ChannelFlow: Open Channel Flow Basics
The Chezy-Manning equation for open channel flow is shown below, together with the general formulas that go with it. The ChannelFlow program uses these formulas to find the depth given a channel flow, using an iterative approach. 
The general channel parameters are the slope, the flow, and Manning's friction factor. The slope refers to the longitudinal channel slope in absolute terms (ft/ft), not as a percentage. Values for Manning's friction factor "n", are listed in the table below for quick reference.
Channel Lining | n | Channel Lining | n |
Concrete | 0.012 to 0.016 | Rock, smooth uniform | 0.025 to 0.035 |
Earth, straight uniform | 0.017 to 0.025 | Rock, jagged irregular | 0.035 to 0.045 |
Grass, below 1 foot hight | 0.09 to 0.18 | Corrugated metal | 0.024 |
Stone stream bed with weedy banks | 0.025 to 0.040 | Cast iron | 0.013 |
Floodplain with trees | 0.10 to 0.16 | Asphalt | 0.013 to 0.016 |
Floodplain with brush | 0.05 to 0.16 | VDOT EC-3 ditch matting | 0.05 |