Runoff Limits
An ecologically based stormwater management program
By
Seth J Wenger,
Tim L. Carter,
R. Alfred Vick,
Laurie A. Fowler
A recent article by David Briglio highlighted the evolution of stormwater management in the Atlanta region, culminating with an overview of the stormwater “runoff limits” program introduced as part of the proposed Etowah Aquatic Habitat Conservation Plan (Briglio 2007). The runoff limits program addresses both water-quality and -quantity issues in a single volume-based performance standard that provides an incentive for low-impact development (LID) and stormwater infiltration. In this article, we recount the genesis of the runoff limits and describe the program in detail.
Background
In the fall of 2004, the counties and municipalities of the Etowah River Basin faced a dilemma. As a global hotspot of aquatic biodiversity, the Etowah Basin (Figure 1) supported 73 native fish species, including three that were federally protected (Burkhead et al. 1997). However, the watershed—on the northern edge of the Atlanta, GA, metropolitan area—was also rapidly developing. Recent analyses had provided evidence that at least one of the protected species was extremely sensitive to increases in impervious surface cover (Wenger et al. in review). The problem appeared to be stormwater runoff: Contaminants and increased storm flows were predicted to cause habitat loss and potential extinction of the species unless these problems were somehow controlled. It seemed that the region was headed for an acrimonious collision between development and environmental protection.
The local governments were already midway through the development of a habitat conservation plan (HCP) designed to avoid precisely this kind of conflict. According to the US Endangered Species Act (ESA), the “take” (killing, harming, harassing, and even habitat alteration) of federally listed species by non-federal entities is prohibited unless the take is incidental to otherwise lawful activities and an HCP is prepared and implemented. The idea behind the proposed Etowah Aquatic HCP was for participating counties and municipalities to adopt a set of policies and regulations to minimize the impacts of new development on the protected fish species [the Etowah darter, amber darter, and Cherokee darter (Figure 2)]. This would, in turn, protect developers—including local governments engaged in development activities—from additional enforcement actions by the federal government as long as they complied with the policies and regulations. By late 2004, the participating governments that comprised the Etowah Aquatic HCP Steering Committee had agreed on standards for managing erosion from construction sites and for stream buffer protection and on certain other measures. But the key issue of stormwater runoff from impervious surfaces, considered by scientists the greatest threat to the species (Wenger and Freeman 2007), remained unresolved.
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| Figure 1. Location of the Etowah River Basin |
The Search for Workable Stormwater Policies
We were part of a team of scientists, lawyers, landscape architects, and stormwater engineers at the University of Georgia charged with advising the Etowah Aquatic HCP Steering Committee on an appropriate stormwater policy. The goal was a policy that would ensure that runoff from new development would not jeopardize the survival and recovery of the protected fish populations. We first considered whether existing regulations would meet this goal. Many of the participating jurisdictions had recently adopted post-construction stormwater ordinances consistent with the National Pollutant Discharge Elimination System (NPDES) Phase II stormwater program, under the guidance of the Metropolitan North Georgia Water Planning District (Metro District). The Metro District model ordinance (Metropolitan North Georgia Water Planning District 2004) specified four performance standards:
- Water-quality protection: Capture and treat runoff from all storm events 1.2 inches or less, as well as the first 1.2 inches of runoff for all larger storm events (together, this constitutes 85% of runoff). The treatment standard is 80% removal of total suspended solids.
- Channel protection volume: Provide 24 hours of extended detention for runoff generated by the one-year, 24-hour storm event. This provision is designed to reduce hydrologic alteration.
- Overbank flood protection: Reduce the post-development 25-year, 24-hour storm event peak discharge rate to no more than the predevelopment discharge rate.
- Extreme flood protection: Design all stormwater management facilities to safely convey the runoff from the 100-year, 24-hour storm event.
These rules are typical of the current generation of stormwater ordinances across the US. However, it was clear that although these standards could reduce the impact of runoff, they could not guarantee that those impacts would remain within the thresholds of what the most sensitive species could tolerate.
We next considered whether impervious cover itself could be limited, either through the use of impervious surface limits or through changes to zoning regulations. Impervious limits have been adopted by a number of local governments and states, primarily to protect highly sensitive watersheds and regions. For example, the City of Durham, NC, limits impervious surfaces in designated watershed protection districts to 6% to 24% of lot area (Durham Zoning Ordinance § 5.5.6), and Maryland limits imperviousness of designated Resource Conservation Areas and Limited Development Areas to 15% as part of its efforts to protect the Chesapeake Bay (Annotated Code of Maryland § 8-1808). However, research indicated that impervious limits would need to be set at less than 10% total imperviousness to protect the Etowah darter (Wenger et al. in review). This would effectively prohibit nearly all development except low-density residential, which was politically unacceptable for such a large area. Attempts to limit densities through zoning regulations would suffer from the same problem. Furthermore, on its own this approach does not provide an incentive for better stormwater management: The developer who manages the stormwater runoff from a site so that it has virtually no impact is constrained by the same impervious area limits as the developer who provides minimal stormwater management. A different approach was clearly necessary.
