Post-Construction
Ordinance Development
EXECUTIVE SUMMARY
N
LAKE
NORMAN
CENTRAL CATAWBA
WESTERN CATAWBA
YADKIN-SOUTHEAST CATAWBA
GOOSE CREEK
HUNTERSVILLE
ROCKY
RIVER
UPPER
MTN
ISLAND
CLARKE
McDOWELL
LOWER
CLARKE
GAR
LOWER
MTN
ISLAND
MALLARD
LONG
BACK
CATAWBA
PAW
IRWIN
REEDY
BRIAR
UPPER
LITTLE
SUGAR
BEAVERDAM
CLEAR
SUGAR
GOOSE
McMULLEN
LAKE
WYLIE
STEELE
McKEE
CALDWELL
LOWER
LITTLE
SUGAR
McALPINE
FOUR MILE
CROOKED
TWELVE MILE
SIX MILE
CLEM
5
0
5
10 Miles
Prepared by:
Mecklenburg County Water Quality Program
and
Charlotte Storm Water Services
and
3200 Chapel Hill-Nelson Hwy.
Research Triangle Park, NC 27709
September 2005
Table of Contents
1. Introduction....................................................................................................................1
1.1.
1.2
1.3
Background and Purpose ...............................................................................................................1
Ordinance Goals ............................................................................................................................1
Stakeholder Involvement ...............................................................................................................3
2. Technical Approach .......................................................................................................4
2.1
2.2
Modeling Framework Description.................................................................................................5
Development of Management Recommendations .........................................................................6
3. Baseline Analysis...........................................................................................................8
3.1
3.2
3.3
3.4
Existing vs. Predicted Future Land Use.........................................................................................8
Pollutant Loading Analysis..........................................................................................................10
Catawba Watersheds Baseline Pollutant Loading Results...........................................................11
South Catawba Watersheds Baseline Pollutant Loading Results ................................................11
Yadkin Watersheds Baseline Pollutant Loading Results.............................................................12
Stream Stability Analysis ............................................................................................................13
Overall Baseline Assessment.......................................................................................................14
4. Alternatives Development ...........................................................................................15
4.1
4.2
Management Options...................................................................................................................15
Three Management Scenarios......................................................................................................15
Scenario 1 ....................................................................................................................................15
Scenario 2 ....................................................................................................................................17
Scenario 3 ....................................................................................................................................18
5. Evaluation of Alternatives ...........................................................................................21
5.1
5.2
5.3
Evaluation Criteria.......................................................................................................................21
Initial Evaluation of Scenarios 1 and 2........................................................................................22
Comparison Across the Three Alternatives .................................................................................24
6. Ordinance Development ..............................................................................................28
7. Implementations Steps and Considerations .................................................................30
8. References....................................................................................................................32
Acknowledgements
Charlotte Storm Water Services and the Mecklenburg County Water Quality Program
would like to recognize and thank all of the members of the Post-Construction
Stakeholders’ Group and members of staff whose tireless effort through 30 full group
meetings and numerous additional subcommittee meetings is directly responsible for
successful completion of a consensus based set of ordinance recommendations.
1. Introduction
1.1 Background and Purpose
Under North Carolina’s implementation of the Phase II stormwater
regulations for the National Pollution Discharge Elimination System
(NPDES), the City of Charlotte, Mecklenburg County, and the county’s
Phase II partners (Cornelius, Davidson, Huntersville, Matthews, Mint
Hill and Pineville) are required to develop and implement postconstruction ordinances. The intent of the post-construction ordinances
is to address the impacts of stormwater runoff in areas undergoing new
development or redevelopment. Many studies indicate that prior
planning and design for the minimization of pollutants in postconstruction stormwater discharges is the most cost-effective approach
to stormwater quality management (USEPA, 2000). The eight
jurisdictions comprising the county are therefore working together to
establish a consistent means of implementing stormwater management.
These partners agreed to use the county’s watersheds and basin
boundaries as the planning framework, both to protect the county’s
water resources and to build on previous important efforts such as
Mecklenburg County’s Surface Water Improvement and Management
(S.W.I.M.) program. In March 2004, the City and County contracted
with Tetra Tech, Inc., an environmental and engineering consulting
firm, to support the post-construction ordinance development process.
This Executive Summary provides an overview of the ordinance
development process and results. More detailed descriptions of project
steps, methods, and results are available in Tetra Tech’s project Phase 1
(February 2005) and Phase 2 (available Fall 2005) reports.
1.2 Ordinance Goals
The intent of the
post-construction
ordinances is to
address the impacts
of stormwater runoff
in areas undergoing
new development or
redevelopment.
Managing Post-Construction Runoff
There are generally two forms of post-
At the outset, the City and County staff adopted four primary
construction runoff to manage: 1) an
increase in the type and quantity of
goals to guide development of the ordinance. The first goal of
pollutants, and 2) an increase in the
this effort is to develop an ordinance for each jurisdiction that
quantity of water delivered to streams and
complies with state and federal regulatory stormwater
lakes during storms.
management requirements. Second, the local jurisdictions
want the ordinances to satisfactorily address the guidelines to
mitigate the cumulative and secondary impacts to aquatic and terrestrial
wildlife resources specified by the NC Wildlife Resources Commission
(WRC) and U.S. Fish and Wildlife Service (USFWS) for Goose Creek
and the Yadkin River watershed. Species such as the Carolina
Heelsplitter mussel are highly sensitive to degradation of physical and
water quality conditions, and the WRC and USFWS are hesitant to
provide regulatory certifications needed to build water, sewer, and road
projects in these watersheds. In fact, issues surrounding the cumulative
September 2005
Executive Summary p. 1
and secondary impact concerns have prevented the City and County
from expanding its wastewater treatment facilities and extending its
sewer service area into certain areas, affecting the type of economic
development that can occur. The ordinance development process
therefore provides an opportunity to promote a consistent, uniform
approach to protection in sensitive watersheds that can meet the intent
of mitigation guidelines and provide a basis for the WRC and USFWS
to approve projects.
Nutrients in post-construction runoff contribute to algae
blooms in downstream lakes.
Another priority is to enhance current
efforts to protect Water Quality
throughout the County. The North
Carolina Division of Water Quality
(DWQ) has put many streams in the
County on its 303(d) listing of impaired
waters for which Total Maximum Daily
Loads (TMDLs) and restoration plans
need to be developed. Since restoration
is more costly than prevention and
oftentimes full restoration is impossible
to achieve, the City and County have
set the third goal for the ordinance as
minimizing the risk of additional
stream segments being impaired and
put on the 303(d) list.
The fourth goal is to satisfactorily
detain stormwater to control larger volumes and peaks associated with
new construction. In recent years, the City and County have examined
detention and its effect on controlling flooding and environmental
impacts. The post-construction ordinance provides an opportunity to
place the detention requirement within the context of overall watershed
planning and stormwater management.
Four Primary Goals for Post-Construction Ordinance Development:
1. Achieve compliance with Phase I and Phase II NPDES Stormwater permit requirements.
2. Address cumulative and secondary impacts to aquatic and terrestrial resources and water quality in
Goose Creek and the Yadkin River watersheds.
3. Protect water quality, minimizing the risk of additional 303(d) listings and complementing restoration of
existing impaired waters.
4. Address detention of the control of stormwater volumes and peaks.
Executive Summary p. 2
September 2005
1.3 Stakeholder Involvement
The City and County formed a diverse Post-Construction Stakeholders’
Group (15 different constituencies represented) to review information,
discuss policies, and develop ordinance language. The group used a
consensus building process with consensus defined as when “all
members present can live with the outcome or proposal being made.” A
select panel of City and County staff was tasked with working with the
Stakeholders’ Group to provide information and support group
discussions. Tetra Tech was asked to attend 10 of the group’s meetings
to provide information and facilitate discussion for certain key topics.
