Rational Method Calculator

What is the Rational Method?

The Rational Method is a simple and widely used hydrologic technique for estimating peak discharge from small drainage basins. It relies on the principle that the maximum rate of runoff occurs when the entire watershed is contributing to the flow at the outlet.

Formula: Q = C × I × A

• C: Runoff coefficient (dimensionless, depends on land use and surface type)
• I: Rainfall intensity (mm/h)
• A: Drainage area (km² or hectares)

This method is highly useful for designing storm sewer systems, culverts, and other small hydraulic structures where the drainage area does not exceed certain limits (typically up to a few square kilometers).

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