WRC Stormwater Volumes (Modified Rational)

Compute required volumes per WRC Stormwater Standards (April 2025) using Equation 3 and Table 2 depths.

Reference only—verify against the current manual, site soils, and your composite C calculations.

Composite C helper

Add subareas and weight their C values; sends the result to the main input.

MDOT Table 3-1 preset

MDOT 7th Ed Table 3-1 (R 280.9). Lower values for flat/permeable soils, higher for steep/impermeable (2:1 or steeper).

Area to add (ac)

Label	Area (ac)	C	Action

Total contributing area (ac)

Composite runoff coefficient C

Time of concentration Tc (min)

Used for Water Quality Rate (Eq. 18). TR-55 minimum of 5 minutes is a common lower bound.

Water Quality depth Pwq (in)

Channel Protection Volume Pcpvc (in)

Channel Protection Rate Control Pcprc (in)

Forebay depth Pfb (in)

100-year (24-hr) depth P100 (in)

Equation 3 (manual): V = 3,630 × P(in) × C × A(ac). Table 2 depths above are pre-filled from the April 2025 manual.

Manual links: WRC Stormwater Standards (oakgov.com) · Local PDF copy

Formulas: Eq. 3 (volumes) and Eq. 18 (Water Quality Rate). Confirm release rates, infiltration feasibility, and any agency-specific MEP decisions.