In total, the Group met 30 times between April 2004 and September
2005, not including additional subcommittee meetings.
The City and County
formed a diverse
Post-Construction
Stakeholders’ Group
(15 different
constituencies
represented) to
review information,
discuss policies, and
develop the
ordinance.
The Post-Construction Stakeholders’ Group met 30
times between April 2004 and September 2005.
September 2005
Executive Summary p. 3
The greatest threats
to water quality from
newly developed or
redeveloped land
are likely to come
from physical
changes such as
flow volumes/
velocities and
temperature, and
increased pollutant
loading for
contaminants such
as sediment,
nutrients, and
pathogens (e.g.,
fecal coliform
bacteria).
2. Technical Approach
Tetra Tech’s primary role was to provide technical support to staff and
the Stakeholders’ Group. Tetra Tech’s project support was divided into
two phases:
Phase 1 involved comparing existing water quality and habitat
conditions to predicted future conditions, using existing
management policies and programs. This is called the “baseline
analysis.”
Phase 2 involved working with staff and the Stakeholders’
Group to develop and evaluate management options (referred to
as Scenarios) which could achieve the four primary ordinance
development goals.
The first task under Phase 1 was to establish which post-construction
runoff pollutants and impacts to include in the assessment, and what
modeling tools would be used to assess these parameters. Review of
existing past watershed data and assessments indicated that the greatest
threats to water quality from newly developed or redeveloped land will
likely come from physical changes such as flow volumes/velocities and
temperature, and increased pollutant loading for contaminants such as
sediment, nutrients, and pathogens (e.g., fecal coliform bacteria). Tetra
Tech developed modeling tools for
predicting future watershed
conditions so that the stakeholders
and state could better understand how
water quality and habitat would be
impacted without further action or
additional regulation, and how
proposed ordinance requirements
would address these issues. The
project team built on existing
information and modeling tools to
develop a technical basis sufficient to
evaluate the county’s 33 named
watersheds that cover over 500
square miles of drainage area.
The indicators selected for modeling
were stream stability (comparison of
Streambank collapse and channel erosion is common in
specific stream power and velocity to
Mecklenburg County where post-construction runoff
critical threshold values), sediment,
controls are lacking.
and nutrients (phosphorus and
nitrogen). Stream stability is indicative of the impact that stormwater
runoff quantity can cause, whereas sediment and nutrients are
indicative of the impact that contaminated stormwater runoff can have
Executive Summary p. 4
September 2005
on water quality. In addition to reflecting some of the greatest concerns
regarding stress on the watersheds, these parameters could be readily
modeled using the following available tools and information:
•
•
•
•
•
Calibrated HSPF model for the McDowell Creek Watershed
USGS Mecklenburg County Unit Hydrograph Study
Tetra Tech dynamic BMP modeling tool
The Mecklenburg County Site Evaluation Tool (SET)
The Mecklenburg County Flood Insurance Study (FIS)
HEC models
2.1 Modeling Framework
Description
The indicators
selected for modeling
were:
stream stability
sediment
total phosphorus
total nitrogen
The components of the models were woven together to conduct both
pollutant loading and stream stability analysis for the county’s
watersheds. GIS databases and municipal planning input were used to
define modeling land use classes, delineate watershed and
subwatershed evaluation and reporting units, and establish the existing
and future buildout land use databases. For pollutant analyses,
representative site design assumptions were developed and the SET
was applied to generate runoff pollutant loading rates (e.g., lb/ac/yr) for
each land use category.
Information was then aggregated to the watershed scale using the land
use database to produce average loading rates for the watershed. This
was done separately, first for the baseline analysis (comparing existing
conditions to future conditions with and without current regulations),
and later during Phase 2 for the evaluation of alternative management
scenarios.
For the stream stability analysis, hydrographs were developed for each
land use category and aggregated to the subwatershed scale using the
land use databases. The subwatershed hydrographs were input to the
HEC models to route flow and predict channel velocity and specific
stream power. The modeling results for the baseline analysis and
alternative management scenarios were then compared to thresholds
defined from field reconnaissance to predict the percent of watersheds
at risk for severe stream channel instability.
What is a Hydrograph?
A hydrograph is a representation of how stormwater runoff changes over time. The shape of the
hydrograph is important. For example, too much runoff in a short amount of time can lead to flooding or
damage to the stream channel.
September 2005
Executive Summary p. 5
Municipal Planning
Input
City/County GIS
y Modeling watershed delineations
y Land use spreadsheet databases
Stream Stability Analysis
Pollutant Runoff Analysis
y Land use type site designs for each
scenario
y Runoff rates (lb/ac/yr) for sediment,
phosphorus, and nitrogen for each land
use type for each scenario
y Per-acre runoff hydrographs for each
land use category
y Subwatershed hydrographs
y Stream flow estimates throughout
stream network
y Predicted channel velocity and specific
stream power
Key
Indicator
Key
Indicator
Runoff rates (lb/ac/yr)
for sediment,
phosphorus and
nitrogen for 33
named watersheds
for each scenario
Figure 2-1.
Key
Indicator
Key
Indicator
Percent of modeled
stream miles at risk for
severe instability
Flow Diagram Showing Integration of Modeling Tools
2.2 Development of Management
Recommendations
The Stakeholders’ Group and staff were first presented with the
baseline modeling results for each of the key indicators, along with
information on various management options (e.g., structural and nonstructural integrated management practices) and their relative
effectiveness for addressing the key hydrologic and water quality
stressors. Next, stakeholders and staff crafted two alternative
management scenarios (referred to as Scenarios 1 and 2) combining
multiple management practices to address the gaps in existing
regulations. The project team modeled the two alternatives and
presented the results along with relative costs. The stakeholders then
evaluated each alternative for its compatibility with other community
values, political feasibility, and administrative feasibility.
Executive Summary p. 6
September 2005
After reviewing the results for the first two scenarios, the Stakeholders’
Group indicated that there were several issues to be addressed before
the Group would be willing to support specific measures as the basis
for ordinance development. A series of nine meetings were held to
systematically work through the issues so that the Group could reach
consensus on Scenario 3. The stakeholders and staff then used the
results of the management evaluations to begin the Phase 3 ordinance
language development and adoption process.
Stakeholder
Input
Develop
Scenarios
Model
Scenarios
Stakeholder
Discussion
Refine
Alternative
Ready to
Select
Alternative?
Figure 2-2.
No
Stakeholder
Review
Yes
Stakeholders and
staff initially crafted
two alternative
management
scenarios (referred
to as Scenarios 1
and 2). Based on
evaluation and
revision of these
options, the Group
developed and
adopted consensus
Scenario 3.
Proceed to
Ordinance
Language
Development
Stakeholder Process for Selecting Management Options
for Inclusion with the Ordinance
September 2005
Executive Summary p. 7
3. Baseline Analysis
The modeling framework developed for the project was used to
conduct a “Baseline Analysis.” The analysis consists of applying the
models under existing conditions, and then comparing the results to
model predictions for future conditions. The future is modeled two
ways, first with no management controls and then with existing
management regulations applied. The baseline analysis helps answer
two important questions:
1. How would stream stability and pollutant loading conditions
change in the future if there were no management requirements?
2. How effective are existing regulations in mitigating any
predicted negative impacts to stream stability and water quality?
The answer to the first question illustrates how significant the changes
caused by new development could be (i.e., the potential magnitude of
additional stream instability and pollutant loading impacts directly
attributable to future development). The answer to the second question
identifies gaps in existing management strategies and provides the
baseline for what the post-construction ordinance should achieve for
these indicators.
