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.