Storm Water Compliance and Characteristics for Five Coastal
Counties in Southeastern North Carolina
Jim Rabon
Pls521
November2005
Abstract
Introduction
This paper will examine the state of North Carolinas Stormwater Program in five coastal
counties. It will define the criteria of the program including state stormwater management
background, permitting, methodology, analysis, results, and compliance of permitted state
stormwater projects.
The purpose for the North Carolina State Stormwater Program is to reduce water quality
degradation resulting from increases of polluted stormwater runoff generated by the construction
of impervious surfaces. The Program was implemented in 1988 and issues permits for new
development and construction activities located in coastal areas.
Permits are required for development activities when either a CAMA (Coastal Area Management
Act) or Sedimentation and Erosion Control Plan is required. Stromwater permits place limits on
the allowable amount of impervious surface coverage and installation of stormwater treatment
system.
The effectiveness of this program relies heavily on the compliance of these permits
including the proper installation and maintenance of stormwater treatment systems. As with
other state regulatory agencies resources for conducting site evaluations of compliance and
enforcement are limited due to budgetary restraints.
In an effort to improve, efficiency of limited resources a study to identify trends in noncompliance was conducted as to better focus education, inspection, enforcement efforts and
permitting requirements. The study investigated five hundred and twenty four projects
approximately fifteen percent of the permitted projects issued by the Division of Water Quality
for Brunswick, New Hanover, Pender, Onslow, and Carteret counties from 1998 through 2002
according to state stormwater management rules. (NCEMC.1995a)
N.C. Stormwater Management Background
Rules for the state stormwater management plans (15A NCAC 2H.1000) apply to
watersheds that drain to High Quality Waters and Outstanding Resources Waters. These rules
specifically address and apply to development located within twenty coastal (CAMA) counties.
Stromwater Management rules apply to projects that are required to modify and obtain an
Erosion and Sedimentation Control Plan. Which, are issued by the Division of Land Resources
or approved local permitting program or a (CAMA) permit issued by the Division of Coastal
Management. That includes any land disturbing activity that increases the amount of impervious
surface or other wise decrease infiltration of precipitation into the soil.
There are two permitting options available: low density and high density. Both types of
permits have prerequisites for the allowable built-upon area (BUA). Built upon area is any
portion of a development project that is covered by impervious or partially pervious surface.
Low-density permits require passive stormwater controls, deed restrictions, and protective
covenants. High-density permits require engineered structural stormwater controls. A more
concise description of the permitting process is covered below.
N.C. Stromwater Management Permitting
There were three thousand six hundred and forty eight state stormwater permits for
development activities that have occurred in Brunswick, New Hanover, Pender, Onslow, and
Carteret Counties from 1998 through 2002. These permits were issued through the North
Carolina Department of Environmental and Natural Resources, Wilmington Regional Office
(WiRO). The types of permitted stormwater control options included Detention Ponds, Low-
Density, Curb Outlet, and Infiltration Systems. Different development activities are able to use
different stormwater control measures to meet prerequisite requirements stated by the North
Carolina Stormwater Management Rules. The state’s “Stormwater Best Management Practices”
manual (April 1999) provides both engineered and public officials guidance for use in the design
and maintenance requirements of the state stormwater rules. (NCDWQ 1999)
The Stormwater management Rules include provisions for the delegation of permitting to
local governments. At this time there are no delegated local programs approved to implement
these rules. All projects in this investigation received stormwater permits issued by the Division
of Water Quality.
Detention Pond Permitting
The most popular engineered stormwater treatment system for development activities
within the twenty coastal counties is the detention ponds. This is automatically required when
impervious surface exceeds thirty percent. These facilities must be designed to treat a one inch
precipitation event, unless the development activity is both high-density and within one-half mile
of and draining to tidal salt waters (NCEMC. 1995b, NCEMC. 2004). In these cases, highdensity projects are required to install infiltration systems that treat one and half inch events from
all impervious surfaces.
