t_ch = time of concentration (minutes)

K = a units conversion factor taken as 0.0195 for Si units or 0.0078 for US units

L = channel flow length (m)

S = dimensionless main channel slope

A correction factor has been proposed to extend the use of the Kirpich equation outside the original studies catchment characteristics.

• 0 for natural grass surfaces
• 4 for flow over concrete or asphaltic surfaces
• 2 for flow in concrete channels

Applications

This method includes for both overland flow and channel flow, though an additional estimation of overland flow may be required where this is expected to be significant. As the formula is based on well-defined channels, the overland portion may be underestimated. This can lead to conservative high design rainfall intensities.

For this reason, this method is not widely used in the UK, though it can be useful as a check on a more modern method, or where the catchment parameters closely align with the original study. For this reason it has been included in the CivilWeb Rainfall & Runoff Calculator package. The calculated time of concentration can be taken forward into a rainfall intensity calculation and then used in a surface runoff calculation tool such as the rational method.

A nomograph which is often used to apply the Kirpich method is included below;