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Runoff Reduction Standard The Runoff Reduction Standard requires SCMs to infiltrate, evaporate, or evapotranspire a quantity of water equal to 60% of the WQCV if all of the applicable development site discharges without infiltration. This standard can be met using a variety of methods including infiltration in receiving pervious areas (RPAs) such as grass buffers, swales, vegetated filter strips, and permeable pavements as well as SCMs such as bioretention and sand filters designed to infiltrate the WQCV without surface discharge (typically less than 5 acres, above which extended detention basins may be considered). Stormwater Management of Impervious and Pervious Areas to Meet Municipal Separate Storm Sewer System (MS4) Base Design Standards INTRODUCTION The Colorado Discharge Permit System (CDPS) General Permit for Stormwater Discharges Associated with Municipal Separate Storm Sewer Systems (MS4) requires management of post-construction runoff using stormwater control measures (SCMs) that are selected, designed, implemented, and maintained in accordance with good engineering, hydrologic, and pollution control practices (COR090000 Part For new development and redevelopment, SCMs must meet one of the base design standards in the MS4 permit (COR090000 Part I.E.4): Water Quality Capture Volume (WQCV) Standard Pollutant Removal Standard Runoff Reduction Standard Regional WQCV Standards Constrained Redevelopment Standard These standards apply to “applicable development sites” which are sites that result in land disturbance of greater than or equal to one acre, including smaller sites that are part of a larger common plan of development or sale. Many of these standards recognize that it may not be feasible to grade a site so that 100 percent of the site drains to SCMs and allow exclusions of up to 20% (not to exceed an acre) for areas that cannot practically drain to a SCM. A 50% exclusion may be Figure 1. Illustration of traditional versus runoff reduction approaches. ---PAGE BREAK--- WQCV for Pervious Areas By simple calculation, it is evident that if the imperviousness of an area is zero percent, the associated WQCV would also be zero. Even if there are incidental landscaping stones or other small disconnected impervious areas, a landscaped area will not typically generate runoff in a water quality event (0.6 inches of rainfall) unless there is run-on from a larger drainage area. This is because the depression storage provided by the vegetation and landscaped surface is on the order of 0.3 inches or more and the top 2 inches of topsoil would absorb 0.5 inches or more. Receiving pervious area (RPA) providing runoff reduction for small roof area draining to downspout. allowed for constrained sites for the WQCV and Pollutant Removal Standards given the site conditions. This fact sheet discusses approaches to meeting these base design standards using one or more SCMs and how to address pervious and impervious areas that do not drain to SCMs, either through treatment or exclusion. DETERMINING THE WQCV The WQCV is calculated using Equation 3.1 in Chapter 3 of the MHFD Manual: WQCV = a(0.91I3 – 1.19I2 + 0.87I) Where, WQCV = water quality capture volume in watershed inches a = coefficient corresponding to the type of stormwater control measure (ranging from 0.8 to 1.0) I = imperviousness of area draining to the SCM, percent expressed as a decimal When designing an SCM, the WQCV is determined based on the imperviousness of the portion of the applicable development site that drains to the SCM and would not include portions of the site that are excluded or drain to other SCMs. The WQCV is calculated for all of the standards listed above except for the Pollutant Removal Standard, which is based on treating runoff to a median event mean concentration of 30 mg/L or less. Note that the WQCV is based on the portions of the applicable development site draining to SCM and does not include excluded areas of the site. ---PAGE BREAK--- Quick Reference Sizing for RPAs including Grass Buffers HSG Required UIA:RPA Ratio1 60% WQCV Reduction A 7.2:1 B 3.4:1 C/D 2:1 1 Based on WQCV precipitation of 0.6 inches and slopes up to 33%. EXCLUDED AREAS For standard sites, up to 20% (not to exceed 1 acre) of the applicable development site may be excluded if analysis demonstrates that these areas cannot feasibly drain to SCMs. Excluded areas may include drives, sidewalks, and other similar features that drain directly to the street system or offsite. The excluded area is based on the total area that cannot be drained to