financial arrangements at which time the maintenance responsibilities for the BMPs shall revert to the Homeowners Association, property owner or other party responsible for long term maintenance as specified in the
Maintenance Covenant. It shall be expressly stated within the restrictive covenants or homeowners association documents that it will be the responsibility of the owner or assigns to maintain BMPs until such time as maintenance responsibilities have been transferred to the Homeowners Association Board of Directors, property owner or other party responsible for long term maintenance of the BMPs. It shall be the sole responsibility of the owner or assigns to correct any deficiencies prior to said transfer of maintenance responsibilities.
.21 Maintenance.
The owner of each BMP installed pursuant to this ordinance shall maintain and operate it so as to preserve and continue its function is controlling storm water quality and quantity at the degree or amount of function for which the BMP was designed. BMPs shall not be constructed on public land, within public rights-of-way, and/or within public easements without written approval from the public body with ownership/jurisdiction of the subject property.
The following requirements shall be met for all BMPs that have been constructed on privately-owned property and not within a public easement. a) Operation and Maintenance Agreement. Prior to the issuance of an Occupancy
Permit for any building within a permitted development served by a BMP, the applicant or owner of the BMP shall establish a formal Operation and
Maintenance Agreement approved by the Storm Water Administrator and recorded in the Office of the Register of Deeds in which the owner acknowledges the duty of the owner and all subsequent owners of the property to maintain the
BMP in accordance with the terms of the Agreement, including repairing and, if necessary, reconstruction of the BMP. Until the transference of all property, sites,
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or lots served by the structural BMP, the original owner or applicant shall have primary responsibility for carrying out the provisions of the Operation and
Maintenance Agreement. A maintenance plan shall be included as part of the
Agreement that shall state the terms, conditions, and schedule of maintenance for the structural BMP as well as describe the mechanism for funding maintenance and repairs. The Operation and Maintenance Agreement shall also specify the
Property Owners Association or other party responsible for maintenance of the
BMP. A Property Owners Association or similar legal entity shall have the power to compel contributions from residents of a development to cover their proportionate shares of the costs associated with BMP maintenance. At the discretion of the Storm Water Administrator, certificates of occupancy may be withheld pending receipt of an operation and maintenance agreement.
Standard Operation and Maintenance Agreements for BMPs shall be developed by the Storm Water Administrator and made available in the Administrative
Manual. The Operation and Maintenance Agreement must be approved by the
Storm Water Administrator prior to plan approval, and it shall be referenced on the final plat and shall be recorded by the applicant or owner with the
Mecklenburg County Register of Deeds upon final plat approval. A copy of the recorded maintenance agreement shall be given to the Storm Water Administrator within fourteen (14) days following its recordation. b) Inspection and Maintenance Requirements. All BMPs installed pursuant to the requirements of this ordinance shall be inspected at a minimum of annually by a qualified registered North Carolina professional engineer or landscape architect performing services only in their area of competence. The purpose of these inspections is to identify maintenance and repair needs and to ensure compliance with the requirements of this ordinance. The person responsible for BMP maintenance shall submit to the Storm Water Administrator a completed inspection report beginning one year from the date of as-built certification and each year thereafter on or before the anniversary date of the as-built certification.
All inspection reports shall be on forms supplied by the Storm Water
Administrator and contained in the Administrative Manual. Any identified maintenance and/or repair needs shall be addressed in a timely manner. The inspection and maintenance requirement may be increased as deemed necessary by the Storm Water Administrator to ensure proper functioning of the BMP. c) Records of Installation and Maintenance Activities. Parties responsible for the inspection, operation, and maintenance of a BMP shall make records of the installation of all the maintenance and repairs and shall retain the records for at least five (5) years. Those records shall be made available to the Storm Water
Administrator upon request and/or as specifically outlined in the Maintenance
Covenant. c) Failure to Maintain Practices. It is unlawful for a property owner to fail to meet the requirements of the Operation and Maintenance Agreement. Any person or
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association that fails to meet the requirements of the Operation and Maintenance
Agreement shall be subject to a civil penalty as described in Section 8.17.25. d) Maintenance Easement. Every structural BMP installed pursuant to this ordinance shall be made accessible for adequate inspection, maintenance, reconstruction and repair by a maintenance easement. The easement shall be recorded with the
Mecklenburg County Register of Deeds Office and its terms shall specify who may make use of the easement and for what purposes.
.22 Deed Recordation and Indications on Plat
The approval of the storm water management permit shall require an enforceable restriction on property usage that runs with the land, such as plat, recorded deed restrictions or protective covenants, to ensure that future development and redevelopment maintains the site consistent with the approved project plans. Stream buffers as described in Section 8.25 including the delineation of each buffer zone must be specified on all surveys and record plats. The Operations and Maintenance Agreement pertaining to every structural BMP shall be referenced on the final plat and shall be recorded with the
Mecklenburg County Register of Deeds Office upon final plat approval. If no subdivision plat is recorded for the site, then the Maintenance Covenant shall be recorded with the
Mecklenburg County Register of Deeds Office so as to appear in the chain of title of all subsequent purchasers under generally accepted searching principles. A copy of the recorded Maintenance Covenant shall be provided to the Storm Water Administrator within fourteen (14) days following receipt of the recorded document. A maintenance easement shall be recorded for every structural BMP to allow sufficient access for adequate maintenance. The specific recordation and deed restriction requirements as well as notes to be displayed on final plats and deeds shall be contained in the Administrative
Manual.
.23 Inspection of BMPs. a) Inspections. As described in Section 8.17.21(b), the owner of a BMP shall inspect said BMP at a minimum of annually. Additional inspections may be conducted by the Storm Water Administrator on any reasonable basis, including but not limited to: routine inspections; random inspections; inspections based upon complaints or other notice of possible violations; inspections of drainage basins or areas identified as higher than typical sources of sediment or other contaminants or pollutants; inspections of businesses or industries of a type associated with higher than usual dischargers of contaminants or pollutants or with discharges of a type which are more likely than the typical discharge to cause violations of State or Federal water quality standards or the NPDES Storm Water
Permit; and joint inspections with other agencies inspecting under environmental and safety laws. Inspections may include, but are not limited to: reviewing maintenance and repair records; sampling discharges, surface water, groundwater, and material or water in BMPs; evaluating the condition of BMPs and storm water management practices.
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b) Right-of-Entry for Inspection. When any new BMP is installed on private property, the property owner shall grant to the Storm Water Administrator the right to enter the property at reasonable times and in a reasonable manner for the purpose of inspection. This includes the right to enter a property when the Storm
Water Administrator has a reasonable basis to believe that a violation of this regulation is occurring or has occurred, and to enter when necessary for abatement of a public nuisance or correction of a violation of this regulation.
.24 Standards for Storm Water Control Measures. a) Evaluation According to Contents of Design Manual
All storm water control measures and storm water treatment practices (also referred to as Best Management Practices, or BMPs) required under this ordinance shall be evaluated by the Storm Water Administrator according to the policies, criteria, and information, including technical specifications, standards and the specific design criteria for each storm water best management practice contained in the Design Manual. The Storm Water Administrator shall determine whether these measures will be adequate to meet the requirements of this ordinance.
(b) Determination of Adequacy; Presumptions and Alternatives
Storm water treatment practices that are designed, constructed, and maintained in accordance with the criteria and specifications in the Design Manual will be presumed to meet the minimum water quality and quantity performance standards of this ordinance. Whenever an applicant proposes to utilize a practice or practices not designed and constructed in accordance with the criteria and specifications in the Design Manual, the applicant shall have the burden of demonstrating that the practice(s) will satisfy the minimum water quality and quantity performance standards of this ordinance before it can be approved for use. The Storm Water Administrator may require the applicant to provide such documentation, calculations, and examples as necessary for the Storm Water
Administrator to determine whether such an affirmative showing is made.
(c) Submittal of Digital Records
Upon submittal of as-built plans, the location of storm drainage pipes, inlets and outlets as well as the location of all BMPs must be delivered to the Storm Water
Administrator in the digital format specified in the Administrative Manual.
.25 Civil Penalties
(a) Violations of Ordinance
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A violation of any of the provisions of this ordinance or rules or other orders adopted or issued pursuant to this ordinance may subject the violator to a civil penalty. A civil penalty may be assessed from the date the violation occurs. No penalty shall be assessed until the person alleged to be in violation has been notified of the violation of this ordinance. Refusal to accept the notice or failure to notify the Storm Water Administrator of a change of address shall not relieve the violator’s obligation to comply with the ordinance or to pay such a penalty.
(b) Amount of Penalty
The maximum civil penalty for each violation of this ordinance is $5,000.00.
Each day of continuing violation shall constitute a separate violation.
(c) Notice of Assessment of Civil Penalty
The Storm Water Administrator shall determine the amount of the civil penalty and shall notify the violator of the amount of the penalty and the reason for assessing the penalty. This notice of assessment of civil penalty shall be served by any means authorized under G.S. 1A-1, Rule 4 and shall direct the violator to either pay the assessment or file an appeal within 30 days of receipt of the notice as specified in Section 8.17.25(e) below.
(d) Failure to Pay Civil Penalty Assessment
If a violator does not pay a civil penalty assessed by the Storm Water
Administrator within 30 Days after it is due, or does not request a hearing, the
Storm Water Administrator shall request the initiation of a civil action to recover the amount of the assessment. The civil action shall be brought in Mecklenburg
County Superior Court or in any other court of competent jurisdiction. A civil action must be filed within three (3) years of the date the assessment was due. An assessment that is appealed is due at the conclusion of the administrative and judicial review of the assessment.
(e) Appeal of Remedy or Penalty
The issuance of an order of restoration and/or notice of assessment of a civil penalty by the Storm Water Administrator shall entitle the responsible party or entity to an appeal before the Zoning Board of Adjustment of the Town of
Huntersville if such Person submits written demand for an appeal hearing to the
Town of Huntersville within 30 days of the receipt of an order of restoration and/or notice of assessment of a civil penalty. The appeal of an order of restoration and/or notice of assessment of a civil penalty shall be conducted as described in Section 11.3 of this ordinance.
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8.25
S.W.I.M. (Surface Water Improvement and Management) Stream Buffers
.1 Purpose.
.2 have the primary natural functions of protecting water quality by filtering pollutants, providing intermittent storage for floodwaters, allowing channels to meander naturally, and providing suitable habitat for wildlife.
Definitions.
For the purposes of this section, the following words and phrases shall be defined as specified below:
Best Management Practices (BMPs). A structural or nonstructural management based practice used singularly or in combination to reduce non-point source input to receiving waters in order to achieve water quality protection goals.
Buffer. A vegetated area through which storm water runoff flows in a diffuse manner so that the runoff does not become channelized and which provides for infiltration of the runoff and filtering of pollutants.
Buffer Zones. Buffer widths are measured in three (3) zones as shown below. The buffer width is measured horizontally and must be surveyed by a licensed land surveyor on a line parallel to the surface water, landward from the top of the bank on each side of the stream.
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.3
Drainage Basin. The area of land that drains to a given point on a body of water.
Floodway. The channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than the allowable surcharge (currently one foot).
Flood Fringe. The land area located between the limits of the floodway and the maximum elevation subject to inundation by the base (1% chance) flood.
Floodplain. The low, periodically flooded lands adjacent to rivers and lakes. For land use planning purposes, the regulatory floodplain is usually viewed as all land alongside a watercourse that would be inundated by the base (1% chance) flood; the floodway plus the flood fringe.
Mitigation. Actions taken on-site and/or off-site to offset the effects of temporary or permanent loss of a buffer.
Top of Bank. The landward edge of the stream channel during high water, bankfull conditions at the point where water begins to overflow onto the floodplain.
Applicability. a) All properties shall comply with the buffer requirements of this Section except those which, as of the effective date of October 19, 1999, have previously secured a right to proceed by one of the following methods, and have received written authorization to disturb the buffer from the Town’s Planning Staff:
• Having been issued a Certificate of Building Code Compliance;
• Being subject to an approved major subdivision preliminary plan and/or recorded final plat;
• Being subject to a minor subdivision plat, exempt plat listed under the
• definition of subdivision in the Subdivision Ordinance, or described by metes and bounds in a recorded deed which:
• If to be used for residential purposes, are one (1) acre or less in size.
If to be used for nonresidential purposes or mixed use purposes, are four (4) acres or less in size if located on a non-FEMA regulated floodway, or are seven (7) acres or less in size if located
• on a FEMA regulated floodway.
Being subject to a site specific development plan defined under Section
2.2.2
of these zoning regulations; or
• Having otherwise secured a vested property right under state law. b) Redevelopment or expansion of uses and structures included in a), above, shall comply with the buffer requirements of this Section, however uses and structures previously approved and constructed in a buffer may remain.
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.4
.5 c) A site specific development plan amended by action of the Board of
Commissioners subsequent to adoption of this Section shall comply, in its amended form, with the S.W.I.M. buffer requirements, however uses and structures previously approved for construction in a buffer may remain. d) Where stream buffers are also required as part of the Lake Norman or Mountain
Island Lake Watershed Overlay Districts, the more stringent of the stream buffer requirements shall apply.
Buffer Delineation.
S.W.I.M. Stream Buffers, throughout the jurisdiction of the Town of Huntersville shall be delineated by Mecklenburg County through its geographic information system (GIS) using the most current digital elevation model (DEM) of no greater than 10-foot cells.
This stream buffer delineation including buffer widths shall be periodically updated as new data becomes available. The most recent delineation shall be provided for public use through Mecklenburg County’s website.
Minimum Buffer Widths.
All perennial and intermittent streams draining less than 50 acres shall have a minimum
30-foot vegetated buffer including a 10-foot zone adjacent to the bank. Disturbance of the buffer is allowed; however, any disturbed area must be revegetated and disturbance of the 10-foot zone adjacent to the bank shall require stream bank stabilization using bioengineering techniques as specified in the Design Manual. All perennial and intermittent streams draining greater than or equal to 50 acres and less than 300 acres shall have a 35-foot buffer with two (2) zones, including a 20-foot stream side and 15foot upland zone. Streams draining greater than or equal to 300 acres and less than 640 acres shall have a 50-foot buffer with three (3) zones, including a 20-foot stream side, 20foot managed use and 10-foot upland zone. Buffers for streams draining greater than or equal to 640 acres shall be 100 feet in width or include the entire floodplain, whichever is greater. This buffer shall consist of a 30-foot stream side, 45-foot managed use and 25foot upland zone or the entire FEMA floodplain, whichever is greater. All buffers shall be measured from the top of the bank on both sides of the stream. A summary of minimum buffer widths is provided in the table below.
Table of Minimum Buffer Widths by Basin Size and Buffer Zone
Area
Designation
Stream
Side
Zone
Managed
Use Zone
< 50 acres N/A N/A
> 50 acres 20 feet None
Upland
Zone
30 feet
15 feet
> 300 acres 20 feet 20 feet 10 feet
> 640 acres 30 feet 45 feet 25 feet or balance of floodplain,
Total Buffer Width each side of Stream
30 feet
35 feet
50 feet
100 feet or entire floodplain, whichever is greater
Notes
(1), (2)
(2)
(2)
(2), (3)
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whichever is greater
Notes:
(1) All perennial and intermittent streams draining less than 50 acres shall have a minimum 30-foot vegetated buffer including a 10-foot zone adjacent to the bank.
Disturbance of the buffer is allowed; however, any disturbed area must be revegetated and disturbance of the 10-foot zone adjacent to the bank shall require stream bank stabilization using bioengineering techniques as specified in the
Design Manual.
(2) Buffer widths are surveyed horizontally on a line parallel to the surface water, landward from the top of the bank on each side of the stream.
(3) Floodplain and buffer calculations will be based upon the FEMA flood fringe and floodway encroachment lines, as locally adopted and as may be amended from time to time.
.6
Buffer Description.
Buffer function, vegetation and use vary according to the different buffer zones and are described in the following table.
Table of Buffer Treatment by Buffer Zone
Function
Stream Side Zone Managed Use Zone Upland Zone Notes
Protect the integrity of the ecosystems
Provide natural filter; provide distance between upland development and the stream side zone
Prevent encroachment and filter runoff
Vegetative
Requirements
Undisturbed (no cutting, clearing or grading).
If existing tree density is inadequate, reforestation is required.
Limited clearing (no Herbaceous grading).
Existing tree ground cover , density must be retained to a minimum of 8 healthy trees of a including grass, is allowed; maintenance of minimum 6 inch caliper existing forest or per 1000 square feet. If reforestation is encouraged. existing tree density is inadequate, reforestation is encouraged.
(1)
Uses Very restricted.
Permitted uses limited to flood control structures and bank stabilization (where permitted) as well as installation of parallel or near perpendicular
Restricted.
Permitted uses limited to those allowed in the Stream
Restricted.
Permitted uses limited to those
Side Zone, as well as bike paths and allowed in Stream
Side and Managed greenway trails up to 10 Use Zones, as well feet in width. as gazebos, noncommercial
(2), (3),
(4)
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(75°) water and sewer utilities and near perpendicular road crossings (75°) with stabilization of disturbed areas as specified in Section
8.25.9. storage buildings less than 150 square feet, limited grading that does not change the functionality or extent of the floodplain, and storm water structural best management practices (BMPs) if approved in accordance with
Section 8.25.10 b), as a condition of a buffer width variance.
Notes:
(1) When reforestation of disturbed buffers is required, tree planting shall be as specified in the Charlotte-Mecklenburg SWIM Stream Buffer Implementation Guidelines.
(2) Fill material cannot be brought into any required buffer. In the Upland Zone only, limited grading that does not change the extent or functional characteristics of the floodplain is permitted. Uses permitted in the buffer zones should be coordinated to ensure minimal disturbance of the buffer system. For example, if it is necessary to install utilities within the buffer, then if Greenway Trails are built they should follow these cleared areas instead of necessitating additional clearing.
(3) Notwithstanding the uses and structures permitted in the “Upland Zone”, the stricter standards of floodway regulations, if applicable, shall apply.
(4) Greenway Trails referenced in this table refer exclusively to those approved by and
.7 dedicated to the Town of Huntersville or Mecklenburg County Parks and Recreation
Departments. Other paths or trails in the buffer shall be in accordance with the Charlotte-
Mecklenburg SWIM Stream Buffer Implementation Guidelines.
Diffuse Flow Requirement.
Diffuse flow of runoff shall be maintained in the buffer by dispersing concentrated flow through the use of level spreaders or other such devices to create sheet flow and by reestablishing vegetation. Techniques for providing diffuse flow are specified in the Huntersville Water Quality Ordinance and the Charlotte-Mecklenburg
Land Development Standards Manual.
• Maximum drainage area size shall be no greater than 10 acres for all outfalls discharging directly into a stream buffer.
• When practical, a drop structure should be installed prior to the last section of outfall pipe discharging to a buffer. A short length of outfall pipe should be laid
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•
• flat (0% Slope Energy Dissipater), prior to the riprap apron or other energy dissipater.
Concentrated runoff from ditches or other manmade conveyances shall be diverted to diffuse flow before the runoff enters the buffer.
Periodic corrective action to restore diffuse flow shall be taken by the property owner as necessary to impede the formation of erosion gullies.
.8
Ponds.
Ponds that intersect the stream channel shall have the same buffers as the original stream. Buffer requirements do not apply to wet ponds used as structural BMPs.
.9 Buffer Delineation.
The following buffer delineations are required: a) Buffer boundaries including all buffer zones must be clearly delineated on all sitespecific plans for Board of Commissioner approval, on all construction plans, including grading and clearing plans, erosion and sediment control plans, and site plans. b) The surveyed outside buffer boundary, including all buffer zones, must be clearly marked on-site with orange “tree-protection” or “high-hazard” fence prior to any land disturbing activities. Tree protection is required by Section 7.4(3) of this
Ordinance. Where existing trees are to be preserved in a buffer zone, the limits of grading shall equal the drip line of those trees plus an additional five (5) feet on the upland side of the buffer. All Specimen and Heritage trees require a tree survey prior to land-disturbing activity and shall be saved in all buffer zones. c) The surveyed outside boundary of the buffer must be permanently marked with an iron pin at the intersection of the watershed buffer and each property line on each parcel following the completion of land disturbing activities and prior to occupancy. Properties greater than 200’ in width shall be marked at a maximum of 100’ intervals. d) Separate buffer zones must be permanently marked at highway stream crossings. e) Buffer boundaries including all buffer zones as well as all buffer requirements must be specified on the record plat, on individual deeds, and in property association documents for lands held in common.
.10 Buffer Impacts Permitted under Section 8.25, S.W.I.M.
The following buffer impacts are permitted, but design and construction shall comply with the specifications provided in the Charlotte-Mecklenburg Land Development
Standards Manual and/or Design Manual for stabilization of disturbed areas to minimize negative effects on the quality of surface waters.
• Near perpendicular (75° or greater) road crossings for connectivity or transportation links where the Town of Huntersville has granted site plan approval.
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•
• Near perpendicular (75° or greater) utility crossings as approved by Charlotte-
Mecklenburg Utilities.
Parallel water and sewer utility installation as approved by Charlotte-
Mecklenburg Utilities, where a logical and appropriate basis for the impact is demonstrated, where disturbance of the Stream Side Zone is minimized to the maximum extent practicable, and where guidelines for restoring vegetation within buffers disturbed as a result of parallel utility installation are met. These guidelines are specified in the Charlotte-Mecklenburg Land Development
•
•
•
Standards Manual.
Public paths and trails parallel to the creek outside the Stream side Zone and near perpendicular stream crossings in any zone. Pathways must use existing and proposed utility alignments or previously cleared areas and minimize tree cutting to the maximum extent practicable. To the extent possible, pathways shall preserve existing drainage patterns and avoid drainage structures that concentrate storm water.
Incidental drainage improvements/repairs for maintenance.
Individual pedestrian paths connecting homeowners to the stream in the form of narrow, pervious footpaths with minimal tree disturbance.
•
• New domesticated animal trails (farming) where existing trails are lost as a result of action beyond the farmer’s control. Stream crossings should be constructed to minimize impacts to the Stream Side Zone and be maintained with fencing perpendicular to and through the buffer to direct animal movement.
Mitigation approved by a state or federal agency acting pursuant to Sections 401 or 404 of the federal Clean Water Act.
.11 Appeals and Variances. a) An appeal to reverse or modify the order, decision, determination, or interpretation of the Zoning Administrator shall comply with the procedures and standards of Section 11.3 of these regulations. b) Special Variance Provisions/Mitigation Techniques. When “practical difficulties or unnecessary hardships”, as defined in Section 11.3.2 f, would result from strict adherence to the buffer width requirements and/or buffer treatment standards, a petition for variance may be filed with the Huntersville Board of Adjustment in compliance with the procedures and standards of Section 11.3. Site specific mitigation plans using the mitigation techniques set out below and approved by the designated agency shall be construed by the Board of Adjustment to be evidence responsive to Section 11.3.2 f), subparagraphs 1) (b) and 1) (c) – consistency with adopted plans and protection of public safety and welfare.
Specifications for these mitigation techniques are provided in the Charlotte-
Mecklenburg Land Development Standards Manual (for Structural BMPs). The techniques below are not construed to offset the requirement of Section 8.25.6 for diffuse flow.
(1) Installation of Structural BMPs. The installation of an on-site structural
BMP designed to achieve specified pollutant removal targets will allow
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for all proposed stream buffer impact on the specific site. The BMP must remain outside of the Stream Side Zone and Manage Use Zone. A detailed BMP design plan must be submitted to the Mecklenburg County
Department of Environmental Protection for approval based on specifications contained in the Charlotte-Mecklenburg Land Development
Standards Manual. This plan must also include a long-term maintenance strategy for the BMP, complete with the establishment of adequate financing to support the proposed maintenance practices.
(2) Stream Restoration. The owner may restore and preserve the buffer area on any stream of equivalent or greater drainage area the condition of which is determined to be qualified for restoration by the Mecklenburg
County Department of Environmental Protection on a 1:1 basis in linear feet of stream. This restoration shall include stream bank improvements and Stream Side and Managed Use Zone re-vegetation, in accordance with the Charlotte-Mecklenburg Land Development Standards Manual, and receive approval by Mecklenburg County Department of Environmental
Protection.
(3) Stream Preservation. The owner may purchase, fee simple, other stream segments at equivalent or greater drainage area on a 1:1 linear foot basis and convey fee simple and absolute title to the land to the Town of
Huntersville, Mecklenburg County, or conservation trust, with a plan approved by the Mecklenburg County Department of Environmental
Protection.
(4) Wetlands Restoration. On a 2:1 acreage basis for disturbed stream and buffer area (2 acres of wetland for each acre of disturbed area), the owner may provide a combination of the preservation and/or restoration of wetlands with protective easements and the implementation of structural or non-structural BMPs to achieve specific pollutant removal targets within the impacted area. Restoration plan must be approved by the
Mecklenburg County Department of Environmental Protection.
(5) Bottom Land Hardwood Preservation. On a 2:1 acreage basis for impacted stream and buffer area 2 acres of bottomland hardwood for each acre of disturbed area), the owner may provide a combination of the preservation of existing bottom land hardwood forest or other specifically approved natural heritage area by conservation easement or other legal instrument, and the implementation of structural or non-structural BMPs to achieve specific pollutant removal targets within the impacted area.
Plan to be approved by Mecklenburg County Department of
Environmental Protection.
(6) Controlled Impervious Cover for Disturbance Landward of Stream Side
Zone. The owner may commit to, and provide, a specific site development plan for the parcel with requested buffer disturbance. The plan shall limit overall site impervious cover to less than or equal to 24%.
Preservation of the Stream Side Zone is still required. Plan to be approved by Mecklenburg County Department of Environmental Protection.
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(7) Open Space Development. The submission of a site-specific development plan that preserves 50% of the total land area as undisturbed open space.
Plan to be approved by Mecklenburg County Department of
Environmental Protection.
(8) Mitigation Credits: The purchase of mitigation credits through the Stream
Restoration Program on a 1:1 basis, utilizing linear feet of stream impacted and the prevailing rate of purchase as established by the
Charlotte-Mecklenburg Land Development Standards Manual. Mitigation credits purchased under any other program (i.e., U.S. Army Corp of
Engineers) shall not cover this requirement unless the issuing agency agrees to relinquish the funds to the appropriate local government agency.
(9) Alternative mitigation. The list of mitigation techniques shall not prevent the creative development of alternative mitigation plans that achieve the purposes of this section.
.12 Posting of financial security required.
When structural BMPs (wet detention ponds and other BMPs) are approved for mitigation of a buffer disturbance, the approval will be subject to the owner filing a surety bond or letter of credit or making other financial arrangements which are acceptable to the Mecklenburg County Department of Environmental Protection, in a form which is satisfactory to the County Attorney, guaranteeing the installation and maintenance of the required structural BMPs until the issuance of certificates of occupancy for seventy-five percent (75%) of all construction which might reasonably be anticipated to be built within the area which drains into the BMPs, allowing credit for improvements completed prior to the submission of the final plat. At such time that this level of occupancy is achieved, written notice thereof must be submitted by the owner to the Mecklenburg County Department of Environmental Protection. The owner must also verify the adequacy of the maintenance plan for the BMPs including the necessary financing to support the proposed maintenance practices. The Mecklenburg County
Department of Environmental Protection will inspect the structural BMPs and verify the effectiveness of the maintenance plan; if both are found to be satisfactory, the department will notify the owner in writing within 30 days of the date of notice.
.13 Maintenance responsibilities for structural BMPs – Civil Penalties.
Maintenance of all structural BMPs will be the responsibility of the property owner or his designee. Any person who fails to maintain the required BMPs in accordance with the approved maintenance plan will be subject to a civil penalty of not more than $5,000.
Each day that the violation continues shall constitute a separate violation. No penalties shall be assessed until the person alleged to be in violation has been notified in writing of the violation by registered or certified mail, return receipt requested, or by other means which are reasonably calculated to give actual notice. The notice shall describe the nature of the violation with reasonable particularity, specify a reasonable time period within which the violation must be corrected, and warn that failure to correct the
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violation within the time period will result in assessment of a civil penalty or other enforcement action.
.14 Request for Determination of Buffer Requirements
When a landowner or other affected party believes that the S.W.I.M stream buffer delineation maps described in Section 8.25.4 inaccurately depict buffer requirements, he or she shall request a determination from the Storm Water Administrator. Such determinations shall be made by the Storm Water Administrator based on an on-site evaluation using the U.S. Army Corps of Engineers and N.C. Division of Water Quality methodology for stream delineation as well as information from databases maintained for stream delineation by Mecklenburg County. Such determinations can also be made at the discretion of the Storm Water Administrator in the absence of a request from a landowner or other concerned party. The buffer requirements of this ordinance shall apply based on determinations made by the Storm Water Administrator. Surface waters that appear on the maps shall not be subject to this ordinance if an on-site determination by the Storm
Water Administrator shows that they fall into one of the following categories.
(1) Ditches and manmade conveyances other than modified natural streams.
(2) Manmade ponds and lakes that are not intersected by a buffered stream segment and that are located outside natural drainage ways.
(3) Ephemeral (storm water) streams.
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6.400 PRELIMINARY PLAN REQUIREMENTS
The preliminary subdivision plan must be drawn to the following specifications and must contain or be accompanied by the information listed below. No processing or review of a preliminary plan will proceed without all of the information listed. Detailed standards and specifications for construction of state standard road are contained in the Land Development Standards Manual available from the Mecklenburg County Engineering Department. For Town Streets, the standards of Article 5 of the Huntersville Zoning Ordinance shall control.
1. The boundary of the area to be subdivided and the location within the area, or contiguous
2.
3. to it, of any existing streets, railroad line, water courses, easements or other significant features of the tract.
The location, size, elevations of existing sanitary sewers, storm drains, and culverts within the tract and immediately adjacent thereto.
Original contours, including tree lines, shown at intervals of not less than 4 feet for the entire area to be subdivided and extended into adjoining property for a distance of 300 feet at all points where street rights-of-way connect to the adjoining property. These contours shall be referenced to mean sea level datum established by the U.S. Coast and
Geodetic survey. Proposed contours for the full width of all street rights-of-way, along open drainage channels and in all other portions of the subdivision where extensive
4.
5.
6.
7. grading is proposed must be shown. These requirements shall not apply where the size of the subdivision and the topography make such information unnecessary.
The location of proposed streets, alleys, easements, lots, parks or other open spaces, reservations, other property lines, front build-to lines and rear and side yard dimensions for each lot, street dimensions, tentative building locations, and the location of any building restriction flood lines required by Section 7.280. The location and area calculations of constraining features including wetlands, slopes over 25%, watercourses, intermittent streams and floodways, S.W.I.M. buffers (outside of floodways), watershed buffers, and all rights-of-way and easements (current and future)
The location of all proposed storm drains and appurtenances with grades, inverts, and sizes indicated, together with a map of the drainage area or areas tributary to the proposed storm drains, a copy of the data used in determining the sizes of drainage pipes and structures, and the Building Restriction Floodline and Flood Protection Elevation for each lot subject to flooding as defined in Section 7.280.
The name of the subdivision; the name and signature of the owner or the owner's duly authorized agent; the name of the surveyor, engineer or designer; the names of proposed streets; the names of adjoining subdivisions or property owners. The name assigned to the subdivision and the names assigned to streets at this time will be used throughout the review and approval process for preliminary and final plats and may not be changed without approval of the Planning Director and or Designated Administrative Agent.
The scale of the plan which shall not be smaller than 100 feet to the inch, north point, date.
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8.
9.
Typical cross sections of internal or abutting streets showing width, sidewalk, and planting details and proposed construction of roadways.
Proposed profiles of roadways. Where a proposed street is an extension of an existing street the profile shall be extended to include 300 feet of the existing roadway and storm drains if present and a cross section of the existing street shall be shown. Where a proposed street within the subdivision abuts a tract of land that adjoins the subdivision and where said street may be expected to extend into said adjoining tract of land, the profile shall be extended to include 300 feet of the said adjoining tract.
10. The proposed method of water supply and sewer disposal.
11. A small scale vicinity map showing the location of the subdivision with respect to adjacent streets and properties.
12. The location of any existing LCID landfills on the site and the location of any proposed
LCID landfills on the site.
13. A timetable for estimated project completion of the area covered by the preliminary plan.
14. The zoning district(s) in which the project is located.
15. For projects in the Mountain Island Lake Watershed Overlay District, the calculated built-upon area permitted for each building lot, taking into account permanently preserved open space.
16. For subdivisions within which open space is required, a draft of the documents by which irrevocable preservation of open space shall be assured.
17. A Landscape/Preservation Plan is required for all subdivisions. Items to be included on the plan are as follows:
(a) Project Name
(b) Owner Name
(c) Landscape Architect’s seal and signature
(d) Tree save calculations
(e) Tree save areas – must also be shown on all sheets to be included within the
Preliminary Plan
(f) Street Trees Proposed
(g) Additional trees to be planted to meet minimum tree save requirements and appropriate table (if any)
(h) Individual residential lot trees and a table showing total amount by lot type
(i) Internal/Perimeter landscaping for parking lots
(j) Landscape easements properly labeled – must also be shown on the site plan and final plat
(k) Notes on plan related to maintenance of street trees by individual homeowner or homeowners association – must also be noted on all preliminary plans and final plats to be recorded and documented within the homeowner’s covenants and restrictions.
(l) Proposed utilities in relation to tree save areas.
18. Storm Water Permit Application. A Storm Water Permit Application shall be submitted along with preliminary plans for each proposed development. The permit application shall demonstrate compliance with Section 8.17.6, Performance Criteria of the
Huntersville Zoning Ordinance, unless otherwise exempted. The storm water permit application shall contain SET outputs, supporting computations, drawings, soil analyses,
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calculations for each BMP, and overall site hydrology calculations as well as other information sufficient to describe the manner, location, and type of measures for managing storm water from the development in compliance with Section 8.17.6. In addition, the permit application shall specify those parties responsible for long-term maintenance of all BMPs. Section 9 of the Huntersville Water Quality Design Manual contains specific submission requirements. The Mecklenburg County Land Use and
Environmental Services Agency shall review the permit application to determine compliance with the Performance Criteria listed in Section 8.17.6. Approval of the storm water permit application by the Mecklenburg County Land Use and Environmental
Services Agency is required prior to the initiation of land disturbing activities and said storm water permit application shall serve as the basis for all subsequent construction.
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7.300 DESIGN STANDARDS FOR S.W.I.M. (SURFACE WATER IMPROVEMENT
AND MANAGEMENT) STREAM BUFFERS
1. PURPOSE.
The purpose of a stream buffer network is to filter pollutants, store floodwaters, provide habitat, and contribute to the “green infrastructure”. Stream systems are comprised of each stream and its respective drainage basin.
• Streams have the primary natural functions of conveying storm and ground water, storing floodwater, and supporting aquatic life.
• Vegetated lands adjacent to the stream channel in the drainage basin serve as “buffers” to protect the stream’s ability to fulfill its natural functions. Buffers have the primary natural functions of protecting water quality by filtering pollutants, providing intermittent storage for floodwaters, allowing channels to meander naturally, and providing suitable habitat for wildlife.
2. DEFINITIONS.
For the purposes of this section, the following words and phrases shall be defined as specified below:
Best Management Practices (BMPs)
A structural or nonstructural management based practice used singularly or in combination to reduce non-point source input to receiving waters in order to achieve water quality protection goals.
Buffer.
A vegetated area through which storm water runoff flows in a diffuse manner so that the runoff does not become channelized and which provides for infiltration of the runoff and filtering of pollutants.
Buffer Zones.
Buffer widths are measured in three (3) zones as shown below. The buffer width is measured horizontally on a line parallel to the surface water, landward from the top of the bank on each side of the stream.
Drainage Basin.
The area of land which drains to a given point on a body of water.
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Floodway.
The channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than the allowable surcharge (currently one foot).
Flood Fringe.
The land area located between the limits of the floodway and the maximum elevation subject to inundation by the base (1% chance) flood.
Floodplain.
The low, periodically flooded lands adjacent to rivers and lakes. For land use planning purposes, the regulatory floodplain is usually viewed as all alongside a watercourse that would be inundated by the base (1% chance) flood; the floodway plus the flood fringe.
Mitigation.
Actions taken on-site and/or off-site to offset the effects of temporary or permanent loss of a buffer.
Top of Bank.
The landward edge of the stream channel during high water, bankfull conditions at the point where water begins to overflow onto the floodplain.
3. APPLICABILITY.
(a) All properties shall comply with the buffer requirements of this Section except those which, as of the effective date of October 19, 1999, have previously secured a right to proceed by:
•
•
Being subject to a recorded subdivision plat;
Being subject to a subdivision sketch plan approved by the Board of
Commissioners prior to the effective date of this Section;
• Being subject to a site specific development plan defined under Section 2.2.2 of
• these zoning regulations; or
Having otherwise secured a vested property right under state law.
(b) Redevelopment or expansion of development projects included in a), above, shall comply with the buffer requirements of this Section, however uses and structures previously approved and constructed in a buffer may remain.
(c) A site specific development plan amended by action of the Board of Commissioners subsequent to adoption of this Section shall comply, in its amended form, with the
S.W.I.M. buffer requirements, however uses and structures previously approved for construction in a buffer may remain.
(d) Where stream buffers are also required as part of the Lake Norman or Mountain Island
Lake Watershed Overlay Districts, the more stringent of the stream buffer requirements shall apply.
4. BUFFER DELINEATION.
S.W.I.M. Stream Buffers, throughout the jurisdiction of the Town of Huntersville shall be delineated by Mecklenburg County through its geographic information system (GIS) using the most current digital elevation model (DEM) of no greater than 10-foot cells. This stream buffer
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delineation including buffer widths shall be periodically updated as new data becomes available.
The most recent delineation shall be provided for public use through Mecklenburg County’s website.
5. MINIMUM BUFFER WIDTHS.
All perennial and intermittent streams draining less than 50 acres shall have a minimum 30-foot vegetated buffer including a 10-foot zone adjacent to the bank. Disturbance of the buffer is allowed; however, any disturbed area must be revegetated and disturbance of the 10-foot zone adjacent to the bank shall require stream bank stabilization using bioengineering techniques as specified in the Design Manual. All perennial and intermittent streams draining greater than or equal to 50 acres and less than 300 acres shall have a 35-foot buffer with two (2) zones, including a 20-foot stream side and 15-foot upland zone. Streams draining greater than or equal to 300 acres and less than 640 acres shall have a 50-foot buffer with three (3) zones, including a
20-foot stream side, 20-foot managed use and 10-foot upland zone. Buffers for streams draining greater than or equal to 640 acres shall be a 100 feet in width or include the entire floodplain, whichever is greater. This buffer shall consist of a 30-foot stream side, 45-foot managed use and
25-foot upland zone or the entire FEMA floodplain, whichever is greater. All buffers shall be measured from the top of the bank on both sides of the stream. A summary of minimum buffer widths is provided in the table below.
Table of Minimum Buffer Widths by Basin Size and Buffer Zone
Area
Designation
Stream
Side
Zone
< 50 acres N/A
Managed
Use Zone
N/A 30
Upland Zone Total Buffer Width each side of Stream
30 feet
Notes
(1), (2)
> 50 acres 20 feet None 15 feet
> 300 acres 20 feet 20 feet 10 feet
> 640 acres 30 feet
35 feet
50 feet
(2)
(2)
45 feet 25 feet or balance 100 feet or entire of floodplain, floodplain, whichever whichever is greater is greater
(2), (3)
Notes:
(1) All perennial and intermittent streams draining less than 50 acres shall have a minimum
30-foot vegetated buffer including a 10-foot zone adjacent to the bank. Disturbance of the buffer is allowed; however, any disturbed area must be revegetated and disturbance of the 10-foot zone adjacent to the bank shall require stream bank stabilization using bioengineering techniques as specified in the Design Manual.
(2) Buffer widths are surveyed horizontally on a line parallel to the surface water, landward from the top of the bank on each side of the stream.
(3) Floodplain and buffer calculations will be based upon the FEMA flood fringe and floodway encroachment lines, as locally adopted and as may be amended from time to time.
6. BUFFER DESCRIPTION.
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Buffer function, vegetation and use vary according to the different buffer zones and are described in the following table.
Table of Buffer Treatment by Buffer Zone
Function
Stream Side Zone Managed Use Zone
Protect the integrity of the ecosystems
Provide natural filter; provide distance between upland development and the stream side zone
Upland Zone Notes
Prevent encroachment and filter runoff
Land
Disturbance/
Vegetative
Requirements
Undisturbed (no cutting, clearing or grading).
If existing tree density is inadequate, reforestation is required.
Limited clearing (no Herbaceous ground grading).
Existing tree cover , including grass,
(1) density must be retained to a minimum of 8 healthy trees of a minimum 6-inch is allowed; maintenance of existing forest or reforestation is encouraged. Limited caliper per 1000 square grading that does not feet. If existing tree change the density is inadequate, reforestation is encouraged. functionality or extent of the floodplain is permitted.
Development
Uses
Very restricted.
Restricted.
Limited to Restricted.
Limited to (2), (3),
Limited to flood control those allowed in the those allowed in Stream (4) structures and bank Stream Side Zone, as Side and Managed Use stabilization (where permitted) as well as installation of parallel well as bike paths and greenway trails up to
10 feet in width.
Zones, and to storm water structural best management practices or near perpendicular (³
75°) water and sewer utilities and near perpendicular road
(BMPs) if approved in accordance with
Section 7.300 10. b), as a condition of a buffer width variance. crossings (³ 75°) with stabilization of disturbed areas as specified in Section
7.300, 9.
Notes:
(1) When reforestation of disturbed buffers is required, tree planting shall be as specified in the Charlotte-Mecklenburg SWIM Stream Buffer Implementation Guidelines.
(2) Fill material cannot be brought into any required buffer. In the Upland Zone only, limited grading that does not change the extent or functional characteristics of the floodplain is permitted. Uses permitted in the buffer zones should be coordinated to ensure minimal disturbance of the buffer system. For example, if it is necessary to install utilities within the buffer, then if Greenway Trails are built they should follow these cleared areas instead of necessitating additional clearing.
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(3) Notwithstanding the uses and structures permitted in the “Upland Zone”, the stricter standards of floodway regulations, if applicable, shall apply.
(4) Greenway Trails referenced in this table refer exclusively to those approved by and dedicated to the Town of Huntersville or Mecklenburg County Parks and Recreation
Departments. Other paths or trails in the buffer shall be in accordance with the Charlotte-
Mecklenburg SWIM Stream Buffer Implementation Guidelines.
7. DIFFUSE FLOW REQUIREMENT.
Diffuse flow of runoff shall be maintained in the buffer by dispersing concentrated flow and reestablishing vegetation. Techniques for providing diffuse flow are specified in the Charlotte-
Mecklenburg Land Development Standards Manual.
• Concentrated runoff from ditches or other manmade conveyances shall be diverted to diffuse flow before the runoff enters the buffer.
• Periodic corrective action to restore diffuse flow shall be taken by the property owner as necessary to impede the formation of erosion gullies
8. PONDS THAT INTERSECT THE STREAM CHANNEL.
Ponds that intersect the stream channel shall have the same buffers as the original stream. Buffer requirements do not apply to wet ponds used as structural BMPs.
9. BUFFER DELINEATION.
The following buffer delineations are required: a) Buffer boundaries including all buffer zones must be clearly delineated on all site-specific plans for Board of Commissioner approval, on all construction plans, including grading and clearing plans, erosion and sediment control plans, and site plans. b) Buffer boundaries including all buffer zones must be clearly marked on-site prior to any land disturbing activities. Where existing trees are to be preserved in a buffer zone, limits of grading shall maintain a minimum 20’ separation from the base of each tree on the upland side of the buffer. c) The outside boundary of the buffer must be permanently marked on each parcel following the completion of grading activities and prior to occupancy. d) Separate buffer zones must be permanently marked at highway stream crossings. e) Buffer boundaries including all buffer zones as well as all buffer requirements must be specified on the record plat, on individual deeds, and in property association documents for lands held in common.
10. BUFFER IMPACTS PERMITTED UNDER SECTION 7.300.
The following buffer impacts are permitted, but design and construction shall comply with the specifications provided in the Charlotte-Mecklenburg Land Development Standards Manual and/or Design Manual for stabilization of disturbed areas to minimize negative effects on the quality of surface waters.
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• Near perpendicular (75° or greater) road crossings for connectivity or transportation links where the Town of Huntersville has granted site plan approval.
• Near perpendicular (75° or greater) utility crossings as approved by Charlotte-
Mecklenburg Utilities.
• Parallel water and sewer utility installation as approved by Charlotte-Mecklenburg
Utilities, where a logical and appropriate basis for the impact is demonstrated, where disturbance of the Stream Side Zone is minimized to the maximum extent practicable, and where guidelines for restoring vegetation within buffers disturbed as a result of parallel utility installation are met. These guidelines are specified in the Charlotte-
Mecklenburg Land Development Standards Manual.
• Public paths and trails parallel to the creek outside the Stream side Zone and near perpendicular stream crossings in any zone. Pathways must use existing and proposed utility alignments or previously cleared areas and minimize tree cutting to the maximum extent practicable. To the extent possible, pathways shall preserve existing drainage patterns and avoid drainage structures that concentrate storm water.
• Incidental drainage improvements/repairs for maintenance.
• Individual pedestrian paths connecting homeowners to the stream in the form of narrow, pervious footpaths with minimal tree disturbance.
• New domesticated animal trails (farming) where existing trails are lost as a result of action beyond the farmer’s control. Stream crossings should be constructed to minimize impacts to the Stream Side Zone and be maintained with fencing perpendicular to and through the buffer to direct animal movement.
• Mitigation approved by a state or federal agency acting pursuant to Sections 401 or 404 of the federal Clean Water Act.
11. APPEALS AND VARIANCES. a) An appeal to reverse or modify the order, decision, determination, or interpretation of the
Zoning Administrator shall comply with the procedures and standards of Section 4.000 of these regulations as well as the requirements of the Huntersville Zoning Ordinance b) Special Variance Provisions/Mitigation Techniques.
• When a difficulty or hardship would result from adherence to the buffer width requirements and/or buffer treatment standards, a petition for variance may be filed with the Huntersville Planning Staff in compliance with the procedures of Section
4.300.
• The Standards for Granting a Variance, as set out in Section 4.400
of these regulations shall be met. Site specific mitigation plans using the techniques below, and approved by the designated agency, shall constitute conditions relating to the intent and standards of this ordinance, and may be attached to variance approval by the Town Board. Specifications for these mitigation techniques are provided in the
Charlotte-Mecklenburg Land Development Standards Manual. The techniques below are not construed to offset the requirement of Section 7.300 6. for diffuse flow.
(1) Installation of Structural BMPs. The installation of an on-site structural BMP designed to achieve specified pollutant removal targets will allow for all proposed stream buffer impact on the specific site. The BMP must remain outside of the
Stream Side Zone and Managed Use Zone. A detailed BMP design plan must be
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submitted to the Mecklenburg County Department of Environmental Protection for approval based on specifications contained in the Charlotte-Mecklenburg
Land Development Standards Manual. This plan must also include a long-term maintenance strategy for the BMP, complete with the establishment of adequate financing to support the proposed maintenance practices.
(2) Stream Restoration. The owner may restore and preserve the buffer area on any stream of equivalent or greater drainage area the condition of which is determined to be qualified for restoration by the Mecklenburg County Department of
Environmental Protection on a 1:1 basis in linear feet of stream. This restoration shall include stream bank improvements and Stream Side and Managed Use Zone re-vegetation, in accordance with the Charlotte-Mecklenburg Land Development
Standards Manual and receive approval by Mecklenburg County Department of
Environmental Protection.
(3) Stream Preservation. The owner may purchase, fee simple, other stream segments at equivalent or greater drainage area on a 1:1 linear foot basis and convey fee simple and absolute title to the land to the Town of Huntersville, Mecklenburg
County, or conservation trust, with a plan approved by the Mecklenburg County
Department of Environmental Protection.
(4) Wetlands Restoration. On a 2:1 acreage basis for disturbed stream and buffer area
(2 acres of wetland for each acre of disturbed area), the owner may provide a combination of the preservation and/or restoration of wetlands with protective easements and the implementation of structural or non-structural BMPs to achieve specific pollutant removal targets within the impacted area. Restoration plan must be approved by the Mecklenburg County Department of Environmental
Protection.
(5) Bottom Land Hardwood Preservation. On a 2:1 acreage basis for impacted stream and buffer area 2 acres of bottomland hardwood for each acre of disturbed area), the owner may provide a combination of the preservation of existing bottom land hardwood forest or other specifically approved natural heritage area by conservation easement or other legal instrument, and the implementation of structural or non-structural BMPs to achieve specific pollutant removal targets within the impacted area. Plan to be approved by Mecklenburg County
Department of Environmental Protection.
(6) Controlled Impervious Cover for Disturbance landward of Stream Side Zone.
The owner may commit to, and provide, a specific site development plan for the parcel with requested buffer disturbance. The plan shall limit overall site impervious cover to less than or equal to 24%. Preservation of the Stream Side
Zone is still required. Plan to be approved by Mecklenburg County Department of Environmental Protection.
(7) Open Space Development. The submission of a site-specific development plan that preserves 50% of the total land area as undisturbed open space. Plan to be approved by Mecklenburg County Department of Environmental Protection.
(8) Mitigation Credits: The purchase of mitigation credits through the Stream
Restoration Program on a 1:1 basis, utilizing linear feet of stream impacted and the prevailing rate of purchase as established by the Charlotte-Mecklenburg Land
Development Standards Manual. Mitigation credits purchased under any other
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program (i.e., U.S. Army Corp of Engineers) shall not cover this requirement unless the issuing agency agrees to relinquish the funds to the appropriate local government agency.
(9) Alternative mitigation. The list of mitigation techniques shall not prevent the creative development of alternative mitigation plans that achieve the purposes of this section.
12. POSTING OF FINANCIAL SECURITY REQUIRED.
When structural BMPs (set detention ponds and other BMPs) are approved for mitigation of a buffer disturbance, the approval will be subject to the owner filing a surety bond or letter of credit or making other financial arrangements which are acceptable to the Mecklenburg County
Department of Environmental Protection, in a form which is satisfactory to the County Attorney, guaranteeing the installation and maintenance of the required structural BMPs until the issuance of certificates of occupancy for seventy-five percent (75%) of all construction which might reasonably be anticipated to be built within the area which drains into the BMPs, allowing credit for improvements completed prior to the submission of the final plat. At such time that this level of occupancy is achieved, written notice thereof must be submitted by the owner to the
Mecklenburg County Department of Environmental Protection. The owner must also verify the adequacy of the maintenance plan for the BMPs, including the necessary financing to support the proposed maintenance practices. The Mecklenburg County Department of Environmental
Protection will inspect the structural BMPs and verify the effectiveness of the maintenance plan; if both are found to be satisfactory, the department will notify the owner within 30 days of the date of notice.
13. MAINTENANCE REQUIREMENTS FOR STRUCTURAL BMP’S – CIVIL
PENALTIES.
Maintenance of all structural BMPs will be the responsibility of the property owner or his designee. Any person who fails to maintain the required BMPs in accordance with the approved maintenance plan will be subject to a civil penalty of not more than $5000. Each day that the violation continues shall constitute a separate violation. No penalties shall be assessed until the person alleged to be in violation has been notified in writing of the violation by registered or certified mail, return receipt requested, or by other means which are reasonably calculated to give actual notice. The notice shall describe the nature of the violation with reasonable particularity, specify a reasonable time period within which the violation must be corrected, and warn that failure to correct the violation within the time period will result in assessment of a civil penalty or other enforcement action.
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Prepared for
Mecklenburg County Land Use and
Environmental Services Agency
Prepared by
Tetra Tech, Inc.
P.O. Box 14409
3200 Chapel Hill – Nelson Highway, Suite 105
Research Triangle Park, NC 27709
(919) 485-8278
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ACKGROUND AND
NTRODUCTION
Tetra Tech, Inc. compared the costs of conventional and Low Impact Development (LID) designs for a medium density residential site in Huntersville, North Carolina. Results of this cost analysis were presented to the Huntersville Planning Board and Town Council members on
September 4, 2002. This memo provides the detailed documentation for how the analysis was performed and the results.
For purposes of conducting this cost comparison, the Town of Huntersville in cooperation with the DMC Land Development Company and Alliance Engineering provided site plans and information for the subject development. Tetra Tech then provided redesigns of the property using conventional and LID approaches based on the Town’s proposed density zoning changes with new open space provisions. The overall lot and street layout were the same for the conventional and LID designs (Figure 1). However, the designs differed in several aspects. The conventional design had two 5-foot wide sidewalks, and the streets were drained with curb and gutter and storm drain systems. The LID plan had one 4-foot sidewalk, and instead of curb and gutter, the LID plan used bioretention swales to drain the streets and rain gardens to treat stormwater from each roof and lawn. This analysis showed that a developer would save construction costs by choosing the LID development instead of the conventional development.
Figure 1.
Low Impact Design Site Plan for Subject Development
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The most significant difference between the two designs is the chosen goal for stormwater management. The goal of the conventional design was to effectively move stormwater off the development site, through stormwater conveyance systems in the development and nearby streams (i.e. efficient drainage). In contrast, the goal of the LID design was to retain stormwater onsite (specifically to retain the runoff generated by the development during the 2-year storm or
3.1-inch storm event).
The previous cost analysis was expanded in this memo to consider a wet detention pond in the conventional design. This pond would remove a similar amount of sediment when compared to
LID but was not as effective at managing nutrients and water volume. In addition, the conventional design that included the pond was more expensive than the LID and initial conventional design.
Please note that a few corrections were made to the original analysis. Catch basins were added to provide more realistic costs of the conventional development. A range of costs was estimated for the infrastructure as well as the LID to account for variation in design. Curb and gutter and storm drain frontage improvements were added to the conventional development to equal the length of the LID frontage improvements. Driveway culverts were added to the swale costs so that the swales would have the potential to convey large volumes of runoff.
STIMATION
OURCES AND
ETHODS
The rain gardens and swales used in the LID plan are among several Best Management Practices
(BMPs) used as residential stormwater controls. Sources exist to estimate the costs of BMPs, but the estimates vary widely in both form and value. Two frequently used methods are the cubicfoot and unit price methods.
In the cubic-foot method, the estimator uses an equation that relates cubic-foot capacity to cost for each BMP. Wiegand et al. (1986) and Brown and Schueler (1997) estimate equations that predict the cost of BMPs per cubic-foot capacity. Cubic-foot estimates return about a 15% margin of error and are most appropriate for preliminary estimates (RS Means, 1990). For example, CH2M-Hill used cubic-foot estimates to report general costs within a watershed, and
The Center for Watershed Protection used cubic-foot estimates to compute the difference in costs between development types. EPA (1999) and Rouge River (1997, 2001) also report cubic-foot estimates as guidelines for planning.
Unlike the cubic-foot method, the unit price method is accurate to about a 5% margin of error and is most appropriate when site plans are available (RS Means, 1990). To obtain unit prices, an estimator uses a cost data manual that lists average unit prices of construction materials, labor, and equipment. The Robert Snow Means Company (RS Means) publishes construction cost data manuals with BMP components. The cost estimator can take the design of a BMP, separate it into components, and estimate the total cost using RS Means data. The cost estimator also can convert national averages to local prices with RS Means city indexes. Southeastern Wisconsin
Regional Planning Commission (SEWRPC, 1991) estimated BMP costs with the unit price method; their work is cited in Baker (2001), Center for Watershed Protection (1998), EPA
(1999), and Rouge River (1997, 2000).
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The unit price method was chosen for the majority of this estimation. Unit prices were obtained from RS Means Manuals, North Carolina Agricultural Extension, US EPA, and the Low Impact
Development Center. The cubic-foot method was used for estimating the cost of the wet detention pond.
OST
STIMATION
ROCEDURE
This study was designed to use a single development site as an example for potential, future development of Huntersville. Because of this, it was important to estimate the costs as conservatively as possible. Minimum and maximum prices were chosen for each component, and a minimum, midpoint, and maximum were reported for the total costs. All prices, both national averages and non-local prices, were converted to Charlotte prices for the year 2002.
Conventional Infrastructure
Price ranges for conventional infrastructure were chosen to capture the variation in choice of design (Table 1). Sidewalk prices varied depending on the extent of the gravel base. The curb and gutter costs varied by contractor and method of installation (e.g. pre-cast or cast in place).
The street drainage was chosen as a typical system but not designed for a specific storm event; this drainage included storm drain piping and catch basins. The storm drain prices varied by width of pipe; the minimum price refers to the minimum diameter required by NCDOT, and the maximum price refers to the pipe that would carry a similar flow to the LID swale during a bank full event. The average number of catch basins was assumed to be 20, and catch basin depth was varied from 4 to 8 feet. Catch basin inspection and cleaning was not included because it would cost the developer less than $60 for the first year (Rouge River, 2001).
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Table 1. Prices for Infrastructure
Component
Sidewalk
(per SF)
Curb and Gutter
(per LF)
Storm Drain
(per LF)
Minimum
$2.41
$12.88
$13.36
Maximum Source
$3.06 RS Means, 2001
Notes
4" thick, 3000 psi, CIP,
6 x 6 - W1.4 x W1.4 mesh, broomed finish
$15.91 Barrus Construction and RS Means, 2001
Straight with 6-inch high curb and 6-inch thick gutter, wood forms; 30-inch wide,
0.066 CY per LF
$38.94 RS Means, 2001 Porous wall concrete underdrain, standard strength, 15" to 30” diameters
Catch Basins
(per unit)
$586.21 $1045.35 RS Means, 2001 Concrete, pre-cast, 4 feet I.D., 4 to 8-feet deep
Conventional Stormwater Control
A wet detention pond was added to the conventional development as a conventional stormwater control. This pond would detain the first inch of storm runoff and is designed to remove 85%
Total Suspended Solids (TSS) from runoff during a long-term average storm (NCDENR, 1999).
The cost of the pond was varied by choice of depth, from 6 to 8 feet, and the pond volume ranged from about 200,000 to 230,000 cubic feet as calculated from North Carolina state guidelines (NCDENR, 1999). The pond cost was estimated with cubic foot equations and ranged
1
from about $150,000 to $190,000 (Table 2). Pond maintenance, ranged from $5,000 to
$11,000 per year and included lawn care, sediment and litter removal, inspection, and administration (EPA, 1999 and SEWRPC, 1991).
1 -If this pond was sized to retain the runoff from the 2-year storm, it would cost significantly more and require significantly more space within the development.
168

