Small stream flowing through a valley near the French Pyrenees mountains Water pouring out of a culvert and flowing downhill within a paved channel

"Roughness" of a stream channel increases due to the presence of rocks, vegetation, and debris. Channelizing a stream by doing such things as removing vegetation and lining the stream bed with concrete will reduce the roughness. The roughness factor has a direct impact on how quickly the water will move in the channel and how high the peak stage will be. Manning's equation is often used in hydrology to account for the roughness factor.

comparative hydrographs for smooth surface and rough channels

The greater the roughness, the more turbulent the flow. More turbulent flow results in slower runoff and streamflow velocities. This allows more time for infiltration, and it also results in a broader flood wave with lower peak discharges than in rapid runoff situations.

Conversely, reducing channel roughness results in faster streamflow velocities and greater peak flows.

Additional Resources

Refer to the Streamflow Routing module, Section Four: Stream Properties and Manning's Equation for more information on Manning's Equation.