---
title: "Friend's Equation"
canonical: "https://help.innovyze.com/space/infodrainage2020v2/16416897/Friend's%20Equation"
format: markdown
---
### Parameters

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

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

**Catchment Roughness - **Horton’s Roughness value.

### Calculation


![image](media://22455a19-34fe-4e1d-8635-1f598440c3d2)


where:

 B = Const (107 metric)

 S = Average catchment slope (m/m)

 L  = Length of overland flow (m)

 n  = Horton's roughness (similar but not identical to Manning’s n)

 Paved 0.015

 Bare Soil .0275

 Poorly Grassed .035

 Average Grass .045

<span style="color: #707070">Dense grass 0.06</span>

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