Table 2. Cost Estimation of Wet Detention Pond
Type of Pond Equation* Design and
Engineering
Ponds greater than 100,000 CF
C = 40.6V
0.64
25%
Cost
$154,361
Equation Source
SEWRPC, 1991
All Ponds C = 30.6V
0.70
32% $186,805
Brown and
Schueler, 1997
*C = cost; V = cubic foot volume, including permanent and temporary pool.
Low Impact Design
The two stormwater controls employed in the subject development LID plan were bioretention swales and rain gardens. Construction, maintenance, design, and engineering costs were considered. Design and engineering costs were estimated as 25% of the total BMP construction costs (EPA, 1999).
Bioretention Swales
The cost estimate of the bioretention swales was separated into grass swale and trench components (Table 3). The grass swales had a width of 10 feet, and their price included grading, seeding, topsoil stockpiling and spreading, straw spreading, and watering. A breakdown of component costs was not necessary for the grass swales because the description for the linearfoot price matched the subject development swale design. No variation was assumed for the grass swales cost. Culverts were included for each 9-foot wide driveway.
The cost for the trench included excavation, hauling, gravel, drainage pipes, and conditioned soil
(Table 4). The cost per cubic yard of excavation and hauling varied by type of machinery used and hauling distance. Prices for all other components were assumed fixed.
Table 3. Total Prices for Bioretention Swale ($ per linear foot)
BMP
Trench
Grass Swale*
Total for Swale
Minimum
$3.64
$3.07
$6.71
Maximum
$3.07 RS Means, 1997
$9.30
Source
$6.23 See Table 3
Driveway
Culvert**
$13.83 $40.10 RS Means, 2001
* Includes 10’ wide, grading, seeding, topsoil stockpiling and spreading, straw, and watering.
**Price of conventional storm drain piping plus excavation.
169