3.1 Existing vs. Predicted Future
Land Use
Land use information was obtained from each jurisdiction with the help
of City and County staff, and planning staff from the partner
jurisdictions. Parcels were assigned to one of twenty-three specific
land use categories established for the countywide analysis. Categories
included eight different residential classifications ranging from very
low to very high densities, six nonresidential classes (commercial,
institutional, and industrial) with different ranges of imperviousness
represented, and three mixed use categories reflecting different levels
of imperviousness. The remaining categories included classes such as
parks, forest, meadow, agriculture, golf courses, and interstate
highway. Future land use projections were based on all of the available
master plans and best professional judgment of local planning staff.
These projections considered the existing and planned future transit
corridors. Very detailed descriptions of the processes to build the
existing and future land use databases are contained in the Phase 1
project report (Tetra Tech, 2005a)
Executive Summary p. 8
September 2005
Figures 3-1 and 3-2 graphically display condensed summaries of the
existing and future land use, respectively. Visual comparison reveals
that the largest increase in future development is expected for singlefamily residential development, largely replacing forest and rural use.
Table 3-1 summarizes the changes for the condensed categories.
Figure 3-1.
Existing Land Use
Table 3-1.
Summary of Mecklenburg County Predicted Land Use Change
Land Use
Existing (ac)
Figure 3-2.
Future (ac)
Future Land Use
Net Change (ac)
Net Change (%)
Agricultural/Rural
45,850
2,370
- 43,480
- 95%
Commercial
16,690
23,420
+ 6,730
+ 40%
Institutional
10,870
16,150
+ 5,280
+ 49%
7,570
8,390
+ 820
+ 11%
Multi-Family Residential
13,950
18,720
+ 4,770
+ 34%
Office / Industrial
33,470
42,900
+ 9,430
+ 28%
6,570
15,160
+ 8,590
+ 131%
108,840
197,160
+ 88,320
+ 81%
95,350
14,150
- 81,200
- 85%
Interstate Hwy
Park
Single-Family Residential
Vacant/Forest
September 2005
Executive Summary p. 9
3.2 Pollutant Loading Analysis
Baseline pollutant loading analyses were performed to estimate average
runoff rates by watershed for total suspended solids (TSS) and total
phosphorus (TP). Results are presented below for these two focal
parameters of concern, and grouped by three areas for presentation:
Catawba watersheds (draining to drinking water supplies), South
Catawba watersheds (draining to South Carolina), and Yadkin River
Basin watersheds. Figure 3-3 illustrates the relative locations of the
named watersheds and corresponding groupings.
LAKE NORMAN
CATAWBA
WATERSHED
DISTRICT
ROCKY
RIVER
McDOWELL
UPPER
MTN ISLAND
YADKIN
WATERSHED
DISTRICT
CLARKE
LOWER
CLARKE
GAR
LOWER
MTN ISLAND
MALLARD
LONG
BACK
CATAWBA
IRWIN
PAW
UPPER
LITTLE
SUGAR
BEAVERDAM
STEELE
Figure 3-3.
Executive Summary p. 10
SOUTH
CATAWBA
WATERSHED
DISTRICT
BRIAR
McKEE CALDWELL
CLEAR
McMULLEN
SUGAR
LAKE
WYLIE
REEDY
GOOSE
LOWER
LITTLE
SUGAR
McALPINE
CROOKED
FOUR MILE
TWELVE
MILE
CLEM
SIX MILE
Baseline Analysis Watershed Groupings
September 2005
Catawba Watersheds Baseline Pollutant Loading Results
Results for TSS and TP average loading rates in the Catawba
watersheds are presented in Figures 3-4 and 3-5. Comparison of
existing loading rates (blue bars) and loading rates in the future with no
stormwater controls (purple bars) shows that changed land use in the
future will significantly increase pollutant generation rates for both TSS
and TP. However, comparison of the future loading rates under
existing management strategies (yellow bars) for these watersheds
indicates that the current Huntersville ordinance requirements should
be quite effective at mitigating increases in loads (affected watersheds
include McDowell, Upper Mountain Island, and a good portion of Gar).
Additionally, the water supply regulations that affect a sizeable portion
of the other Catawba watersheds are expected to do well in mitigating
TSS, but be slightly less effective in mitigating future TP loading rate
increases.
TP Loading Rates, Catawba Watershed District
TSS Loading Rates, Catawba Watershed District
Future, No Controls
Future, Current Regs
Existing
Future, No Controls
0.60
Existing
1.60
0.50
1.40
0.40
1.20
lb/ac/yr
0.30
1.00
0.80
0.60
0.20
0.40
0.10
0.20
Upper Mtn
Island
Paw
McDowell
Lower Mtn
Island
Long
Lake Wylie
Lake
Norman
Gar
Upper Mtn
Island
Paw
McDowell
Lower Mtn
Island
Long
Lake Wylie
Lake
Norman
Gar
Catawba
Beaverdam
Figure 3-4. TSS Loading in Catawba Grouping
Catawba
0.00
0.00
Beaverdam
ton/ac/yr
Future, Current Regs
1.80
Figure 3-5. TP Loading in Catawba Grouping
South Catawba Watersheds Baseline Pollutant Loading Results
Results for TSS and TP average loading rates in the South Catawba
watersheds are presented in Figures 3-6 and 3-7. Comparison of
existing loading rates (blue bars) and loading rates in the future with no
controls (purple bars) shows that pollutant loading for changed land use
in the future will increase, but not as much as for the Catawba
watersheds. This can be attributed to the fact that watersheds in the
South Catawba are already more built-out, so existing loading rates are
higher and there is less land available for conversion from forested or
rural uses to urban uses. Additionally, comparison of the future loading
rates under existing management strategies (yellow bars) for these
watersheds indicates that existing requirements in these watersheds are
not very effective at mitigating future increases. The South Catawba
watersheds do not have the water supply protection regulations applied
to them, nor do they have protection measures like those recently
established for Huntersville.
September 2005
Executive Summary p. 11
TP Loading Rates, South Catawba Watershed District
TSS Loading Rates, South Catawba Watershed District
Future, No Controls
Future, Current Regs
Existing
Future, No Controls
0.60
Existing
1.60
0.50
1.40
0.40
1.20
lb/ac/yr
0.30
0.20
1.00
0.80
0.60
0.40
0.10
0.20
0.00
Figure 3-6. TSS Loading in South Catawba
Sugar
Upper Little
Sugar
Steele
Six Mile
McMullen
McAlpine
Lower Little
Sugar
Irwin
Four Mile
Briar
Upper Little
Sugar
Sugar
Steele
Six Mile
McMullen
McAlpine
Lower Little
Sugar
Irwin
Four Mile
Clem
Briar
0.00
Clem
ton/ac/yr
Future, Current Regs
1.80
Figure 3-7. TP Loading in South Catawba
Yadkin Watersheds Baseline Pollutant Loading Results
Results for TSS and TP average loading rates in the Yadkin watersheds
are presented in Figures 3-8 and 3-9. Comparison of existing loading
rates (blue bars) and loading rates in the future with no controls (purple
bars) shows that changed land use in the future will greatly impact
average loading rates in the Yadkin watersheds. Relative to the
Catawba and South Catawba watersheds, these are the least developed
under existing conditions. Therefore, the high rate of conversion from
forested and rural uses to suburban and some nonresidential uses is
expected to substantially increase rates of pollutant generation. With
the exception of the Clarke Creek watershed which is protected by the
Town of Huntersville ordinance provisions, existing regulations are not
expected to mitigate future pollutant loading rate increases
(demonstrated by yellow bars indicating loading rates almost the same
as those reflecting no controls).