Although infiltration may be used in settings other than high-density for the purpose of this paper
are treated separately. The State Stormwater Management Rules allows for provisions that
provide for the permitting of other control options such that the alternative design provides equal
or better stormwater treatment or the protection of surface waters. This allows combinations of
treatment options and alternative designs available for use in certain landscapes.
Wet Detention ponds
These treatment systems are used for the temporary holding of stormwater runoff. In
order to allow at least eighty five percent of the total suspended solids (TSS) to settle out of the
water column and then slowly discharge stored water over a period of at least two and no more
than five days in order to prevent downstream erosion. This is achieved by having two levels of
storage within the storage basin. The first level is considered permanent storage. The level in the
permanent pool is the area from the base of the storage pond to the discharge orifice elevation
located on the outlet structure. The second area of the pond is considered the temporary pool.
This is the column of water above the permanent pool. Once the first inch and one half of
stormwater capacity has been met the excess water is by passed by an overflow weir. The storm
water located in the temporary
Pool is allowed to discharge through an approved outlet structure. The purpose of the detention
pond outlet structure is to slowly release the stored runoff that is held in the detention pond
directly to a water body this is for ponds designed to remove ninety percent (TSS). Ponds that are
designed to meet the minimum requirement of eighty five percent (TSS) are required to
incorporate a vegetative swale of thirty feet between the outlet structure and the receiving water
body. The design of the detention pond has evolved to work more effectively and in the
permitting, program two distinct periods: pre September 1995 and post September 1995. Ponds
built pre September 1995 were less complex in design and consisted of a basin with a three to
one grass to covered slope ratio. With an outlet structure located at a position away from the
point where water enters. This design allows (TSS) to settle out before reaching the outlet
structure. After September 1995, detention ponds are required to meet two additional design
standards including a fore bay and an aquatic shelf. A fore bay is located at the top of the pond
and is used to settle out suspended solids and reduce the amount of sediment entering the
temporary holding pond. A weir located between the fore bay and pond separates the two areas
of the basin. This design increases the life of the detention pond. The implementation of an
aquatic shelf, which is located just below the detention ponds upper banks that are sloped three
to one. The aquatic shelf is sloped six to one and slopes to a depth of two feet under the
permanent pool depth. This shelf allows for the placement of aquatic plants species in the pond
to further reduce the suspended solid levels from the discharge.
Infiltration Systems
The design of infiltration systems allows for a more natural replenishment of
groundwater, by holding the stormwater and allowing it to sink into the soil. The systems meet
the eighty five percent (TSS) removal by allowing the first inch to inch and one half of
stormwater runoff to enter the ground leaving the sediment trapped in the basin. Although
infiltration systems and trenches function alike, the designs take different approaches in treating
stormwater. The Infiltration basin is a depression in the ground in which water filters through the
soil. An infiltration trench is constructed underground and is lined and filled with gravel. The
runoff is stored in the trench and slowly seeps out to the surrounding soil leaving sediment
trapped in the trench. Both systems require that stormwater in excess of the design storm bypass
through vegetated filters without entering the stormwater control system to bypass stormwater
when inch and half of stormwater storage is met. Both use pretreatment to filter out large debris
in order to prevent clogging of the bypass structure. In the case of the infiltration basin, a
spreader mechanism is used to evenly spread the flow of water to prevent erosion damage to the
basin and surrounding banks. Maintenance issues related to infiltration basins and a trench is due
to vegetated swales that drain into them. Maintenance of these swales is necessary to prevent
them from contributing sediment to the system through erosion.
Low Density Permitting
This is permitting for development activities within the twenty coastal counties that have
impervious surfaces thirty percent or less. Coastal low-density projects are required to secure a
permit under state stormwater management rules. Stormwater runoff is required to be treated and
transported primarily by vegetated conveyances. Discrete conveyance systems: conduit and
piped systems used to transport stormwater which, are not stormwater treatment measures are not
allowed. The conveyance system must have a thirty foot vegetative buffer to surface waters is
required. All low-density facilities must be designed to treat a one-inch precipitation event,
unless the proposed development activities are within one-half mile of draining into tidal salt
waters or unnamed tributary. The built upon area must not exceed twenty five percent
(NCEMC.1995b, NCEMC.2004). Low-density stormwater treatment in low-density
development is primarily handled through vegetated conveyance systems. Roadside swales are
one way of collecting runoff in these areas. Roadside swales, which run parallel with the streets,
located through out the site. The swales are designed with a three to one rise over run slope that
reduces flow rates and hold runoff to allow for infiltration. This in turn reduces the amount of
suspended solids leaving the site. Streets are designed to direct runoff into the swales; runoff not
directed to the swales must be treated by sheet flow across grassed areas.