Table 4. Unit Prices for Trench Components
Component
#57 Stone
4” HDPE Underdrain
$8.00 per CY
$1.00 per LF
Price
Excavation and Hauling Minimum: $2.00 per CY
Maximum: $5.38 per CY
Source
Weinstein, 2001
Weinstein, 2001
Weinstein, 2001
Rouge River, 1997
Conditioned Soil $15.00 per CY Weinstein, 2001
Rain Gardens
The cost estimate for rain gardens was separated into 9 components (Table 5). Vegetation accounted for 60% of the variation in cost for rain gardens. The minimum vegetation price represents vegetation transplanted on-site, and the maximum price is for gardens planted with mature nursery plants. Excavation and hauling varied by type of soil and hauling distance, and this variation accounted for about 20% of the total variation. Soil, mulch, and stone varied slightly by either brand or price source. Stone, pipes, weirs, and end sections were assumed to not vary significantly.
Table 5. Unit Prices for Rain Garden Components
Components
Excavation (per CY)
Hauling (per CY)*
Pipes -- 4" HDPE Underdrain
-- 15" outlet
Stone (per CY)
Soil (per CY)
Stone Weirs (per unit)
End Section (per unit)
Vegetation (per SF)**
Minimum
$3.74
$0.00
$1.00
$10.00
$8.00
$10.80
$125.00
$125.00
$0.30
Maximum
$4.70
$5.17
$1.00
$10.00
$8.00
$15.00
$125.00
$125.00
$2.00
Mulch: 2” of wood chips (per SF) $0.16 $0.28
*Ranges from dumping on-site to 5 miles round trip.
**Ranges from on-site transplanting to mature nursery plants.
Maintenance
For the bioretention swales, the cost of cutting grass with a tractor was used as the minimum cost of maintenance (Table 6). The maximum cost included grass cutting, sediment removal, drainpipe inspection, and reseeding. Grass cutting was assumed to occur biweekly. This was a
170