TSS Loading Rates, Yadkin Watershed District
Future, No Controls
TP Loading Rates, Yadkin Watershed District
Future, Current Regs
Existing
Future, No Controls
Existing
1.60
0.50
1.40
0.40
1.20
lb/ac/yr
0.30
0.20
1.00
0.80
0.60
0.40
0.10
0.20
0.00
Figure 3-8. TSS Loading in Yadkin
Executive Summary p. 12
Twelve Mile
Rocky River
Reedy
McKee
Mallard
Lower Clarke
Goose
Crooked
Clear
Clarke
Caldwell
Twelve Mile
Rocky River
Reedy
McKee
Mallard
Lower Clarke
Goose
Crooked
Clear
Clarke
Caldwell
Back
0.00
Back
ton/ac/yr
Future, Current Regs
1.80
0.60
Figure 3-9. TP Loading in Yadkin
September 2005
3.3 Stream Stability Analysis
Stream stability analyses were performed for 23 of the 33 named
watersheds where sufficient information and modeling tools were
available. Results for the three watershed groupings are shown in
Figures 3-10 through 3-12. For all watersheds, the risk of channel
degradation is predicted to increase in the future if no stormwater
controls are implemented for new development (purple bars compared to
blue bars). Except for the Beaverdam, Briar, and Little Sugar
watersheds, the current regulations are not predicted to provide much
mitigation of future stream instability increases (yellow bars compared
to purple bars). Additionally, in the remaining three watersheds, further
analysis revealed that the duration of time above channel erosion
thresholds is expected to increase substantially, thereby increasing the
risk of channel instability for these streams as well. Figure 3-13
provides an example of this for the Little Sugar watershed. The graph
shows how future peak flow, although similar in magnitude to existing
peak flow, exceeds the threshold for erosion for twice the amount of
time.
Catawba Watershed District
Existing
Future, No Controls
Figure 3-10. Stream Instability in Catawba
10%
Upper Little
Sugar
Sugar
Steele
Six Mile
0%
McMullen
Upper Mtn
Island
Paw
McDowell
Lower Mtn
Island
Long
Lake Wylie
Lake
Norman
Gar
Catawba
0%
20%
McAlpine
10%
30%
Irwin
20%
40%
Lower Little
Sugar
30%
50%
Four Mile
40%
Existing
60%
Clem
50%
Future, Current Regs
70%
Briar
60%
Percentage of Channel Length at Risk
Future, Current Regs
70%
Beaverdam
Percentage of Channel Length at Risk
Future, No Controls
South Catawba Watershed District
Figure 3-11. Stream Instability in South Catawba
Yadkin Watershed District
Existing Conditions
Future, Current Regs
Future, Current Regs
Threshold Discharge
Existing
70%
4000
60%
3500
50%
3000
40%
Flow (cfs)
2500
30%
20%
2000
1500
10%
1000
Figure 3-12. Stream Instability in Yadkin
September 2005
Twelve Mile
Rocky River
Reedy
McKee
Mallard
Lower
Clarke
Goose
Crooked
Clear
Clarke
Caldwell
0%
Back
Percentage of Channel Length at Risk
Future, No Controls
500
0
200
300
400
500
600
700
800
900
1000
Flow Duration (min)
Figure 3-13. Influence of Flow Duration on
Channel Stability in Little Sugar Watershed
Executive Summary p. 13
3.4 Overall Baseline Assessment
Additional insight was provided by generating a composite assessment
for existing conditions using four environmental stress indicators (TSS,
TP, TN and Channel Instability Risk) along with watershed condition
indicators (percent imperviousness and the county’s biological support
rating). Watershed composite indicator scores were sorted into quartile
ranges (i.e., top quarter reflected lowest level of environmental stress
and the bottom quarter reflected the highest level).
The composite assessment provided some very interesting results. In
the top quartile (least stressed), watersheds were characterized by
relatively lower levels of imperviousness (ranging from 5 to 15 percent
with an average of 9 percent). Additionally, three of the four
watersheds with current overall biological assessment ratings of fullysupporting (FS) were found in the upper quartile, indicating that
watersheds with these lower stressor values are much more likely to
support healthy aquatic communities. The average loading rates for the
fully-supporting watersheds are compared to rates for the lower
quartiles in Table 3-2.
Table 3-2. Comparison of Existing Environmental Stressors By Watershed Rank
Watershed Rank
Category
Average TN
Loading Rate
(lb/ac/yr)
Average TP
Loading Rate
(lb/ac/yr)
Average TSS
Loading Rate
(tons/ac/yr)
Existing Risk of
Channel
Instability (%)
Existing
Impervious
Surface (%)
Fully Supporting
3.5
0.55
0.24
< 1%
9%
Quartile
4.5
0.71
0.29
8%
18%
3 Quartile
5.2
0.82
0.32
15%
22%
4th Quartile
7.1
1.13
0.4
27%
30%
nd
2
rd
Overall, the baseline analyses demonstrated that water quality and
aquatic habitat protection are more likely to occur in watersheds
characterized by lower imperviousness and lower rates of stress from
stream instability and pollutant loading. Therefore, as new development
occurs and imperviousness increases within the watersheds,
management should be aimed at reducing stressor loads to levels more
indicative of existing high quality watersheds. The rates shown in the
Fully Supporting use category in Table 3-2 provide a target to aim for
in meeting the primary water quality objective.
In most cases, existing regulations are not sufficient to mitigate the
impacts of future development. Exceptions include the current
Huntersville ordinance regulations that were already developed to
address post-construction runoff impacts. The post-construction
ordinance is therefore needed to address the gaps in current
management regulations.
Executive Summary p. 14
September 2005
4. Alternatives Development
4.1 Management Options
Stakeholders were asked to support development of management
options to consider as the basis for the ordinance. To help the
Stakeholders’ Group better understand which management options and
BMPs are most effective, Tetra Tech developed a Management Options
Matrix that displayed a consumer reports-type effectiveness rating
based on a review of available technical research. The options were
divided into the following categories:
•
Development Site Performance Standards for water quality and
hydrology (e.g., reduce phosphorus loading by 70%, capture
and detain runoff from 1-yr, 24-hr storm event).
•
Structural BMPs such as bioretention, stormwater wetlands, and
wet ponds.
•
Non-structural BMPs such as reducing imperviousness and
requiring vegetated buffers along streams.
The Stakeholders’
Group screened
different management
options, and then
combined promising
options into 3 different
management
scenarios for
evaluation.
Other management options evaluated included land use regulation (e.g.,
open space requirements and imperviousness caps), education, and
monitoring. The evaluation showed the linkage between each of these
management options and the study’s primary hydrology and water
quality indicators. The Stakeholders’ Group used the matrix to help
construct the scenarios, or what combination of management measures
seem promising in meeting the management goals and therefore should
be evaluated in more detail.
4.2 Three Management Scenarios
Scenario 1
Scenario 1 represents a proposal by the City of Charlotte and
Mecklenburg County staff based on the results from the Project’s Phase
1 modeling. In Phase 1, the modeled pollutant loading rates taken from
the upper quartile of watersheds in Mecklenburg County were used to
set thresholds representative of conditions that allow the streams in
these watersheds to meet their designated uses.
Scenario 1 varies the performance criteria between two Watershed
Districts, the Yadkin and the Catawba, to fulfill the four ordinance
goals and meet the threshold criteria established through the Phase 1
modeling.
September 2005
Scenario 1 featured
uniform stormwater
technology control
requirements for
two watershed
districts divided
along major river
basin boundaries.
Executive Summary p. 15
For the Yadkin District, the performance criteria were derived from the
proposed Mint Hill post-construction ordinance, which was specifically
developed to fulfill the mitigation measures of protection of the
Carolina Heelsplitter from the cumulative and secondary impacts of
land development activities developed by NCWRC and USFWS.
