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Eastern Washington Phase II Municipal Slormwaler Permit <br />Treatment Facility Selection <br />Treatment facilities must be selected in accordance with the guidance in Chapter 5 of the <br />Stormwater Management Manual for Eastern Washington (2004), or another technical <br />stormwater manual approved by the Department, to meet the treatment requirements identified <br />for the project's proposed land use and site conditions. <br />Treatment Facility Sizing <br />Each treatment BMP is sized based on a water quality design volume, or a water quality design <br />flow rate. Permittees shall adopt criteria to provide for consistent sizing of treatment facilities. <br />Computational methods for predicting runoff volumes and flow rates for a proposed <br />development condition are included in Chapter 4 of the Stormwater Management Manual for <br />Eastern Washington (2004). Specific design criteria for treatment facilities may be taken from <br />Chapter 5 of the Stormwater Management Manual for Eastern Washington (2004) or another <br />technical stormwater manual approved by the Department. Specifically, public road projects <br />may be designed using BMPs in the current version of the Washington State Department of <br />Transportation Highway Runoff Manual approved by the Department. <br />Water quality design volume: Each Permittee shall specify which of the following methods will <br />be used to determine treatment volumes in their jurisdiction. Different methods may be specified <br />for different types of projects. Volume -based treatment BMPs are sized the same whether <br />located upstream or downstream from detention facilities. <br />Runoff Volume Method 1: The volume of runoff predicted for the proposed development <br />condition from the regional storm with a 6 -month return frequency. An alternative to this <br />method is the modified Type IA storm described in Chapter 4.2 of the Stormwater <br />Management Manual for Eastern Washington (2004); this alternative method is intended <br />for use on small projects where the designer's software does not accept storms longer than <br />24 hours. <br />• Runoff Volume Method 2: The volume of runoff predicted for the proposed development <br />condition from the SCS Type IA 24-hour storm with a 6 -month return frequency. <br />• Runoff Volume Method 3: In Regions 2 and 3, volume -based facilities may be sized for 0.5 <br />inch predicted runoff produced for the proposed development condition from all <br />impervious surface areas that contribute flow to the treatment facility. (This method may be <br />modified for design of BMP T5.30 Bio -infiltration swale in Chapter 5 of the Stormwater <br />Management Manual for Eastern Washington (2004).) <br />• Runoff Volume Method 4: The volume of runoff predicted for the proposed development <br />condition from the SCS Type II storm with a 6 -month return frequency. <br />• Runoff Volume Method 5: Another sizing approach and criteria based on peer-reviewed <br />methods and supported by local data that meet the objective of treating at least 90% of the <br />annual volume of runoff from the site. Snowmelt should be considered in determining the <br />water quality design volume if this method is selected. <br />Appendix 1- Minimum Technical Requirements <br />Eastern Washington Phase II Municipal Stormwater Permit —August 1, 2014 <br />Page 15 <br />