WMS:OC Rational Equation

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Flows for the Orange County rational method are calculated from the following equation:

 Q = 0.90(I - F_m)A


  • Q – peak flow (ft3/s or m3/s)>
  • I – rainfall intensity (in/hr, mm/hr)
  • Fm – maximum loss rate, where Fm = ap * Fp
  • A – catchment area (acres)

This equation is very similar to the traditional Rational Method equation. The term I - Fm is often referred to as the effective precipitation.

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