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Keywords:

  • Runoff reduction;
  • site design;
  • regulations

Abstract:

  1. Top of page
  2. Abstract:
  3. Overview of the Runoff Reduction Method
  4. Runoff Coefficients and Treatment Volume
  5. Documenting BMP Performance
  6. Runoff Reduction Practices
  7. Pollutant Removal Practices
  8. Accountability for Better BMP Design
  9. Peak Flow Reduction
  10. Transferability of the Runoff Reduction Concept
  11. Conclusion
  12. Author Bio and Contact Information
  13. References

The Runoff Reduction Method (RRM) provides an innovative approach to crediting the total performance of stormwater best management practices (BMPs). Total BMP performance refers to the pollutant removal and runoff volume reduction capabilities. The RRM tracks the implementation of stormwater BMPs, including Low Impact Development (LID) strategies such as permeable pavement, rainwater harvesting, downspout disconnection, soil amendments, etc., and credits the appropriate runoff volume reduction, pollutant concentration reduction, or both towards compliance with stormwater quality and quantity requirements. By providing a mechanism to credit the volume reduction associated with these LID strategies the RRM also documents an allowable reduction of the overall size and footprint of structural detention practices, thereby providing an economic incentive for the development community to implement LID providing a better overall solution for minimizing the impact of development on the hydrologic cycle.

The Runoff Reduction Method (RRM) was developed in early 2008 by the Center for Watershed Protection and the Chesapeake Stormwater Network as a compliance tool for the proposed Virginia stormwater regulations. The RRM includes incentives for minimizing the increase in runoff associated with developed lands, while also providing a measure of the capability of both conventional and innovative stormwater Best Management Practices (BMPs) (e.g., permeable pavement, green roofs, rainwater harvesting, bioretention, downspout disconnection) to reduce the increased volume of runoff associated with developed pervious and impervious land cover. The RRM also accounts for the capability of certain BMPs to remove pollutants from stormwater runoff. The result is a comprehensive compliance tool that promotes better site design as the first step in compliance with both stormwater quality and quantity requirements, and strives to properly account for overall BMP effectiveness.

Most stormwater quality regulatory programs require a reduction in the post-developed pollutant load defined as an annual pollutant load measured in pounds per acre per year (lb/ac/yr). The annual pollutant load is typically the product of the annual runoff volume and the concentration of pollutant expected to be present in the developed condition runoff. The RRM includes incentives for better site design through the use of volumetric runoff coefficients that reflect the hydrologic response characteristics of both impervious and pervious cover conditions including soil type, forested land, managed turf, etc. Developing a design that reflects a low volumetric runoff coefficient (i.e. less impervious cover, less overall site disturbance, less managed turf, and more forested or open space areas) will result in a water quality benefit through a reduced volume of runoff in the pollutant load calculation.

The RRM further accounts for volume reduction through the use of various BMPs that have a demonstrated capability to reduce the overall volume of runoff based on the post-development condition. Runoff can be reduced via canopy interception, soil infiltration, evaporation, transpiration, rainfall harvesting, engineered infiltration, or extended filtration. The use of these practices in conjunction with the site design incentives noted above will reduce the volume of runoff used to compute the annual pollutant load generated by the site.

Additional BMPs that serve to remove the pollutants from stormwater through settling, filtering, adsorption, biological update, or other mechanisms can be combined with the volume reduction strategy to further reduce the pollutant load. The RRM strategy therefore includes BMPs that achieve runoff reduction, pollutant removal, or both.

In addition to water quality, the RRM provides a metric for computing the volume reduction benefit when calculating compliance with water quantity requirements. These requirements typically include the control of relatively large storms: 1 to 2-year frequency design storms for channel protection, and the 10-year frequency storm for localized flood protection. The volume reduction that is achieved through the use of RRM practices and strategies, along with any uniformly distributed retention storage can be utilized to satisfy the quantity requirements through a curve number adjustment, or if desired, the hydraulic routing of individual facilities.