A Performance Standard to Encourage Infiltration and LID
Perhaps the most effective solution to the ecological impacts of stormwater runoff is to infiltrate the excess (beyond what would occur in natural conditions) back into the soil as close as possible to where it is generated, particularly for small storm events. Infiltration practices can achieve pollutant removal rates that are among the highest of any stormwater treatment best management practices (BMPs) (Walsh et al. 2004). For example, studies have shown nearly 100% removal of metals by bioretention areas (Davis et al. 2003). Maintaining near-natural levels of infiltration also minimizes hydrologic alteration to streams. Infiltration has been used extensively both in the US and abroad for many years (Ferguson 1998). Along with the minimization of impervious cover, it is a critical component of LID approaches (Williams and Wise 2006).
We reasoned that what was needed was a performance standard to limit the total volume of runoff that can leave a site as surface runoff to natural or near-natural levels. To meet this standard, developers would use infiltration and evapotranspiration BMPs. Such a standard would also encourage good site design practices that maintain forest cover and minimize both imperviousness and high-runoff lawns, because the less runoff that is generated from a site, the less that needs to be handled by structural BMPs. Most importantly, the allowable runoff could be set at a level shown to meet the needs of the most sensitive fish species. This approach would give developers and builders great flexibility, because they would be free to design sites of any density and type otherwise allowed by law, as long as they complied with the performance standard. We called this approach “runoff limits.”
Description of the Runoff Limits Program
The runoff limits program was developed between 2005 and 2006 through a series of meetings involving the project team and local engineers, developers, builders, and officials. The program was finalized in the summer of 2006 and approved by the Etowah Aquatic HCP Steering Committee in July 2006.
The performance standards of the runoff limits program are strictest in areas that support the most sensitive endangered fish species, designated Priority 1 areas (Figure 3). Areas inhabited by a less sensitive federally listed fish species are designated Priority 2, and areas that do not provide essential habitat for any of these species are designated Priority 3 and exempted from the program. Priority 1 and 2 areas have the following performance standards:
- Priority 1: For storms up to the two-year, 24-hour recurrence interval, the volume of runoff that leaves a site must not exceed the volume that would occur from the site under fully forested condition, given the soils present.
- Priority 2: For storms up to the two-year, 24-hour recurrence interval, the volume of runoff that leaves a site must not exceed the volume that would occur from the site under 5% impervious cover and 95% forested cover, given the soils present.
Because of the strictness of these standards, local governments are allowed under the HCP to designate a limited number of “development nodes” for commercial, industrial, and other high-intensity uses. Development nodes have a less strict performance standard: 50% of actual imperviousness. For example, a site with 60% total imperviousness must reduce its volume of runoff to the amount that would come from the site if it had only 30% imperviousness and the remainder forested. Nodes are selected by local governments based primarily on existing zoning and future land-use maps, subject to the constraint that they do not cause excessive impacts to the protected fish species. This is tested by running a predictive model of fish occurrence and abundance under future watershed build-out and comparing the results to a set of standards developed by the Etowah Aquatic HCP Scientific Advisory Committee. (Details are available in the Draft Etowah HCP, available at www.etowahhcp.org.) Local governments are free to adjust development nodes in the future as long as modeling indicates these standards will be met.
Volume calculations are made by determining curve numbers using the TR-55 method (Soil Conservation Service 1975), which was selected by the technical committee because it is widely and easily used by the engineering community. Applying the performance standard to a site is a three-step process:
- Calculate the volume of runoff from the site using the curve number of a forest in good condition for the regional two-year, 24-hour design storm.
- Calculate the volume of runoff from the site using the curve numbers of the post-development conditions for the regional two-year, 24-hour design storm.
- Subtract the forested volume from the post-development volume. This is the volume of runoff that must be managed through BMPs for infiltration and evapotranspiration.
The above procedure applies to Priority 1 areas. The same procedure is used for Priority 2 areas and development nodes, except that the calculation in step 1 is modified accordingly.
In addition, the runoff limits program includes two auxiliary requirements for the runoff generated by small storm events. The first is that all post-development runoff generated by impervious surfaces must be directed through stormwater BMPs. Without this requirement, the primary performance standard could be met by over-infiltrating stormwater from a portion of the site and allowing stormwater from another portion of the site to flow off untreated. With this requirement, all runoff must be directed to BMPs and given the opportunity to infiltrate, eliminating bypass of small storms (there will still be overflow from large and back-to-back storms). The second requirement is that BMPs should be distributed throughout the site to the extent practicable, to avoid concentrating excessive volumes in one or a few locations.