conservative estimate because grass cutting should occur less frequently, given that swales require a grass height of about 4 to 6 inches (Rouge River, 1997).
Mulching comprised the majority of rain garden maintenance costs (Table 6). Mulching was assumed to occur twice per year as a conservative estimate, but it is likely to occur less frequently. Mulch prices varied depending on how the labor is performed. The cost of machine spreading was used for the minimum price, and the cost of hand spreading was used for the maximum price. Pipe unclogging and plant replacement were not considered because the developer is unlikely to face these costs in the first year.
Table 6. Yearly prices for Maintenance of Low Impact Design
Component Minimum Maximum Source
Grass swales $0.22 / LF $0.51 / LF RS Means, 2001 and Rouge River, 2001
Rain Gardens $72 per unit $135 per unit RS Means, 2001 and Hunt, 2002
Discounting
The developer’s maintenance costs were calculated for one year after construction, assuming that homeowners and the government would maintain the BMPs after the first year. These maintenance costs were discounted because they occur one year after construction. During that one year’s time, the money allocated to maintenance can earn an average return of 6% in another investment (Helfert, 1997). The developer’s maintenance costs were decreased by 6% and compounded at the end of each year.
Purpose of Spreadsheets
Spreadsheets were used to organize the price and quantity data (Table 7). Each development design had a separate set of spreadsheets. This series of spreadsheets allows for ease of movement between stages of estimation, and the spreadsheets have room for expansion by adding components, increasing maintenance years, or considering property value and growth rates.
171

Table 7. Description of Spreadsheets
Sheet Title
P_Mnt
P_Cnst
Dim
P_Min
P_Max.
Description
Lists minimum and maximum maintenance prices, with specifications and sources.
Lists minimum and maximum construction prices, with specifications and sources.
Lists dimensions of all components in the same units as the prices.
Organizes minimum prices (from P_mnt and P_Const) in a table with room for price changes over 10 years.
Organizes maximum prices (from P_mnt and P_Const) in a table with room for price changes over 10 years
Q Organizes dimensions from Dim in a table by timing of construction and maintenance over 10 years.
CashFlow_Min Displays product of Q and P_Min tables: the cost of each component, subtotals for each category, total cost, and the present value of total cost (maintenance is discounted). This sheet also has a separate cash flow table used for calculating the distribution of costs (all costs are discounted separately in this table).
CashFlow_Max Same as CashFlow_Min but uses P_Max instead of P_Min.
Results Summarizes the total costs for each category; shows incremental cost between
LID and conventional; shows distribution of costs, both for the design in question and incrementally.
Results
Net Cost savings
The LID development provides a net cost savings for the developer over conventional design.
The total cost of LID stormwater controls includes the cost of 94 rain gardens, 10,740 feet of bioretention swales, and 1 year of discounted maintenance by the developer (Table 8). The net savings for choosing LID over conventional design was calculated by subtracting the LID costs from the infrastructure savings (Table 9). This net savings ranged from about $190,000 to
$410,000 with a midpoint of $300,000. When a wet detention pond is included in the conventional site, the net cost savings for LID ranged from about $350,000 to $610,000 with a midpoint of $480,000 (Table 10). Since the cost range for LID was less than the conventional cost range, LID is likely to be the efficient choice for developments similar to the subject development (Figure 2).
172

Table 8. Total Costs of Low Impact Design for Subject Development
Component Minimum Maximum Midpoint
Rain Gardens
Bioretention Swales
Design and Engineering
Maintenance (2002 dollars)
Total
$78,678
$83,727
$40,601
$8,268
$211,274
$137,146
$133,793
$67,735
$16,549
$355,223
$107,912
$108,760
$54,168
$12,409
$283,249
Table 9. Cost Savings to the Developer for Choosing LID over Conventional Design.
Component Minimum Maximum Midpoint
Curb and Gutter
(11,586 minus 1,100 ft)
Sidewalks: Conv. minus LID
(57,930 minus 23,172 ft
2
)
Storm Drain Piping
(12,686 minus 1,100 ft)
Catch Basins
(20 units)
Subtotal
LID costs
Net Savings
$149,265
$83,782
$154,836
$11,724
$399,608
($211,274)
$188,334
$184,283
$106,295
$451,209
$20,907
$762,694
($355,223)
$407,471
$166,774
$95,039
$303,023
$16,316
$581,151
($283,249)
$297,902
Table 10. Cost Savings to the Developer for Choosing LID over Conventional Design with a
Stormwater Pond.
Component Minimum Maximum Midpoint
Subtotal from Table 9
Wet Detention Pond
(cost plus 1-year maint.)
Subtotal
LID costs
Net Savings
$399,608
$158,993
$558,601
($211,274)
$347,327
$762,694
$198,013
$960,707
($355,223)
$605,484
$581,151
$178,503
$759,654
($283,249)
$476,406
173

$1,200,000
$1,000,000
$800,000
$600,000
Maximum
Midpoint
Minimum
$400,000
$200,000
$0
Conventional Conventional with
Figure 2. Total Cost Ranges for the Three Development Designs
LID
Limitations
The net cost savings reported here does not include maintenance costs for Huntersville town government or the homeowner’s association. Once the developer sells a majority of the homes, the homeowners would be responsible for the rain garden maintenance. The town government most likely would maintain the swales, and swale maintenance is typically more expensive than maintenance of storm drain systems (SEWRPC, 1991). However, LID maintenance would be similar in cost to the maintenance of the wet detention pond.
The conservative estimates in this study provide a probable range of cost savings for developments similar to the subject development. Each site will differ in the feasibility and cost of LID, and the developers need to evaluate which BMPs are most economic for their specific sites.
174

References
Brown, W., and T. Schueler. 1997. The Economics of Stormwater BMPs in the Mid-Atlantic
Region, Final Report. Center for Watershed Protection. Chesapeake Research Consortium.
Center for Watershed Protection. 2000. The Economics of Stormwater Treatment: An Update.
Technical Note #90. Watershed Protection Techniques, 2(4): 395-499. [Online] Available: http://www.stormwatercenter.net/
Baker, S. Paige. 2001. Management Options Preliminary Cost Estimate, Upper Neuse River
Watershed Management Plan. CH2M-Hill.
Helfert, Erich A. 1997. Techniques of Financial Analysis, 9th edition. McGraw-Hill, New
York.
Hunt, William F. 2002. Urban Waterways: Designing Rain Gardens (Bio-retention Areas).
North Carolina Cooperative Extension, AG-588-3. [Online] Available: http://www.bae.ncsu.edu/people/faculty/hunt/index.html
NCDENR. 1999. Stormwater Best Management Practices. North Carolina Department of
Environment and Natural Resources Division of Water Quality Water Quality Section. [Online]
Available: http://h2o.enr.state.nc.us/su/PDF_Files/SW_Documents/BMP_Manual.PDF
NCDOT. 2001. Subdivision Roads Minimum Construction Standards. North Carolina
Department of Transportation Division of Highways. [Online] Available: http://www.doh.dot.state.nc.us/preconstruct/highway/dsn_srvc/value/manuals/newsubdiv3-23-
00.pdf
RS Means. 1997. Environmental Restoration Unit Cost Book. Robert Snow Means Company,
Inc., Kingston, Massachusetts.
RS Means. 2001. Facilities Construction Cost Data. Robert Snow Means Company, Inc.,
Kingston, Massachusetts.
RS Means. 1990. Means Estimating Handbook. Robert Snow Means Company, Inc., Kingston,
Massachusetts.
Rouge River. 1997. Planning and Cost Estimating Criteria for Best Management Practices, 1st edition. Rouge River National Wet Weather Demonstration Project, Wayne County, Michigan.
Rouge River. 2001. Planning and Cost Estimating Criteria for Best Management Practices
(Update). Rouge River National Wet Weather Demonstration Project, Wayne County,
Michigan. April 2001. TR-NPS25.00.
SEWRPC. 1991. Costs of Urban Nonpoint Source Water Pollution Control Measures.
Southeastern Wisconsin Regional Planning Commission.
175

US EPA. 1999. Preliminary Data Summary of Urban Stormwater Best Management Practices.
Report EPA-821-R-99-012. United States Environmental Protection Agency, Office of Water.
Washington, DC. [Online] Available: http://www.epa.gov/OST/stormwater/
Weinstein, Neil. 2001. Columns Subdivision LID cost estimates. Low Impact Design Center.
Beltsville, Maryland.
Weinstein, Neil. 2002. Low Impact Design Center. Beltsville, Maryland (personal communication).
Wiegand, C., T. Schueler, W. Chittenden and D. Jellick. 1986. "Cost of Urban Runoff
Controls." pp. 366-380. In: Proceedings of an Engineering Foundation Conference. Urban Water
Resources. ASCE. Henniker, New Hampshire June 23-27, 1986.
176

Prepared for
Mecklenburg County Land Use and
Environmental Services Agency
Prepared by
Tetra Tech, Inc.
P.O. Box 14409
3200 Chapel Hill – Nelson Highway, Suite 105
Research Triangle Park, NC 27709
(919) 485-8278
177

ACKGROUND AND
NTRODUCTION
Tetra Tech, Inc. compared the costs of conventional and Low Impact Development (LID) designs for a proposed high density residential development in Huntersville, North Carolina.
This memo presents the preliminary design and cost comparison.
For purposes of conducting this cost comparison, the Town of Huntersville provided conceptual site plans and information about the subject development. Tetra Tech, with design support from
Soil and Environmental Consultants (S&EC), then provided conceptual designs of conventional and LID stormwater systems for the site (Figures 1 and 2). It should be noted that the LID design was constrained by the Town and County to work with existing building footprints, road configurations, parking areas, and use of curb and gutter. The conceptual nature of the designs also limited the precise sizing of stormwater systems.
The LID design achieves more stringent water quantity and quality guidelines than the conventional design. The wet detention pond would meet the existing Mecklenburg County requirement to remove 85% of Total Suspended Solids (TSS) from the runoff of the first inch of rainfall. Due to spatial constraints, the conventional design would not meet the existing
Mecklenburg County requirements to approximate pre-development peak discharge runoff rates for the 2-year and 10-year storms. If additional detention capacity were added to meet the peak discharge runoff rate requirements, the infrastructure cost of the conventional design would substantially increase. In addition, the site may need to be redesigned to accommodate the added detention capacity, potentially reducing parking or landscaping area. These additional infrastructure and redesign costs have not been included in this analysis.
The LID design complies with Huntersville’s proposed water quality regulations for the Transit-
Oriented Development, Town Center, and Highway Commercial Zoning Districts. The regulations for these high-density districts are to meet the existing requirements described above and to retain the post-development minus pre-development runoff volume for the 1-year, 24-hour storm. The LID design achieves these requirements with bioretention areas. By complying with more stringent guidelines, the LID design retains far more runoff onsite and removes considerably more pollutants than the conventional design.
2
This analysis shows that the current LID design costs about $25,000 less than the conventional design. The conventional design would cost more if it were designed to meet the peak flow discharge requirements. Therefore, the net LID savings would be higher than reported here.
However, the conventional design cost may be reduced if a shared, offsite pond is used. The accuracy of this study is limited due to the conceptual nature of the designs.
2 Tetra Tech will provide a detailed comparison of pollutant removal and runoff retention.
178

Figure 1.
Conventional Site Plan for the Subject Development
Buildings 0.74 AC
Sidewalks 0.15 AC
Parking 0.15 AC
Landscaping 1.30 AC
Roads 0.54 AC
Wet Detention Pond 0.12 AC
Figure 2. Low Impact Development Plan for Subject Development
Buildings 0.74 AC
Sidewalks 0.15 AC
Parking 0.15 AC
Landscaping 1.18 AC
Roads 0.54 AC
Bioretention Cells 0.24 AC
Drainage area boundary
STIMATION
OURCES AND
ETHODS
179

This study was designed to use a development site as an example for potential, future development of Huntersville. All prices, both national averages and non-local prices, were converted to Charlotte prices for the year 2003. Unit prices were obtained from S&EC, RS
Means Manuals, and Yarbrough-Williams and Houle, Inc., a development engineering firm local to Huntersville. The sites did not differ in pavement or curb and gutter.
The designs differed, in part, by storm drain length and inlet type (Table 1). The storm drain price was estimated from trends in the Yarbrough-Williams and Houle cost data and agreed with prices in RS Means (2001). It was assumed that both designs would employ poured-in-place storm drain inlets. Due to the use of bioretention, the LID design would require fewer inlets than the conventional design.
Table 1. Unit Prices and Quantities for Conventional and LID Storm Systems
Component Price Quantity
Conventional LID
Storm Drain, 12”
Reinforced Concrete
$12 per LF
Source
775 LF 540 LF Yarbrough-Williams and Houle, Inc.
1
Catch Basins, Curb Inlets and Yard Inlets
$1,100 per unit
15 units 5 units Yarbrough-Williams and Houle, Inc.
1
Price was extrapolated from Yarbrough-Williams and Houle, Inc. cost data and confirmed with RS Means (2001).
The costs of the wet detention pond included excavation, hauling, fill, and outlet structures
(Table 2). Since this analysis was conducted at a conceptual level, S&EC based the price of the pond outlet structure
3
on similar project experience and not site-specific routing data. Therefore, the price of the outlet may vary considerably depending on the stormwater routing of the final site design.
The costs of the bioretention cells included excavation, hauling, soil fill, and subdrain piping
(Table 3). The LID design would contain five bioretention cells of varying sizes, and catch basins would function as outlet structures for the bioretention cells. It was assumed that pond and bioretention landscaping would fulfill some of the landscaping requirements of the development, and, therefore, planting costs were not considered. For both the LID and conventional stormwater controls, design and engineering costs were estimated as 25% of the total construction costs (US EPA, 1999).
3
The pond outlet structure would include the following components: riser, barrel, anchor slab, flared end section (or head wall), riprap outlet protection apron, erosion protection blanket, and seepage control device.
180

Table 2. Cost Estimation of Wet Detention Pond (Conventional Design)
Component
Excavation (CY)
Quantity Unit Price Total Source
1,275 $3.74 $4,769 RS Means, 2001
Hauling
1
(CY) 765 $3.56 $2,723 RS Means, 2001
Outlet (Unit)
Backfill (CY)
2
Compaction (CY)
2
Design and Engineering
1.00 $35,000 $35,000 S&EC
382.5 $0.90
382.5 $2.00
$345
$765
RS Means, 2001
RS Means, 2001
25% $10,900 USEPA, 1999
Total $54,502
1
Quantity assumes 60% of excavated material is dumped offsite; price refers to hauling distance of 2 miles.
2
Quantity assumes 30% of excavated material is used in earthworks, and 10% is spread elsewhere onsite.
Table 3. Cost Estimation for Bioretention Areas (LID Design)
Components
Excavation (CY)
Quantity Price Total Source
1,650 $3.74 $6,171 RS Means, 2001
Hauling
1
(CY) 1,485 $3.56 $5,287 RS Means, 2001
Soil (CY)
Subdrain (LF)
Design and Engineering
1,460 $15.00 $21,900
350 $3.50 $1,225
25% $8,646
S&EC
S&EC
USEPA, 1999
Total $43,228
1
Quantity assumes 90% of excavated material is dumped offsite; price refers to hauling distance of 2 miles.
181