Curb-Outlet Permitting (Low Density Option)
Title 15A North Carolina Administrative Code 02H .01008 (g) provides for additional
low-density stormwater control option known as Curb- Outlet (NCEMC.1995c). These lowdensity projects may choose to use curb and gutter with outlets to convey stormwater to grassed
swales or vegetated areas prior to the runoff discharging to wetlands or waters.
Curb-Outlet Systems
These treatment measures use street curbs and catch basins or flumes to divert runoff to
specially designed swales located at strategic locations. These measures differ from roadside
swales in that curb-outlet requires a shallower five to one slopes. This is because curb-outlets
receive drainage from large areas. These two requirements along with the required grass
coverage, helps to reduce the flow rates in order to reduce potential erosion. Curb-outlets swales
are required to have a minimum linear length of 100 feet and be longitudinal graded at less than
0.05%. The design is intended to reduce flow rate, hold runoff for infiltration, and reduce the
amount of suspended solids leaving the site. This treatment measure is often chosen over
roadside swales for the aesthetics.
Methods
The focus of this study was on both high and low density permitted sites in five coastal
counties within the Division of Water Quality Wilmington Regional Office District: Brunswick,
Carteret, Pender, New Hanover, and Onslow Counties. A list of all permits issued between 1988
through 2002 was compiled. Control measures types separated the projects: detention ponds, low
density, and infiltration systems and grouped by county. Approximately 15% of detention ponds
and low-density projects were inspected. Statistical analysis was preformed on randomly selected
sites to enable inference about the rest of the permitted sites. In order to best target new
developments only projects of one acre or more were studied. For every detention pond, low
density, curb-outlet, and infiltration system investigated, a file review was conducted. This
enabled a close review of data that included total project area (ac), the percent of built upon area,
and the area of impervious surfaces permitted. The percent impervious acreage was calculated by
multiplying the total project area by the percent built upon area.
Analysis
For this analysis the total project area, built upon area, and impervious areas were
analyzed and sample statistics were summarized. In addition, frequency distributions for these
parameters were also provided.
The Mann- Whitney U-test (Zar 1996) was applied to the results. The hypothesis used
assumed that the respective distributions underlying the groups are the same. The hypothesis was
tested using a nonparametric version of two sample t-tests. This analysis was performed in order
to compare total project area, with the built upon area, and the impervious areas permitted.
Analysis was also preformed to compare characteristics of low-density and curb-outlet projects.
A simple linear regression of total project area versus impervious area permitted were produced
for detention pond projects as well as for low-density and curb-outlet systems (Zar 1996).
Full compliance percentages were also provided for all projects. In addition full
compliance percentages were calculated for each stormwater control measure type and for each
county investigated. The percentage of projects found in violation for reporting requirements,
built upon area violations, control measures installation violations, and maintenance violations
were provided.
Results
The Division of Water Quality of the Department of Environment and natural Resources,
Wilmington Regional Office issued approximately 3648 State Stormwater permits for
development projects from 1988 through 2002 according to state stormwater management rules
(NCEMC. 1995a). This does not take into account projects that were reviewed by the Division of
Water Quality staff and determined not to trigger state stormwater permitting requirements. 524
of the 3648 state stormwater projects permitted or 14.4% were investigated for this study. From
1988 through 2002 the mean number of permits issued by the Division of Water Quality for
Brunswick, New Hanover, Pender, Onslow, and Carteret Counties was 243.2 permits per year.