The field of stormwater management has recently seen many new terms being adopted to describe various related and overlapping design approaches and philosophies such as low impact development, environmental site design, green infrastructure, etc. Runoff reduction describes a process or function of BMPs that can be described as bringing many of these strategies into a single method. As such, it is a collection of methods that has been derived to provide a simple and readily implementable compliance tool that incentivizes the use of minimization and avoidance of impacts to achieve the stormwater quality and quantity goals. The RRM approach also moves beyond managing stormwater solely for peak rate and water quality treatment by attempting to replicate a more natural (or pre-development) hydrologic condition. More specifically, it represents a more comprehensive approach that addresses runoff volume, duration, velocity, frequency, groundwater recharge, and protection of stream channels. When used in conjunction with site pollutant load limits, the approach can help meet the pollutant reduction goals (i.e. nutrient strategies, TMDLs) of the larger watershed.

Overview of the Runoff Reduction Method

  1. Top of page
  2. Abstract:
  3. Overview of the Runoff Reduction Method
  4. Runoff Coefficients and Treatment Volume
  5. Documenting BMP Performance
  6. Runoff Reduction Practices
  7. Pollutant Removal Practices
  8. Accountability for Better BMP Design
  9. Peak Flow Reduction
  10. Transferability of the Runoff Reduction Concept
  11. Conclusion
  12. Author Bio and Contact Information
  13. References

The RRM relies on a three-step compliance procedure, as described below:

Step 1: Reduce Stormwater Runoff by Design

Step one focuses on implementing better site planning and design techniques during the early phases of site layout. The goal is to minimize impervious cover and mass grading, and maximize retention of forest cover, natural areas and undisturbed soils (especially those most conducive to landscape-scale infiltration – Hydrologic Soil Groups A and B). The RRM assigns runoff coefficients for forest, disturbed soils, managed pervious areas, and impervious cover to calculate a site-specific target treatment volume.

Step 2: Reduce Volume of Post-Construction Stormwater Runoff

In this step, the designer uses combinations of small-scale, distributed, and conventional practices that are effective at reducing runoff from the site. In each case, the designer estimates the area to be treated by each runoff reduction practice to incrementally reduce the required treatment volume for the site. The designer is encouraged to use these practices in series within individual drainage areas (such as rooftop disconnection leading to a grass swale leading to a bioretention area) in order to achieve a higher level of runoff reduction.

Step 3: Capture and Treat Remaining Stormwater Runoff

In this step, the designer applies pollutant reduction values to the runoff reduction practices used in Step 2. If the target pollutant limits are not reached, the designer can select additional, conventional BMPs – such as filtering practices, wet ponds, and stormwater wetlands – to meet the remaining pollutant reduction requirements.

The three-step process is iterative for most sites. When compliance cannot be achieved on the first try, designers can return to prior steps to explore alternative combinations of site planning, site design, runoff reduction practices, and pollutant removal practices to achieve compliance. A possible Step 4 would involve paying an offset fee (or fee-in-lieu payment) to compensate for any pollutant load that cannot feasibly be met on particular sites. A related, but simpler option would be to allow a developer to conduct an off-site mitigation project in lieu of full on-site compliance. Table 1 includes a list of site design and stormwater practices that can be used for each step.

Table 1.  Practices included in the runoff reduction method. Thumbnail image of

Runoff Coefficients and Treatment Volume

  1. Top of page
  2. Abstract:
  3. Overview of the Runoff Reduction Method
  4. Runoff Coefficients and Treatment Volume
  5. Documenting BMP Performance
  6. Runoff Reduction Practices
  7. Pollutant Removal Practices
  8. Accountability for Better BMP Design
  9. Peak Flow Reduction
  10. Transferability of the Runoff Reduction Concept
  11. Conclusion
  12. Author Bio and Contact Information
  13. References

