Watershed Retrofit and Management Evaluation for Urban Stormwater Management Systems in North Carolina



In response to water quality concerns in the Jordan Lake Reservoir and state and federal mandates, several cities in North Carolina are being required for the first time to reduce nutrient loads in stormwater from previously developed lands; that is, install retrofits. It is anticipated that similar requirements will become necessary for other urban areas in North Carolina. The goal of this study is to evaluate the feasibility of alternative approaches to stormwater management for existing developments within North Carolina cities. Geographic coverage of the study included a portion of the New Hope Creek watershed, located within the City of Durham in central North Carolina. The watershed was analyzed to identify potential retrofit opportunities that could be implemented to reduce pollutant loadings entering New Hope Creek and, ultimately, Jordan Lake. Current pollutant loadings generated by the watershed, as well as reductions in annual loadings of total suspended solids, total nitrogen and total phosphorus that could be achieved by implementing the identified retrofits, were estimated. Trends and relationships between land use type and the quantity and type of retrofit opportunities were identified and conclusions were drawn as to the most appropriate types of retrofits for certain land uses.

In 2005, North Carolina was the 5th fastest growing state in the country, with populations increasing by 1.7 percent between July 2004 and July 2005 and by 7.9 percent between 2000 and 2005 (U.S. Census Bureau online). Furthermore, the number of people living in the state in 2000 is projected to increase by approximately 33 percent by the year 2010 (U.S. Census Bureau online). An increase in population leads to an increase in development and infrastructure, which is directly related to the quality of surface waters draining these areas. Increases in runoff volume and velocities, coupled with the substantial amount of land disturbance required by construction, greatly increases the amount of sediment introduced to surface water bodies via erosion and channel incision (Colosimo and Wilcock 2007; Meyer 2005). In addition, numerous studies have demonstrated elevated concentrations of nutrients, such as nitrogen and phosphorus, as well as other substances such as chlorine, sulfate, and ammonium in streams draining urban areas (Biggs et al. 2004; Phillips and Bode 2004; Wheeler et al. 2005).

North Carolina is a unique state in that the barrier islands lining the coast have lead to the formation of the second largest estuary system in the United States, which drains seven of North Carolina's 17 watersheds. Estuaries are complex and fragile ecosystems, are home to a large variety of aquatic life and support the shellfish and seafood markets that are important to North Carolina's economy. Increases in urbanization throughout North Carolina and surrounding areas have led to water quality degradation in the estuaries due to increased nutrient and sediment concentrations. Excess nutrients have caused eutrophication and low oxygen levels, as well as stimulated Pfiesteria piscicida, the combination of which led to major fish kills in the 1990s. The fish kills were viewed as a threat to human health and to North Carolina's economy and prompted immediate action to address the declining quality of rivers and estuaries within the state. Strict laws and regulations were put into place to govern the quality and quantity of stormwater leaving newly developed or redeveloped sites.

While these regulations have been successful in improving water quality within the state, the ever-increasing amount of urbanization and its threat to water quality have led regulators to stricter regulations regarding stormwater runoff.

Jordan Lake was constructed in 1983 and has been classified as eutrophic or hypereutrophic ever since. New regulations established in August 2009 require all communities within the Jordan Lake watershed to reduce their nitrogen and phosphorus loadings to the lake by establishing best management practices (BMPs) to new and existing development (NCDENR 2009). It is anticipated that these new regulations will be emulated in many urban areas across the state of North Carolina. As such, it is important to understand the feasibility, both physically and economically, of implementing such practices.

The goal of this project was to evaluate the feasibility and cost-effectiveness of treating stormwater in urban areas that have already been developed. The City of Durham was chosen as the study location due to its location within the Jordan Lake watershed. The high degree of urbanization within the City of Durham makes it difficult to implement retrofit BMPs due to spatial, structural and financial constraints. Therefore, an urbanized watershed within the Jordan Lake watershed seemed an ideal place to investigate retrofit possibilities for stormwater volume and nutrient loading reductions.


With assistance from the City of Durham, a watershed of approximately 188 ha was selected for analysis. The selected watershed drains to New Hope Creek, which is located within the Cape Fear River Basin. New Hope Creek was listed on the 2006 North Carolina 303d impaired streams list due to fecal coliform bacteria, turbidity, low dissolved oxygen and biological integrity (NCDENR 2007).