ESULTS
The net savings for choosing LID over conventional design was calculated by subtracting the
LID costs from the infrastructure costs (Table 4). The net savings was estimated at about
$25,000. About 45% of the net savings was due to the difference in cost between the bioretention and a conventional stormwater pond. Differences in storm drain piping and inlets
(including catch basins) represented 11% and 44% respectively of the net savings for LID.
Table 4. Net Savings to the Developer for Choosing LID over Conventional Design.
Component Conventional LID Net
Storm Drain Piping
$ 9,300 $6,480 $2,820
Storm Drain Inlets
$16,501 $5,500 $11,000
Wet Detention Pond
$54,502 - $54,502
Bioretention
- $43,228 $(43,228)
Net Savings
$25,094
S&EC considered this site as a separate development, as requested by the County, and estimated the dimensions of an on-site wet detention pond that would remove 85% of TSS from the runoff generated during the first inch of rainfall. An on-site pond was not designed to meet the peak discharge runoff rate requirements due to spatial constraints. If on-site ponds were to be employed, the site would need to be redesigned to accommodate more detention capacity. The additional detention capacity and site redesign would increase the costs of the conventional design, and the net savings for LID would be greater than $25,000.
It should be noted that, in the proposed design, the subject development is part of a larger development, and the entire site will drain to a larger, more economical pond than the one considered in this analysis. Tetra Tech was unable to compare the off-site pond to the LID design because the data for the off-site pond was not available. The conventional design cost may be reduced if the shared, off-site pond is used as a part of the larger development.
EFERENCES
RS Means. 2001. Facilities Construction Cost Data. Robert Snow Means Company, Inc.,
Kingston, Massachusetts.
US EPA. 1999. Preliminary Data Summary of Urban Stormwater Best Management Practices.
Report EPA-821-R-99-012. United States Environmental Protection Agency, Office of Water.
Washington, DC. [Online] Available: http://www.epa.gov/OST/stormwater/
182

Prepared for
Mecklenburg County Land Use and
Environmental Services Agency
Town of Huntersville, NC
Prepared by
Tetra Tech, Inc.
P.O. Box 14409
3200 Chapel Hill – Nelson Highway, Suite 105
Research Triangle Park, NC 27709
(919) 485-8278
183

ACKGROUND AND
NTRODUCTION
Tetra Tech, Inc. compared the costs of conventional and Low Impact Development (LID) designs for a proposed commercial development site in Huntersville, North Carolina. This memo presents the preliminary design and cost comparison.
For purposes of conducting this cost comparison, the Town of Huntersville provided conceptual site plans and information for the subject development (Figure 1). Mecklenburg County estimated the dimensions of the conventional stormwater system for this site. Tetra Tech, with design support from Soil and Environmental Consultants (S&EC), then provided a conceptual design of an LID stormwater system (Figure 2). It should be noted that the LID design was constrained by the Town and County to work with existing building footprints, road configurations, parking areas, and use of curb and gutter. The conceptual nature of the designs also limited precise sizing of stormwater systems.
The LID design achieves more stringent water quantity and quality guidelines than the conventional design. The conventional site complies with the existing Mecklenburg County requirements, which are to remove 85% of Total Suspended Solids (TSS) from the runoff of the first inch of rainfall and to approximate pre-development peak discharge runoff rates for the 2year and 10-year storms. The LID design complies with Huntersville’s proposed water quality regulations for the Transit-Oriented Development, Town Center, and Highway Commercial
Zoning Districts. The proposed requirements for these high-density districts are to meet the existing requirements described above and to control and treat the post-development minus predevelopment runoff volume for the 1-year, 24-hour storm. By complying with more stringent guidelines, the LID design controls nearly 1.4 acre-feet more runoff volume and removes more pollutants than the conventional design.
4
This analysis shows that the LID design would cost about $10,000 more than the conventional design. The difference in cost is due, in part, to the difference in stormwater criteria and the design constraints. The accuracy of this study is limited due to the conceptual nature of the designs.
4 Tetra Tech will provide a detailed comparison of pollutant removal and runoff retention.
184

Figure 1.
Conventional Site Plan for the Subject Development
Buildings 2.79 AC
Sidewalks 1.48 AC
Parking 8.41 AC
Landscaping 5.75 AC
Roads 8.67 AC
Wet Detention 0.49AC
Figure 2. Low Impact Development Plan for the Subject Development
Buildings 2.79 AC
Sidewalks 1.48 AC
Parking 3.41 AC
Landscaping 4.81 AC
Roads 8.67 AC
Dry Detention 0.49AC
Bioretention 0.94 AC
Vegetated Filter Basins
185

STIMATION
OURCES AND
ETHODS
This study was designed to use a development site as an example for potential, future development of Huntersville. All prices, both national averages and non-local prices, were converted to Charlotte prices for the year 2003. Unit prices were obtained from S&EC, RS
Means Manuals, and Yarbrough-Williams and Houle, Inc., a development engineering firm local to Huntersville. The sites did not differ in pavement or curb and gutter.
Storm drain prices were obtained from Yarbrough-Williams and Houle, Inc. and RS Means
(2001) (Table 1). Mecklenburg County designed a conceptual layout of the conventional storm drain system. The County then estimated that bioretention in the LID Design would reduce flow through the storm drain system by about 5% to 15%, depending on the size and slope of each parking lot; therefore, storm drain pipe costs for LID were estimated as a median of 10% less than conventional pipe costs. The cost of manholes, inlets, end sections, and head walls were assumed to be equal for the LID and conventional designs.
Table 1. Cost Estimation of Storm Drain Systems
Component
15-inch Reinforced Concrete
Pipe (RCP)
18-inch RCP
24-inch RCP
30-inch RCP
36-inch RCP
48-inch RCP
60-inch RCP
Inlets/ Manholes
Quantity Unit Price Conventional LID
1
2090 $15/ LF
1550
880
610
1020
$18/ LF
$24/ LF
$30/ LF
$36/ LF
150
50
$55/ LF
$88/ LF
80/ 4 $1,100/ unit
Source
$31,350 $28,215 Yarbrough-Williams and Houle, Inc.
$27,900 $25,110 Yarbrough-Williams and Houle, Inc.
$21,120 $19,008 Yarbrough-Williams and Houle, Inc.
$18,300 $16,470 Yarbrough-Williams and Houle, Inc.
2
$36,720 $33,048 Yarbrough-Williams and Houle, Inc.
$8,250 $7,425
$4,400 $3,960
RS Means, 2001
RS Means, 2001
$92,400 $92,400 Yarbrough-Williams and Houle, Inc.
24-inch Flared End Section
36-inch Flared End Section
1 $400/ unit
2 $675/ unit
$400 $400 Yarbrough-Williams and Houle, Inc.
$1,350 $1,350 Yarbrough-Williams and Houle, Inc.
$1,600 $1,600 Yarbrough-Williams and Houle, Inc.
2
60-inch Head Wall 1 $1,600/ unit
Total Cost $243,790 $228,986
1
LID pipes are assumed at 90% of the conventional cost based on a median 10% reduction in flow.
2
Price was extrapolated from Yarbrough-Williams and Houle, Inc. cost data and confirmed with RS Means (2001).
The costs of the stormwater ponds and bioretention included excavation, hauling, soil fill, and outlet structures. Since this analysis was conducted at a conceptual level, S&EC based the price of the pond outlet structures
5
on similar project experience and not site-specific routing data. As
a conservative estimate, the outlets were assumed to be equal in cost; however, in the final site design, the LID design outlet is likely to cost less than the conventional design outlet.
5
The pond outlet structures would include the following components: riser, barrel, anchor slab, flared end section
(or head wall), riprap outlet protection apron, erosion protection blanket, and seepage control device.
186

It was assumed that pond and bioretention landscaping would fulfill some of the landscaping requirements of the development, and, therefore, planting costs were not considered. Design and engineering costs were estimated as 25% of the total construction costs (US EPA, 1999).
The cost estimations of the stormwater ponds and bioretention are presented in Tables 2 through
4. The conventional design achieves existing requirements with a 1-acre wet detention pond
(Table 2). The original conventional plan allocated 0.5 acres for a stormwater pond; therefore, the street and building layout would need to be altered to accommodate a 1-acre pond.
Effectively, this redesign would likely result in less building, parking, or landscaping square footage, or the creation of a more urban design with increased building height and/or parking decks. The cost of this redesign is not considered in this estimation.
In the LID design, bioretention areas (Table 3) retain the volume of the 1-year storm postdevelopment minus pre-development volume and partially control peak discharge. A dry detention pond controls the additional peak discharge in the LID design (Table 4).
Table 2. Cost Estimation of Wet Detention Pond (Conventional Design)
Component Quantity Unit Price Total Source
Excavation (CY)
Hauling
1
(CY)
Backfill (CY)
2
Compaction (CY)
2
Outlet (Unit)
Design and Engineering
22,741
13,644
6,822
6,822
1
$3.74
$3.56
$0.90
$2.00
$100,000
25%
$85,050 RS Means, 2001
$48,574 RS Means, 2001
$6,152 RS Means, 2001
$13,644 RS Means, 2001
$100,000 S&EC
$63,355 USEPA, 1999
Total $316,776
1
Quantity assumes 60% of excavated material is dumped offsite; Price refers to hauling distance of 2 miles.
2
Quantity assumes 30% of excavated material is used in earthworks, and 10% is spread elsewhere onsite.
Table 3. Cost Estimation for Bioretention Areas (LID Design)
Components Quantity Price Total Source
Excavation (CY)
Hauling
1
(CY)
6,825
6,143
$3.74
$3.56
$25,526 RS Means, 2001
$21,867 RS Means, 2001
Soil (CY) 6,066 $15.00 $90,990 S&EC
Subdrain (LF) 2,300 $3.50 $8,050 S&EC
Design and Engineering 25% $36,608 USEPA, 1999
Total $183,041
1
Quantity assumes 90% of excavated material is dumped offsite; price refers to hauling distance of 2 miles.
187

Table 4. Cost Estimation for Dry Detention Pond (LID Design)
Component Quantity Unit Price Total Source
Excavation (CY)
Hauling
1
(CY)
Backfill (CY)
2
Compaction (CY)
2
3,950
2,370
1,185
1,185
$3.74
$3.56
$0.90
$2.00
$14,773 RS Means, 2001
$8,437 RS Means, 2001
$1,069 RS Means, 2001
$2,370 RS Means, 2001
Outlet (Unit)
Design and Engineering
1 $100,000
25%
$100,000 S&EC
$31,662 USEPA, 1999
Total $158,311
1
Quantity assumes 60% of excavated material is dumped offsite; price refers to hauling distance of 2 miles.
2
Quantity assumes 30% of excavated material is used in earthworks, and 10% is spread elsewhere onsite.
ESULTS
The net cost for choosing LID over conventional design was calculated by subtracting the LID costs from the conventional costs. The net cost was estimated at about $10,000 (Table 5). The difference in cost is due, in part, to the difference in stormwater criteria and the design constraints. The accuracy of this study is limited due to the conceptual nature of the designs.
Table 5. Net Cost to the Developer for Choosing LID over Conventional Design
Component Conventional LID Net
Storm Drain System
Dry Detention Pond
Bioretention Areas
$243,790
$316,776
$0
$228,986
$158,311
$183,041
$14,804
$158,465
($183,041)
Total $560,566 $570,338 ($9,772)
In essence, the LID design costs more than the conventional design because the LID design controls nearly 1.4 acre-feet more runoff than the conventional design. The conventional design controls about 1.5 acre-feet while the LID design controls about 2.9 acre-feet.
It also should be noted that the conventional costs do not include the cost of allocating an additional one-half acre to a wet detention pond to meet the existing requirements. This cost to the developer may be substantial since the pond would likely displace parking, building, and/or landscape square footage.
A constraint on the size of the dry detention pond also contributed to the net cost of the LID design. The dry detention pond was used to control the peak discharge of the 2-year and 10-year storms. Since the dry detention pond was limited to one-half acre by the original design, the bioretention cells had to be designed to handle more volume to help control peak discharge for the 10-year storm event.
Several small areas of the site will need to be graded to ensure proper drainage. Alternatively, in the LID design, S&EC suggested the possible use of vegetated filter basins for these areas
(Figure 2).
188

References
RS Means. 2001. Facilities Construction Cost Data. Robert Snow Means Company, Inc.,
Kingston, Massachusetts.
US EPA. 1999. Preliminary Data Summary of Urban Stormwater Best Management Practices.
Report EPA-821-R-99-012. United States Environmental Protection Agency, Office of Water.
Washington, DC. [Online] Available: http://www.epa.gov/OST/stormwater/
189

Prepared for
Mecklenburg County Land Use and
Environmental Services Agency
Prepared by
Tetra Tech, Inc.
P.O. Box 14409
3200 Chapel Hill – Nelson Highway, Suite 105
Research Triangle Park, NC 27709
(919) 485-8278
190

ACKGROUND AND
NTRODUCTION
Tetra Tech, Inc. compared the costs of traditional and Low Impact Development (LID) designs for a proposed elementary school in Huntersville, North Carolina. This memo presents the preliminary design and cost comparison.
For purposes of conducting this cost comparison, the Town of Huntersville provided site plans and information for the proposed school site using a conventional stormwater approach. Tetra
Tech, with design support from Soil and Environmental Consultants (S&EC), then provided a redesign of the property using an LID approach. It should be noted that the LID design was constrained by the Town and County to work with existing building footprints, road configurations, parking areas, and use of curb and gutter.
The LID design achieves stricter water quantity and quality guidelines than the traditional design. The traditional site is designed to comply with the existing Mecklenburg County requirements, which are to remove 85% of Total Suspended Solids (TSS) from the first inch of runoff and to approximate pre-development peak discharge runoff rates for the 2-year and 10year storms. The traditional plan achieves these requirements with a curb and gutter system that directs runoff into a stormwater pond.
The LID design complies with Huntersville’s proposed regulation to retain the post-development minus pre-development runoff volume for the 2-year storm. By complying with stricter guidelines, the LID design retains far more runoff onsite and removes considerably more pollutants than the traditional design.
6
While the LID plan uses curb and gutter, the runoff is directed into bioretention areas throughout the site. The bioretention areas retain the excess volume of the 2-year storm post-development minus pre-development volume and partially control peak discharge. A dry detention pond controls the additional peak discharge.
This analysis shows that the current LID design costs about $50,000 more than the traditional design. The higher cost is due, in part, to the high amount of impervious surface onsite and the resulting volume of runoff. A medium or low-density development would offer more options for reducing infrastructure costs than a highly impervious development like the proposed school site.
The LID design also costs more than the conventional design because the proposed regulations require more expensive stormwater controls than the existing ordinance.
6 Tetra Tech will provide a detailed comparison of pollutant removal and runoff retention.
191

Figure 1. Traditional Site Plan for Proposed School
Buildings 2.1 AC
Sidewalks 0.7 AC
Play Areas 0.3 AC
Parking 1.0 AC
Roads 1.0 AC
Wet Detention Pond 0.6 AC
Remaining Natural Areas 2.0 AC
Landscaped Areas 8.6 AC
Figure 2. Low Impact Development Plan for Proposed School
Buildings 2.1 AC
Sidewalks 0.7 AC
Play Areas 0.3 AC
Parking 1.8 AC
Roads 1.0 AC
Dry Detention Basin 0.4 AC
Remaining Natural Areas 2.5 AC
Landscaped Areas 7.5 AC
Bioretention Cells 0.8 AC
Drainage area boundary
192

STIMATION
OURCES AND
ETHODS
This study was designed to use a development site as an example for potential, future development of Huntersville. All prices, both national averages and non-local prices, were converted to Charlotte prices for the year 2002. Infrastructure prices (Table 1) were obtained from Yarbrough-Williams and Houle, Inc., a development engineering firm local to Huntersville.
Pond and bioretention costs were estimated with unit prices obtained from S&EC and RS Means
Manuals.
Table 1. Prices for Infrastructure
Component Price Source Notes
Street and Parking Lot
Paving (per SY)
$9.50 Yarbrough-Williams and Houle, Inc.
8” & 2 ½”
Storm Drain (per LF)
$24.00 Yarbrough-Williams and Houle, Inc.
24” Reinforced Concrete Pipe
Catch Basins (per unit)
$1,100 Yarbrough-Williams and Houle, Inc.
Single
The costs of the stormwater ponds and bioretention areas included excavation, hauling, soil fill, and outlet structures (Tables 2 through 4). Since this analysis was conducted at a conceptual level, S&EC based the price of the pond outlet structure
7
on similar project experience and not
site-specific routing data. Therefore, the price of the outlet may vary considerably depending on the stormwater routing of the final site design. It was assumed that pond and bioretention landscaping would fulfill some of the landscaping requirements of the development, and, therefore, planting costs were not considered. Design and engineering costs were estimated as
25% of the total construction costs (US EPA, 1999).
Table 2. Cost Estimation of Wet Detention Pond (Conventional)
Component
Excavation (CY)
Quantity Unit Price
9,680
Total Source
$3.74 $36,203 RS Means, 2001
Hauling* (CY)
Outlet (Unit)
4,840
1
$3.56
$75,000
$17,230 RS Means, 2001
$75,000 S&EC
Design and Engineering 25% $32,108 US EPA, 1999
Total $160,542
*Quantity assumes half of excavated material is dumped offsite; Price refers to hauling distance of 2 miles.
7
The pond outlet structures would include the following components: riser, barrel, anchor slab, flared end section
(or head wall), riprap outlet protection apron, erosion protection blanket, and seepage control device.
193

Table 3. Cost Estimation for Bioretention Areas (LID)
Components Quantity Price Total Source
Excavation (per CY)
Hauling* (per CY)
Soil (per CY)
5,082
2,541
4,518
$3.74 $19,007 RS Means, 2001
$3.56 $9,046 RS Means, 2001
$15.00 $67,763 S&EC
Subdrain (per LF) 1,278 $3.50 $4,473 S&EC
Total $100,288
*Quantity assumes half of excavated material is dumped offsite; Price refers to hauling distance of 2 miles.
Table 4. Cost Estimation for Dry Detention Pond (LID)
Component Unit Quantity Unit Price Total Source
Excavation (CY) CY 1,500 $3.74 $5,610 RS Means, 2001
Hauling* (CY)
CY 750 $3.56 $2,670 RS Means, 2001
Backfill, Structural Common Earth CY 1250 $0.90 $1,127 RS Means, 2001
Compaction CY 1250 $2.00 $2,500 RS Means, 2001
Outlet (Unit) unit 1 $60,000 $60,000 S&EC
Total $71,907
*Quantity assumes half of excavated material is dumped offsite; Price refers to hauling distance of 2 miles.
194

ESULTS
The net cost for choosing LID over conventional design was calculated by subtracting the LID costs (Table 5) from the infrastructure costs (Table 6). The net cost was estimated at about
$50,000.
Table 5.Total Costs of Low Impact Design for Proposed School
Component Cost
Bioretention
Dry Detention
Design and Engineering
Total
$100,288.14
$71,907.21
$43,048.84
$215,244.18
Table 6. Net Cost to the Developer for Choosing LID over Conventional Design.
Component LID Traditional Net
Street and Parking Lot Paving
Storm Drain Piping
$128,744
$55,392
$128,744
$57,216
-
$1,824
Catch Basins
Stormwater Pond
Subtotal
LID costs
Net Cost
$19,800
-
$203,936
$215,244
$24,200
$160,542
$370,702
-
$4,400
$160,542
$166,766
$(215,244)
$(48,478)
195

References
RS Means. 2001. Facilities Construction Cost Data. Robert Snow Means Company, Inc.,
Kingston, Massachusetts.
US EPA. 1999. Preliminary Data Summary of Urban Stormwater Best Management Practices.
Report EPA-821-R-99-012. United States Environmental Protection Agency, Office of Water.
Washington, DC. [Online] Available: http://www.epa.gov/OST/stormwater/
196