To meet the standards, developers can choose from a large menu of BMPs for infiltration and (to a lesser extent) evapotranspiration described in the draft Etowah HCP Runoff Limits Manual (Carter et al. 2007). These include bioretention areas, infiltration trenches, subsurface infiltration beds, green roofs, dry wells, infiltration basins, porous pavements, and variations of these (Figure 4). The cost of implementing these structural stormwater BMPs, while not necessarily higher than for conventional BMPs, creates a significant incentive for developers to design or redesign sites to minimize impervious surfaces and to maximize forest cover. For this reason, the runoff limits program can be viewed as a performance-based approach to low-impact development. A developer can choose to create either a low-impact site design that requires fewer structural BMPs or a conventional site design that requires more structural BMPs. The feasibility of the runoff limits was demonstrated to local stakeholders through a design study, which showed that a development of typical density could meet the requirements through the use of BMPs with no loss of buildable lots (Vick 2006).
Many of the local governments participating in the Etowah HCP are obligated by state law to adopt ordinances consistent with the Metro District model ordinance, which was described above. For this reason, the runoff limits performance standard was integrated into the Metro District ordinance as a fifth standard that must be met for development within Priority 1 and Priority 2 areas. This generates some redundancy, because the runoff limit standard plays the same role as the contaminant removal and channel protection volume standards. Under most conditions, however, a stormwater system designed to meet runoff limits standards will also meet the other performance standards, without the need for additional structures or measures (except in development nodes, where a hybrid system is required). The Etowah HCP model stormwater ordinance incorporating the runoff limits program is available at www.etowahhcp.org.
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| Figure 3. Priority areas and development nodes in the Etowah Basin |
Issues and Concerns
The runoff limits program was developed with extensive participation of consulting engineers, local government officials, developers, and other interested stakeholders. In meetings held over the course of a year, participants raised concerns and identified real or perceived barriers to implementation of the program, which were subsequently addressed. Many of these are general issues likely to surface in other localities where a volume control performance standard is under consideration.
Soils
The first response of many engineers to a volume-based standard is that “infiltration is impossible with our soils.” Certainly, soils vary greatly in their properties, and some are relatively poor for infiltration. However, the proposed performance standard takes into consideration existing soil type in the calculation of predevelopment runoff volumes. The less pervious the soil type, the higher the predevelopment runoff volume and the lower the volume that must be infiltrated. Of course, there are some conditions that preclude the use of infiltration: bedrock very close to the surface, water table very close to the surface, or soils with infiltration rates less than 0.1 inch per hour. Infiltration BMPs should not be sited in such locations, and the runoff limits program provides a variance procedure for sites where 80% or more of the area is characterized by such conditions. Under the program, this is the only grounds for a variance, however—a necessary condition considering the need to protect federally listed fish.
Cost
In some cases, the cost of a system of infiltration BMPs can be substantially more than that of a conventional stormwater detention pond (Nelson 2006). Conversely, the cost advantage of an infiltration system is that it can eliminate the need for much of the conveyance network. Piping can be a major cost of a stormwater management system, and once this is taken into account, a system of infiltration BMPs can be less costly than a conventional system with a detention pond and extensive piping (Conservation Research Institute 2005). We estimated that under a worst-case scenario where both infiltration BMPs and conventional BMPs are constructed side by side, adding infiltration may add about $1,100 per home for residential development (Nelson 2006).
The Question of Baseline for Calculating Runoff Volumes
A significant topic of debate among the consulting engineers and developers who helped guide the creation of the runoff limits program was whether to use existing condition or forested condition as the baseline for calculating runoff volumes. In the area covered by the runoff limits program, forest was the dominant land cover prior to intensive cultural modification, although substantial portions of the basin have been in agricultural use for two centuries. Many stakeholders questioned whether it was fair to use forested condition as the baseline, because this would require a developer of pasture or cropland to increase the amount of infiltration over what was occurring under existing conditions. Ultimately the decision was made to use a forested baseline because it was more protective of the species and eliminated many potential avenues for abuse, such as speculative clearing. However, if a site contains sections that will not be disturbed, the runoff limits program allows developers to draw the boundaries of the development area around these locations and exclude them from calculations. For example, an existing farmhouse and a section of pasture to be maintained as greenspace could be excluded from the project area, eliminating the need to reduce runoff from these parts of the property.
Inspections and Maintenance
A central concern of local governments was identifying who would be responsible for inspection and maintenance of the many small, distributed stormwater BMPs encouraged by the runoff limits program. Government staff expressed skepticism over the additional workload, particularly in jurisdictions already strapped for staffing resources. On the other end of the spectrum was concern that the private individual or homeowners’ association would not adequately maintain the BMPs. The committee ultimately chose to allow local governments to decide which option would work best in each of their jurisdictions. Therefore, the runoff limits program allows either local governments or private entities such as homeowners’ associations to hold maintenance responsibility for the BMPs. The committee decided that inspections would be performed by local jurisdictions, with the frequency conditional on the size of the BMP: BMPs that serve small areas are to be inspected every three years, and larger BMPs are to be inspected on an annual schedule. Inspection programs will be supported by regional staff funded by an $8-per-disturbed-acre HCP implementation fee.