The negative impacts of increased impervious cover on receiving water bodies have been well documented (Center for Watershed Protection 2003; Walsh et al. 2004; Shuster et al. 2005; Bilkovic et al. 2006). Due to widespread acceptance of this relationship, impervious cover has frequently been used in watershed and site design efforts as a chief indicator of stormwater impacts. More recent research, however, indicates that other land covers, such as disturbed soils and managed turf, also impact stormwater quality (Law et al. 2008). Numerous studies have documented the impact of grading and construction on the compaction of soils, as measured by increase in bulk density, declines in soil permeability, and increases in the runoff coefficient (Ocean County Soil Conservation District et al. 2001; Pitt et al. 2002; Schueler and Holland 2000). These areas of compacted pervious cover (lawn or turf) have a much greater hydrologic response to rainfall than forest, meadow, or pasture. The hydrologic effects of compaction are significant enough that some jurisdictions require a downgrading of the hydrologic soil type and corresponding runoff curve number (RCN) based solely on soil disturbance. For example, a type A Hydrologic Soil Group (HSG) soil (highly permeable, and therefore generating little runoff) that is impacted by clearing and grading operations will be automatically considered a type B soil (less permeable, and therefore generating more runoff) in the developed condition runoff calculations (Anne Arundel County, MD 2006).

In addition to the hydrologic response, managed turf can contribute to elevated nutrient loads due to excessive management in the early years of establishing a dense turf cover after construction activities, as well as long term maintenance by homeowners. Typical turf management activities include mowing, active recreational use, and fertilizer and pesticide applications (Robbins and Birkenholtz 2003). The combination of greater than predicted runoff volumes due to soil compaction and higher concentrations of pollutants resulting from management activities yields a significantly higher pollutant load. It should be noted that properly managed native grasses, turf, or meadow on an undisturbed soil profile are fundamentally different than managed turf and should be considered a beneficial strategy in terms of both runoff volume and quality.

The runoff coefficients provided in Table 2 were derived from research by Pitt et al. (2005), Lichter and Lindsey (1994), Schueler (2001a), Schueler (2001b), Legg et al (1996), Pitt et al. (1999), Schueler (1987), and Cappiella et al. (2005). As shown in this table, the effect of grading, site disturbance, and soil compaction greatly increases the runoff coefficient compared to forested areas.

Table 2.  Site cover runoff coefficients (Rv). Thumbnail image of

The RRM uses runoff coefficients to calculate a site-specific target treatment volume (Tv). A site's Tv is calculated by multiplying the “water quality” rainfall depth by the runoff coefficients that correspond to the site cover conditions (forest, disturbed soils, and impervious cover), as shown in Table 3. The “water quality” rainfall depth is often defined as the rainfall depth associated with 90 percent of runoff-producing storm events on an average annual basis. Use of this depth as a design requirement for stormwater BMPS targets 90 percent of all storm events for treatment, as well the first portion of all larger storm events. The 90 percent storm event approach to defining the treatment volume is widely accepted and is consistent with several state stormwater manuals (Maryland Department of the Environment 2000; Atlanta Regional Commission 2001; NYDEC 2001; Vermont Department of Environmental Conservation 2002; Ontario Ministry of the Environment 2003; Minnesota Pollution Control Agency 2005).

Table 3.  Determining the stormwater treatment volume. Thumbnail image of

Using a site specific Tv has several distinct advantages when it comes to evaluating runoff reduction practices and sizing BMPs. In effect, Tv provides an objective measure to gage the aggregate performance of runoff reduction practices and conventional BMPs using a common currency: runoff volume. Calculating the Tv explicitly acknowledges the difference between forest and turf cover, and disturbed and undisturbed soils, which creates incentives to conserve forests and reduce mass grading. Further, since runoff is a direct function of impervious cover and disturbed soils, the Tv computation provides designers with an incentive to minimize both at a development site.

Additionally, using the 90th percentile rainfall depth for the Tv computation ensures that stormwater practices are adequately sized to capture runoff from a wide range of storm events. This includes the small and more frequent runoff producing events, as well as the equivalent runoff volume under the rising limb of the runoff hydrograph from larger storms. Managing this volume through the RRM, which totals approximately 90 percent of the annual runoff volume, is important since the first flush effect in terms of total load has been found to be modest for many pollutants (Pitt et al. 2005).

