The rainfall intensity used in the rational method is based on a specific rainfall duration and recurrence interval. The recurrence used depends on the importance of
Rational Method of Calculating Peak Rate of Runoff 237 Table 17.1. Table of runoff coefficients (C).
Topography, vegetation and slope Sandy loam Clay and silt loam Tight clay
Woodland Flat 0-5% Rolling 5-10% Hilly 10-30% Pasture Flat 0-5% Rolling 5-10% Hilly 10-30% Cultivated Flat 0-5% Rolling 5-10% Hilly 10-30%
the project. Terraces and waterways are designed for a 10-year recurrence, whereas spillways for dams may require a design based on a recurrence interval of 100 years or more. The duration of rainfall used in the rational method is determined by the time of concentration of the watershed.
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