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| Figure 4. Examples of infiltration best management practices described in the Runoff Limits Manual. Above: Bioretention in a residential landscape. Below: Bioretention in a commercial landscape |
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| Above: Infiltration trench. Below: Green roof |
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Development Node Standards
Development nodes are a critical component of the runoff limits program because they allow for designated high-density locations to have less restrictive performance standards. There was some debate as to how to set these standards, however. The “50% reduction” standard was selected because modeling results showed that given the distribution of nodes selected by local governments, this standard would not jeopardize the survival and recovery of protected fish species. If future research indicates otherwise, the adaptive management program of the Etowah Aquatic HCP allows the node standard and the size and locations of undeveloped nodes to be modified as necessary, with consent of the local governments. Other approaches to managing runoff from development nodes were considered but were not adopted due to lack of evidence that they would be sufficiently protective of fish populations. For example, it is possible that the Metro District standards alone may provide sufficient reduction of runoff impacts within nodes, but this has yet to be demonstrated. Another option proposed by consulting engineers was to allow developers within nodes to pay “in-lieu-of” fees rather than meet the runoff limits standards; these fees would go toward retrofitting existing development or protecting land. Both of these options are included in the Etowah Aquatic HCP Adaptive Management Plan as potential modifications of the runoff limits program, should future research demonstrate that they can be implemented without jeopardizing survival and recovery of the protected fish species.
Similar Programs and Recommendations for Programs
This is not the first volume control performance standard to be developed. One of the earliest was a provision within Maryland’s Chesapeake Bay Critical Area Act, adopted in 1984. Among other requirements, the act mandates that, within designated development areas and resource conservation areas, “development may not cause downstream property, watercourses, channels or conduits to receive stormwater runoff at a higher volume or rate than would have resulted from a two- or ten-year storm, whichever is more restrictive, were the land in its predevelopment state” (Code of Maryland Regulations § 27.01.02). To properly meet this standard requires matching predevelopment runoff volumes. Another example is from Huntersville, NC. Huntersville requires that LID or a combination of LID and conventional stormwater BMPs be used to control and treat the increase in runoff volume from predevelopment conditions for the two-year, 24-hour storm in “rural” and “transitional” zoning districts (City of Huntersville, NC, Zoning Ordinance § 8.17). The number of such programs is also increasing: The Commonwealth of Pennsylvania recently adopted a recommended volume control standard to its statewide stormwater manual (Pennsylvania Department of Environmental Protection 2006).
If there are protected species present in a watershed, a program such as the runoff limits can provide assurance that the impacts of future development will not exceed the tolerances of the species. We believe, however, that a volume control standard has wide applicability for stormwater management nationwide, even in locations without imperiled species concerns. This type of standard keeps stormwater regulations simple by creating one small-storm standard encompassing both water-quality and -quantity concerns. This, in turn, can reduce the hydrologic calculations, paperwork, and plan review time. Additionally, the volume control approach allows flexibility for both the developer and the local jurisdiction. Because the actual volume of runoff is targeted, rather than the amount of development, there are virtually unlimited options for the development community to manage stormwater onsite. Local jurisdictions may choose to set the volume standard based on the sensitivity of the watershed or its fauna or based on the desired state of ecological integrity. In the case of the runoff limits program, forested conditions, 5% impervious area, and 50% of actual impervious area were used as baseline standards. Other programs may wish to employ a standard equivalent to the oft-cited threshold of 10% impervious cover.
As of this writing, the Etowah Aquatic HCP is still under review by the Fish and Wildlife Service (FWS), so local governments have not yet implemented the runoff limits program. However, the FWS has adopted the general requirements of the runoff limits program as the “best available science” for protecting the listed fish species of the Etowah. Since the development of the standards, the FWS has reviewed more than 100 development projects in the region and typically recommends stormwater management measures consistent with the runoff limits program as part of the “reasonable and prudent measures” to prevent or minimize impacts to listed species (Eric Prowell, USFWS, personal communication). These recommendations are generally followed by those who consult with FWS, providing a partial implementation of the runoff limits program while the Etowah Aquatic HCP goes through its review process.
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Conclusions
The runoff limits program of the Etowah Aquatic HCP is a unique integration of science, policy, and practice. Extensive field-collected data and subsequent analysis established the tolerance of protected fish species to the impacts of human development, in this case increased runoff from impervious surfaces in the watershed. An inclusive process was used to develop the runoff limits program that meets the ecological needs of the species and unites stakeholders around achievable, flexible standards. The support of local governments and oversight by the federal government will ensure implementation.
The required standards are achievable using proven stormwater management practices, most of which have already been successfully implemented in the metro-Atlanta region and all of which have been successfully implemented in other regions of the country. The crafting of the program as performance standards allows for flexibility and innovation in the design and implementation, rewarding stormwater professionals and land planners for implementing low-impact development strategies throughout the design process, not just as an afterthought to manage runoff. The runoff limits program is already shifting the practice of stormwater management in the region to a more sustainable approach, demonstrating that it is possible to develop the land without sacrificing fish species.
Author's Bio: Seth J. Wenger, Ph.D., is with the University of Georgia River Basin Center.
Author's Bio: Tim L. Carter, Ph.D., is with the University of Georgia River Basin Center.
