🌧️ Modified Rational Method (Q = CIA Peak Flow & Detention Storage Acre-Feet)

Stormwater Detention Basin Volume Calculator

Size civil stormwater detention basins and dry retention ponds under local municipal drainage criteria using the Modified Rational Method: calculate peak pre-development vs post-development runoff (Q = CIA), determine required live detention storage volume (Acre-Feet or Cubic Feet to attenuate 10-year and 100-year storm hydrographs), and earthwork profits.

1. Project & Operating Inputs

Development Drainage Area (Acres e.g. 15.0 Acre Commercial Site)

Post-Development Impervious Runoff Coefficient C (e.g. 0.85 Paved/Roofs)

Customer Bid Price for Basin Excavation & Outflow Structure ($)

Heavy Earthmoving, Concrete Riser, Riprap Spillway & Seed ($)

2. Financial Breakdown & Yield

MODIFIED RATIONAL METHOD DETENTION

Turnkey Stormwater Basin Contract Profit

Civil Earthwork Contractor Margin

0.0%

Gross Revenue / Bill

Total Project CapEx

Operating / Material Cost