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Universities Council on Water Resources
Journal of Contemporary Water Research & Education Issue 146, Pages 11-21, December 2010
The Runoff Reduction Method Joseph Battiata, Kelly Collins, David Hirschman, and Greg Hoffmann Center for Watershed Protection, Mechanicsville, VA
Abstract: 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. Keywords: Runoff reduction, site design, regulations
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he 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 Journal of Contemporary Water Research & Education
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 UCOWR