Author's Bio: R. Alfred Vick, ASLA, LEED AP, is with the University of Georgia School of Environmental Design.
Author's Bio: Laurie A. Fowler, J.D., L.L.M., is with the University of Georgia River Basin Center.
March-April 2008
Runoff Limits
An ecologically based stormwater management program
By
Seth J Wenger,
Tim L. Carter,
R. Alfred Vick,
Laurie A. Fowler
A recent article by David Briglio highlighted the evolution of stormwater management in the Atlanta region, culminating with an overview of the stormwater “runoff limits” program introduced as part of the proposed Etowah Aquatic Habitat Conservation Plan (Briglio 2007). The runoff limits program addresses both water-quality and -quantity issues in a single volume-based performance standard that provides an incentive for low-impact development (LID) and stormwater infiltration. In this article, we recount the genesis of the runoff limits and describe the program in detail.
Background
In the fall of 2004, the counties and municipalities of the Etowah River Basin faced a dilemma. As a global hotspot of aquatic biodiversity, the Etowah Basin (Figure 1) supported 73 native fish species, including three that were federally protected (Burkhead et al. 1997). However, the watershed—on the northern edge of the Atlanta, GA, metropolitan area—was also rapidly developing. Recent analyses had provided evidence that at least one of the protected species was extremely sensitive to increases in impervious surface cover (Wenger et al. in review). The problem appeared to be stormwater runoff: Contaminants and increased storm flows were predicted to cause habitat loss and potential extinction of the species unless these problems were somehow controlled. It seemed that the region was headed for an acrimonious collision between development and environmental protection.
The local governments were already midway through the development of a habitat conservation plan (HCP) designed to avoid precisely this kind of conflict. According to the US Endangered Species Act (ESA), the “take” (killing, harming, harassing, and even habitat alteration) of federally listed species by non-federal entities is prohibited unless the take is incidental to otherwise lawful activities and an HCP is prepared and implemented. The idea behind the proposed Etowah Aquatic HCP was for participating counties and municipalities to adopt a set of policies and regulations to minimize the impacts of new development on the protected fish species [the Etowah darter, amber darter, and Cherokee darter (Figure 2)]. This would, in turn, protect developers—including local governments engaged in development activities—from additional enforcement actions by the federal government as long as they complied with the policies and regulations. By late 2004, the participating governments that comprised the Etowah Aquatic HCP Steering Committee had agreed on standards for managing erosion from construction sites and for stream buffer protection and on certain other measures. But the key issue of stormwater runoff from impervious surfaces, considered by scientists the greatest threat to the species (Wenger and Freeman 2007), remained unresolved.
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| Figure 1. Location of the Etowah River Basin |
The Search for Workable Stormwater Policies
We were part of a team of scientists, lawyers, landscape architects, and stormwater engineers at the University of Georgia charged with advising the Etowah Aquatic HCP Steering Committee on an appropriate stormwater policy. The goal was a policy that would ensure that runoff from new development would not jeopardize the survival and recovery of the protected fish populations. We first considered whether existing regulations would meet this goal. Many of the participating jurisdictions had recently adopted post-construction stormwater ordinances consistent with the National Pollutant Discharge Elimination System (NPDES) Phase II stormwater program, under the guidance of the Metropolitan North Georgia Water Planning District (Metro District). The Metro District model ordinance (Metropolitan North Georgia Water Planning District 2004) specified four performance standards:
- Water-quality protection: Capture and treat runoff from all storm events 1.2 inches or less, as well as the first 1.2 inches of runoff for all larger storm events (together, this constitutes 85% of runoff). The treatment standard is 80% removal of total suspended solids.
- Channel protection volume: Provide 24 hours of extended detention for runoff generated by the one-year, 24-hour storm event. This provision is designed to reduce hydrologic alteration.
- Overbank flood protection: Reduce the post-development 25-year, 24-hour storm event peak discharge rate to no more than the predevelopment discharge rate.
- Extreme flood protection: Design all stormwater management facilities to safely convey the runoff from the 100-year, 24-hour storm event.
These rules are typical of the current generation of stormwater ordinances across the US. However, it was clear that although these standards could reduce the impact of runoff, they could not guarantee that those impacts would remain within the thresholds of what the most sensitive species could tolerate.
We next considered whether impervious cover itself could be limited, either through the use of impervious surface limits or through changes to zoning regulations. Impervious limits have been adopted by a number of local governments and states, primarily to protect highly sensitive watersheds and regions. For example, the City of Durham, NC, limits impervious surfaces in designated watershed protection districts to 6% to 24% of lot area (Durham Zoning Ordinance § 5.5.6), and Maryland limits imperviousness of designated Resource Conservation Areas and Limited Development Areas to 15% as part of its efforts to protect the Chesapeake Bay (Annotated Code of Maryland § 8-1808). However, research indicated that impervious limits would need to be set at less than 10% total imperviousness to protect the Etowah darter (Wenger et al. in review). This would effectively prohibit nearly all development except low-density residential, which was politically unacceptable for such a large area. Attempts to limit densities through zoning regulations would suffer from the same problem. Furthermore, on its own this approach does not provide an incentive for better stormwater management: The developer who manages the stormwater runoff from a site so that it has virtually no impact is constrained by the same impervious area limits as the developer who provides minimal stormwater management. A different approach was clearly necessary.