Hydrology, topography, land use, tax parcel, stormwater infrastructure and aerial photography GIS data were compiled for the watershed and an initial analysis of potential stormwater retrofit opportunities was conducted. Stormwater BMPs considered in the analysis were: bioretention, wet ponds, stormwater wetlands, sand filters, green roofs, cisterns, level spreader/filter strip combinations and permeable pavement. Aerial photographs were examined in conjunction with elevation data and stormwater infrastructure to determine flow paths of stormwater. Areas that could be converted to a BMP for stormwater treatment were identified and recorded. Each potential retrofit site was visited to ground-truth the GIS data and to analyze the feasibility, estimated size and specific location for the BMP. These site visits were also used to characterize the watershed and neighborhoods and identify potential retrofit locations that were not identified via GIS analyses. The contributing drainage area for each potential BMP was estimated using in-field observations and stormwater infrastructure and topography GIS data.

The SCS Curve Number method was used to compute runoff volumes for the entire watershed and for each area draining to a potential BMP. A literature review was conducted to establish representative pollutant loadings for each land use type and these values are shown in Table 1. Expected removal rates for total suspended solids (TSS), total nitrogen (TN) and total phosphorus (TP) for each BMP type were established based on those published in the North Carolina Stormwater BMP Manual (NCDENR 2008). Using the collected data and representative loadings and removals, annual pollutant loadings entering each BMP and annual pollutant removal loads were estimated.

Table 1.  Representative pollutant loadings rates and SCS curve numbers for each land use type analyzed in the New Hope Creek watershed.
 Representative Loading RatesSCS Curve Number Method
Land Use TypeTSS (mg/L)TP (mg/L)TN (mg/L)Corresponding SCS Land Use*Curve Number
  1. *All pervious land use curve numbers based on hydrologic soil group C.

  2. Sources: Bannerman et al. (1993); Line et al. (2002); Line and White (2007); Moran (2004); NCDENR (2008); Passeport et al. (2009); Skipper (2008); Wu et al. (2008).

   Driveway1731.161.43Impervious Area98
   Roof270.151.63Impervious Area98
   Lawn26.50.442.24Open Space – Good Condition74
   Parking lot580.161.44Impervious Area98
   Roof270.151.63Impervious Area98
   Open/Landscaped26.50.442.24Open Space – Good Condition74
   Parking lot3120.391.44Impervious Area98
   Roof270.151.63Impervious Area98
   Open/Landscaped26.50.442.24Open Space – Fair Condition79
   Parking lot3261.072.02Impervious Area98
   Roof270.151.63Impervious Area98
   Open/Landscaped26.50.442.24Open Space – Good Condition74
   High density (interstate, main)2830.433.67Impervious Area98
   Low density (secondary, feeder)930.521.4Impervious Area98
   Rural300.471.14Impervious Area98
     Sidewalks1731.161.43Impervious Area98
   Woods4180.435.44Woods – Fair Condition73
   Maintained grass200.593.06Meadow – Good Condition71
   Pasture1512.144.76Pasture – Fair Condition79

Results and Discussion

Watershed Characteristics

The existing land use within the watershed is split fairly evenly among roads (25 percent), commercial/industrial (25 percent), residential (19 percent) and institutional (29 percent). Figure 1 shows the spatial distribution of different land uses throughout the watershed. The category ‘institutional’ refers to the Duke University campus, which is concentrated in the northwest part of the watershed. Parcels tagged as residential are grouped together in three major sections with commercial/industrial areas scattered throughout. New Hope Creek is piped through much of the watershed and only daylights in a few locations, the longest section being 66 m.

Figure 1.

Land use map for the New Hope Creek watershed in the City of Durham, NC.

The application of the SCS Curve Number method and the representative pollutant loadings shown in Table 1 resulted in the estimated annual stormwater volume and pollutant loadings displayed in Table 2. A total of approximately 1,076,000 L of stormwater runoff is estimated to leave the watershed and enter New Hope Creek every year, carrying with it an estimated 125,800 kg, 1,900 kg and 320 kg of TSS, TN and TP, respectively. These values equate to loadings of 670 kg/ha/yr, 10 kg/ha/yr and 1.7 kg/ha/yr of TSS, TN and TP, respectively. The Jordan Lake Nutrient Strategy rules governing pollutant loadings from existing development may require the City of Durham to reduce these loadings by up to 8 percent for TN and 5 percent for TP (NCDENR 2009).