(Table 1 below)
Permits Per Year (Table 1)
Mean 243.200
Std.Dev 76.957
Minimum 64.000
Maximum 422.000
Variance 5922.314
Sum 3648.000
Median 242.000
Characteristics of State Stormwater Projects
When grouped together both detention pond and low-density projects the mean and
median total project area for a development activity are 30.3 and 12.3 acres. The median percent
built upon area permitted and the impervious acreage authorized for development activities in the
examined coastal counties are 30.0% and 3.9 acres respectively. (Table 2)
Total Project Area
Percent Build Upon Area
Impervious Area Permitted
Mean
30.372
39.803
8.389
Std. Dev
73.568
22.515
18.600
Std. Error 3.556
1.090
0.900
Count
428
427
427
Min
0.590
0.020
0.120
Max
844.000
100.000
265.200
Median
12.350
30.00
3.960
(Please note that annual acreage of Total project Area and Impervious Surface Acreage is
only a ruff estimate due to the omission of small projects below an acre).
For Brunswick, New Hanover, Pender, Onslow, and Carteret Counties, when considering
the median number of state stormwater permits issued annually (242) and the median total
project area (12.4 acres), development activities affected approximately 3000 acres of land
annually. Further, by multiplying the median impervious area by the number of permits issued,
results suggest that 968 acres of impervious surfaces are added annually to the landscape of the
southeastern coastal counties.
A frequency analysis was conducted to depict the frequency distribution. A frequency
distribution records the numerical value of a variable and how often each value occurs. An
examination of the overall shape of the distribution determines whether the sample is symmetric
or skewed and shows if there are any outliers of the distribution pattern. The frequency analysis
revealed that most projects have a total project area and impervious area of 25 acres or less.
There was a spike in areas with permitted facilities with a built upon area of 25%. These elevated
occurrences were due the fact that requirements for advanced stormwater control measures were
implemented. For projects located within one-half mile and or draining into coastal waters or
unnamed tributaries to coastal waters the low-density projects may not exceed 25% impervious
requirements.
Characteristics of Permitted State Stormwater Projects
Of the 3648 state stormwater, projects permitted by the Division of Water Quality, a total
of 524 or 14.4% were investigated for this study. Accordingly, when both detention ponds and
low-density projects are grouped together, the mean and median total project areas for a
development activity are 30.3 acres and 12.3 acres. The median percent built upon area permitted
and the impervious acreage authorized for development are 30.0% and 3.9 acres respectively. (
Table 3)
Mean
Std.Dev Std.Error Count
30.373
73.568
3.556
428
0.590
844.000
12.350
% Build upon Area 39.803
22.515
1.090
427
0.020
100.000
30.00
Impervious Area
18.600
0.900
427
0.120
265.20
3.960
Total Project Area
8.389
Min
Max
Med
For Brunswick, New Hanover, Pender, Onslow, and Carteret Counties when considering
the median number of state stormwater permits issued annually (242) and the median total
project area of 12.4 acres, development activities affect approximately 3000 acres of land
annually. By multiplying the median impervious area by the number of permits issued, results in
approximately 968 acres of impervious surfaces are added annually to the landscape of the
southeastern coastal counties.
Compliance of Permitted Stormwater Projects
(All Control System Types Combined)
For detention-ponds, low-density, curb-outlet option (low-density), and infiltration
systems combined 30.7% of the 524 projects investigated were in full compliance. These
projects were determined to have complied with permit requirements by providing The Division
of Water Quality with copies of deed restrictions and designer’s certification or reporting
requirements. Further, the site inspection revealed that the project adhered to built upon area
limitations, and stormwater control measures were satisfactorily maintained.