A Performance Standard to Encourage Infiltration and LID
Perhaps the most effective solution to the ecological impacts of stormwater runoff is to infiltrate the excess (beyond what would occur in natural conditions) back into the soil as close as possible to where it is generated, particularly for small storm events. Infiltration practices can achieve pollutant removal rates that are among the highest of any stormwater treatment best management practices (BMPs) (Walsh et al. 2004). For example, studies have shown nearly 100% removal of metals by bioretention areas (Davis et al. 2003). Maintaining near-natural levels of infiltration also minimizes hydrologic alteration to streams. Infiltration has been used extensively both in the US and abroad for many years (Ferguson 1998). Along with the minimization of impervious cover, it is a critical component of LID approaches (Williams and Wise 2006).
We reasoned that what was needed was a performance standard to limit the total volume of runoff that can leave a site as surface runoff to natural or near-natural levels. To meet this standard, developers would use infiltration and evapotranspiration BMPs. Such a standard would also encourage good site design practices that maintain forest cover and minimize both imperviousness and high-runoff lawns, because the less runoff that is generated from a site, the less that needs to be handled by structural BMPs. Most importantly, the allowable runoff could be set at a level shown to meet the needs of the most sensitive fish species. This approach would give developers and builders great flexibility, because they would be free to design sites of any density and type otherwise allowed by law, as long as they complied with the performance standard. We called this approach “runoff limits.”
Description of the Runoff Limits Program
The runoff limits program was developed between 2005 and 2006 through a series of meetings involving the project team and local engineers, developers, builders, and officials. The program was finalized in the summer of 2006 and approved by the Etowah Aquatic HCP Steering Committee in July 2006.
The performance standards of the runoff limits program are strictest in areas that support the most sensitive endangered fish species, designated Priority 1 areas (Figure 3). Areas inhabited by a less sensitive federally listed fish species are designated Priority 2, and areas that do not provide essential habitat for any of these species are designated Priority 3 and exempted from the program. Priority 1 and 2 areas have the following performance standards:
- Priority 1: For storms up to the two-year, 24-hour recurrence interval, the volume of runoff that leaves a site must not exceed the volume that would occur from the site under fully forested condition, given the soils present.
- Priority 2: For storms up to the two-year, 24-hour recurrence interval, the volume of runoff that leaves a site must not exceed the volume that would occur from the site under 5% impervious cover and 95% forested cover, given the soils present.
Because of the strictness of these standards, local governments are allowed under the HCP to designate a limited number of “development nodes” for commercial, industrial, and other high-intensity uses. Development nodes have a less strict performance standard: 50% of actual imperviousness. For example, a site with 60% total imperviousness must reduce its volume of runoff to the amount that would come from the site if it had only 30% imperviousness and the remainder forested. Nodes are selected by local governments based primarily on existing zoning and future land-use maps, subject to the constraint that they do not cause excessive impacts to the protected fish species. This is tested by running a predictive model of fish occurrence and abundance under future watershed build-out and comparing the results to a set of standards developed by the Etowah Aquatic HCP Scientific Advisory Committee. (Details are available in the Draft Etowah HCP, available at www.etowahhcp.org.) Local governments are free to adjust development nodes in the future as long as modeling indicates these standards will be met.
Volume calculations are made by determining curve numbers using the TR-55 method (Soil Conservation Service 1975), which was selected by the technical committee because it is widely and easily used by the engineering community. Applying the performance standard to a site is a three-step process:
- Calculate the volume of runoff from the site using the curve number of a forest in good condition for the regional two-year, 24-hour design storm.
- Calculate the volume of runoff from the site using the curve numbers of the post-development conditions for the regional two-year, 24-hour design storm.
- Subtract the forested volume from the post-development volume. This is the volume of runoff that must be managed through BMPs for infiltration and evapotranspiration.
The above procedure applies to Priority 1 areas. The same procedure is used for Priority 2 areas and development nodes, except that the calculation in step 1 is modified accordingly.
In addition, the runoff limits program includes two auxiliary requirements for the runoff generated by small storm events. The first is that all post-development runoff generated by impervious surfaces must be directed through stormwater BMPs. Without this requirement, the primary performance standard could be met by over-infiltrating stormwater from a portion of the site and allowing stormwater from another portion of the site to flow off untreated. With this requirement, all runoff must be directed to BMPs and given the opportunity to infiltrate, eliminating bypass of small storms (there will still be overflow from large and back-to-back storms). The second requirement is that BMPs should be distributed throughout the site to the extent practicable, to avoid concentrating excessive volumes in one or a few locations.
