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.

m
m
m
m
m/m
m per sec
m3 per sec

Method

  1. Choose a straight reach of channel with a length at least 5 times the width, with a constant slope.
  2. Survey at least 3 cross sections to determine average channel dimensions.
  3. Survey the channel bed (as an estimation of the water surface level) to determine the energy gradient (slope S) of the reach.
  4. Estimate a value for Manning's Roughness Coefficient (𝜂) from the table below.
  5. Enter channel dimensions into calculator to determine velocity (V) and unit discharge (Q).

Mannings roughness coefficients

DescriptionCoefficient (𝜂)
Earth channel
Fairly regular section0.022
Irregular section with pools, slight channel meander0.037
Gravelly channel
Fairly regular section0.025
Irregular section with pools, slight channel meander0.040
Stony, cobbles channel
Fairly regular section0.035
Irregular section with pools, slight channel meander0.050
Some grass or weeds, little or no brush
Fairly regular section0.030 – 0.035
Irregular section with pools, slight channel meander0.045 – 0.050
Dense growth of weeds, little or no brush
Fairly regular section0.035 – 0.050
Irregular section with pools, slight channel meander0.050 – 0.065
Some weeds, light brush on banks
Fairly regular section0.035 – 0.050
Irregular section with pools, slight channel meander0.050 – 0.065
Some weeds, heavy brush on banks
Fairly regular section0.050 – 0.070
Irregular section with pools, slight channel meander0.065 – 0.085

Maximum permissible velocities for various materials

Surface MaterialPermissible Velocity (m/s)
Sand0.4
Sandy loam0.5
Silty loam0.6
Fine gravel0.8
Stiff clay1.2
Coarse gravel1.2
Grass mixtures0.8 – 1.5
Jute matting1.5 – 1.8
Grass: Couch, Bent, Fescue1.5 – 2.2
Grass: Kikuyu1.8 – 2.5
Stones 150 mm2.5 – 3.0
Stones 300 mm4 - 5
Gabions/Reno mattresses4 - 6

Mannings Formula

V Velocity
R Hydraulic radius
S Energy gradient (Slope)
𝜂 Roughness coefficient
Q Unit discharge
A Cross-sectional area of flow