---
title: "Izzard's Formula"
canonical: "https://help.innovyze.com/space/infodrainage2021v1/16547888/Izzard's%20Formula"
format: markdown
---
### Parameters

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

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

**Event Intensity - **The intensity of the rainfall.

**Catchment Retardance - **Retardance coefficient.

### Calculation

![image](media://90fdae1d-c106-4789-8db9-28dfa6d9a456)


where:

![image](media://90d657a9-1850-4a7a-bfde-c256a6025fec)

L  = Length of overland flow (ft)

S = Average catchment slope (ft/ft)

i  = Rainfall Intensity (in/hr)

C<sub>r</sub>  = Retardance Coefficient

Very smooth asphalt 0.007

Tar and sand pavement 0.0075

Concrete 0.012

Tar and gravel pavement 0.017

Short grass 0.046

Dense grass 0.06

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