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TOOL-003 / STORMWATERReleased calculator

Rational Method Peak Runoff Calculator

Estimate preliminary peak runoff from catchment area, runoff coefficient and design rainfall intensity.

Method & assumptionsWorked example
Preliminary design. Select rainfall intensity and runoff coefficient from an appropriate design basis. This result does not establish code or permit compliance.
01

Runoff inputs

Define your catchment and design event.

Input units

A reference label for this calculation.

Contributing area at the design point.

Dimensionless catchment response.

Use the selected design event and duration, commonly informed by time of concentration.

SI and US inputs represent the same catchment. Switching units converts the values entered.

02

Peak runoff result

Your estimate appears here.

Ready for calculation

Review the inputs, then select Calculate peak runoff to see your preliminary flow estimate.

Q = C × i × A
03 / METHOD & DESIGN BASIS

Rational Method peak runoff

Understand the estimate before carrying it into your design.

Q = C × i × APeak flow = runoff coefficient × rainfall intensity × catchment area

TOOL-003 calculates peak runoff with Q = C × i × A. Internally, rainfall intensity is converted to m/s and catchment area to m², so the canonical result is m³/s.

Input guidance

Catchment area · A
Use the contributing drainage area for the design point.
Runoff coefficient · C
Select an appropriate coefficient for the catchment surface and design basis.
Rainfall intensity · i
Use the design-event intensity for an appropriate duration, commonly informed by time of concentration.

Applicability and limitations

The Rational Method is intended for peak-flow estimation under its accepted assumptions. This calculator does not derive an IDF curve, time of concentration, hydrograph, detention routing, pipe capacity, or network hydraulics. It does not automatically establish code or permit compliance.

Method reference

FHWA HEC-22 Urban Drainage Design Manual, represented in the WaterEngineerLab Source Registry as SRC-RATIONAL-METHOD-TOOL003-001.

CONTINUE YOUR WORKFLOW
Before estimating runoffTime of concentration

Check the catchment response time.

After estimating runoffStorm-drain pipe capacity

Evaluate conveyance capacity.