Documenting BMP Performance

  1. Top of page
  2. Abstract:
  3. Overview of the Runoff Reduction Method
  4. Runoff Coefficients and Treatment Volume
  5. Documenting BMP Performance
  6. Runoff Reduction Practices
  7. Pollutant Removal Practices
  8. Accountability for Better BMP Design
  9. Peak Flow Reduction
  10. Transferability of the Runoff Reduction Concept
  11. Conclusion
  12. Author Bio and Contact Information
  13. References

After implementing environmental site design and site planning techniques to minimize the site's treatment volume, the next steps of the RRM involve the selection of BMPs to reduce the Tv, and consequently, the pollutant load from the site. Center for Watershed Protection and CSN conducted an extensive literature search to identify the capabilities of various BMPs to reduce overall runoff volume (runoff reduction) and pollutant concentrations (pollutant removal). Historically, BMP performance has been evaluated according to only the pollutant removal efficiency of a practice. Further, removal efficiencies do not always address runoff volume reductions in BMPs (Strecker et al. 2004; Jones et al. 2008). Therefore, the lack of volume reduction data can lead to the assumption that the total load reduction was the result of the pollutant concentration (EMC) reduction, leading to an over-estimation of the pollutant removal capabilities, while ignoring the additional benefits of volume reduction.

More recent BMP performance research has focused on runoff reduction as well as overall pollutant removal. In order to isolate the pollutant removal mechanisms of a BMP apart from runoff reduction, both EMC-based pollutant removal efficiencies and volume reduction efficiencies were researched. For several practices, there were a limited number of studies available that reported runoff reduction or EMC-based nutrient removal efficiencies. As a result, some of the values listed in Table 4: Runoff Reduction, and Table 5: EMC-Based Pollutant Removal will be subject to change as more studies and data become available. The values represent the authors’ best judgment based on currently available information. Further documentation on the derivation of these values can be found in Hirschman et al. (2008).

Table 4.  Runoff reduction for various BMPs (based on average rainfall in the Virginia Piedmont areas).
PracticeRunoff Reduction (percent)*
  1. *Range of values is for median (Level 1) and 75th percentile (Level 2) designs.

Green Roof45–60
Rooftop Disconnection25–50
Raintanks and Cisterns40
Permeable Pavement45–75
Grass Channel10–20
Bioretention40–80
Dry Swale40–60
Wet Swale0
Infiltration50–90
ED Pond0–15
Soil Amendments50–75
Sheetflow to Open Space50–75
Filtering Practice0
Constructed Wetland0
Wet Pond0
Table 5.  Approximate Number of CARA M.Sc. Students Trained (or currently in program).
PracticeTotal Phosphorus Pollutant Removal (percent)*Total Nitrogen Pollutant Removal (percent)*
  1. *Range of values is for median (Level 1) and 75th percentile (Level 2) designs.

Green Roof00
Disconnection00
Rainwater harvesting and Cisterns00
Permeable Pavement2525
Grass Channel1520
Bioretention25–5040–60
Dry Swale20–4025–35
Wet Swale20–4025–35
Infiltration2515
ED Pond1510
Soil Amendments00
Sheetflow to Open Space00
Filtering Practice60–6530–45
Constructed Wetland50–7525–55
Wet Pond50–7530–40

Runoff Reduction Practices

  1. Top of page
  2. Abstract:
  3. Overview of the Runoff Reduction Method
  4. Runoff Coefficients and Treatment Volume
  5. Documenting BMP Performance
  6. Runoff Reduction Practices
  7. Pollutant Removal Practices
  8. Accountability for Better BMP Design
  9. Peak Flow Reduction
  10. Transferability of the Runoff Reduction Concept
  11. Conclusion
  12. Author Bio and Contact Information
  13. References

As described earlier in this paper, various BMPs are capable of reducing the overall volume of runoff based on the post-development condition. Runoff can be reduced via canopy interception, soil infiltration, evaporation, transpiration, rainfall harvesting, engineered infiltration, or extended filtration. Extended filtration includes bioretention or dry swales with underdrains that delay the delivery of stormwater from small sites to the stream system by six hours or more.