The Catawba District includes those watersheds draining into the
Catawba River. This district contains the drinking water supply
watersheds as well as the majority of
the impaired stream length in
LAKE
Mecklenburg County. This district is
NORMAN ROCKY
YADKIN DISTRICT
RIVER
CATAWBA DISTRICT
an area of concern due to elevated
nutrient level discharges to Lake
UPPER
CLARKE
MTN
McDOWELL
ISLAND
Wateree in South Carolina. The
LOWER
CLARKE
performance standards selected for this
GAR
LOWER
district were derived from an
MTN
MALLARD
ISLAND
LONG
assessment of the modeling data from
BACK
Phase 1 for healthy, unimpaired
PAW
McKEE
IRWIN
CATAWBA
REEDY
CALDWELL
watersheds. These criteria are
UPPER BRIAR
specifically targeted toward minimizing
LITTLE
CLEAR
BEAVERDAM
SUGAR
increases in nutrient loads and channel
SUGAR
GOOSE
McALPINE
degradation from future development
McMULLEN
LAKE
LOWER
STEELE
WYLIE
activities.
LITTLE
CROOKED
FOUR MILE
N
SUGAR
TWELVE MILE
SIX MILE
CLEM
5
0
5
10 Miles
Features of Scenario 1
2 Management Districts
Catawba
LID required for development with greater than 12% imperviousness
Water quality performance criteria: 70% removal of TP, 85% removal of TSS
Runoff peak and volume control requirements
50 ft. buffer requirements
Yadkin
LID required for development with greater than 6% imperviousness
Water quality performance criteria: 85% removal of TSS
Runoff peak and volume control requirements
100 ft. buffer requirements
Executive Summary p. 16
September 2005
Scenario 2
Over a series of meetings, the Stakeholders’ Group discussed the
environmental and economic implications of the proposed requirements
for Scenario 1 of the post-construction ordinance. The general
consensus of the Stakeholders’ Group after these meetings was that the
differing needs of individual watersheds should be addressed, instead
of grouping all watersheds into only two categories (i.e., the Catawba
and Yadkin watershed districts).
Scenario 2 varied
requirements across
five watershed
districts to reflect
varying levels of
protection needs,
with endangered
species and water
supply protection
receiving extra
attention.
The principal differentiators among watershed protection needs
identified by the Stakeholders’ Group include:
•
The presence of state and federally listed rare, threatened, and
endangered species.
•
Drinking water protection.
•
Degree of the watershed’s existing urbanization or buildout.
Using these criteria, a spectrum of requirements was developed ranging
from those watersheds with the lowest need for additional protection to
those with the greatest need. The goal of varying the level of
requirements was to appropriately target water quality and ecological
protection measures without overly burdening development
opportunities. Thus, watersheds that were the most urbanized, that
contained no listed species and did not drain to the Charlotte drinking
water supply were assigned the lowest need for additional protection
whereas watersheds containing listed species or draining to the City’s
drinking water supply were given the greatest additional protection.
As a result of the various levels of
protection, the two Watershed Districts
proposed in Scenario 1 were segregated
into five districts under Scenario 2: Central
Catawba, Western Catawba, YadkinSoutheast Catawba, Huntersville, and
Goose Creek. The spectrum of additional
protection ranges from the lowest need in
the Central Catawba District and
progresses through additional levels of
protection in the Western Catawba,
Yadkin-Southeast Catawba, Huntersville,
and Goose Creek districts.
Another management option included in
Scenario 2 was preservation of undisturbed
open space during and after development.
This requirement varied by the degree of
imperviousness of the development.
September 2005
N
LAKE
NORMAN
CENTRAL CATAWBA
WESTERN CATAWBA
YADKIN-SOUTHEAST CATAWBA
GOOSE CREEK
HUNTERSVILLE
ROCKY
RIVER
UPPER
MTN
ISLAND
CLARKE
McDOWELL
LOWER
CLARKE
GAR
LOWER
MTN
ISLAND
MALLARD
LONG
BACK
CATAWBA
PAW
IRWIN
REEDY
BRIAR
UPPER
LITTLE
SUGAR
BEAVERDAM
CLEAR
SUGAR
GOOSE
McMULLEN
LAKE
WYLIE
STEELE
McKEE
CALDWELL
LOWER
LITTLE
SUGAR
McALPINE
FOUR MILE
CROOKED
TWELVE MILE
SIX MILE
CLEM
5
0
5
10 Miles
Executive Summary p. 17
Features of Scenario 2
5 Management Districts: Central Catawba, Western Catawba, Yadkin-Southeast
Catawba, Huntersville, and Goose Creek
Vary requirements according to watershed needs
LID encouraged (except in Huntersville and Goose Creek where required)
Water quality performance criteria: 70% removal of TP; 85% removal of TSS all
districts (except Central Catawba where there is only a TSS standard)
Runoff peak and volume control requirements vary by district
Stream buffer requirements vary in stringency by district (from existing S.W.I.M.
buffer plus 30 ft. up to 200 ft)
Undisturbed open space preservation required for all development less than
85% imperviousness
25% open space for development with < 24% impervious area
17.5% open space for development with ≥ 24% and < 50% impervious area
10% open space for development with ≥ 50% and < 85% impervious area
Scenario 3
After an evaluation of Scenario 1 and 2, the Stakeholders’ Group
reached consensus that Scenario 2 was preferred over Scenario 1.
However, significant concerns remained about Scenario 2. To reach a
consensus alternative, several key areas needed to be resolved:
Scenario 3
represents
consensus
modifications to
Scenario 2 to
address key areas of
stakeholder concern.
Executive Summary p. 18
•
Cost of open space and water quality performance standards.
•
Peak controls.
•
Maintenance responsibility.
•
Low impact development requirements.
Building on Scenario 2, Scenario 3 was crafted to address each of these
outstanding issues. Most of the Committee’s effort centered on
reducing the cost to the development community of meeting Scenario 2
requirements. For the TP water quality performance standard, a
flexible “buy-down” option was provided that allows a development
applicant to “buy down” from 70% TP removal to 50% (the minimum
required under the state’s Phase II regulations), and allows the City or
County to use the revenue to construct BMP retrofits off-site to “make
up” the difference in phosphorus loading (i.e., between 50 and 70%).
Alternatively, an applicant could also build a BMP off site to mitigate
this difference.
September 2005
To reduce the cost of meeting the open space requirements, Scenario 3
provides several mechanisms: off-site mitigation and on-site mitigation
techniques, as well as payment-in-lieu. All five districts have a uniform
volume control requirement. To meet all of these hydrology and water
quality requirements, developers are allowed (rather than required) to
use LID. Finally, Scenario 3 recommends that the City and County
assume BMP maintenance responsibility for single-family residential
development, but require all other developments to provide for longterm maintenance. Page 20 provides the details of the consensus
Scenario 3.
Features of Scenario 3
5 Management Districts: Central Catawba, Western Catawba, Yadkin-Southeast
Catawba, Huntersville, and Goose Creek
LID allowed (excepts in Huntersville where it is required)
Water quality performance criteria: 70% removal of TP; 85% removal of TSS all
districts (except Central Catawba where there is only a TSS standard)
Buy-down allowed for development with greater than 60% imperviousness;
off-site mitigation also allowed.
Uniform volume control requirement across districts
Stream buffer requirements vary in stringency by district (from existing S.W.I.M.
buffer plus 30 ft. up to 200 ft)
Undisturbed open space preservation required for all development
25% open space for development with < 24% impervious area
17.5% open space for development with ≥ 24% and < 50% impervious area
10% open space for development with ≥ 50% and < 85% impervious area
On-site, off-site, and payment-in-lieu mitigation allowed
September 2005
Executive Summary p. 19
Table 4-1.