SCMs, including impervious and pervious areas. For some street-side features that do not drain to the primary SCMs on a site, it may still be feasible to provide treatment meeting the Runoff Reduction Standard through the use of RPAs. RPAs could be as simple as a well-vegetated grass areas adjacent to impervious areas such as tree lawns adjacent to sidewalks. RPAs also are well suited for infiltration of runoff from downspouts. To infiltrate 60% of the WQCV, RPAs must be designed with the appropriate ratio of unconnected impervious area (UIA, defined as the impervious area that drains to the RPA) to RPA. This ratio varies by hydrologic soil group (HSG) as shown in the Quick Referencee Sizing table, which is appropriate for initial sizing of RPAs. Design RPAs in accordance with Chapter 4 of Volume 3 of the Mile High Flood District’s (MHFD’s) Urban Storm Drainage Criteria Manual (MHFD Manual). MHFD’s UD-BMP Workbook includes a Runoff Reduction worksheet that can be used for sizing and designing RPAs to meet the Runoff Reduction Standard. In cases where a designer is struggling to get the excluded area to less than or equal to 20%, RPAs provide a means for stormwater treatment for small areas that cannot easily drain to the primary SCMs on a site. APPLICATION OF MULTIPLE BASE DESIGN STANDARDS To meet the MS4 base design standards, it is common to use multiple SCMs on a site. Depending on the type of SCM and treatment mechanisms, different base design standards may apply. For example, SCMs including bioretention, sand filters, permeable pavements, and extended detention ponds are designed based on storing and releasing the WQCV and generally satisfy the WQCV Standard, while Separate pervious areas (SPAs) like this grass strip that do not receive runoff from other areas will infiltrate or evapotranspire at least 60% of the water quality design depth and do not require additional treatment. ---PAGE BREAK--- infiltration-based SCMs including RPAs and bioretention or sand filters designed to infiltrate the WQCV into underlying soils are well-suited for meeting the Runoff Reduction Standard. Some proprietary devices are designed to provide the required treatment without significant storage of the WQCV, and these types of SCMs are best-suited to meet the Pollutant Removal Standard. On any given site, a combination of these SCMs may be used to demonstrate compliance with MS4 base design standards, and the key to such an approach is to demonstrate on an SCM-by- SCM basis that the proposed design meets one of more of the base design standards. For example, a site could include the following: SCMs meeting the WQCV Standard such as extended detention basins, bioretention, sand filters, permeable pavements, or other SCMs that provide the WQCV and either drain or infiltrate the WQCV over the drain times specified in the MHFD Manual. These SCMs are sized based on the tributary drainage area to the SCM and the overall imperviousness of the drainage area. RPAs such as buffers, swales, or filter strips that treat runoff from smaller impervious areas that can be directed to pervious areas for infiltration with appropriate ratios of UIA:RPA to meet the Runoff Reduction Standard. Separate Pervious Areas (SPAs) that do not receive runoff from impervious areas by definition. To the extent that these SPAs fall outside of areas treated by WQCV-based SCMs, these areas should not require additional control measures because at least 60 percent of the WQCV direct rainfall of 0.6 inches falling on the SPA) will infiltrate, assuming a suitable topsoil layer and absence of a shallow restrictive layer, thereby meeting the Runoff Reduction Standard. Manufactured treatment devices such as high rate media filters that are capable of meeting the Pollutant Removal Standard and have been verified using independent testing to meet the 30 mg/L median effluent event mean concentration. These types of SCMs are often used to treat street and parking lot runoff. A combination of these practices can be used to meet the requirements to treat at least 80% of the applicable development site for unconstrained sites (50% for constrained sites). In some cases, it may be possible to address MS4 requirements by relying on only one of the MS4 base design standards; however, if the topography of the applicable development site requires multiple SCMs, it may be necessary to apply multiple base design standards to use the types of SCMs most appropriate for each area. This information must be thoroughly documented in the drainage report for a site to demonstrate compliance with MS4 permit requirements.