While runoff reduction data were limited for many practices, runoff reduction rates were assigned to the various BMPs based on the research findings, as shown in Table 4. A range of values represents the median and 75th percentile runoff reduction rates based on the literature search. Several BMPs reflected moderate to high capabilities for reducing annual runoff volume. Others – including filtering, wet swales, wet ponds, and stormwater wetlands – were found to have a negligible effect on runoff volumes, and were not assigned runoff reduction rates. The runoff reduction performance of any given BMP can often depend upon the specific BMP characteristics (i.e., geometry, sizing, etc.), therefore specific design criteria are important to define for each BMP type.

Pollutant Removal Practices

  1. Top of page
  2. Abstract:
  3. Overview of the Runoff Reduction Method
  4. Runoff Coefficients and Treatment Volume
  5. Documenting BMP Performance
  6. Runoff Reduction Practices
  7. Pollutant Removal Practices
  8. Accountability for Better BMP Design
  9. Peak Flow Reduction
  10. Transferability of the Runoff Reduction Concept
  11. Conclusion
  12. Author Bio and Contact Information
  13. References

In order to isolate the pollutant removal mechanisms of a BMP and offer a better approach to assessing BMP pollutant removal performance (as distinct from runoff reduction), EMC-based pollutant removal rates for nitrogen and phosphorus were researched. EMC-based pollutant removal is accomplished via processes such as settling, filtering, adsorption, and biological uptake. This is separate from changes in the overall volume of runoff entering and leaving the BMP.

Based on the research findings, EMC-based pollutant removal rates were assigned to various BMPs, as shown in Table 5. The range of values represents the median and 75th percentile runoff reduction rates based on the literature search. It should be noted that the data used to estimate pollutant removal were derived from practices in good condition; most studies focused on BMPs that were monitored within three years of construction. As with runoff reduction, since pollutant removal rates are dependent on site characteristics and BMP geometry, these EMC-based pollutant removal numbers are associated with specific design criteria.

Accountability for Better BMP Design

  1. Top of page
  2. Abstract:
  3. Overview of the Runoff Reduction Method
  4. Runoff Coefficients and Treatment Volume
  5. Documenting BMP Performance
  6. Runoff Reduction Practices
  7. Pollutant Removal Practices
  8. Accountability for Better BMP Design
  9. Peak Flow Reduction
  10. Transferability of the Runoff Reduction Concept
  11. Conclusion
  12. Author Bio and Contact Information
  13. References

A range of performance values are provided for certain BMPs in Tables 4 and 5. These values are associated with specific design criteria and are identified as Level 1 and Level 2 designs. A standard, or Level 1 design can be expected to achieve the median value of Runoff Reduction and Pollutant Removal derived from the research, while an enhanced, or Level 2 design achieves the 75th percentile values.

Based on the evaluation of BMP performance in the literature, design factors – such as sizing, pretreatment, flow path geometry, vegetative condition, and treatment processes – that enhance nutrient pollutant removal and runoff reduction of BMPs were identified. These design factors were then applied to Level 1 and Level 2 BMPs. The specified design factors associated with these levels must be met in order to achieve credited runoff reduction and pollutant removal rates.

The scientific rationale and assumptions used to assign sizing and design features to the standard and enhanced BMP levels involved identifying the “standard” or basic functional design features that should be included in all designs (i.e., not directly related to differential nutrient removal or runoff reduction rates). A review of individual BMP studies and corresponding modifications of design point tables in the Stormwater Retrofit Manual, Appendix B (Schueler et al. 2007) were then utilized to reflect the more specific goals of reducing runoff and nutrient removal for BMPs.