Table 2.  Estimated annual stormwater volume and various pollutant loadings for the New Hope Creek watershed in Durham, NC.
New Hope Creek Watershed 
Total Watershed Area (ha)188
Annual Stormwater Volume (L)1,076,000
Annual TSS Loading (kg)125,800
Annual TN Loading (kg)1,900
Annual TP Loading (kg)320

Retrofit Indentification

As shown in Table 3, the most abundant type of retrofit opportunity identified was permeable pavement, which includes the replacement of existing impermeable parking lots. Permeable pavement was the preferred retrofit opportunity when bioretention would not work due to grading, utility, infrastructure or spatial constraints. When identifying potential sites for a permeable pavement retrofit, several factors were considered to determine feasibility. The slope of the parking lot was the first characteristic considered. As Durham is located within the piedmont region of North Carolina and has soils with significant clay content, permeable pavement applications must include underdrain systems. As such, a steeper slope results in greater difficulty and cost for installing a retrofit permeable pavement system. Additionally, the North Carolina Department of the Environment and Natural Resources (NCDENR) establishes a maximum slope of 0.5 percent for permeable pavement applications (NCDENR 2008). The apparent use of the parking lot was the second factor considered. The location was ruled out as a potential retrofit if the facility requires the regular presence of large trucks or heavy equipment, as permeable pavement is not designed to withstand frequent use by heavy vehicles (Hunt and Collins 2008). Finally, maintenance requirements were considered. A large amount of overhanging trees and vegetation eliminated the location from consideration, as the vegetation debris would result in intensive maintenance requirements and make the pavement more prone to clogging (Hunt and Collins 2008).

Table 3.  Number of potential retrofit BMPs identified in the New Hope Creek watershed.
BMP TypeNumber Identified
Permeable Pavement41
Green Roof15
Water Harvesting System9
Proprietary Devices4
Stormwater Wetland2
Sand Filter1
Underground Detention1
Vegetated Swale1

As bioretention provides more water quality improvement and peak flow reduction than permeable pavement, it was the preferred retrofit where physical constraints allowed for its use. The most common applications of bioretention retrofits in the New Hope Creek watershed included parking lots and commercial land uses. Bioretention areas could be placed in parking lot median strips, along the periphery of the parking lot or in place of several parking spaces. Existing utilities, grading, stormwater infrastructure and the possibility of removing parking spaces were all factors considered when determining the feasibility of implementing bioretention retrofits. It was recognized that many bioretention retrofits may be undersized due to site constraints; however, this factor was not considered when calculating potential pollutant loading reductions.

The downtown (also referred to as ultra-urban) portion of the New Hope Creek watershed offered little opportunity for retrofits due to spatial constraints. Retrofits most applicable to these areas included green roofs, underground detention and proprietary systems. Only buildings with flat roofs and initial construction dates after 1970 were considered due to the structural requirements of green roof systems. Applicable building codes prior to 1970 did not allow for the additional weight required to support a green roof system. Additionally, building deterioration may prohibit the adding of weight for a green roof. This resulted in 15 possible green roof retrofits in the watershed. Proprietary systems were suggested primarily for parking garages, where the large amount of impervious area offered good opportunities for capturing runoff, but the presence of large amounts of TSS, oil, grease, and other automobile-associated pollutants eliminated the use of water harvesting systems without some form of additional treatment. Due to high construction costs, underground detention was only considered in areas where all other BMPs were eliminated as possibilities and water quality treatment was not a priority.

Water harvesting BMPs (cisterns) were most applicable in institutional areas, as water harvested from rooftops can be used for irrigating open spaces and lawns. Potential stormwater wetland sites were identified in low-lying areas or in areas adjacent to New Hope Creek where flood waters could be captured and treated. Sand filters were considered for high traffic areas where permeable pavement and bioretention were eliminated as retrofit options due to spatial constraints.

Water Quality Benefits

Figure 2 shows the area of the New Hope Creek watershed, approximately 34 ha (18 percent), that can potentially be treated by some type of retrofit. The land use breakdown for each drainage area was determined and using the data displayed in Table 1, total annual pollutant loadings entering each BMP were calculated.