Watershed
District
Central
Catawba
Detailed Description of Consensus Scenario 3 Management Scenario
Structural
Water Quality BMPs
>24% BUA requires 85%
TSS removal for runoff
from 1st inch of rainfall;
LID optional
Buffers(1)
30 ft. no build zone on
intermittent streams
35 ft. (2 zones) on perennial
streams draining <300 acres
50 ft (3 zones) on streams
draining >300 acres
100 ft + 50% of flood-fringe on
streams draining >640 acres
Volume & Peak Control
Open Space Requirements
Volume (Commercial & Residential):
Open space is undisturbed area
>24% BUA control entire volume for 1-yr, 24hr storm
<24% BUA = 25% open space
Peak for Residential: >24% BUA perform a
downstream flood analysis to determine
whether peak control is needed and if so, for
what level of storm frequency (i.e., 10, 25, 50
or 100-yr, 6-hr) OR if a downstream analysis
is not performed, control the peak for the 10yr and 25-yr, 6-hr storms
>24% and <50% BUA = 17.5%
open space
>50% BUA = 10% open space
Peak for Commercial: >24% BUA control the
peak for the 10-yr, 6-hr storm AND perform a
downstream flood analysis to determine
whether additional peak control is needed
and if so, for what level of storm frequency
(i.e., 25, 50 or 100-yr, 6-hr) OR if a
downstream analysis is not performed,
control the peak for the 10-yr and 25-yr, 6-hr
storms
Western
Catawba
>12% BUA requires 85%
TSS and 70% TP removal
for runoff from 1st inch of
rainfall; LID optional; BUA
area caps apply in water
supply watersheds
30 ft. no build zone on
intermittent streams
Same as Central Catawba except use >12%
BUA as the threshold for application
Same as Central Catawba
Same as Central Catawba except use >10%
BUA as the threshold for application
Same as Central Catawba
35 ft. (2 zones) on perennial
streams draining <300 acres
50 ft (3 zones) on streams
draining >300 acres
100 ft + 50% of flood-fringe on
streams draining >640 acres
YadkinSoutheast
Catawba
>10% BUA requires 85%
TSS and 70% TP removal
for runoff from 1st inch of
rainfall; LID optional
Executive Summary p. 20
50 ft undisturbed forested
buffers on intermittent streams
100 ft undisturbed forested
buffers on perennial streams,
plus remainder of floodplain
September 2005
5. Evaluation of Alternatives
5.1 Evaluation Criteria
A “consumer report” Scenario Evaluation Matrix was used to evaluate
and convey how effective Scenario 1 and 2 were in meeting the
project’s goals. Four measurable water quality and hydrology goals
were used and four feasibility goals were used, based on input from the
Stakeholders’ Group. Each goal has specific evaluation criteria. For the
quantitative goals and criteria, numeric ranges or thresholds were
established and then compared to modeling analysis results for each
watershed district. The other goals, which were more subjective,
required input and evaluation by the Stakeholders’ Group and the staff
technical panel. Tetra Tech used facilitated discussion and a survey
with the Group members to evaluate the goals and criteria related to
community values, political feasibility, and administrative feasibility.
Criteria were
developed for four
measurable water
quality and
hydrology goals
and four feasibility
goals to evaluate
and convey how
effective Scenario
1 and 2 were in
meeting the
project’s goals.
Each scenario was evaluated based on whether it met, partially met, or
failed to meet the following goals (shown in bold) and criteria (denoted
with bullets). Note that the 30 evaluation criteria are condensed and
summarized below. For more details, please refer to the Phase II Report
(Tetra Tech, 2005b).
Table 5-1.
Goals and Criteria for Scenario Evaluation
Meets State NPDES Phase 1 and 2 Post-Construction Stormwater Regulations
Meets housing density, imperviousness, and stormwater control requirements
Addresses Cumulative and Secondary Impacts to Aquatic Life and Water Quality in Watersheds
with Listed Species
Mitigates pollutant loading based on rates that support good habitat conditions
Minimizes risk of stream bank and channel instability through maintaining dominant
stormwater flow at existing conditions
Protects Water Quality through Minimizing Additional 303(d) Listings
Mitigates increases in pollutant loading
Minimizes risk of streambank and channel instability through managing increase in
the dominant stormwater flow
Addresses Detention for the Control of Stormwater Volume and Peaks
Mitigates increased risk of geomorphic instability for dominant discharge
Minimizes additional flooding potential
September 2005
Executive Summary p. 21
Table 5-1. (continued) Goals and Criteria for Scenario Evaluation
Minimizes the Uncertainty or Risk of Not Meeting Targets
Does not cause significant, irreversible land use changes related to risk of
downstream impairment
Cost
Cost of mitigation relative to other scenarios
Other Community Values
Improve air quality, discourage sprawl, provide connectivity
Encourage attractive development
Improve housing affordability
Enhance quality of life
Remain economically competitive
Political Feasibility
Meets local governments’ existing policies and plans for growth
Provides for flexibility in development design
Does not pose a public health or safety risk
Favorable response from the Stakeholders’ Group
Administrative Feasibility
Does not cause a significant administrative burden
5.2 Initial Evaluation of Scenarios
1 and 2
Scenario 1 and 2
were equal in
meeting
environmental
criteria, but Scenario
1 cost more and
provided less
flexibility in
requirements.
Executive Summary p. 22
Based on a comparison of the evaluation criteria and environmental/
cost modeling results, Tetra Tech provided a scenario evaluation matrix
for each district, allowing the Stakeholders’ Group to see how well
each scenario met overall goals.
In its evaluation of criteria, Tetra Tech concluded that for all districts
both scenarios:
•
Meet Phase II NPDES stormwater requirements.
•
Minimize the risk of additional 303(d) listings.
•
Mitigate the risk of geomorphic instability.
•
Mitigate additional flooding potential.
September 2005
It should be noted that both scenarios rely heavily on BMPs to mitigate
impacts, and therefore should be accompanied by programs that assure
that BMPs are installed and working properly.
At the site-scale,
loading rates for new
development under
Scenarios 1 & 2 are
predicted to be
equivalent to those
in watersheds fully
supporting their
uses.
Additionally, for the Goose Creek Watershed, Tetra Tech concluded
that both scenarios effectively meet the U.S. Fish and Wildlife
guidelines for addressing cumulative and secondary impacts to aquatic
and terrestrial resources and water quality in watersheds with
endangered species.
For the quantitative evaluation criteria, both scenarios were equal in
meeting environmental criteria. Figures 5-1 and 5-2 display
comparative results for the TSS and TP indicators at the watershed
district scale. At the site-scale for new development and redeveloped
areas loading rates are predicted to be even lower and equivalent to
those associated with the watersheds fully-supporting their uses.
TSS Loading Rates by Scenario #2 Watershed Districts
Future, No Controls
Future, Current Regs
Scenario #1
Scenario #2
TP Loading Rates by Scenario #2 Watershed Districts
Existing
Future, No Controls
0.60
Future, Current Regs
Scenario #1
Scenario #2
Existing
1.60
0.50
1.40
1.20
lb/ac/yr
ton/ac/yr
0.40
0.30
0.20
1.00
0.80
0.60
0.40
0.10
0.20
Figure 5-1.
TSS Loading Rate Comparisons
Figure 5-2.
Huntersville
Goose
Creek
TP Loading Rate Comparisons
The estimated cost for Scenario 2 was slightly lower than Scenario 1,
although the only significant cost difference was in the South Catawba
(others districts were within 10% cost of each other). The Stakeholders’
Group therefore decided which scenario was more favorable based on
how the scenarios compared in political and administrative feasibility
and overall community values.
Based on a survey and facilitated discussion, the Stakeholders’ Group
reached consensus that Scenario 2 was preferred over Scenario 1 in
meeting the overall goals and criteria. However, the Group still had
significant concerns about Scenario 2. Seven key areas needed to be
resolved: open space/imperviousness caps; low impact development
requirements; peak controls; stream buffers; maintenance
responsibility; districts; and costs. The consensus alternative,
Scenario 3, was crafted to address each of these issues.
September 2005
YadkinSoutheast
Catawba
Western
Catawba
0.00
Central
Catawba
Huntersville
Goose Creek
YadkinSoutheast
Catawba
Western
Catawba
Central
Catawba
0.00
Scenario 3 was
crafted using
stakeholder
consensus to
address seven key
issue areas
associated with
Scenarios 1 and 2.