For example, the bioretention Level 2 design provides for an increase in both runoff reduction and pollutant removal credit as compared to Level 1 through an increase in the storage volume (provided as either surface or subsurface storage) and the minimum soil media depth, and the ability to infiltrate into the underlying soils or provide a deep sump. In addition, expanded design criteria related to pre-treatment techniques, the landscaping plan, and the overall bioretention basin geometry such as minimum flow path lengths for each inflow point, also serves to support an increase in total credited performance. Another example of a Level 1 and Level 2 distinction is the wet pond design criteria. The Level 2 design provides for increased pollutant removal through a 50 percent increase in the treatment volume (which can be in the form of the normal pool and an extended detention volume above the normal pool). More importantly, the Level 2 design also includes requirements for shallow marsh areas and more stringent geometry criteria such as minimum flow path lengths and multiple treatment cells.

It is important to note that the assigned rates are based on the assumption that BMP designs will meet certain minimum “eligibility criteria” for the designated level. That is, the BMPs will be located and designed based on appropriate site conditions and limitations with regard to soils, slopes, available head, drainage area size, and other factors. The specific design factors and eligibility criteria for various BMPs are detailed in the Runoff Reduction Technical Memo (Hirschman et al, 2008).

Peak Flow Reduction

  1. Top of page
  2. Abstract:
  3. Overview of the Runoff Reduction Method
  4. Runoff Coefficients and Treatment Volume
  5. Documenting BMP Performance
  6. Runoff Reduction Practices
  7. Pollutant Removal Practices
  8. Accountability for Better BMP Design
  9. Peak Flow Reduction
  10. Transferability of the Runoff Reduction Concept
  11. Conclusion
  12. Author Bio and Contact Information
  13. References

RRM utilizes an “annual” volume reduction with which to calculate the “annual” pollutant load coming out of the practice. In principle, when runoff reduction practices are used to capture and retain or infiltrate runoff, downstream stormwater management practices should not have to detain, retain or otherwise treat the volume that is removed. In other words, the volume of runoff reduction provided should be subtracted from the volume calculated by stormwater runoff peak flow computations. The challenge lies in how to accurately credit the annual volume reduction to the computation of the peak rate of runoff from larger single event storms for purposes of channel or flood protection. Peak flow reduction is accomplished by providing watershed storage and runoff attenuation.

Many of the volume-based BMPs used in the RRM also provide some amount of storage and runoff attenuation. While one could apply hydraulic routing to a volume-based BMP, the response characteristics of the practice may not follow the traditional detention/retention design parameters. Routing of BMPs can be a difficult and complex task, given all the hydrologic and hydraulic variables associated with volume and peak rate reduction, such as evapo-transpiration, storage within the soil media, infiltration, and extended filtration.

The RRM points to a simpler method for crediting specific runoff reduction values toward peak flow reduction. The method utilizes the Natural Resource Conservation Service runoff equations 2–1 through 2–4 provided in Urban Hydrology for Small Watersheds (USDA 1986) to derive a curve number adjustment that reflects the reduced runoff volume. A simplified derivation of the computational procedure starts with the combined Runoff Equations 2–1 and 2–2 in order to express the runoff depth in terms of rainfall and potential maximum retention, Equation 2–3. In addition, the potential maximum retention, S, is related to soil and cover conditions of the watershed through the runoff curve number (CN) as described by Equation 2–4.

  • image(Eq. 2–1, TR-55)
  • image(Eq. 2–2, TR-55)
  • image(Eq. 2–3, TR-55)
  • image(Eq. 2–4, TR-55)
  • image(Modified Eq. 2–3)
  • image

The retention storage depth equivalent to the runoff reduction values assigned by the RRM and any additional retention storage provided on the site (expressed in terms of retention storage R) are subtracted from the total runoff depth associated with the developed condition CN, which then will provide for a new value of S (Modified Equation 2–3). A new CN is then back-calculated from the new value of S using Equation 2–4 (Koch 2005).