Detention-Pond: Stormwater Characteristics and Compliance
After review and inspection of 249 permitted detention-ponds projects in Brunswick,
New Hanover, Pender, Onslow, and Carteret Counties, an analysis of the detention-pond
permitting age revealed that the mean median permit age in this study was 5.7 years and 5.1
years respectively. (Descriptive Statistics: Stormwater Projects- Detention Pond)
Mean
Std.Dev Std.Error Count
15.377
48.685
3.434
201
% Build upon Area 57.252
20.310
1.436
Impervious Area
9.739
Total Project Area
6.517
0.689
Min
Max
Med
0.590
654.000
5.120
200
2.110
100.000
58.39
200
0.350
68.140
2.805
The mean total project area for permitted projects with detention- ponds was 15.4 acres and the
mean impervious area was 6.5 acres. The facilities with detention ponds typically have
impervious surfaces of 30% or greater. A development may be issued a high-density permit with
less than 30% impervious surface in certain circumstances. This typically occurs due to local
government jurisdiction requirements and or because the tract is an “out-parcel” to an already
existing high-density development, thereby having to comply with high-density treatment
measures. Or an alternative design that provides equal or better stormwater treatment for projects
with detention ponds the median percent built upon area was 58.4% for the five coastal counties
studied.
Detention-Pond: Compliance and Violations
Compliance for projects permitted with detention-ponds, full compliance for all 249
projects investigated were 26.9%. These projects complied with permit requirements by
providing The Division of Water Quality with copies of deed restrictions and designer’s
certification report requirements. Further, field inspections revealed whether the sites were in full
compliance with the project built upon area limitations and if stormwater measures were
properly installed under permit requirements and were satisfactorily maintained. Violations
included the requirements of the permitted project to provide The Division of Water Quality
copies of the deed restrictions and designer’s certification for the subject project. The reporting
compliance was the most common deficiency encountered. Of the 249 permitted projects 51% of
the projects were in violation of these condition requirements. Another violation area was the
built upon area violation which included projects that have built or added impervious surfaces to
a project beyond that authorized by the permit. The investigation revealed the highest rate of
compliance with built upon area requirements for detention-pond projects. Approximately 4.8%
were in violation of the built upon area limitations. The low violation percentage is attributed to
the review process in place because of the state’s stormwater management rules, and The
Division of land Resources Erosion and Sediment Control permitting requirements along with
local government approvals. These requirements generate projects that are well organized;
consultants are used for technical support. Encounters with local and state regulators are routine
due to expectations of design requirements that have prerequisites for a project to move forward.
Another violation was in the control measures installation. These measures include projects that
have either failed to install a stormwater control measure or where control measures were
installed improperly. Example a detention pond may have been constructed, but the width to
length ratio or inlet location was improperly placed, causing treatment measures to be shortcircuited. Detention pond projects studied for this violation 8.8% were in violation of control
measure installation requirements. It is anticipated that the enhanced compliance in the report
requirements will result in fewer control measures violations. For violations due to maintenance
infractions, this included projects that failed to properly, maintain stormwater control measures.
Examples were failure to remove vegetation and or removal of sediment to ensure proper
operation. For the detention-pond projects investigated, approximately 32.5% were in violation
of maintenance requirements.
Low Density (with curb-outlet option)
Stormwater characteristics and compliance
This investigation studied 269 permitted low-density projects in Brunswick, New
Hanover, Pender, Onslow, and Carteret Counties. The mean and median permit age was 6.1
years and 5.8 years respectfully. These projects included sites that have 30% or less impervious
surface, projects that chose the curb outlet option, and development have 25% or less impervious
surfaces and are located within one-half mile to coastal waters.
(Stormwater Low-density and Low-density curb outlet options)
Mean
Std.Dev Std.Error Count
Min
44.085
88.987
5.986
221
1.380
844.000
20.760
% Build upon Area 23.889
8.250
0.555
221
0.20
68.000
25.000
Impervious Area
23.979
1.623
221
0.120
265.200
4.930
Total Project Area
10.060
Max
Med
The mean total project area for low-density permitted projects was 44.1 acres and the
mean impervious area was 10.1 acres. For low-density projects, the median percent built area
was 25% for the five coastal counties studied.
