Water Quality Criteria (WQv)

Goal: capture and treat 90% of the annual runoff volume

• WQv = [(P)(Rv)(A)]/12

  where:
  
  P = 90% rainfall event in inches
  Rv = volumetric runoff coefficient (based on %I) = 0.05+0.009 x (I) where I is percent impervious cover
  A = site area (in acres)
  
  • P ranges from 0.8 to 1.2 inches in most regions of country
  • Minimum WQv of 0.2 in/ac when I is less than 15%
  • Criteria strives to achieve 80% TSS and 40% TP removal
  • Specific practices, design features or greater WQv are needed for higher pollutant removal

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The goal of the water quality criteria is to capture and treat 90% of the average annual stormwater runoff volume. This is equivalent to the 90th percentile rainfall multiplied by the volumetric runoff coefficient and site area. This criteria strives to achieve 80% TSS removal and 40% TP removal on an annual basis (assuming appropriate treatment practices are used and constructed, designed, and maintained correctly). Higher removal rates can be obtained using a greater water quality volume or specific design features.