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5.2.2 REQUIREMENTS FOR USE OF BMP CREDITS 5.2.2 REQUIREMENTS FOR USE OF BMP C DITS Projects that implement flow control BMPs,whether required or optional,may use the flow control BMP credits described in this section subject to the requirements in Sections 5.2.2.1 and 5.2.2.2. Two kinds of credits are available. First, any impervious surface served by a flow control BMP that meets the design specifications for that BMP in Appendix C may be modeled as indicated in Table 5.2.2.A (below). Such credits may be used in the following situations: 1. To compute post-development runoff time series when sizing required flow control facilities. 2. To compute post-development 100-year peak flows when assessing any of the 0.1-cfs exceptions from the area-specific flow control facility requirement in Sections I.2.3.1A,B,and C. Second,any impervious or non-native pervious surface that is fully dispersed per the full dispersion criteria in Section 1.2.3.2C is not considered a target surface of the area-specific flow control facility requirement(Section 1.2.3.1)or the area-specific water quality facility requirement(Section 1.2.8.1). TABLE 5.2.2.A FLOW CONTROL BMP FACILITY SIZING CREDITS(1) Flow Control BMP Type Facility Sizing Credit Full dispersion Model fully dispersed surface as forest12l Full infiltration(3) Subtract impervious area that is fully infiltrated Limited infiltration Model tributary impervious surface as 50%impervious,50%grass Basic dispersion Model dispersed impervious surface as 50%impervious,50%grass Rain garden Model tributary impervious surface as 50%impervious,50%grass 8,;, Permeable pavement(non-grassed) Model permeable pavement area as 50%impervious,50%grass Grassed modular grid pavement Model permeable pavement as all grass Rainwater harvesting Subtract area that is fully controlled Vegetated roof Model vegetated roof area as 50%impervious,50%grass Restricted footprint Model footprint as restricted Wheel strip driveways Model credited area as 50%impervious, 50%grass Minimum disturbance foundation Model foundation area as 50%impervious, 50%grass Open grid decking over pervious area Model deck area as 50%impervious,50%grass Native growth retention credit Model credited impervious area as 50%impervious,50%grass Perforated pipe connection None Notes: (1) These credits do not apply when determining eligibility for exemptions from Core Requirement#3 or exceptions from the flow control facility requirement unless otherwise noted in the exemption or exception. (2) Surface shall be modeled using the soil type found at that location on the site, except for vegetated roofs, where the soil shall be assumed to be till. 13) For any project subject to Small Project Drainage Review, and for any single family residential project subject to Full or Large Project Drainage Review,the design requirements and specifications in Appendix C,Section C.2.2 may be used for design of full infiltration. For all other projects,full infiltration must be designed in accordance with infiltration facility standards in Section 5.4. 5.2.2.1 USE OF CREDITS BY SUBDIVISION PROJECTS If a proposed project is a subdivision project,6 implementation of flow control BMPs on the individual lots of the subdivision may be deferred until a permit is obtained for construction on each lot. Therefore, 6 For purposes of applying flow control BMPs,the term subdivision or subdivision project refers to any project that is a short plat, plat,or binding site plan. 2009 SurFace Water Design Manual 1/9/2009 5-13