197

Latin Name
Acer rubrum
Betula nigra
Taxodium distichum
Myrica cerifera
Persea borbonia
Chamaecyparis thyoides
Liquidambar styraciflua
Nyssa aquatica
Plantanus occidentalis
Quercus bicolor
Quercus laurifolia
Quercus michauxii
Quercus palustrus
Thuja occidentalis
Diospyros virginiana
Fraxinus americana
Fraxinus pennsylvanica
Gleditsia triacanthos
Ulmus americana
Ilex opaca
Magnolia viriginiana
Juniperus virginiana
Koelreuteria paniculata
Quercus phellos
Nyssa sylvatica
Oxydendrum arboreum
Morus rubra
Cornus florida
Cercis canadensis
Ilex vomitoria 'Pendula"
Crataegus viridis
Carpinus caroliniana
Chionanthus virginicus
Cyrilla racemiflora
Illicium parviflorum
Cladrastis kentukea
Aesculus pavia
Common Name
Red Maple
River Birch
Bald Cypress
Southern Wax Myrtle
Sweet Bay or Red Bay
Atlantic white cedar
Sweetgum
Water tupelo
Sycamore
Swamp white oak
Swamp laurel oak
Swamp chestnut oak
Pin oak
White cedar
Persimmon
White Ash
Green Ash
Honeylocust
American Elm
American Holly
Sweet bay magnolia
Eastern red cedar
Golden raintree
Willow oak
Sour gum
Sourwood
Red mulberry
Flowering dogwood
Red Bud Forest Pansy
Weeping yaupon
Green hawthorn
American hornbeam
Fringetree
Leatherwood
Small anise tree
American yellowwood
Red buckeye
Bloom Time
Fall Color
April-May
Evergreen
Evergreen
April-May
Evergreen
April-May
March-April
April
April
April
April
April
Evergreen
May-June
April-May
April-May
Fall Color
Fall Color
Evergreen
May-June
Evergreen
June-July
April
May-June
June-July
Fall Color
April-May
April-May
April-May
May
Fall Color
May-June
Evergreen
April-May
May
April-May
Color
Red
Height
40'-60'
Brown
Green
50'-70'
50'-75'
Green 15'-25'
Greenish 30'-40'
Green
Greenish 75'-100'
White 100'
Yellow 75'-100'
Yellow
Green
Red
Yellow
Green
White
Purple
Purple
50'-60'
60'-70'
60'-80'
50'-70'
30'-50'
35'-60'
60'-80'
50'-70'
Yellow 50'-75'
Yellow 75'-125'
Green
White
30'-50'
10'-35'
Green
Yellow
30'-65'
30'-40'
Yellow 40'-75'
Greenish 30'-50'
White
Yellow
20'-50' white
Red
15'-30'
15'-20'
White
White
Scarlet
White
8'-15'
25'-35'
20'-30'
12'-20'
White
White
White
Red
15'
6'-12'
30'-50'
12'-15'
198

Latin Name
Alnus serrulata
Aronia arbutifolia
Aronia melanocarpa
Clethra alnifolia
Cornus amomum
Fothergilla gardenii
Hydrangea quercifolia
Ilex verticillata
Myrica pensylvanica
Rhododendron maximum
Rhodo periclymenoides
Rhododendron viscosum
Rhododendron austrinium
Rhododendron canescens
Sambucus canadensis
Vaccinium corymbosum
Viburnum nudum
Amelancher canadensis
Itea virginica
Aesculus sylvatica
Leucothoe racemosa
Ilex decidua
Lyonia lucida
Physocarpus opulifolius
Hydrangea arborescens
Ilex serrata
Callicarpa americana
Viburnam dentatum
Ilex glabra
Aesculus parviflora
Spirea alba
Cornus stolonifera
Cephalantus occidentalis
Ilex vomitoria
Myrica cerifera
Euonymus americanus
Hamamelis virginiana
Viburnum lentago
Calycanthus floridus
Common Name
Common alder
Red chokeberry
Black chokeberry
Sweet pepperbush
Silky dogwood
Fothergilla
Oakleaf hydrangea
Winterberry
Bayberry
Great rhododendron
Pinxter azalea
Swamp azalea
Florida Azalea
Piedmont azalea
Common elderberry
Highbush blueberry
Possum-haw viburnum
Shadblow serviceberry
Virginia sweetspire
Painted buckeye
Swamp sweetbells
Possumhaw
Fetterbush
Ninebark
Smooth hydrangea
Sparkleberry holly
Beautyberry
Arrowwood viburnum
Inkberry
Bottlebrush buckeye
Meadowsweet
Red twig dogwood
Buttonbush
Yaupon
Wax myrtle
Strawberry bush
Witch-hazel
Nannyberry
Carolina allspice
March-June
May-June
June-Sept.
June-Sept.
June-Aug.
May-June
May-June
July-Sept
June-Aug.
April-May
June-July
May-June
March
Sept-Oct
Oct.-Dec.
May-June
April-July
Bloom Time
Feb-May
April
May
July-Aug.
May-July
April-May
May-July
April-May
May
June-July
April-May
March-Dec.
March-April
March-April
May-Aug.
May
April-May
April-May
May-June
April-May
April-May
April-May
Height
15'-20'
6'-8'
3'-6'
3'-8'
6'-10'
1.5' - 3.0'
4'-6'
3'-12'
5'-10'
8'-15'
3'-6'
5'-8'
8'-10'
10'-15'
5'-12'
6'-12'
5'-7'
25'-30'
3'-4'
15'-20'
10'-12'
7'-15'
3'-9'
5'-8'
3'-5'
10'-12'
3'-6'
6'-10'
3'-5'
6'-12'
3'-4'
10'-12'
20'-25'
15'-20'
25'
4'-6'
15'-20'
15'-20'
6'-10'
Color
Yellow
White
White
White
White
White
White
White
Yellowish
White
Pink
White
Yellow
White
White
White
White
White
White
Yellow
White
White
Pink
White
White
White
Pink
White
White
White
White
White
White
White
Greenish
Red
Yellow
White
Brown
199

Latin Name
Andropogon glomeratus
Andropogon virginicus
Carex crinita
Carex lurida
Common Name
Bushy bluestem
Broomsedge
Long hair sedge
Sallow sedge
Carex stricta Tussock sedge
Dichanthelium clandestinum Deer-tongue
Juncus effsus
Juncus tenuis
Soft rush
Path rush
Saccharum ravennae
Panicum virgatum
Sparganium americanum
Spartina pectinata
Giant plumegrass
Switch grass
American burreed
Prairie cordgrass
Chasmanthium latifolium
Scripus atrovirens
River Oats
Green Bulrush
Spartina bakeri Sand cordgrass
Schizachyrium scoparium Little Bluestem
Bloom Time
Sept.-Dec.
Aug-Nov
May-July
May-Sept.
May-Sept.
May-Sept.
July-Aug.
May-Sept.
Sept.-Oct.
June-Oct.
May-Sept.
June-Oct.
Aug.-Sept.
June-Aug.
April-Nov
Aug.-Feb
Color
Pinkish
Gold
Green
Green
Green
Green
Green
Green
Purple
Burgundy
Green
Green
Green
Green
Green
Bronze
Height
2'
3'
2'
1'-3'
2-3.5'
2'
2'-4'
0.5'-2'
2.5'-5'
5'
3'-6'
2.5'-3'
2'-2.5'
3'-8'
4'-6'
1.5'-5'
200

Aster laevis
Boltonia asteroides
Crinium americanum
Monarda didymas
Liatris pycnostachya
Heliopsis helianthoides
Eupatorium purpureum
Eupatorium dubium
Phlox subulata
Iris sibirica 'Sparkle"
Baptista australis
Zephyranthes atamasco
Iris virginica
Viola sororia
Smooth aster
Boltonia
Sept-Oct.
Aug-Sept.
American Crinium Lily
Bee balm
Aug.-Sept.
July-Aug
Prairie blazing star
Oxeye
July-Aug.
June-Aug.
Joe pye weed
Little Joe
Creeping phlox
Siberian iris
Blue false indigo
Atamasco lily
Blue Flag Iris
Wolly blue violet
July-Sept.
July-Sept.
April-May
May-June
May-June
March-Oct.
Feb-March
April-Aug.
Phlox divaricata
Amsonia tabernae Montana
Veronia
Hypericum hypericoides
Solidago rugosa
Hibiscus moscheutos
Hibiscus coccineus
Chrysogonum virginianum
Woodland phlox
Bluestar 'Montana"
Ironweed
St. Andrew's cross
April-May
April-May
July-Sept.
July-Aug
Goldenrod August
Hibiscus Blue River II July-Sept.
Scarlet hibiscus
Green and Gold
July-Sept.
May-Oct.
Impatiens capensis
Mimulus ringens
Bidens aristosa
Peltandra virginica
Jewelweed
Allegheny monkey flower
June-Sept.
June-Sept.
Bearded beggarticks
Green arrow arum
July-Aug.
Mar-Sept.
Osmunda cinnanomea Cinnamon fern
Polystichum acrostichoides Christmas fern
Rudbeckia fulgida “Goldstrum” Blackeyed Susan
Solidago rugosa Goldenrod (Fireworks)
Mar-Nov
Evergreen
Sept.-Oct.
Aug-Oct.
Aster 'Celeste'
Chelone glabra
Liriope muscari
Lobelia siphiltica
Aster
Turtlehead
Lilyturf
Great lobelia
Lobelia cardinalis
Phlox paniculata
Cardinal flower
Garden phlox
Rudbeckia hirta Summerblaze
Senecio cineraria Dusty Miller
Hyericum calycinum
Hypericum frondosum
Baptista alba
Penstemon digitalis
Aster novae-angliae
Aster oblongifolius
Senecio aureus
Salvia nemorosa
Helianthus salicifolius
Zenobia pulverulenta
Asclepias tuberosa
Atamasca
St. John's Wort
Golden St. John's Wort
White false indigo
Smooth penstemon
New England Aster
Fall Aster
Golden Ragwort
Salvia "May Night"
First Light Sunflower
Dusty zenobia
Butterfly weed
Atamasco Lily
Aug.-Sept.
Aug.-Oct.
Aug.-Sept.
July-Sept.
July-Sept.
July-Sept.
July-Sept.
Mar-Dec.
July-Aug.
July-Aug.
June-July
June-July
Sept.-Oct.
Oct-Nov
April-May
July-Nov
Sept.-Oct.
May-June
June-August
201
Violet
White
Lavender
Blue
Scarlet
Salmon
Yellow
Silver
Yellow
Yellow
White
White
Pink
Violet
Golden
Violet
Yellow
White
Orange
Rose
Blue
Red
Yellow
Yellow
White
Red
Yellow
Orange
Lilac
Yellow
Green
Green
Green
Golden
Yellow
Violet
White
White
Red
Lilac
Yellow
2.5'-3'
5'-6'
1.5'
2'-4'
2'-5'
2'-4'
Mauve
Lavender
Pink
Blue
5'-7'
3'-4'
0.25'-0.5'
2.5'-3'
Indigo
White
3'-4'
1'-1.5'
Blue 4"
Blue/White 0.5'-0.75'
0.75'-1'
1'-1.5'
4'-6'
6"-12"
3'-4'
2.5'-3'
4'
0.5'-1.5'
2'-5'
0.75'-1'
4'
2'-3'
2'-5'
2'-3'
24"-36"
2.5'-3'
2'-2.5'
2'-3'
1'-1.5'
2'-3'
2'-4'
1.5'-2'
2'-2.5'
6" -18"
6"-12"
3'-4'
3'-5'
2'-3'
3'-4'
3'-4'
1'-3'
1.5'-2'
3'-4'
2'-4'
1'-2.5'

202

(This form for use with property owner associations.)
( INSERT PROJECT NAME must match plat title)
DECLARATION OF COVENANTS
For Maintenance of Water Quality Control Structures
Town of (INSERT JURISDICTION)
THIS DECLARATION OF COVENANTS, made this day of , 20 , by
hereinafter referred to as the “Developer” to and for the benefit of the Town of (INSERT
JURISDICTION) and its successors and assigns.
WITNESSETH:
WHEREAS, the Town of (INSERT JURISDICTION) is authorized to prevent surface water quality degradation from development or redevelopment activities within its jurisdiction as set forth in the Town of (INSERT JURISDICTION) Post-Construction Storm Water Ordinance: and
WHEREAS, the Developer is the owner of a certain tract or parcel of land more particularly described as: being all or part of the land which it acquired by deed dated
from grantors, and recorded with the Mecklenburg County Register of Deeds
Office , in Book at Page such property being hereinafter referred to as the “the property;” and
WHEREAS, the Developer desires to construct certain improvements on its property regulated by the Town of (INSERT JURISDICTION) Post-Construction Storm Water
Ordinance; and
WHEREAS, in order to construct certain improvements on its property, the Developer desires to build and maintain at its expense, a water quality control structure more particularly described and shown on plans titled: and further identified under Storm Water
Management Permit Number ; and
Form #PCO19 203 (Revision Jan. 08)

WHEREAS, the Town of (INSERT JURISDICTION) or its designee have reviewed and approved the plans associated with the Storm Water Management Permit subject to the execution of this agreement.
NOW THEREFORE, in consideration of the benefits received by the Developer as a result of approval by the Town of (INSERT JURISDICTION) or its designee of these plans, the
Developer, with full authority to execute deeds, mortgages, other covenants, and all rights, title and interest in the property described above, does hereby covenant with the Town of (INSERT
JURISDICTION) as follows:
1. The Developer shall develop and attach to this “DECLARATION OF COVENANTS” for recording at the Mecklenburg County Register of Deeds Office a “MAINTENANCE
PLAN” that has been reviewed and approved by the Town of (INSERT JURISDICTION) or its designee. This Maintenance Plan shall describe the specific maintenance practices to be performed for the above referenced water quality control structure and include a schedule for implementation of these practices. The Plan shall indicate that the water quality control structure shall be inspected by a qualified professional at least annually to ensure that it is operating properly. The Plan shall specify the name, mailing address and phone number of the party responsible for the fulfillment of the Maintenance Plan and describe the mechanism by which the funding for the performance of this maintenance shall be secured through the use of an (INSERT "ESCROW ACCOUNT" OR OTHER
FUNDING SOURCE AS APPROVED BY THE STORM WATER
ADMINISTRATOR).
2.
The Developer shall establish an (INSERT "ESCROW ACCOUNT" OR OTHER
FUNDING SOURCE AS APPROVED BY THE STORM WATER
ADMINISTRATOR), which can be spent solely for sediment removal, structural, biological or vegetative replacement, major repair, or reconstruction of the above referenced water quality control structure. If the water quality control structure is not performing adequately or as intended or is not properly maintained, the Town of
(INSERT JURISDICTION) , in its sole discretion, may remedy the situation, and in such instances the Town of (INSERT JURISDICTION) shall be fully reimbursed from the
(INSERT "ESCROW ACCOUNT" OR OTHER FUNDING SOURCE AS APPROVED
BY THE STORM WATER ADMINISTRATOR). Funds may be spent by the Developer for sediment removal, structural, biological or vegetative replacement, major repair, and reconstruction of the water quality control structure, provided that the Town of (INSERT
JURISDICTION) shall first consent to the expenditure.
3. Both contributions by the Developer and contributions to an annual sinking fund from either the Developer or a property owners’ association shall fund the (INSERT
"ESCROW ACCOUNT" OR OTHER FUNDING SOURCE AS APPROVED BY THE
STORM WATER ADMINISTRATOR). Prior to plat recordation or issuance of construction permits, whichever shall first occur, the Developer shall pay into the
(INSERT "ESCROW ACCOUNT" OR OTHER FUNDING SOURCE AS APPROVED
BY THE STORM WATER ADMINISTRATOR) an amount equal to fifteen (15) per cent of the initial construction cost of the water quality control structure. Two-thirds (2/3)
Form #PCO19 204 (Revision Jan. 08)

of the total amount of the sinking fund budget shall be deposited into the (INSERT
"ESCROW ACCOUNT" OR OTHER FUNDING SOURCE AS APPROVED BY THE
STORM WATER ADMINISTRATOR) within the first five (5) years and the full amount shall be deposited within ten (10) years following initial construction of the water quality control structure. Funds shall be deposited each year into the (INSERT "ESCROW
ACCOUNT" OR OTHER FUNDING SOURCE AS APPROVED BY THE STORM
WATER ADMINISTRATOR). A portion of the annual assessments of the property owners’ association shall include an allocation into the (INSERT "ESCROW
ACCOUNT" OR OTHER FUNDING SOURCE AS APPROVED BY THE STORM
WATER ADMINISTRATOR). Any funds drawn down from the (INSERT "ESCROW
ACCOUNT" OR OTHER FUNDING SOURCE AS APPROVED BY THE STORM
WATER ADMINISTRATOR) shall be replaced in accordance with the schedule of anticipated work used to create the sinking fund budget.
4. The percent of Developer contribution and lengths of time to fund the (INSERT
"ESCROW ACCOUNT" OR OTHER FUNDING SOURCE AS APPROVED BY THE
STORM WATER ADMINISTRATOR) may be varied by the Town of (INSERT
JURISDICTION) depending on the design and materials of the water quality control structure.
5. The Developer shall construct and perpetually operate and maintain, at its sole expense, the above-referenced water quality control structure in strict accordance with the attached
Maintenance Plan approved by the Town of (INSERT JURISDICTION) or its designee.
6. The Developer shall, at its sole expense, make such changes or modifications to the water quality control structure as may, at the discretion of the Town of (INSERT
JURISDICTION) or its designee, be determined necessary to ensure that the facility and system is properly maintained and continues to operate as designed and approved.
7.
The Town of (INSERT JURISDICTION) , its agents, employees and contractors shall have the perpetual right of entry to inspect, monitor, maintain, repair and reconstruct the water quality control.
8. The Developer agrees that should it fail to correct any defects in the above described water quality control structure within ten (10) days from the issuance of written notice, or shall fail to maintain the structure in accordance with the attached Maintenance Plan and with the law and applicable executive regulation or, in the event of an emergency as determined by the Town of (INSERT JURISDICTION) or its designee in its reasonable discretion, the Town of (INSERT JURISDICTION) or its designee is authorized to enter the property to make all repairs, and to perform all maintenance, construction and reconstruction as the Town of (INSERT JURISDICTION) or its designee deems necessary. The Town of (INSERT JURISDICTION) or its designee shall then recover from the Developer any and all costs the Town of (INSERT JURISDICTION) expends to maintain or repair the water quality control structure or to correct any operational deficiencies. Failure to pay the Town of (INSERT JURISDICTION) or its designee all of its expended costs, after forty-five days written notice, shall constitute a breach of the
Form #PCO19 205 (Revision Jan. 08)

agreement. The Town of (INSERT JURISDICTION) or its designee shall thereafter be entitled to bring an action against the Developer to pay, or foreclose upon the lien hereby authorized by the agreement against the property, or both. Interest, collection costs, and attorney fees shall be added to the recovery.
9. The Developer shall not obligate the Town of (INSERT JURISDICTION) to maintain or repair any water quality control structure, and the Town of (INSERT JURISDICTION) shall not be liable to any person for the condition or operation of any water quality control structure.
10. The Developer shall not in any way diminish, limit, or restrict the right of the Town of
(INSERT JURISDICTION) to enforce any of its ordinances as authorized by law.
11. The Developer shall indemnify, save harmless and defend the Town of (INSERT
JURISDICTION) or its designee from and against any and all claims, demands, suits, liabilities, losses, damages and payments including attorney fees claimed or made by persons not parties to this Declaration against the Town of (INSERT JURISDICTION) or its designee that are alleged or proven to result or arise from the Developer’s construction, operation, or maintenance of the water quality control structure that is the subject of this Covenant.
12.
The covenants contained herein shall run with the land and the Developer further agrees that whenever the property shall be held, sold and conveyed, it shall be subject to the covenants, stipulations, agreements and provisions of this Declaration, which shall apply to, bind and be obligatory upon the Developer hereto, its heirs, successors and assigns and shall bind all present and subsequent owners of the property served by the water quality control structure. Upon the sale and conveyance by the owner of the Property (or any portion thereof) of its entire interest therein, such owner shall automatically be deemed to be released of all future obligations thereafter arising under this Declaration; and as to any future owner of the Property, or any portion thereof, such future owner shall automatically be subject and bound by the terms and provisions of this Declaration upon its acquisition of fee simple title to the Property (or portion thereof) in the same manner as the owner of the Property as of the date hereof is presently bound under this
Declaration.
13.
The Developer shall promptly notify the Town of (INSERT JURISDICTION) or its designee when the Developer legally transfers any of the Developer’s responsibilities for the water quality control structure. The Developer shall supply the Town of (INSERT
JURISDICTION) or its designee with a copy of any document of transfer, executed by both parties.
14. The provisions of this Declaration shall be severable and if any phrase, clause, sentence or provisions is declared unconstitutional, or the applicability thereof to the Developer is held invalid, the remainder of this Covenant shall not be affected thereby.
Form #PCO19 206 (Revision Jan. 08)