Figure 2.

New Hope Creek watershed with areas draining to potential retrofit BMPs.

In the North Carolina Stormwater BMP Manual, NCDENR establishes percent removal credits for TSS, TN and TP for bioretention, stormwater wetlands, sand filters, and vegetated swales. While research has proven there are water quantity and quality benefits associated with the other BMPs considered in this study, only the removal credits acknowledged by NCDENR were used to calculate potential benefits from the suggested retrofits. Exceptions to this are green roofs and permeable pavement, which were assigned a pollutant removal credit of 50 percent based on NCDENR's reported reduction in impervious area. NCDENR has not published quantitative runoff volume reduction percentages for stormwater BMPs; therefore, hydrologic benefits were not considered.

By applying the removal percentage to the annual incoming pollutant load, the total annual reduction in load for each pollutant was calculated. Table 4 summarizes the load reductions for BMPs with an assigned removal credit. By implementing all suggested retrofit BMPs, a total of approximately 10,309 kg of sediment, 117 kg of total nitrogen and 19 kg of total phosphorus may be intercepted before exiting the New Hope Creek watershed, resulting in a 8.2 percent, 6.0 percent and 6.0 percent decrease in annual TSS, TN and TP, respectively.

Table 4.  Assigned removal rates and resulting mass removals of TSS, TN and TP by potential retrofit BMPs.
 Percent RemovalMass of Pollutant Removed (kg)
Green Roof50/50/50167.76.70.9
Permeable Pavement50/50/501,
Sand Filter85/45/3529.50.60.1
Vegetated Swale35/20/20201.21.30.5
Total (kg) removed 10,30911719

Land Use Types vs. Quantity of Retrofits

As shown in Figure 3, there is a distinct correlation between the type of land use and the quantity of feasible retrofits. Commercial and institutional areas offer the most opportunities for retrofits, while open space and roadways offer the least.

Figure 3.

Map of New Hope Creek watershed indicating land use types and location of potential retrofit BMPs.

It is ideal that commercial areas offer the most retrofit opportunities, as they often produce the greatest pollutant loads. Treating runoff from commercial parking lots can significantly reduce the amount of pollutants leaving a given watershed. Institutional areas are not characteristically high pollutant-producing areas; however, they offer great opportunities to capture rooftop runoff and reapply it to the landscape, resulting in decreased runoff volumes and peak flows.

While it may seem that open spaces would be ideal places for implementing BMPs, they often have small drainage areas and runoff that does reach them is relatively clean due to filtration by grass and vegetation. These factors do not allow for much water quantity or quality improvement and the cost of the BMP is generally not worth the benefit. Roadways in urban areas offer many constraints, including lack of space, the presence of utilities and sanitary sewers and high traffic volumes, and are very difficult to retrofit. Within the New Hope Creek watershed, industrial areas were not easily treated with retrofit BMPs due to the lack of space and existing grading constraints. This is not expected to be a characteristic of all watersheds, especially those where industrial areas are grouped together and not interspersed among other land uses.

Land Use Types vs. Type of Retrofits

The analyses of this watershed revealed several relationships between the type of land use and the most applicable retrofit BMPs. There tends to be a large number of retrofit opportunities in commercial areas. Permeable pavement and bioretention cells are the most common choice for retrofit BMPs, as they can be fitted into a typical commercial site rather well. Sites with little to no slope are ideal for permeable pavement, as steep slopes decrease the functionality of the system and cost more to construct. Steeper slopes are treated best with bioretention.

Additionally, bioretention cells are preferred over permeable pavement for parking lots that are dilapidated. This is due to the high potential for lack of maintenance and upkeep, as well as the higher costs of permeable pavement systems. Figure 4 shows a typical shopping mall site. At this site, it was suggested that permeable pavement be considered for the stalls in the parking lot. Additionally, median strips located at the bottom of the slope (to the left of the picture) could be converted to bioretention strips.

Figure 4.

A shopping mall site that is representative

In some areas entire developments drain to one stormwater retention pond. Converting these ponds to wetlands, or incorporating wetland features, could improve their pollutant removal capabilities.