Compliance and Violations for Low-Density and Curb-Outlet Options
For all 269 projects permitted with low-density and curb-outlet options, there was a
34.9% full compliance. These fully compliant projects fulfilled permit requirements by providing
The Division of Water Quality with copies of deed restrictions and reporting requirements. In
addition, field inspections reveled the sites were fully within the project built upon limitations
and that stormwater measures permitted were satisfactorily maintained. Reporting violations
included failure of the permitted to provide The Division of Water Quality with deed restrictions.
The reporting compliance parameter was the most common deficiency reported. For the 269
projects permitted for low-density 45.4% were in violation of the permit condition requirements.
Violations for projects that have built or added impervious surfaces to a project beyond that
permitted by The Division of Water Quality was 4.5% for low-density projects. Again, the low
rate is believed to be a result of the permitting review processes that are in place. Control
measure installation violations include projects that have either failed to install a stormwater
control measure or the control measure was installed improperly. Examples would be vegetated
swales were not maintained periodically or eroding had occurred due to lack of vegetative
maintenance control. For the low-density projects investigated approximately 17.3% violated
control measure installation requirements. Compared to the low-density projects without the
curb-outlet option in which approximately 14.3% were observed to be in violation of control
measure maintenance requirements.
Infiltration Systems
Infiltration systems were included in this study. Unfortunately, only six sites were
reviewed. Of the six sites, the median total project area was 11.2 acres, and the median
impervious acreage permitted was approximately 4.0 acres. These results are similar to detention
pond project characteristics. (Stormwater Infiltration Systems)
Mean
Total Project Area
Std.Dev Std.Error Count
Min
Max
Med
27.612
40.091
16.36
6
1.140
107.000
11.205
% Build upon Area 44.300
19.554
7.983
6
19.300
75.400
43.700
6
0.620
31.790
4.070
Impervious Area
9.228
11.720
4.785
None of the infiltration system projects were observed to be in full compliance. In
addition, reporting built upon area, installation, and maintenance violations were the highest of
any of the control measure categories investigated. The level of non-compliance is a significant
concern and merits specific attention to better understand the unique constraints associated with
infiltration system stormwater control measures.
Discussion
Development projects that disturb an acre or greater of land are required to secure a
Division of Land Resources issued Erosion and Sedimentation Control Plan. For development
activities within the twenty coastal counties, the applicant must also secure a State Stormwater
Management permit. Therefore, most development projects that are one acre or greater request
and receive project review pursuant to State Stormwater Management Rules (15A NCAC 2H.
1000).
A review of the percent built upon area frequency distribution indicates that the State
Stormwater Management Rules have a significant influence on the impervious surfaces that are
installed in association with development activities. Specifically, the large number of projects
that are developed at the 25% and 30% impervious surface thresholds. For high-density projects,
no limit exists on impervious acreage amounts, provided that the state stormwater control
measures are properly engineered and installed. The results revealed that the total project areas
of low-density sites are nearly four times larger than high-density projects. These observations,
coupled with the notable drop-off in the number of projects exceeding low density thresholds,
indicates that the State Stormwater Management Rules affect development densities within the
coastal counties of North Carolina. Without these rules, impervious surface areas associated with
low-density projects would increase, and the cumulative impact on coastal waters would
increase.
Compliance
The results revealed that the full compliance percentages for state stormwater projects are
low. Only 30.7% of the projects investigated were in full compliance. A closer look at the source
of the violations indicated that reporting violations, maintenance violations and stormwater
control installation violations represented the bulk of non-compliance. As far as detention ponds,
32.5% of the projects investigated contained maintenance violations, where as the low density
and low density curb-outlet projects violation rate was nearly half that amount 4.5%. Excluding
infiltration systems, approximately 5% of the projects contained built upon area violations. The
low violation percentage is likely attributed to the permitting review processes that are in place
from the State Stormwater Management Rules. These requirements force projects to be well
organized, requiring reliance on consultants and early dialogue with state and local regulators. In
turn processes ensure that expectations for design requirements are satisfied before development
projects go forward.
Infiltration systems had the highest rate of non-compliance. The deficiency merit specific
attention to better understand the unique constraints specific to infiltration system stormwater
control measures. Unfortunately the systematic random sample design