Roughness Coefficients in Open Channel Flows with Submerged Rigid Vegetation

Abstract

This paper investigates the roughness coefficients in open channels with submerged rigid vegetation. In this study a series of laboratory experiments were undertaken at NIT, Rourkela to investigate the effect of submerged vegetation on flow resistance. The vegetation consisted of rigid rods replicating stem of a tree. Velocities were measured in 3-D Acoustic Doppler Velocimeter with both downward facing and upward facing probes. Vegetations have higher roughness value due to surface drag to resist the flow which decreases the conveyance capacity. The total flow depth in the channel is divided in to two different zones such as vegetation zone and non-vegetation zone or surface layer. The results showed that the longitudinal velocities decreased within and near the vegetation zones and the flow increased towards the non-vegetated zones. Also, the velocity fluctuations and Reynolds stresses are observed to have higher values in the transitions zones between vegetated and non-vegetated zone or near the top of the vegetation in open channels. New expression for predicting roughness coefficients in open channel flows with submerged rigid vegetation has been attempted.