Developing a blanket statement as to which retrofit BMPs are most suitable for residential land uses is quite problematic, as the type of neighborhood has a significant impact on which BMPs would be most likely accepted and applicable. Newly built townhomes and houses on small lots (0.1–0.2 ha) are extremely hard to retrofit, as there is very little pervious area in which a BMP can be placed. Figure 5a is an example of such a neighborhood. Large houses, wide concrete driveways, wide streets with curb and gutter and small lawns generate a large amount of runoff, but leave very little room to incorporate retrofit BMPs. Houses on large lots (0.3–0.5 ha) located within older neighborhoods have large lawns that may be used to incorporate BMPs, as shown in Figure 5b.

Figure 5(a).

A new townhome community (b) A house on a large lot in an older neighborhood (c) Houses located on small lots in an older neighborhood.

In residential areas, the general affluence of the neighborhood is a factor that must be considered when evaluating what types of BMPs would be most suitable. Upper class neighborhoods tend to have paved driveways, while middle and lower class neighborhoods often have gravel driveways. A neighborhood where the majority of driveways are paved would be an ideal candidate for a rigorous, but expensive, permeable pavement incentive program. Older middle-class neighborhoods would be more suited to a rain garden implementation incentive program. Figure 5c is an example of a middle- to upper-class neighborhood that would benefit from permeable pavement and rain garden incentive programs. The majority of residents in lower-income neighborhoods may be discouraged by construction costs and maintenance requirements of retrofit BMPs and therefore would probably not be good candidates for BMP implementation programs.

An evaluation of the appearance of properties within a neighborhood can offer good indications as to the general acceptance of retrofit BMPs. Houses with elaborate and well-kept flowerbeds were considered to be more likely to embrace the implementation of a rain garden. Homes where the lawns are sparse and rarely mowed would probably not be willing to properly maintain a retrofit BMP. Other indicators of general BMP acceptance include the presence of rain barrels or cisterns, recycling bins and yard art or landscape features.

Large retrofit BMPs are typically not feasible in residential areas due to parcel size and private ownerships; however, city-sponsored programs offering cost-share options for converting paved driveways to permeable pavement or the installation of small bioretention areas (rain gardens) could be very successful and offer significant improvements in stormwater quantity and quality. While the cumulative benefits may be considerable, this type of retrofit opportunity was not included in potential loading reductions in this study.

Land belonging to a university (example shown in Figure 6) provides unique opportunities for BMP retrofits due to the university's attention to maintenance, commitment to environmental stewardship and the potential of integrating research opportunities and funds with a retrofit project. The most efficient BMP for this type of environment may be the collection of rooftop runoff to be used for irrigation purposes. The large number of buildings, as well as the accessibility to open space, lawn and flower beds makes this a preferable practice when compared to other retrofit options. Bioretention is also a practice that fits in well in an institutional location. These retrofit BMPs can mitigate stormwater while functioning as additional flowerbeds or landscape features.

Figure 6.

The campus of Duke University, representing the institutional land use type.

As mentioned previously, land functioning as open space or a park can sometimes be a good location for large-scale BMPs if there is a significant amount of land draining to it. However, grassed areas provide excellent treatment in terms of allowing infiltration and filtering sediment from runoff. Therefore, in these areas it is rare that retrofit BMPs would be cost- effective. Also, care must taken to ensure that open space areas are not intended for other land uses before designating them for retrofit BMPs.

Conclusions and Future Analyses

The pending repercussions of the Jordan Lake Nutrient Strategy compel North Carolina communities to identify ways of reducing nutrient loadings from lands draining to Jordan Lake. While it is cheapest and easiest to focus on new developments for meeting these new requirements, stringent stormwater regulations on new development alone will not reduce pollutant loads enough to meet the requirements of the Jordan Lake Nutrient Strategy. Therefore, communities must also focus on existing development and identify methods that can be used to reduce pollutant loadings from these areas. The findings and conclusions from this study give communities and regulatory agencies a place to start in the search for cost-effective retrofit opportunities within urban areas. However, there is still much to be discovered about treating stormwater in areas of existing development and this study highlights some important topics that must be addressed in future studies.

Retrofit opportunities in residential areas, including cost-sharing and incentive programs, must be analyzed further to determine the feasibility of implementing such programs. The cumulative benefits of implementing numerous small retrofit BMPs through residential areas should be quantified and included in the potential loadings reductions reported in this study.