15. The Declaration and the exact boundary of all water quality control structures (as shown on final plats prepared by a registered surveyor) shall be recorded at the Mecklenburg
County Register of Deeds Office at the Developer’s expense.
16. In the event that the Town of (INSERT JURISDICTION) or its designee shall determine at its sole discretion at future time that the water quality control structure is no longer required, then the Town of (INSERT JURISDICTION) or its designee shall at the request of the Developer execute a release of this Declaration of Covenants which the
Developer shall record at its expenses.
IN WITNESS WHEREOF, the Developer has executed this Declaration of Covenants as of this day of , 20 .
ATTEST: FOR THE COVENANTER(S)
___________________________ _________________________
(Signature) (Signature)
___________________________ _________________________
(Printed Name) (Printed Name and Title)
STATE OF ______________________________:
COUNTY OF ____________________________:
On this ________ day of _________________, 20_____, before me, the undersigned officer, a Notary Public in and for the State and County aforesaid, personally appeared
_____________________________________, who acknowledged himself to be________________________, of ___________________________, and he as such authorized to do so, executed the foregoing instrument for the purposes therein contained by signing his name as _______________________________ for said _________________________.
WITNESS my hand and Notarial Seal
My commission expires __________________ ____________________
Notary Public
Seen and approved
______________________________________
(Storm Water Administrator)
Form #PCO19 207 (Revision Jan. 08)

(This form for use with commercial establishments under single ownership.)
INSERT PROJECT NAME (must match plat title)
DECLARATION OF COVENANTS
For Maintenance of Water Quality Control Structures
Town of (INSERT JURISDICTION)
THIS DECLARATION OF COVENANTS, made this day of , 20 , by hereinafter referred to as the “Owner” to and for the benefit of the Town of (INSERT
JURISDICTION) and its successors and assigns.
WITNESSETH:
WHEREAS, the Town of (INSERT JURISDICTION) is authorized to prevent surface water quality degradation from development or redevelopment activities within its jurisdiction as set forth in the Town of (INSERT JURISDICTION) Post-Construction Storm Water Ordinance: and
WHEREAS, the Owner is the owner of a certain tract or parcel of land more particularly described as: being all or part of the land which it acquired by deed dated from
grantors, and recorded with the Mecklenburg County Register of Deeds Office , in Book
at Page such property being hereinafter referred to as the “the property;” and
WHEREAS, the Owner desires to construct certain improvements on its property regulated by the Town of (INSERT JURISDICTION) Post-Construction Storm Water
Ordinance; and
WHEREAS, in order to construct certain improvements on its property, the Owner desires to build and maintain at its expense, a water quality control structure more particularly described and shown on plans titled and further identified under Storm Water
Management Permit Number ; and
208

WHEREAS, the Town of (INSERT JURISDICTION) or its designee have reviewed and approved the plans associated with the Storm Water Management Permit subject to the execution of this agreement.
NOW THEREFORE, in consideration of the benefits received by the Owner as a result of approval by the Town of (INSERT JURISDICTION) or its designee of these plans, the Owner, with full authority to execute deeds, mortgages, other covenants, and all rights, title and interest in the property described above, does hereby covenant with the Town of (INSERT
JURISDICTION) as follows:
1. The Owner shall develop and attach to this “DECLARATION OF COVENANTS” for recording at the Mecklenburg County Register of Deeds Office a “MAINTENANCE
PLAN” that has been reviewed and approved by the Town of (INSERT JURISDICTION) or its designee. This Maintenance Plan shall describe the specific maintenance practices to be performed for the above referenced water quality control structure and include a schedule for implementation of these practices. The Plan shall indicate that the water quality control structure shall be inspected by a qualified professional at least annually to ensure that it is operating properly. The Plan shall specify the name, mailing address and phone number of the party responsible for the fulfillment of the Maintenance Plan and describe the mechanism by which the funding for the performance of this maintenance shall be secured through the use of an INSERT "ESCROW ACCOUNT" OR OTHER
FUNDING SOURCE AS APPROVED BY THE STORM WATER
ADMINISTRATOR).
2. The Owner shall construct and perpetually operate and maintain, at its sole expense, the above-referenced water quality control structure in strict accordance with the attached
Maintenance Plan approved by the Town of (INSERT JURISDICTION) or its designee.
3. The Owner shall, at its sole expense, make such changes or modifications to the water quality control structure as may, at the discretion of the Town of (INSERT
JURISDICTION) or its designee, be determined necessary to ensure that the facility and system is properly maintained and continues to operate as designed and approved.
4. The Town of (INSERT JURISDICTION) , its agents, employees and contractors shall have the perpetual right of entry to inspect, monitor, maintain, repair and reconstruct the water quality control structure.
5. The Owner agrees that should it fail to correct any defects in the above described water quality control structure within ten (10) days from the issuance of written notice, or shall fail to maintain the structure in accordance with the attached Maintenance Plan and with the law and applicable executive regulation or, in the event of an emergency as determined by the Town of (INSERT JURISDICTION) or its designee in its reasonable discretion, the Town of (INSERT JURISDICTION) or its designee is authorized to enter the property to make all repairs, and to perform all maintenance, construction and reconstruction as the Town of (INSERT JURISDICTION) or its designee deems necessary. The Town of (INSERT JURISDICTION) or its designee shall then recover
209

from the Owner any and all costs the Town of (INSERT JURISDICTION) expends to maintain or repair the water quality control structure or to correct any operational deficiencies. Failure to pay the Town of (INSERT JURISDICTION) or its designee all of its expended costs, after forty-five days written notice, shall constitute a breach of the agreement. The Town of (INSERT JURISDICTION) or its designee shall thereafter be entitled to bring an action against the Owner to pay, or foreclose upon the lien hereby authorized by the agreement against the property, or both. Interest, collection costs, and attorney fees shall be added to the recovery.
6.
7.
The Owner shall not obligate the Town of (INSERT JURISDICTION) to maintain or repair any water quality control structure, and the Town of (INSERT JURISDICTION) shall not be liable to any person for the condition or operation of any water quality control structure.
The Owner shall not in any way diminish, limit, or restrict the right of the Town of
(INSERT JURISDICTION) to enforce any of its ordinances as authorized by law.
8. The Owner shall indemnify, save harmless and defend the Town of (INSERT
JURISDICTION) or its designee from and against any and all claims, demands, suits, liabilities, losses, damages and payments including attorney fees claimed or made by persons not parties to this Declaration against the Town of (INSERT JURISDICTION) or its designee that are alleged or proven to result or arise from the Owner’s construction, operation, or maintenance of the water quality control structure that is the subject of this
Covenant.
9. The covenants contained herein shall run with the land and the Owner further agrees that whenever the property shall be held, sold and conveyed, it shall be subject to the covenants, stipulations, agreements and provisions of this Declaration, which shall apply to, bind and be obligatory upon the Owner hereto, its heirs, successors and assigns and shall bind all present and subsequent owners of the property served by the water quality control structure. Upon the sale and conveyance by the owner of the Property (or any portion thereof) of its entire interest therein, such owner shall automatically be deemed to be released of all future obligations thereafter arising under this Declaration; and as to any future owner of the Property, or any portion thereof, such future owner shall automatically be subject and bound by the terms and provisions of this Declaration upon its acquisition of fee simple title to the Property (or portion thereof) in the same manner as the owner of the Property as of the date hereof is presently bound under this
Declaration.
10.
The Owner shall promptly notify the Town of (INSERT JURISDICTION) or its designee when the Owner legally transfers any of the Owner’s responsibilities for the water quality control structure. The Owner shall supply the Town of (INSERT
JURISDICTION) or its designee with a copy of any document of transfer, executed by both parties.
210

11. The provisions of this Declaration shall be severable and if any phrase, clause, sentence or provisions is declared unconstitutional, or the applicability thereof to the Owner is held invalid, the remainder of this Covenant shall not be affected thereby.
12. The Declaration and the exact boundary of all water quality control structures (as shown on final plats prepared by a registered surveyor) shall be recorded at the Mecklenburg
County Register of Deeds Office at the Owner’s expense.
13. In the event that the Town of (INSERT JURISDICTION) or its designee shall determine at its sole discretion at future time that the water quality control structure is no longer required, then the Town of (INSERT JURISDICTION) or its designee shall at the request of the Owner execute a release of this Declaration of Covenants which the Owner shall record at its expenses.
IN WITNESS WHEREOF, the Owner has executed this Declaration of Covenants as of this
day of , 20 .
ATTEST: FOR THE COVENANTER(S)
___________________________ _________________________
(Signature) (Signature)
___________________________ _________________________
(Printed Name) (Printed Name and Title)
STATE OF ______________________________:
COUNTY OF ____________________________:
On this ________ day of _________________, 20_____, before me, the undersigned officer, a Notary Public in and for the State and County aforesaid, personally appeared
_____________________________________, who acknowledged himself to be________________________, of ___________________________, and he as such authorized to do so, executed the foregoing instrument for the purposes therein contained by signing his name as _______________________________ for said _________________________.
WITNESS my hand and Notarial Seal
My commission expires __________________ ____________________
Notary Public
Seen and approved
______________________________________
(Storm Water Administrator)
211

212

Overview
In order to ensure the long-term effectiveness and efficiency of Low Impact Development (LID)
Best Management Practices (BMPs), periodic inspections and maintenance should be conducted.
Inspection of BMPs allows for detection of problems and helps to avoid long-term problems.
The development of problems may not coincide with the scheduled inspection date. It is the responsibility of the assigned qualified professional to conduct routine inspections and to ensure necessary maintenance of the BMPs is performed.
Inspections and Maintenance
An initial inspection shall occur at the time that responsibility for the BMP is transferred from the developer/builder to the owner. This will be a joint inspection by Mecklenburg County
Water Quality staff and the owner of the structure. The inspection shall ensure that any structural or material deficiencies are addressed before transfer of responsibility.
Subsequent inspections shall be conducted by a qualified professional using the BMP
Maintenance and Inspection Check List that is specific to the BMP being inspected (i.e., rain garden, grass swale). The purpose of the inspection is to ensure that the facility and system is properly maintained and continues to operate as designed and approved. A separate inspection sheet shall be used for each structure.
Inspections shall be conducted on at least an annual basis. Periodic inspections may also be needed to identify and address maintenance needs. Any identified maintenance and/or repair needs shall be addressed in a timely manner. The responsible party shall make records of all maintenance and repairs and shall maintain the records for the life of the BMP. After inspections and maintenance have been performed, a copy of the Maintenance and Inspection Checklist shall be forwarded to Mecklenburg County Water Quality Program within two weeks of the inspection.
Some problems that may be discovered during an inspection include:
• Unexpected ponding (excluding wet ponds)
• Dead or unhealthy vegetation or growth of unwanted vegetation
• Obstructions of inlet or outlet pipes
• Excessive erosion or sediment deposition
• Cracking or settling of structural components
• Wetness on the downstream side of wet pond embankments
• Erosion and sediment loss of contributing areas or shores of wet ponds
Inspection of underground systems, such as underdrains in rain gardens, can be difficult to conduct. Problems to look for that could indicate a blockage or other problem with the underdrain system include water standing in the facility longer than the design draw down time, obvious signs of excessive sediment build up or debris in or around the underdrain. Check flow at the outfall during or following a rain event to ensure underdrain is functioning properly.
213

[Name of Development Project as indicated on approved plans]
[Jurisdiction]
[Date]
[This document must be recorded as an Addendum to the Operation & Maintenance Agreement]
I.
General BMP Information [Complete this table with each BMP that is planned within the development. Use the same naming system used on the approved plans, ie.
Birkdale Phase I Bioretention 1]
BMP ID Name Street with Block Number Parcel Tax ID
II.
BMP Site Location Map (attached) [Attach a small site plan map coinciding with the table above to show the general location of each BMP within the development.]
III.
Maintenance Annual Budget [Provide a simple annual budget for maintenance and inspection of BMPs and list the source of funding, ie. owner, trust, HOA, etc. Edit chart below as necessary]
Expenses
Budget for BMP Maintenance / Inspections
Estimated Costs
Routine inspections
Sediment removal
Plant management / weed control
Replacement supplies, rock, plants, soil media, mulch
Mowing and litter removal
Seeding
Miscellaneous
[Other]
Total
$
Source
Form #PCO20 214

IV.
Escrow Account Activity
Provide documentation of BMP maintenance escrow account activity. This may be provided in the form of a bank statement which includes the current balance, deposits and withdraws for the previous 12 months.
V.
Maintenance Inspection Reports
As indicated in the Post-Construction Storm Water Ordinance, annual maintenance inspection reports shall be submitted to the Storm Water Administrator. The first report shall be submitted one year following the final approval date of the BMP and each year thereafter on or before the approval anniversary date. All maintenance activities and inspection reports shall be documented using the forms contained in the
Administrative Manual. Annual maintenance inspection reports shall be sealed by a registered North Carolina professional engineer or landscape architect. These inspections shall be discontinued only if the BMPs are accepted for maintenance by the applicable jurisdiction.
VI.
Routine Maintenance Tasks and Schedule [The following pages outline the specific maintenance tasks and frequency for each type of BMP in tables. For the recorded document, simply discard the pages (tables) that are not needed according to the types of BMPs within the development.]
Form #PCO20 215

TASK
Forebay inspection and cleanout
SCHEDULE
Monthly inspection. Remove sediment every 7 years or whenever the sediment volume exceeds 50% of storage volume
Volume measurement Yearly – Dredging needed every 20 years or when 25% of permanent pool volume has been lost
Bank mowing and inspection / stabilization of eroded areas
Monthly
Outlet / inlet inspection and cleanout Monthly
Unwanted vegetation and trash removal Monthly
Visual inspection of water quality
Inspect / exercise all mechanical
Monthly
Yearly devices, valves, etc
Inspect for structural damage, leaks, etc Yearly
Rodent management
Security
As needed
As needed
Form #PCO20 216

TASK
Forebay inspection and cleanout
SCHEDULE
Monthly inspection. Remove sediment every 7 years or when sediment volume exceeds 50% of storage volume
Bank mowing and inspection / stabilization of eroded areas
Outlet / inlet inspection and cleanout
Monthly
Monthly
Unwanted vegetation and trash removal Monthly
Inspect for structural damage, leaks, etc Yearly
Inspect / exercise all mechanical devices, valves, etc
Yearly
Yearly Evaluate sediment level (remove as needed)
Security As needed
Form #PCO20 217

TASK
Forebay cleanout
SCHEDULE
Monthly inspection. Remove sediment every 7 years or when sediment volume exceeds 50% of storage volume
Semi-Annual Invasive species control / vegetation management and replanting to maintain design densities
Bank mowing and stabilization of Monthly eroded areas
Outlet / inlet inspection and cleanout Monthly
Trash removal Monthly
Inspect for structural damage, leaks, etc Yearly
Visual inspection of water quality
Rodent and mosquito management
Evaluate sediment level (remove at 20 yrs. or when plants are being impacted)
Monthly
As needed
Yearly
Security As needed
Form #PCO20 218

TASK
Sedimentation prevention
SCHEDULE
Monthly inspection and watch on surrounding drainage areas such as out parcels and parking lots
Drop box clean off
Perimeter mowing
Monthly
Monthly (maintain 2 – 6 inch height)
Inspect for proper drawdown / clogging Monthly
Stabilization of eroded areas
Trash removal
Pruning
Mulch renewal
Monthly
Monthly
Yearly
Yearly
Mulch replacement
Inspect plants, replace as necessary
Test P Index of soil media and replace if over 50 ppm
Replace pea gravel diaphragm
Remove sediment
Perimeter mowing
Every 3 years
Monthly
Every 2 years
As needed
As needed
Monthly
Form #PCO20 219

TASK
Street sweep parking lot Quarterly
SCHEDULE
Trash removal
Inspect outlet for obstructions
Inspect for clogging
Inspect inlet grates
Skim sand media
Pump oil and grit from sedimentation chamber
Replace sand media
Monthly
Monthly
Monthly
Monthly
Yearly
Yearly or at 50% full
As needed (expect 3 years)
Form #PCO20 220

TASK
Maintain stone or mulch top surface Yearly
SCHEDULE
Clean forebay if present
Trash removal
Remove unwanted vegetation
Check observation wells following precipitation events to ensure proper infiltration
Yearly or at 50% full
Monthly
Monthly
Monthly
Form #PCO20 221

Mowing
TASK SCHEDULE
Weekly – Monthly (as needed to retain 2-6 inch height)
Yearly Inspect condition of dispersion devices and check dams
Trash removal
Reseed
Stabilization of eroded areas
Removal of sediment
Inspect for clogging (enhanced swale)
Inspect pea gravel diaphragm and replace / repair as necessary
Weekly – Monthly (prior to mowing)
Yearly
Monthly
Yearly
Monthly
Monthly
Form #PCO20 222

TASK
Mowing of grass
SCHEDULE
Weekly – Monthly (as needed to retain 2-6 inch height)
Monthly Stabilization of eroded areas throughout the filter strip and below the flow dispersion device.
Inspect gravel diaphragm (if present) and remove sediment
Check outlet pipes on berms (if present) for clogging
Remove debris / unwanted vegetation from behind lip of level spreader (if present)
Repair flow dispersion device to prevent formation of channels in filter strip
Reseeding of grass
Yearly
Monthly
Monthly
Monthly as needed
Yearly
Form #PCO20 223

224

[Note: a separate form must be used for each BMP]
Project Name:
Project Address:
Owner’s Name:
Owner’s Address:
Recorded Book and Page Number of the Lot:
BMP Name and Location:
Inspection Date:
Inspector:
Inspector Address/Phone Number:
Date Last Inspected:
Type of Inspection: Visual
Maintenance Item Comments/Actions Required
1. Debris Cleanout
Clear of trash and debris
2. Vegetation Management
Grass height (maintain 2-6 inch height)
Unwanted vegetation present
Ground cover well established (yearly reseeding needed)
W-M
W-M
M
Q
3. Erosion
Evidence of soil erosion in swale or contributing areas
4. Dewatering
Evidence of standing water
5. Sedimentation
Sediment accumulation
M
M
Y
6. Energy dispersion / check dams
Condition of dispersion devices
Condition of check dams
Inspect pea gravel diaphragm and replace / repair as necessary
Y
Y
M
7. Miscellaneous:
W=Weekly, M=Monthly, Q=Quarterly, Y=Yearly
(revision 6/07)
Form: #PCO21 225

