---
title: "US Federal Aviation Authority"
canonical: "https://help.innovyze.com/space/infodrainage2021v1/16547857/US%20Federal%20Aviation%20Authority"
format: markdown
---
### Parameters

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

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

**Average Runoff Coefficient - **The proportion of the rainfall that runs off of the area.

### Calculation

![image](media://1d5b5f47-672f-4094-bee3-ff4820259059)


where:

L  = Length of overland flow (ft)

A = Catchment area (ha)

S = Slope (%)

C = Runoff coefficient (0 - 1)

Business district 0.70 - 0.95

Single family residential 0.30 - 0.50

Asphalt/Concrete 0.70 - 0.95

Sandy soil lawn 0.05 - 0.20

Heavy soil lawn 0.13 - 0.35

Brick 0.70 - 0.85

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