Executive Summary p. 23
5.3 Comparison Across the Three
Alternatives
The three scenarios were essentially equal in meeting the four primary
water quality and hydrology goals, as well as the administrative
feasibility and uncertainty criteria. Scenario 3 was superior, however,
in meeting the remaining three goals related to community values, cost,
and political feasibility.
Scenario 3 was
superior in meeting
the final three goals
related to
community values,
cost, and political
feasibility.
It should be noted
that applying Phase
II regulations alone
would not meet the
water quality or the
cumulative and
secondary impact
mitigation goals.
Executive Summary p. 24
By providing flexible ways to meet open space requirements onsite,
offsite, or through payment-in-lieu, Scenario 3 better met the
community values criteria of enhancing quality of life, improving air
quality, discouraging sprawl, encouraging attractive development, and
remaining economically competitive. The mitigation provisions also
made the open space requirements less costly for developers to meet
than those in Scenario 2.
The cost of meeting the Scenario 1 and 2 water quality performance
standards was perhaps the biggest issue for the Stakeholders’ Group to
resolve, given the significant cost increase they pose over meeting the
County’s existing requirements and the State’s Phase II regulations. It
should be noted, however, that applying Phase II regulations alone
would not meet the water quality or the cumulative and secondary
impact mitigation goals. By allowing the “buy down” and offsite
mitigation option, Scenario 3 reduced this cost increase while still
meeting the four primary goals. Also under Scenario 3, a portion of the
Single Family Residential land uses does not require peak control,
which noticeably reduced costs for these land uses.
Figure 5-3 compares the total, countywide costs at build-out for each
scenario using the percent of the highest cost (in this case Scenario 1)
as the unit of comparison. The total costs include construction, design,
engineering, open space, and operation and maintenance (O&M) costs.
The costs in Figure 5-3 represent the sum of costs for all changed land
use, i.e., newly developed or redeveloped land. Since the cost
estimates are based on hypothetical development designs, the estimates
should be viewed as approximate estimates of ordinance
implementation costs and should not be used to represent costs for a
specific development. Costs are likely to vary, and with new
technologies and innovation, it is likely that developers could meet
requirements with costs less than reported in this analysis. It is also
possible that implementation could be more costly than reported for
developments with particular site constraints.
The comparison of total costs across scenarios illustrates the
differences between ordinance requirements. Scenario 2 costs
approximately 93 percent of Scenario 1 costs. Scenario 3 reduces the
costs to 76 percent of Scenario 1 and presents a regulatory scenario that
September 2005
is slightly less than twice the cost of Phase II. Again, the major
regulatory change that reduced costs in Scenario 3 was the addition of
the water quality performance standard buy down option, which
considerably reduced the expected cost of developing land uses with
imperviousness above 60 percent.
100%
Percent of Highest Cost
80%
60%
40%
20%
0%
Current Regulations
Figure 5-3.
Phase II
Scenario 1
Scenario 2
Scenario 3
Comparison of Total Costs at Build-Out for Mecklenburg
County Across Five Regulatory Scenarios
The countywide cost summary masks cost differences between land
uses. Therefore, Figure 5-4 presents the total cost per acre of site for
each evaluated land use category, weighted by the total changed land
use area in Mecklenburg County projected for those categories. As in
Figure 5-3, the total costs include construction, design, engineering,
open space, and O&M costs. For each land use and treatment
combination, the cost per acre of a development site was estimated, and
then each cost per acre was area-weighted using the ratio of the area
applicable to the land use-requirement combination to the total area of
future new development or redevelopment in each land use. For
example, the Scenario 3 cost for Medium Density Residential (MDR –
30 percent imperviousness) development ($15,400 per acre) is an areaweighted average of the MDR costs in the Western Catawba, YadkinSoutheast Catawba, Central Catawba, and Goose Creek districts,
accounting for the differences in requirements among watershed
districts. The area-weighted average for MDR also accounts for the
difference in costs between MDR developments that will require peak
control versus those that do not require peak control. The areaweighted average costs provide a reasonable cost estimate for the
majority of developments in each land use.
September 2005
Executive Summary p. 25
$140,000
Current Regulations
$120,000
Phase II
Scenario 1
$100,000
Scenario 2
BMP Cost per Acre
Scenario 3
$80,000
$60,000
$40,000
$20,000
$0
Figure 5-4.
90%
85%
72%
70%
70%
66%
60%
60%
45%
41%
40%
30%
30%
19%
UHMX
COMM-H
OI-H
HMFR
HMX
IND
MX
MFR
COMM-L
HDR
INS
OI-L
MDR
MLDR
Cost Per Acre of Site, Area Weighted by Total Changed Land Use in
Mecklenburg County (excluding Huntersville)
Land Use Codes
MLDR: medium low density
residential
MDR: medium density
residential
OI-L: light office/light
industrial
INS: institutional
HDR: high density residential
COMM-L: light commercial
MFR: multi-family residential
MX: mixed urban
IND: heavy industrial
HMX: high density mixed
urban
HMFR: high density multifamily residential
OI-H: office/industrial
COMM-H: heavy commercial
UHMX: ultra high density
mixed urban
Executive Summary p. 26
The major trends across land uses in Figure 5-4 can be explained by the
major differences between the scenarios. Phase II generally costs more
than Current Regulations due to the 85 percent TSS removal and
volume control requirements. As in Figure 5-3, Scenario 1 is generally
the most expensive scenario, and Scenario 3 tends to be less costly than
Scenarios 1 and 2.
The highest cost per acre is shown for the ultra-high density mixed
urban (UHMX) and heavy commercial (COMM-H) land uses in
Scenario 1, in which these land uses would require 70 percent
phosphorus removal countywide. The UHMX and COMM-H costs
were reduced in Scenario 2 when treatment requirements in the Central
Catawba district were reduced to the minimum Phase II requirements.
Scenario 3 further reduced costs for the urban land uses by allowing
buy-down for water quality treatment requirements.
It is important to note that the land uses above 60 percent
imperviousness account for only a quarter of the future development
(or forecasted changed land use), but the magnitude of difference
September 2005
between scenarios is far greater for the more highly impervious land
uses than for the land use with lower imperviousness. For the highly
impervious land uses, the costs of additional treatment in Scenario 1
were as much as $100,000 per acre above Phase II, while land uses
with low imperviousness were as little as $7,000 per acre above Phase
II costs. With the off-site mitigation option in Scenario 3, this trend
was reduced. The differences between Scenario 3 and Phase II costs
ranged from about $10,000 to about $24,000 per acre, reflecting how
Scenario 3 requirements were chosen to be more equitable across
different land uses and types of development.
Developments with less than 60 percent imperviousness represent
about 75 percent of future projected development. As shown in Figure
5-4, implementation of Scenario 3 will cost an estimated $16,000 to
$33,000 per acre for the majority of planned land uses. The areaweighted costs per acre generally decrease from high to low
imperviousness.
Scenario 3 is the least costly combination of post-construction
requirements considering the total costs for all future development and
redevelopment. Scenario 3 is also the least expensive scenario for most
land uses and provides a more equitable cost per acre across different
land use types. According to the consensus agreement reached in the
Post-Construction Stakeholders’ Group meetings, Scenario 3 provides
the preferred balance between costs and the accomplishment of the
group’s post-construction ordinance goals.
Scenario 3
provides the
preferred balance
between costs
and the
accomplishment of
the group’s postconstruction
ordinance goals.
Since it is a consensus alternative, crafted to address the Stakeholders’
Group’s diverse concerns, Scenario 3 also better meets the goal of
political feasibility. This is essential as communities consider adoption
of the Post-Construction Stormwater Ordinance.