If applicable: Attach to this form documentation of BMP maintenance escrow account activity. This may be provided in the form of a bank statement which includes the current balance, as well as deposits and withdraws for the previous 12 months.
Maintenance Actions Taken: [If any of the above items were marked “U” for unsatisfactory, explain the actions taken and time table for correction. Attach additional pages as necessary.]
Additional Comments:
I do hereby certify that I conducted an inspection of the BMP described herein. I further certify that at the time of my inspection said BMP was performing properly and was in compliance with the terms and conditions of the approved maintenance agreement required by the Post-Construction Storm Water Ordinance.
Certification:
________________________
Date
_____________________________________
Inspectors Signature
(seal)
[Note: The Post-Construction Storm Water Ordinance requires that inspections be conducted of all BMPs beginning within one (1) year from the date of as-built certification and each year thereafter and that these inspections be completed by a North Carolina Professional Engineer or
Landscape Architect. All inspections must be documented and submitted using this form. The inspection form must be signed and sealed by the inspector and mailed to the Storm Water
Administrator at the following address: LUESA, Water Quality Program, 700 North Tryon
Street, Charlotte, NC 28202.]
Form: #PCO21 226

[Note: a separate form must be used for each BMP]
Project Name:
Project Address:
Owner’s Name:
Owner’s Address:
Recorded Book and Page Number of the Lot:
BMP Name and Location:
Inspection Date:
Inspector:
Inspector Address/Phone Number:
Date Last Inspected:
Maintenance Item Comments/Actions Required
1. Debris Cleanout
Clear of trash and debris
2. Vegetation Management
Banks / surrounding areas mowed
Unwanted vegetation present
Condition of wetland plants
3. Erosion
Evidence of soil erosion on banks or contributing drainage areas and outlet
4. Sedimentation
Forebay sediment inspection (cleanout every 7 years or when 50% full)
Pond volume measurement (dredge every
20 years or when 25% of permanent pool volume lost)
5. Energy dissipators
Condition of dissipater at inlets
Condition of dissipater at outfall
6. Inlet
Condition of pipe and / or swale (cracks, leaks, sedimentation, woody vegetation)
7. Outlet
Condition of orifice (drawdown device)
Condition of riser outlet and trash rack
8. Emergency spillway and dam
Form: #PCO21
M
M
Y
M
M
M
M
Y
Y
M
227
M
M

Condition of spillway
Condition of dam (ie. leaks, holes, woody vegetation)
9. Mechanical devices
Inspection of all valves, etc. (should be exercised yearly)
Y
Y
Y
10. Visual Inspection
Appearance of water (ie. sheen, muddy, oily, clear, algae, etc)
Mosquito larvae
11. Forebay embankment
Condition of forebay embankment
(breached?)
12. Water elevation
Is pond at normal pool elevation?
M
M
M
M
13. Miscellaneous:
W=Weekly, M=Monthly, Q=Quarterly, Y=Yearly
(revision 6/07)
If applicable: Attach to this form documentation of BMP maintenance escrow account activity. This may be provided in the form of a bank statement which includes the current balance, as well as deposits and withdraws for the previous 12 months.
Maintenance Actions Taken: [If any of the above items were marked “U” for unsatisfactory, explain the actions taken and time table for correction. Attach additional pages as necessary.]
Additional Comments:
I do hereby certify that I conducted an inspection of the BMP described herein. I further certify that at the time of my inspection said BMP was performing properly and was in compliance with the terms and conditions of the approved maintenance agreement required by the Post-Construction Storm Water Ordinance.
Certification:
__________________________________ ________________________
Inspectors Signature Date
(seal)
[Note: The Post-Construction Storm Water Ordinance requires that inspections be conducted of all BMPs beginning within one (1) year from the date of as-built certification and each year
Form: #PCO21 228

thereafter and that these inspections be completed by a North Carolina Professional Engineer or
Landscape Architect. All inspections must be documented and submitted using this form. The inspection form must be signed and sealed by the inspector and mailed to the Storm Water
Administrator at the following address: LUESA, Water Quality Program, 700 North Tryon
Street, Charlotte, NC 28202.]
Form: #PCO21 229

[Note: a separate form must be used for each BMP]
Project Name:
Project Address:
Owner’s Name:
Owner’s Address:
Recorded Book and Page Number of the Lot:
BMP Name and Location:
Inspection Date:
Inspector:
Inspector Address/Phone Number:
Date Last Inspected:
Maintenance Item Comments/Actions Required
1. Debris Cleanout
Clear of trash and debris
2. Vegetation Management
Banks / surrounding areas mowed
Unwanted vegetation present (replant semiannually to maintain design densities)
Condition of wetland plants
3. Erosion
Evidence of soil erosion on banks or contributing drainage areas and outlet
4. Sedimentation
Forebay sediment inspection (cleanout every 7 years or when 50% full)
Sedimentation level in wetland (cleanout every 20 years or when plants are being impacted)
5. Energy dissipators
Condition of dissipater at inlets
Condition of dissipater at outfall
6. Inlet
Condition of pipe and / or swale (cracks, leaks, sedimentation, woody vegetation)
7. Outlet
Condition of orifice (drawdown device)
Condition of outlet
M
M
M
M
M
M
Y
Y
Y
M
M
M
Form: #PCO21 230

8. Mechanical devices
Inspection of all valves, etc. (should be exercised yearly)
9. Visual water inspection
Appearance of water (ie. sheen, muddy, oily, clear, algae, etc)
Y
M
Water level maintained at permanent pool
Mosquito larvae
10. Dam / Embankment
Seepage through embankment
Woody vegetation on embankment
Y
M
Y
Y
11. Miscellaneous:
W=Weekly, M=Monthly, Q=Quarterly, Y=Yearly
(revision 6/07)
If applicable: Attach to this form documentation of BMP maintenance escrow account activity. This may be provided in the form of a bank statement which includes the current balance, as well as deposits and withdraws for the previous 12 months.
Maintenance Actions Taken: [If any of the above items were marked “U” for unsatisfactory, explain the actions taken and time table for correction. Attach additional pages as necessary.]
Additional Comments:
I do hereby certify that I conducted an inspection of the BMP described herein. I further certify that at the time of my inspection said BMP was performing properly and was in compliance with the terms and conditions of the approved maintenance agreement required by the Post-Construction Storm Water Ordinance.
Certification:
_____________________________________
Inspectors Signature
________________________
Date
(seal)
[Note: The Post-Construction Storm Water Ordinance requires that inspections be conducted of all BMPs beginning within one (1) year from the date of as-built certification and each year thereafter and that these inspections be completed by a North Carolina Professional Engineer or
Landscape Architect. All inspections must be documented and submitted using this form. The inspection form must be signed and sealed by the inspector and mailed to the Storm Water
Administrator at the following address: LUESA, Water Quality Program, 700 North Tryon
Street, Charlotte, NC 28202.]
Form: #PCO21 231

[Note: a separate form must be used for each BMP]
Project Name:
Project Address:
Owner’s Name:
Owner’s Address:
Recorded Book and Page Number of the Lot:
BMP Name and Location:
Inspection Date:
Inspector:
Inspector Address/Phone Number:
Date Last Inspected:
Maintenance Item Comments/Actions Required
1. Debris Cleanout
Clear of trash and debris
2. Vegetation Management
Banks / surrounding areas mowed
Unwanted vegetation present
Condition of plants
Condition of mulch - Must be double hammered hardwood, 3 inches deep
(replace at least every 3 years and renew yearly)
3. Erosion
Evidence of soil erosion on banks or contributing areas
4. Sedimentation
Forebay (if present) sediment inspection
(cleanout when 50% full)
Evidence of sediment in bioretention cell
5. Energy dissipators
Condition of dissipater at inlets
Condition of dissipater at outfall
Inspect pea gravel diaphragm (replace as needed)
6. Inlet
Condition of pipe of swale (cracks, leaks, sedimentation, woody vegetation)
M
M
M
M
M
M
M
M
Y
Y
M
M
Form: #PCO21 232

7. Outlet
Condition of outlet / drop box M
8. Dewatering (drawdown must be between 48 hours and 120 hours)
Evidence of standing water M
9. Overall functionality
Evidence of bypass
P Index test results for soil media (indicate test results and date last tested)
M
Y2
10. Miscellaneous:
W=Weekly, M=Monthly, Q=Quarterly, Y=Yearly, Y2=every 2 yrs.
(revision 6/07)
If applicable: Attach to this form documentation of BMP maintenance escrow account activity. This may be provided in the form of a bank statement which includes the current balance, as well as deposits and withdraws for the previous 12 months.
Maintenance Actions Taken: [If any of the above items were marked “U” for unsatisfactory, explain the actions taken and time table for correction. Attach additional pages as necessary.]
Additional Comments:
I do hereby certify that I conducted an inspection of the BMP described herein. I further certify that at the time of my inspection said BMP was performing properly and was in compliance with the terms and conditions of the approved maintenance agreement required by the Post-Construction Storm Water Ordinance.
Certification:
_____________________________________
Inspectors Signature
(seal)
________________________
Date
[Note: The Post-Construction Storm Water Ordinance requires that inspections be conducted of all BMPs beginning within one (1) year from the date of as-built certification and each year thereafter and that these inspections be completed by a North Carolina Professional Engineer or
Landscape Architect. All inspections must be documented and submitted using this form. The inspection form must be signed and sealed by the inspector and mailed to the Storm Water
Administrator at the following address: LUESA, Water Quality Program, 700 North Tryon
Street, Charlotte, NC 28202.]
Form: #PCO21 233

[Note: a separate form must be used for each BMP]
Project Name:
Project Address:
Owner’s Name:
Owner’s Address:
Recorded Book and Page Number of the Lot:
BMP Name and Location:
Inspection Date:
Inspector:
Inspector Address/Phone Number:
Date Last Inspected:
Maintenance Item Comments/Actions Required
1. Debris Cleanout
Clear of trash and debris
2. Vegetation Management
Banks / surrounding areas mowed
Unwanted vegetation present
3. Erosion
Evidence of soil erosion on banks, contributing drainage areas or bottom of pond
4. Sedimentation
Forebay (if present) sediment inspection
(cleanout every 7 years or when 50% full)
Sediment level in pond
5. Energy dissipators
Condition of dissipater at inlets
Condition of dissipater at outfall
6. Outlet / Inlet
Condition of orifice (drawdown device) / trash rack
Condition of outlet
Condition of inlet
7. Mechanical devices
Inspection of all valves, etc. (exercise yearly)
Form: #PCO21
M
M
M
M
M
Y
Y
Y
M
M
M
Y
234

8. Dewatering
Evidence of standing water
9. Structural Integrity
Evidence of structural damage (leaks, cracks, etc)
M
Y
10. Emergency Spillway & Dam
Condition of spillway
Condition of dam
Y
Y
11. Miscellaneous:
W=Weekly, M=Monthly, Q=Quarterly, Y=Yearly
(revision 6/07)
If applicable: Attach to this form documentation of BMP maintenance escrow account activity. This may be provided in the form of a bank statement which includes the current balance, as well as deposits and withdraws for the previous 12 months.
Maintenance Actions Taken: [If any of the above items were marked “U” for unsatisfactory, explain the actions taken and time table for correction. Attach additional pages as necessary.]
Additional Comments:
I do hereby certify that I conducted an inspection of the BMP described herein. I further certify that at the time of my inspection said BMP was performing properly and was in compliance with the terms and conditions of the approved maintenance agreement required by the Post-Construction Storm Water Ordinance.
Certification:
_______________________________
Inspectors Signature
________________________
Date
(seal)
[Note: The Post-Construction Storm Water Ordinance requires that inspections be conducted of all BMPs beginning within one (1) year from the date of as-built certification and each year thereafter and that these inspections be completed by a North Carolina Professional Engineer or
Landscape Architect. All inspections must be documented and submitted using this form. The inspection form must be signed and sealed by the inspector and mailed to the Storm Water
Administrator at the following address: LUESA, Water Quality Program, 700 North Tryon
Street, Charlotte, NC 28202.]
Form: #PCO21 235

[Note: a separate form must be used for each BMP]
Project Name:
Project Address:
Owner’s Name:
Owner’s Address:
Recorded Book and Page Number of the Lot:
BMP Name and Location:
Inspection Date:
Inspector:
Inspector Address/Phone Number:
Date Last Inspected:
Maintenance Item Comments/Actions Required
1. Debris Cleanout
Clear of trash and debris
2. Street Sweeping
Parking lot street sweeping
3. Erosion
Evidence of soil erosion around contributing areas
4. Sedimentation chamber
Sediment level in chamber (pump yearly or when 50% full)
5. Sand media
Condition of media (skim annually, replace as necessary)
6. Outlet / Inlet
Condition of outlet
Condition of inlets and grates
7. Mechanical devices
Inspection of all valves, etc.
8. Dewatering
Evidence of filter clogging
9. Structural Integrity
Evidence of structural damage (leaks, cracks, etc)
10. Overall functionality
Evidence of odors
Form: #PCO21
M
Q
M
M
M
M
M
Y
M
Y
M
236

Evidence of bypass
11. Miscellaneous:
M
W=Weekly, M=Monthly, Q=Quarterly, Y=Yearly
(revision 6/07)
If applicable: Attach to this form documentation of BMP maintenance escrow account activity. This may be provided in the form of a bank statement which includes the current balance, as well as deposits and withdraws for the previous 12 months.
Maintenance Actions Taken: [If any of the above items were marked “U” for unsatisfactory, explain the actions taken and time table for correction. Attach additional pages as necessary.]
Additional Comments:
I do hereby certify that I conducted an inspection of the BMP described herein. I further certify that at the time of my inspection said BMP was performing properly and was in compliance with the terms and conditions of the approved maintenance agreement required by the Post-Construction Storm Water Ordinance.
Certification:
_____________________________________
Inspectors Signature
(seal)
________________________
Date
[Note: The Post-Construction Storm Water Ordinance requires that inspections be conducted of all BMPs beginning within one (1) year from the date of as-built certification and each year thereafter and that these inspections be completed by a North Carolina Professional Engineer or
Landscape Architect. All inspections must be documented and submitted using this form. The inspection form must be signed and sealed by the inspector and mailed to the Storm Water
Administrator at the following address: LUESA, Water Quality Program, 700 North Tryon
Street, Charlotte, NC 28202.]
Form: #PCO21 237

[Note: a separate form must be used for each BMP]
Project Name:
Project Address:
Owner’s Name:
Owner’s Address:
Recorded Book and Page Number of the Lot:
BMP Name and Location:
Inspection Date:
Inspector:
Inspector Address/Phone Number:
Date Last Inspected:
Maintenance Item Comments/Actions Required
1. Debris Cleanout
Clear of trash and debris
2. Vegetation Management
Banks / surrounding areas mowed
Unwanted vegetation present
3. Erosion
Evidence of soil erosion around contributing areas
4. Sedimentation
Forebay sediment inspection (cleanout yearly or when 50% full)
Evidence of sediment in trench
5. Energy dissipators
Condition of dissipater at inlets
Condition of dissipater at outfall
6. Surface aggregate
Condition of stone or mulch
7. Dewatering
Evidence of standing water
Check water level in observation well
8. Overflow spillway
Condition of spillway
9. Overall functionality
Evidence of bypass
M
M
M
M
M
M
Y
Y
Y
M
M
Y
M
Form: #PCO21 238

10. Miscellaneous:
W=Weekly, M=Monthly, Q=Quarterly, Y=Yearly
(revision 6/07)
If applicable: Attach to this form documentation of BMP maintenance escrow account activity. This may be provided in the form of a bank statement which includes the current balance, as well as deposits and withdraws for the previous 12 months.
Maintenance Actions Taken: [If any of the above items were marked “U” for unsatisfactory, explain the actions taken and time table for correction. Attach additional pages as necessary.]
Additional Comments:
I do hereby certify that I conducted an inspection of the BMP described herein. I further certify that at the time of my inspection said BMP was performing properly and was in compliance with the terms and conditions of the approved maintenance agreement required by the Post-Construction Storm Water Ordinance.
Certification:
_____________________________________
Inspectors Signature
________________________
Date
(seal)
[Note: The Post-Construction Storm Water Ordinance requires that inspections be conducted of all BMPs beginning within one (1) year from the date of as-built certification and each year thereafter and that these inspections be completed by a North Carolina Professional Engineer or
Landscape Architect. All inspections must be documented and submitted using this form. The inspection form must be signed and sealed by the inspector and mailed to the Storm Water
Administrator at the following address: LUESA, Water Quality Program, 700 North Tryon
Street, Charlotte, NC 28202.]
Form: #PCO21 239

[Note: a separate form must be used for each BMP]
Project Name:
Project Address:
Owner’s Name:
Owner’s Address:
Recorded Book and Page Number of the Lot:
BMP Name and Location:
Inspection Date:
Inspector:
Inspector Address/Phone Number:
Date Last Inspected:
Maintenance Item Comments/Actions Required
1. Debris Cleanout
Clear of trash and debris
2. Vegetation Management
Grass height (maintain 2-6 inch height)
Unwanted vegetation present
Ground cover well established (yearly reseeding needed)
3. Erosion
Evidence of soil erosion in filter strip and below dispersion device
4. Drainage
Evidence of standing water
Evidence of bypass
Check outlet pipes for clogging
5. Sedimentation
Sediment accumulation
Sediment in gravel diaphragm (if present)
6. Energy dispersion / check dams
Condition / functionality of dispersion devices
Debris on dispersion devices
Condition of check dams
Inspect pea gravel diaphragm (replace as needed)
W-M
W-M
M
Y
M
M
M
M
Y
Y
M
M
M
M
Form #PCO21 240

7. Miscellaneous:
W=Weekly, M=Monthly, Q=Quarterly, Y=Yearly
(revision 6/07)
If applicable: Attach to this form documentation of BMP maintenance escrow account activity. This may be provided in the form of a bank statement which includes the current balance, as well as deposits and withdraws for the previous 12 months.
Maintenance Actions Taken: [If any of the above items were marked “U” for unsatisfactory, explain the actions taken and time table for correction. Attach additional pages as necessary.]
Additional Comments:
I do hereby certify that I conducted an inspection of the BMP described herein. I further certify that at the time of my inspection said BMP was performing properly and was in compliance with the terms and conditions of the approved maintenance agreement required by the Post-Construction Storm Water Ordinance.
Certification:
_____________________________________
Inspectors Signature
________________________
Date
(seal)
[Note: The Post-Construction Storm Water Ordinance requires that inspections be conducted of all BMPs beginning within one (1) year from the date of as-built certification and each year thereafter and that these inspections be completed by a North Carolina Professional Engineer or
Landscape Architect. All inspections must be documented and submitted using this form. The inspection form must be signed and sealed by the inspector and mailed to the Storm Water
Administrator at the following address: LUESA, Water Quality Program, 700 North Tryon
Street, Charlotte, NC 28202.]
Form #PCO21 241

242

Water and Land Resources
Water Quality
SOIL VERIFICATION FORM
I, , acting as a representative for,
(Name of owner or owner’s representative)
,declare that the soil that is being
( Name of the Company supplying the Amended Soils) provided to, , meets the standards set
(Name of Company receiving the Amended Soils) forth in the Town of Huntersville Water Quality Design Manual, dated January 1, 2008.
This the day of , 20 .
(Seal)
(Signature of Owner)
Sworn to and subscribed before me this day of , 20 .
. My commission expires:
Notary Public
243
PEOPLE
●
PRIDE
●
PROGRESS
● PARTNERSHIPS
700 N. Tryon Street
● Suite 205 ● Charlotte, NC 28202
-2236
● (704) 336
-5500
● FAX (704) 336
-4391 http://waterquality.charmeck.org