Roughness Coefficient Determination in Open Channel Flow with Vegetation

The resistance to flow in open channels depends on many flow and channel parameters. Out of the many factors, vegetation is the most important parameter in vegetative channels. Vegetation in an open channel retards the water flow by causing energy loss through turbulence and by exerting additional drag forces on the moving liquid. Vegetation in channels covers from highly flexible low grass blades to dense bushes to firm trees. Presence of vegetation in a channel modifies the velocity profiles, and hence the resistance in terms of roughness coefficients. The roughness coefficients and hence the velocity profiles of such channels change with the flow depths and section to sections. Because of this complex nature, it is hard to develop a flow model based on theoretical calculations and derivations. After many researches, the outcomes have helped to find an analytical model for defining the characteristics of vegetation that determine its effect on flow roughness. The paper presents different characteristics of vegetation that affect roughness coefficient of a vegetated channel. The paper offers review of the most represented methods of roughness coefficient evaluation used in practice especially for channels with vegetative beds.