EngineerHub.orgWhat this calculator does
Calculates Rational Method peak flow and a separate event runoff volume so users do not confuse conveyance sizing with storage volume.
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Background
Optional reading — open any section below.
The Rational Method estimates peak runoff as Q = C i A. It is commonly used for relatively small drainage areas when rainfall intensity is selected for a duration at least as long as the catchment time of concentration.
Peak flow controls conveyance capacity. Event volume is a separate calculation: V = C P A. A pipe may pass the peak while a storage basin is controlled by total event volume and routing.
Method and cautions follow the FHWA Urban Drainage Design Manual (HEC-22). Use local authority criteria and approved rainfall data for design.
Frequently Asked Questions
Practical questions about assumptions, inputs and limitations.
Use locally approved coefficients for the surface and recurrence interval. Composite C may be area-weighted across roofs, pavement, landscaping and other surfaces.
It gives a screening inflow. Hydraulic capacity, inlet control, tailwater, surcharge and network routing require separate analysis.
No. Confirm rainfall data, time of concentration, runoff coefficients and applicable design standards with the responsible authority.