Streamflow calculator (Manning)

Estimate the flow rate Q in m³/s of a channel or river reach using the Manning formula. Pick the section geometry, enter the dimensions, and you'll get the area, hydraulic radius, velocity, and flow rate.

Q = (1/n) · A · R^2/3 · S^1/2

Parameters

Section shape

Width b (m)

Flow depth y (m)

Slope S (m/m)

e.g. 0.001 = 1 m drop per 1000 m.

Manning coefficient n

Results

Wetted area A

0m²

Wetted perimeter P

Hydraulic radius R

Velocity V

0m/s

Flow rate Q

0m³/s

0 L/s

How to interpret the formula

n - Manning roughness coefficient: Measures the friction of the bed and walls. The rougher the surface (vegetation, stones), the higher the n and the lower the velocity for the same slope.
A - wetted area: Cross-sectional area occupied by the water.
P - wetted perimeter: Length of the solid boundary in contact with the water (does not include the free surface).
R - hydraulic radius: R = A / P. Characterizes the hydraulic efficiency of the section: a higher R means lower relative friction.
S - bed slope: Longitudinal slope in m/m. In uniform flow it is assumed equal to the slope of the energy line.
V and Q: V = (1/n) R^2/3 S^1/2; Q = V · A.