To meet the standards, developers can choose from a large menu of BMPs for infiltration and (to a lesser extent) evapotranspiration described in the draft Etowah HCP Runoff Limits Manual (Carter et al. 2007). These include bioretention areas, infiltration trenches, subsurface infiltration beds, green roofs, dry wells, infiltration basins, porous pavements, and variations of these (Figure 4). The cost of implementing these structural stormwater BMPs, while not necessarily higher than for conventional BMPs, creates a significant incentive for developers to design or redesign sites to minimize impervious surfaces and to maximize forest cover. For this reason, the runoff limits program can be viewed as a performance-based approach to low-impact development. A developer can choose to create either a low-impact site design that requires fewer structural BMPs or a conventional site design that requires more structural BMPs. The feasibility of the runoff limits was demonstrated to local stakeholders through a design study, which showed that a development of typical density could meet the requirements through the use of BMPs with no loss of buildable lots (Vick 2006).
Many of the local governments participating in the Etowah HCP are obligated by state law to adopt ordinances consistent with the Metro District model ordinance, which was described above. For this reason, the runoff limits performance standard was integrated into the Metro District ordinance as a fifth standard that must be met for development within Priority 1 and Priority 2 areas. This generates some redundancy, because the runoff limit standard plays the same role as the contaminant removal and channel protection volume standards. Under most conditions, however, a stormwater system designed to meet runoff limits standards will also meet the other performance standards, without the need for additional structures or measures (except in development nodes, where a hybrid system is required). The Etowah HCP model stormwater ordinance incorporating the runoff limits program is available at www.etowahhcp.org.
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| Figure 3. Priority areas and development nodes in the Etowah Basin |
Issues and Concerns
The runoff limits program was developed with extensive participation of consulting engineers, local government officials, developers, and other interested stakeholders. In meetings held over the course of a year, participants raised concerns and identified real or perceived barriers to implementation of the program, which were subsequently addressed. Many of these are general issues likely to surface in other localities where a volume control performance standard is under consideration.
Soils
The first response of many engineers to a volume-based standard is that “infiltration is impossible with our soils.” Certainly, soils vary greatly in their properties, and some are relatively poor for infiltration. However, the proposed performance standard takes into consideration existing soil type in the calculation of predevelopment runoff volumes. The less pervious the soil type, the higher the predevelopment runoff volume and the lower the volume that must be infiltrated. Of course, there are some conditions that preclude the use of infiltration: bedrock very close to the surface, water table very close to the surface, or soils with infiltration rates less than 0.1 inch per hour. Infiltration BMPs should not be sited in such locations, and the runoff limits program provides a variance procedure for sites where 80% or more of the area is characterized by such conditions. Under the program, this is the only grounds for a variance, however—a necessary condition considering the need to protect federally listed fish.
Cost
In some cases, the cost of a system of infiltration BMPs can be substantially more than that of a conventional stormwater detention pond (Nelson 2006). Conversely, the cost advantage of an infiltration system is that it can eliminate the need for much of the conveyance network. Piping can be a major cost of a stormwater management system, and once this is taken into account, a system of infiltration BMPs can be less costly than a conventional system with a detention pond and extensive piping (Conservation Research Institute 2005). We estimated that under a worst-case scenario where both infiltration BMPs and conventional BMPs are constructed side by side, adding infiltration may add about $1,100 per home for residential development (Nelson 2006).
The Question of Baseline for Calculating Runoff Volumes
A significant topic of debate among the consulting engineers and developers who helped guide the creation of the runoff limits program was whether to use existing condition or forested condition as the baseline for calculating runoff volumes. In the area covered by the runoff limits program, forest was the dominant land cover prior to intensive cultural modification, although substantial portions of the basin have been in agricultural use for two centuries. Many stakeholders questioned whether it was fair to use forested condition as the baseline, because this would require a developer of pasture or cropland to increase the amount of infiltration over what was occurring under existing conditions. Ultimately the decision was made to use a forested baseline because it was more protective of the species and eliminated many potential avenues for abuse, such as speculative clearing. However, if a site contains sections that will not be disturbed, the runoff limits program allows developers to draw the boundaries of the development area around these locations and exclude them from calculations. For example, an existing farmhouse and a section of pasture to be maintained as greenspace could be excluded from the project area, eliminating the need to reduce runoff from these parts of the property.
Inspections and Maintenance
A central concern of local governments was identifying who would be responsible for inspection and maintenance of the many small, distributed stormwater BMPs encouraged by the runoff limits program. Government staff expressed skepticism over the additional workload, particularly in jurisdictions already strapped for staffing resources. On the other end of the spectrum was concern that the private individual or homeowners’ association would not adequately maintain the BMPs. The committee ultimately chose to allow local governments to decide which option would work best in each of their jurisdictions. Therefore, the runoff limits program allows either local governments or private entities such as homeowners’ associations to hold maintenance responsibility for the BMPs. The committee decided that inspections would be performed by local jurisdictions, with the frequency conditional on the size of the BMP: BMPs that serve small areas are to be inspected every three years, and larger BMPs are to be inspected on an annual schedule. Inspection programs will be supported by regional staff funded by an $8-per-disturbed-acre HCP implementation fee.