September 2005
Executive Summary p. 27
6. Ordinance Development
As a first step in the ordinance development process, a Stakeholders’
Group representing various interest groups was assembled. The group
included representatives from the development community,
environmental organizations, homeowners, and area residents. The
group met roughly twice a month from April 2004 to September 2005.
Deliberation of the
Ordinance language
occurred from June
9, 2005 through
September 22, 2005
and encompassed
seven full
stakeholders’
meetings and many
subcommittee
meetings.
Among the first items agreed to by the post-construction Stakeholders’
Group was the need to divide Mecklenburg County up into districts. It
was decided that a one size fits all approach to post-construction
ordinance implementation was not appropriate. Therefore, districts
were drawn based upon the need for protection and other criteria. An
example of one of the criteria used was the presence of a federally
endangered species in Goose Creek District and the Yadkin-Southeast
Catawba District, which resulted in more stringent controls on new
development. Areas with a high percentage of existing development,
such as the Central Catawba District, resulted in less stringent controls
on new development. Figure 3-3 shows the configuration of the
districts. Other factors, such as close proximity to drinking water
reservoirs, resulted in more stringent levels of control. An existing
post-construction ordinance in the Town of Huntersville necessitated
breaking out Huntersville’s ETJ as its post-construction controls were
unique. Subsequent to the recognition that certain areas of
Mecklenburg County represented unique characteristics and needs, the
Stakeholders’ Group began debating basic criteria that would provide
the foundation for the post-construction ordinance. In order to meet the
primary goals adopted to guide the ordinance development process
(Section 1.2), the stakeholders established four main categories of
control on new development and redevelopment that would be
addressed by the post-construction ordinance. The categories are
described below:
•
•
•
Executive Summary p. 28
Structural Water Quality BMPs: These controls are intended to
remove water quality pollutants from storm water runoff. The
ordinance targets pollutants such as TSS and TP.
Stream Buffers: These controls require that areas directly
adjacent to streams be set aside as natural areas. Limited
disturbance may be allowed depending on the distance from the
stream.
Volume and Peak Control: These controls require that the
additional stormwater runoff volume and peak flow rates
generated by land development activities be held back and
released slowly over time so as not to cause downstream
erosion or flooding.
September 2005
•
Open Space Requirements: These controls require that a certain
percentage of a developed site be preserved as undisturbed area
unless mitigation is provided.
Throughout the criteria selection process, the Stakeholders’ Group
received modeling analyses showing the effects of each proposed
criteria on simulated water quality conditions (Section 4.2). This
provided the group information in selection of the criteria for each
district and the effectiveness of the criteria at meeting the ordinance
goals (Section 1.2). Each district has a unique combination of the
aforementioned controls, depending upon the criteria mentioned earlier.
Table 4-1 shows the specific controls for each of the districts included
in the final ordinance.
The Post-Construction Storm Water Ordinance language was initially
developed from a model ordinance prepared by the State of North
Carolina. The initial ordinance was presented to the Stakeholders’
Group in April of 2005. Deliberation of the ordinance language
occurred from June 9, 2005 through September 22, 2005 and
encompassed seven full stakeholders’ meetings and many
subcommittee meetings. All aspects of the final draft ordinance were
agreed to by consensus of the full Stakeholders’ Group.
All aspects of the
final draft
Ordinance were
agreed to by
consensus of the
full Stakeholders’
Group.
The Post-Construction Storm Water Ordinance seeks to meet its
general purpose (satisfaction of the Ordinance Goals – Section 1.2)
through several specific objects, several of which are listed below:
•
•
•
•
•
•
•
Establish the decision making processes for new development;
Minimize the changes to site and stream hydrology, which will
help to minimize pollution, site and streambank erosion, and
downstream flooding potential;
Establish minimum post-construction storm water management
standards;
Establish design and review criteria and processes for new
development and redevelopment;
Establish criteria for the use of green-space and conservation
areas;
Establish long-term maintenance responsibility for BMPs; and,
Establish administrative procedures for the plan submission and
review.
The ordinance also defines an appeals and variance hearing process,
which empowers the Storm Water Advisory Committee (SWAC) to
hear and decide upon all appeals and variances to the post-construction
ordinance. Furthermore, the ordinance defines certain buy-down and
payment-in-lieu options, which allow developers to pay into a fund if
circumstances prohibit the onsite implementation of certain ordinance
provisions, such as open space and total phosphorus treatment. The
ordinance designates the jurisdictions responsible for maintenance of
BMPs treating runoff from single family developments.
September 2005
Executive Summary p. 29
7. Implementations Steps
and Considerations
A few of the actions
that will take place
before the
Ordinance becomes
effective include
City and County
legal review,
presentation to
elected officials,
and public hearings.
The consensus building and ordinance development process took
approximately 18 months to complete. However, there is much work to
be completed before the post-construction ordinance can take effect.
The stakeholders plan to adjourn on September 22, 2005. After this
time, City and County staff will move into the implementation phase of
the project and complete key steps in order to put the ordinance into
practice in 2006. A few of the actions that will take place before the
ordinance becomes effective include City and County legal review,
presentation to elected officials, and public hearings. If any significant
revisions to the ordinance language are required by the City and County
legal staff or elected officials, Marc Cramer (representing REBIC) and
Rick Roti (representing the Sierra Club) have agreed to be available to
represent the stakeholders on those adjustments. Additionally, the
entire Stakeholders’ Group will be invited to attend presentations to
elected officials to show support and answer questions regarding the
consensus building process. The following milestones and schedule
have been set in order for the post-construction ordinance to become
effective:
Milestone
Executive Summary p. 30
Scheduled Date
City/County staff and legal perform
extensive ordinance review
October 2005
Presentation to Town staff
October 2005
Reach consensus on any revisions based
on staff review with Marc Cramer, Rick
Roti, and etcetera
November 2005
Presentation to elected officials
December 2005 or January 2006
Public workshops
March 2006
Elected official public hearing
March 2006
Elected official approval
April 2006
Submit to state for approval
April 2006
State approval
May/June 2006
Finalize Best Management Practices
Design Manual
June 2006
Developer/Engineer training
Ongoing beginning June 2006
Contingency
90 days
Effective date
On or before October 1, 2006
September 2005
Among many others, a very important consideration to implementation
of the ordinance will be development of a BMP design manual. The
City and County will be working to deliver a design manual by the time
the ordinance takes effect. The design manual will support
recommendations from the post-construction controls Stakeholders’
Group to meet the goals of the process. The manual will provide design
guidelines for stormwater control of new development and
redevelopment that allows flexibility in meeting watershed protection
goals. This will be accomplished by 1) providing useful information
for sizing water quality BMPs to meet the required removal efficiencies
for pertinent pollutants, 2) providing specifics for plantings, vegetation
establishment, outlet control structures, slopes, and infiltration rates,
where applicable, and 3) providing an inspection and maintenance
schedule to ensure adequate long-term functionality of the facility. The
goal is to develop a useful tool for the development community that
utilizes the best national information for meeting the required
objectives of the ordinance. The City and County plan to provide
training on the use of any design aids created to support
implementation of the ordinance. It is anticipated this training will
facilitate transition into these new regulations to control stormwater
runoff.
September 2005
A very important
consideration to
implementation of
the ordinance will
be development of
a BMP design
manual that
provides design
guidelines for
stormwater control
of new
development and
redevelopment.
Executive Summary p. 31
8. References
USEPA, 2000. Storm Water Phase II Final Rule: Post-Construction
Runoff Control Measure, EPA 833-F-00-009, Fact Sheet 2.7.
Tetra Tech, Inc., 2005a. Post-Construction Ordinance Development
Phase 1 Report.
Tetra Tech, Inc., 2005b. Post-Construction Ordinance Development
Phase 2 Report (in production).
Executive Summary p. 32
September 2005