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Stormwater Performance Requirements Flow Chart

Determine stormwater requirements based on project size, type, and location. Follow the flow chart to assess performance criteria for small, moderate, and large development projects. Consider site design, water quality treatment, runoff retention, and peak management.

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Stormwater Performance Requirements Flow Chart

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  1. Flow Chart to Determine Performance Requirements Start Here Is the project Exempt or does not discharge to MS4? YES NO Project is < 2,500 ft2of new and replaced impervious surface YES NO Project is a detached single-family residence? YES NO SFR > 2,500 ft2 See Figure A4 EXEMPT No Stormwater Requirements All Others See Figure A2 Figure A1. Initial Screening for All Development Projects

  2. Projects ≥ 2,500 ft2 Project is >5,000 ft2net impervious area but < 15,000 ft2of new and replaced impervious surface Project is > 2,500 ft2of new and replaced impervious surface but < 5,000 ft2 of net impervious area Project is >15,000 ft2of new and replaced impervious surface Performance Requirements #1 (Site Design) and #2 (WQ Treatment) Performance Requirement #1 (Site Design) See Figure A3 Figure A2. Requirements for Small to Moderate Development Projects

  3. Projects ≥ 15,000 ft2 New and Replaced Impervious Area See Special Circumstances (Performance Requirement #5) Does the project fall under the Special Circumstances designation? Yes No Determine WMZ and apply Performance Requirement #3 (Runoff Retention) Watershed Management Zone 1 2 3 4 5 6 7 8 9 10 Retain 95th Percentile event via infiltration Retain 95th Percentile event via storage, harvesting, infiltration, and/or evapotranspiration N/A Retain 95th Percentile event via infiltration where overlying groundwater basin Retain 85th Percentile event via infiltration Retain 85th Percentile event via storage, harvesting, infiltration, and/or evapotranspiration Retain 95th Percentile event via infiltration where overlying groundwater basin Retain 85th Percentile event via infiltration Retain 85th Percentile event via storage, harvesting, infiltration, and/or evapotranspiration Retain 95th Percentile event via infiltration where overlying groundwater basin No No additional Stormwater Requirements Project in WMZ 1, 2, 3, 6, or 9 Yes No Project creates > 22,500 ft2 of new and replaced impervious surface Yes • Apply Performance • Requirement #4 • (Peak Management) Figure A3. Requirements for Large Development Projects

  4. DetachedSingle-FamilyResidentialProjects Project is > 2,500 ft2new and replaced impervious surface, but < 15,000 ft2 net impervious area Project is > 15,000 ft2 net impervious area, but < 22,500 ft2 new and replaced impervious surface Project is > 22,500 ft2 new and replaced impervious surface Performance Requirement #s 1, 2, and 3 Performance Requirement #s 1, 2, 3, and 4 Performance Requirement #1 Figure A4. Requirements for Single-Family Residential Projects

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