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Eastern H ashington Phase 11 Municipal Slornnvaler° Permit <br />Water qualfty design flow rate: Flow -rate -based treatment BMPs are sized differently depending <br />on whether they are located upstream or downstream from detention facilities, if detention is <br />required. For runoff treatment facilities sited downstream of detention facilities, the design flow <br />rate is the full 2 -year release rate of the detention facility. For runoff treatment facilities <br />preceding detention facilities or when detention facilities are not required, each Permittee shall <br />specify which of the following methods will be used to determine flow rates in their jurisdiction. <br />Different methods may be specified for different types of projects. For large facilities receiving <br />inflow from multiple sources, the flow rate generated by the regional or Type IA storm should <br />also be checked. <br />Flow Rate Method I: The runoff flow rate predicted for the proposed development <br />condition from the short -duration storm with a 6 -month return frequency. Time intervals <br />for some facilities are specified in the BMP design requirements in Chapter 5 of the <br />Stormwater Management Manual for Eastern Washington (2004). <br />Flow Rate Method 2: The runoff flow rate predicted for the proposed development <br />condition from the SCS Type 1I 24-hour storm with a 6 -month return frequency. Time <br />intervals for some facilities are specified in the BMP design requirements in Chapter 5 of <br />the Stormwater Management Manual for Eastern Washington (2004). <br />Flow Rate Method 3: The runoff flow rate for the proposed development condition <br />calculated by the Rational Method using the 2 -year Mean Recurrence Interval (see Chapter <br />4.7 of the Stormwater Management Manual for Eastern Washington (2004)). This method <br />may only be used to design facilities based on instantaneous peak flow rates. <br />Bypass Requirements <br />A bypass must be provided for all treatment BMPs unless the facility is able to convey the 25 - <br />year short -duration storm without damaging the BMP or dislodging pollutants from within it. <br />Extreme runoff events may produce high flow velocities through BMPs that can damage and or <br />dislodge pollutants from within the facility. The designer must: check the maximum allowable <br />velocity (typically less than 2 ft/s) or shear stress specified for the BMP; and implement a flow <br />bypass as necessary to prevent exceeding these velocities. Bypass is not recommended for wet <br />ponds, constructed wetlands, and similar volume -based treatment facilities; inlet structures for <br />these facilities should be designed to dampen velocities; the pond dimensions will further <br />dissipate the energy. <br />Use of Existing Wetlands to Provide Runoff Treatment <br />Stormwater treatment facilities are not allowed within a wetland or its natural vegetated buffer <br />except for: <br />Necessary conveyance systems approved by the local government; or <br />As allowed in a wetland mitigation plan; or <br />When the requirements below are met. <br />A wetland can be considered for use in stormwater treatment if: <br />Appendix I- Minimum Technical Requirements <br />Eastern Washington Phase II Municipal Stormwater Permit —August I, 2014 <br />Page 16 <br />