Flow velocity is dependent on the amount of friction between the water and the stream channel. Smoother channels will have less friction and, therefore, faster flow. Channel roughness contributes to turbulence, which dissipates energy and reduces flow velocity. The following equation is the Manning Equation, which is used to calculate average flow velocities in open-channel systems:
In this equation, V is the average flow velocity, R is the hydraulic radius, or the ratio of the cross-sectional area of the stream divided by the perimeter of the channel in contact with water (wetted perimeter), S is the slope of the water surface, and n is the Manning roughness coefficient (Note: this form of the Manning equation is for English units only. For metric calculations, 1.49 is replaced by 1). From the mathematics, it follows that a greater n-value will result in a smaller value for flow velocity. Here are some typical n values*:
| Mountain streams with rocky beds | 0.04-0.05 |
| Winding natural streams with weeds | 0.035 |
| Natural streams with little vegetation | 0.025 |
| Straight, unlined earthen channels | 0.020 |
| Smoothed concrete | 0.012 |
The open ditches in SMC Watershed would have a roughness coefficient of 0.020, while Seven Mile Creek itself would be nearly double that, between 0.035 and 0.045. As can be inferred from the Manning Equation, halving the roughness coefficient would double the flow velocity.