Channel flow calculator
This simple program calculates the flow of water in a rectangular or trapezoidal channel using Manning's Formula. The formula is only applicable to channels that have a relatively uniform profile and slope.
Method
- Choose a straight reach of channel with a length at least 5 times the width, with a constant slope.
- Survey at least 3 cross sections to determine average channel dimensions.
- Survey the channel bed (as an estimation of the water surface level) to determine the energy gradient (slope S) of the reach.
- Estimate a value for Manning's Roughness Coefficient (𝜂) from the table below.
- Enter channel dimensions into calculator to determine velocity (V) and unit discharge (Q).
Mannings roughness coefficients
| Description | Coefficient (𝜂) |
|---|---|
| Earth channel | |
| Fairly regular section | 0.022 |
| Irregular section with pools, slight channel meander | 0.037 |
| Gravelly channel | |
| Fairly regular section | 0.025 |
| Irregular section with pools, slight channel meander | 0.040 |
| Stony, cobbles channel | |
| Fairly regular section | 0.035 |
| Irregular section with pools, slight channel meander | 0.050 |
| Some grass or weeds, little or no brush | |
| Fairly regular section | 0.030 – 0.035 |
| Irregular section with pools, slight channel meander | 0.045 – 0.050 |
| Dense growth of weeds, little or no brush | |
| Fairly regular section | 0.035 – 0.050 |
| Irregular section with pools, slight channel meander | 0.050 – 0.065 |
| Some weeds, light brush on banks | |
| Fairly regular section | 0.035 – 0.050 |
| Irregular section with pools, slight channel meander | 0.050 – 0.065 |
| Some weeds, heavy brush on banks | |
| Fairly regular section | 0.050 – 0.070 |
| Irregular section with pools, slight channel meander | 0.065 – 0.085 |
Maximum permissible velocities for various materials
| Surface Material | Permissible Velocity (m/s) |
|---|---|
| Sand | 0.4 |
| Sandy loam | 0.5 |
| Silty loam | 0.6 |
| Fine gravel | 0.8 |
| Stiff clay | 1.2 |
| Coarse gravel | 1.2 |
| Grass mixtures | 0.8 – 1.5 |
| Jute matting | 1.5 – 1.8 |
| Grass: Couch, Bent, Fescue | 1.5 – 2.2 |
| Grass: Kikuyu | 1.8 – 2.5 |
| Stones 150 mm | 2.5 – 3.0 |
| Stones 300 mm | 4 - 5 |
| Gabions/Reno mattresses | 4 - 6 |
Mannings Formula
V Velocity
R Hydraulic radius
S Energy gradient (Slope)
𝜂 Roughness coefficient
Q Unit discharge
A Cross-sectional area of flow