Rational Method Runoff Basics
When this applies
Use this guide for early drainage conversations when you must show assumptions for intensity, area, and coefficient transparently.
Tool to use
Browser-based preliminary check for Rational Method Runoff Calculator — fast estimates, not code or stamped design output.
Open Rational Method Runoff Calculator →Steps
- 1Define catchment boundary and contributing impervious vs pervious fractions.
- 2Pick a design storm intensity and duration appropriate to local guidance.
- 3Choose a runoff coefficient consistent with cover type and slope.
- 4Compute peak flow and sanity-check magnitude with experience and alternate methods.
- 5Move to hydrograph routing when storage and timing effects dominate.
Examples
- Parking lot inlet sizing discussion.
- Roof drain peak for a small building.
What to avoid
- Using rational method for very large basins without checking validity limits.
- Mixing intensities from different return periods without labeling.
- Ignoring downstream constraints while only sizing a pipe segment.
Related tools
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Browse all task guides or see the full list on the Engineering hub.
FAQ
Is this code-compliant?
It is a teaching-oriented screening tool; local codes may require approved hydrology methods.
Snowmelt?
Not included; extend model when winter hydrology matters.