While it is not easy to predict the absolute runoff hydrograph modification provided by reducing stormwater runoff volumes, it is clear that reducing runoff volumes will have an impact on the runoff hydrograph of a development site. Simple routing exercises have indicated that this curve number adjustment approach represents a conservative estimate of peak reduction.

Additional research to compare the results of hydraulic routing of management practices with the measured volume reduction in the field is needed to better predict the ability of these practices to modify the runoff hydrograph and reduce peak discharges. Documenting an accurate volume reduction for the larger storm events can provide a tremendous incentive to further maximize the use of site design techniques as a way to reduce the footprint and long-term operation and maintenance of large BMPs, while also providing better protection of aquatic resources.

Transferability of the Runoff Reduction Concept

  1. Top of page
  2. Abstract:
  3. Overview of the Runoff Reduction Method
  4. Runoff Coefficients and Treatment Volume
  5. Documenting BMP Performance
  6. Runoff Reduction Practices
  7. Pollutant Removal Practices
  8. Accountability for Better BMP Design
  9. Peak Flow Reduction
  10. Transferability of the Runoff Reduction Concept
  11. Conclusion
  12. Author Bio and Contact Information
  13. References

While the RRM was originally developed in tandem with the Virginia Department of Conservation and Recreation (DCR) efforts to update the stormwater regulations and handbook, the concept is widely applicable to other state and local stormwater planning procedures. The focus on runoff volume as the common currency for BMP evaluation is gaining wider acceptance across the county (U.S. EPA 2008). Currently, various state programs are in the process of using or adopting methods similar to the RRM as part of regulatory and non-regulatory programs. With the incorporation of the RRM into these programs, communities are poised for the widespread implementation of runoff reduction practices on development sites.

  • • 
    Virginia — The Virginia Department of Conservation and Recreation is integrating the RRM into proposed stormwater management regulations and an updated stormwater management handbook. The method focuses on compliance to meet site-based pollutant load limits, aimed at limiting the total nutrient load leaving a new development site. A site design compliance spreadsheet was developed to ensure runoff reduction and nutrient loading requirements for the site are met.
    The annual runoff reduction credit (measured as a percentage of the 1-inch, or 2.54 cm, event) is then used to calculate the corresponding curve number adjustment for quantity control compliance for larger storms. Flexibility is provided for designers to account for the actual retention storage provided by volume-based practices rather than the annual reduction credit to calculate individual storm curve number adjustments.
  • • 
    Georgia — The Georgia Department of Natural Resources modified the RRM for a new Coastal Stormwater Management Manual Supplement. The manual requires that the stormwater runoff volume from the 1.2 inch (3.05 cm) rainfall event (85th percentile event) be retained, reused, or reduced to the maximum extent practical, on a new development or redevelopment site.

Additional efforts to incorporate the concept of runoff reduction into updated stormwater regulations and design manuals are underway in the States of Delaware, Maryland, and the District of Columbia (Cappiella et al. 2007; DeBlander et al. 2008; Maryland Stormwater Consortium 2008). The Pennsylvania Stormwater Best Management Practices Manual (PA DEP 2006) already incorporates standards for volume control achieved by structural and nonstructural BMPs.

Conclusion

  1. Top of page
  2. Abstract:
  3. Overview of the Runoff Reduction Method
  4. Runoff Coefficients and Treatment Volume
  5. Documenting BMP Performance
  6. Runoff Reduction Practices
  7. Pollutant Removal Practices
  8. Accountability for Better BMP Design
  9. Peak Flow Reduction
  10. Transferability of the Runoff Reduction Concept
  11. Conclusion
  12. Author Bio and Contact Information
  13. References

The concept of runoff reduction marks an important philosophical milestone that will help define the next generation of stormwater design. The promise of runoff reduction is that the benefits go beyond water quality improvement. The RRM provides an effective and readily implemented compliance tool that serves to incentivize site design strategies that minimize and avoid impacts to the natural or existing hydrologic response of the site. These types of site design strategies have long been promoted, but not necessarily implemented. If site and stormwater designs can successfully implement runoff reduction strategies, then they will do a better job at replicating a more natural (or pre-development) hydrologic response that goes beyond peak rate control to also address runoff volume, duration, velocity, frequency, and groundwater recharge. Important future work will involve continued integration of the runoff reduction concept with stormwater requirements for channel protection and flood control, so that stormwater criteria can be presented in a unified approach.

