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Open water flow rate

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Open water flow rate diagram
The Manning formula is evaluated to give flow speed, using a substitution of A/P for the hydraulic radius. The volumetric flow rate is also given, by use of the discharge formula (Q=AV, where Q=discharge rate and V= velocity).
Cross-sectional area
Wetted perimeter
Slope (linear head loss)
Coefficient of roughness

Flow velocity
Discharge rate
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The Manning equation is used to find the flow or discharge (volume) rate of an open flow such as a river, stream, or irrigation channel. Flow occurs as a consequence of gravity and the slope of the conduit. While the Manning formula is convenient, it can easily give errors of up to 20%.
The Manning coefficient of roughness varies between ~0.02 for smooth, straight sides and ~0.075 for meandering flows with very uneven bottoms and banks.