---
title: "Kerby's Equation"
canonical: "https://help.innovyze.com/space/infodrainage2021v1/16547886/Kerby's%20Equation"
format: markdown
---
### Parameters

**Flowpath Length - **The length of the Sheet flow path.

**Land Slope - **The slope of the hydraulic grade line.

**Catchment Retardance - **Retardance roughness.


### **Calculation**

![image](media://8b7cda53-0659-47a3-904e-e1d5bddf6b2d)

where:

L  = Length of overland flow (ft)

S = Average catchment slope (ft/ft)

n  = Retardance Coefficient

Smooth pavement 0.02

Poor grass, bare soil 0.30

Average grass 0.40

Dense grass 0.80

<span style="color: #707070">t</span><sub>c </sub><span style="color: #707070">= time of concentration (mins)</span>