Non-uniform flow over cobble bed with submerged vegetation strip

Abstract

To better understand the influence of non-uniform flow in coarse-bed streams with a submerged vegetation strip on flow characteristics, data were collected from experiments in a laboratory flume and field investigations in a cobble-bed river reach in central Iran. The results of the experiments and field observations showed that the velocity profiles for non-uniform flows in cobble-bed streams with vegetation strips can be divided into either two or three sub-zones. The velocity profiles of non-uniform flows in the laboratory flume and cobble-bed stream deviated from the log law in the wake and mixing zones, but fitted approximately in the log-law layer zone. The distribution of Reynolds stress for non-uniform flows was influenced by the vegetation strip and showed a non-concave shape. Near the channel bed, sweep motion occurred more frequently than the ejection process. Close to the top of vegetation strip, the correlation coefficient (ru′w′) was negative, indicating downward momentum transport due to sweep and ejection processes. However, ru′w′ was positive near the water surface, indicating upward momentum by the vegetation strip due to inward and outward interactions. The study shows that laboratory results cannot be easily applied to natural rivers without considerable assumptions.