Analysis of different roughness coefficients' variation in an open channel 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. Presence of vegetation in a channel modify the velocity profiles, and hence the resistance in terms of roughness coefficients. The roughness coefficients of such channels change with the flow depths and from sections to sections. Because of this complex nature, it is hard to develop a flow model based on theoretical calculations and derivations. A laboratory study to explore the effect of vegetation in terms of rigid cylindrical roughness on the behaviors of Manning's roughness coefficient n, Chezy's coefficient C and Darcy-Weisbach's friction factor f in an open channel is presented. The study consists of flume experiments for flows with unsubmerged rigid cylindrical stems of a concentration and diameter arranged in a regular staggered configuration. The usual practice in 1D analysis is to select a value of n depending on the channel surface roughness and take it as uniform for the entire surface for all depths of flow. The influences of all the parameters are assumed to be lumped into a single value of n, C and f. Researches have shown that the coefficients not only denote the roughness characteristics of a channel but also the energy loss in the flow. The larger the value of n, the higher is the loss of energy within the flow Different roughness coefficients are found to vary differently with the non-dimensional hydraulic, geometric and surface parameters. Behaviours of different resistance coefficients due to vegetation are discussed and results are summarized and presented. Graphs of aspect ratio vs. n, C and f respectively are presented.