Stormwater Performance Requirements Flow Chart
A detailed flow chart outlining the performance requirements for stormwater management based on the size and type of development projects. The chart guides users through determining the necessary performance criteria for stormwater management based on project size, impervious surface area, and specific circumstances, helping to ensure compliance with stormwater regulations.
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Flow Chart to Determine Performance Requirements Start Here Is the project Exempt or does not discharge to MS4? YES NO Project is < 2,500 ft2 of 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
Projects 2,500 ft2 Project is > 5,000 ft2 net impervious area but < 15,000 ft2 of new and replaced impervious surface Project is > 2,500 ft2 of new and replaced impervious surface but < 5,000 ft2 of net impervious area Project is >15,000 ft2 of 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
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 1. Retain 95th Percentile event via infiltration 2. Retain 95th Percentile event via storage, harvesting, infiltration, and/or evapotranspiration 3. N/A 4. Retain 95th Percentile event via infiltration where overlying groundwater basin 5. Retain 85th Percentile event via infiltration 6. Retain 85th Percentile event via storage, harvesting, infiltration, and/or evapotranspiration 7. Retain 95th Percentile event via infiltration where overlying groundwater basin 8. Retain 85th Percentile event via infiltration 9. Retain 85th Percentile event via storage, harvesting, infiltration, and/or evapotranspiration 10. Retain 95th Percentile event via infiltration where overlying groundwater basin Project in WMZ 1, 2, 3, 6, or 9 No No additional Stormwater Requirements 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
DetachedSingle-FamilyResidentialProjects Project is > 2,500 ft2 new 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