As mentioned previously, many urban retrofit systems must be undersized due to spatial constraints. While it is not yet clear what impact this has on a BMP's ability to sufficiently treat incoming stormwater, current research at NC State University will address this topic and the results will be used in further analyses of the Durham watershed. The potential pollutant load reductions by retrofit BMPs will be updated to account for the allowed sizing of the BMP when compared to the calculated size for a recommended design storm.

The large volumes and velocities of urban stormwater result in high storm flows and flashy flow regimes in urban streams, the detriments of which are well documented in the literature. It is therefore imperative to account for the hydrologic benefits of retrofit BMPs, a task which was not performed in this study due to the lack of established volume reduction percentages by NCDENR. An extensive literature review is planned for the BMPs considered in this study to establish representative hydrologic reduction percentages which will then be used to quantify the hydrologic benefits associated with implementing retrofit BMPs in areas of existing development.

Finally, the biggest concern of communities affected by the Jordan Lake Nutrient Strategy is the economic consequences of having to reduce pollutant loadings to the lake. It is of great interest to these communities to achieve the requirements of the Strategy in the most cost-effective manner. Therefore, a full economic analysis will be performed for the New Hope Creek watershed to determine which retrofit scenarios offer the most water quality and quantity benefit at the lowest cost.

The Jordan Lake Nutrient Strategy is an aggressive, new approach to addressing the negative impacts of urban stormwater runoff. If successful, these regulations may serve as a template for other nutrient-impaired waters throughout North Carolina, making stormwater pollutant reductions in areas of existing development a statewide priority. It is anticipated that the findings from this study and future studies will be critical in helping communities develop strategies for these new regulations.

Author Bios and Contact Information

Kathy M. DeBusk is an extension associate in the Biological and Agricultural Engineering department at North Carolina State University. She received her B.S. and M.S. degrees in Biological Systems Engineering from Virginia Tech and is a certified engineering intern. Miss DeBusk is a member of Dr. Bill Hunt's stormwater engineering research group and conducts research on various stormwater management practices, helps conduct workshops and trainings, and is heavily involved in extension outreach and public education projects related to stormwater. She can be contacted at NC State University, Biological and Agricultural Engineering Department, Campus Box 7625, Raleigh, NC 27695, phone: (919) 515–8595, email: kathy_debusk@ncsu.edu.

William F. Hunt is an associate professor and extension specialist in North Carolina State University's Department of Biological and Agricultural Engineering department. Hunt holds degrees in Civil Engineering (NCSU, B.S., 1994), Economics (NCSU, B.S., 1995), Biological & Agricultural Engineering (NCSU, M.S., 1997) and Agricultural & Biological Engineering, (Penn State, Ph.D., 2003). Dr. Hunt is a registered PE in North Carolina. Since 2000, Hunt has conducted research on many types of stormwater best management practices (BMPs), including bioretention, stormwater wetlands, innovative wet ponds, green roofs, permeable pavement, water harvesting/cistern systems, and level spreaders. He teaches numerous short courses and workshops on stormwater BMP design and function throughout NC and the US. He can be contacted at NC State University, Biological and Agricultural Engineering Department, Campus Box 7625, Raleigh, NC 27695, phone: (919) 515–0185, email: wfhunt@ncsu.edu.

Upton Hatch is research professor in the Department of Agricultural and Resource Economics, North Carolina State University and professor emeritus, Auburn University. He is former president of the National Institutes for Water Resources, acting director of the North Carolina Water Resources Research Institute, director of the Auburn University Environmental Institute, and Alabama Water Resources Research Institute. His research and teaching program has focused on the economics of water resources, particularly water resource management, evaluation of environmental and natural resources, economic damage assessment, and technology assessment. He can be contacted at NC State University, Agricultural and Resource Economics, Box 8109, Raleigh, NC 27695, phone: (919) 513–0185, email: upton_hatch@ncsu.edu.

Olha Sydorovych has completed a Ph.D. in Economics at North Carolina State University. Currently, she is a researcher at the Department of Agricultural and Resource Economics at NCSU. Her areas of research interest include Agricultural, Resource, and Environmental Economics. She can be contacted at NC State University, Agricultural and Resource Economics, Box 8109, Raleigh, NC 27695, phone: (919) 513–0185, email: obsydoro@ncsu.edu.