Author Bio and Contact Information

  1. Top of page
  2. Abstract:
  3. Overview of the Runoff Reduction Method
  4. Runoff Coefficients and Treatment Volume
  5. Documenting BMP Performance
  6. Runoff Reduction Practices
  7. Pollutant Removal Practices
  8. Accountability for Better BMP Design
  9. Peak Flow Reduction
  10. Transferability of the Runoff Reduction Concept
  11. Conclusion
  12. Author Bio and Contact Information
  13. References

Joseph Battiata is a Senior Water Resources Engineer with the Center for Watershed Protection. He has over 25 years of experience in water resources and stormwater management in the private and public sector. Notably, Mr. Battiata developed and managed the implementation of the Virginia Department of Transportation's first Municipal Separate Storm Sewer System (MS4) and Construction General Permit Programs. He also served as the Stormwater Program Manager and Chief Engineer with the Virginia Department of Conservation and Recreation (DCR). Joe started his career as a consultant in the D.C. metro area after graduating from The Catholic University of America with a B.S. in Civil Engineering. He may be contacted at jgb@cwp.org or at Center for Watershed Protection, 7368 Sunshine Court, Mechanicsville VA 23111.

Kelly Collins is a Water Resources Engineer with the Center for Watershed Protection. Kelly is knowledgeable in several areas of watershed and stormwater management, including stormwater retrofit evaluation, planning and design of stormwater management practices, local stormwater and watershed management program development, stormwater monitoring, and watershed management planning. Kelly has a Master of Science in Biological and Agricultural engineering from North Carolina State University and a Bachelor of Science in Biological and Agricultural engineering from Pennsylvania State University. She may be reached at kac@cwp.org.

David Hirschman serves as a Program Director for the Center for Watershed Protection. In this capacity, he works on a variety of stormwater, watershed, and training programs. He has 27 years of experience in the public, private, and non-profit sectors. Prior to working for CWP, he served as Water Resources Manager for Albemarle County, Virginia, where he was responsible for the development and implementation of a stormwater management program. He has a B.A. in Biology from Duke University and a Master of Urban & Regional Planning from Virginia Tech. He may be contacted at djh@cwp.org.

Greg Hoffmann, a Wisconsin native, is a Professional Engineer with a Master of Engineering from Michigan Technological University. Greg began at the Center for Watershed Protection in 2008, and specializes in development of stormwater regulations and guidance documents, stormwater retrofitting, watershed assessment and planning, and training on various stormwater and watershed topics. Prior to joining the Center, Greg worked for a private consulting firm in Port Huron, Michigan. Greg lives in Baltimore with his wife, Elizabeth, and spends his spare time hiking, traveling, and working in his garden. He may be reached at gph@cwp.org.

References

  1. Top of page
  2. Abstract:
  3. Overview of the Runoff Reduction Method
  4. Runoff Coefficients and Treatment Volume
  5. Documenting BMP Performance
  6. Runoff Reduction Practices
  7. Pollutant Removal Practices
  8. Accountability for Better BMP Design
  9. Peak Flow Reduction
  10. Transferability of the Runoff Reduction Concept
  11. Conclusion
  12. Author Bio and Contact Information
  13. References
  • Anne Arundel County, MD. 2006. Stormwater Management Practices and Procedures Manual, Revised Policies and Procedures dated July 3, 2006.
  • Atlanta Regional Commission. 2001. Georgia Stormwater Design Manual, Vol 2: Technical Handbook. Atlanta, GA.
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