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| Figure 4. Examples of infiltration best management practices described in the Runoff Limits Manual. Above: Bioretention in a residential landscape. Below: Bioretention in a commercial landscape |
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| Above: Infiltration trench. Below: Green roof |
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Development Node Standards
Development nodes are a critical component of the runoff limits program because they allow for designated high-density locations to have less restrictive performance standards. There was some debate as to how to set these standards, however. The “50% reduction” standard was selected because modeling results showed that given the distribution of nodes selected by local governments, this standard would not jeopardize the survival and recovery of protected fish species. If future research indicates otherwise, the adaptive management program of the Etowah Aquatic HCP allows the node standard and the size and locations of undeveloped nodes to be modified as necessary, with consent of the local governments. Other approaches to managing runoff from development nodes were considered but were not adopted due to lack of evidence that they would be sufficiently protective of fish populations. For example, it is possible that the Metro District standards alone may provide sufficient reduction of runoff impacts within nodes, but this has yet to be demonstrated. Another option proposed by consulting engineers was to allow developers within nodes to pay “in-lieu-of” fees rather than meet the runoff limits standards; these fees would go toward retrofitting existing development or protecting land. Both of these options are included in the Etowah Aquatic HCP Adaptive Management Plan as potential modifications of the runoff limits program, should future research demonstrate that they can be implemented without jeopardizing survival and recovery of the protected fish species.
Similar Programs and Recommendations for Programs
This is not the first volume control performance standard to be developed. One of the earliest was a provision within Maryland’s Chesapeake Bay Critical Area Act, adopted in 1984. Among other requirements, the act mandates that, within designated development areas and resource conservation areas, “development may not cause downstream property, watercourses, channels or conduits to receive stormwater runoff at a higher volume or rate than would have resulted from a two- or ten-year storm, whichever is more restrictive, were the land in its predevelopment state” (Code of Maryland Regulations § 27.01.02). To properly meet this standard requires matching predevelopment runoff volumes. Another example is from Huntersville, NC. Huntersville requires that LID or a combination of LID and conventional stormwater BMPs be used to control and treat the increase in runoff volume from predevelopment conditions for the two-year, 24-hour storm in “rural” and “transitional” zoning districts (City of Huntersville, NC, Zoning Ordinance § 8.17). The number of such programs is also increasing: The Commonwealth of Pennsylvania recently adopted a recommended volume control standard to its statewide stormwater manual (Pennsylvania Department of Environmental Protection 2006).
If there are protected species present in a watershed, a program such as the runoff limits can provide assurance that the impacts of future development will not exceed the tolerances of the species. We believe, however, that a volume control standard has wide applicability for stormwater management nationwide, even in locations without imperiled species concerns. This type of standard keeps stormwater regulations simple by creating one small-storm standard encompassing both water-quality and -quantity concerns. This, in turn, can reduce the hydrologic calculations, paperwork, and plan review time. Additionally, the volume control approach allows flexibility for both the developer and the local jurisdiction. Because the actual volume of runoff is targeted, rather than the amount of development, there are virtually unlimited options for the development community to manage stormwater onsite. Local jurisdictions may choose to set the volume standard based on the sensitivity of the watershed or its fauna or based on the desired state of ecological integrity. In the case of the runoff limits program, forested conditions, 5% impervious area, and 50% of actual impervious area were used as baseline standards. Other programs may wish to employ a standard equivalent to the oft-cited threshold of 10% impervious cover.
As of this writing, the Etowah Aquatic HCP is still under review by the Fish and Wildlife Service (FWS), so local governments have not yet implemented the runoff limits program. However, the FWS has adopted the general requirements of the runoff limits program as the “best available science” for protecting the listed fish species of the Etowah. Since the development of the standards, the FWS has reviewed more than 100 development projects in the region and typically recommends stormwater management measures consistent with the runoff limits program as part of the “reasonable and prudent measures” to prevent or minimize impacts to listed species (Eric Prowell, USFWS, personal communication). These recommendations are generally followed by those who consult with FWS, providing a partial implementation of the runoff limits program while the Etowah Aquatic HCP goes through its review process.
Conclusions
The runoff limits program of the Etowah Aquatic HCP is a unique integration of science, policy, and practice. Extensive field-collected data and subsequent analysis established the tolerance of protected fish species to the impacts of human development, in this case increased runoff from impervious surfaces in the watershed. An inclusive process was used to develop the runoff limits program that meets the ecological needs of the species and unites stakeholders around achievable, flexible standards. The support of local governments and oversight by the federal government will ensure implementation.
The required standards are achievable using proven stormwater management practices, most of which have already been successfully implemented in the metro-Atlanta region and all of which have been successfully implemented in other regions of the country. The crafting of the program as performance standards allows for flexibility and innovation in the design and implementation, rewarding stormwater professionals and land planners for implementing low-impact development strategies throughout the design process, not just as an afterthought to manage runoff. The runoff limits program is already shifting the practice of stormwater management in the region to a more sustainable approach, demonstrating that it is possible to develop the land without sacrificing fish species.