Flexible and Rigid Vegetation in Overland Flow Resistance

Abstract

<abstract><title><italic>Abstract.</italic></title> Vegetation generates resistance to overland flow through surface friction drag. Flexible vegetation oscillates with flow, which reduces flow momentum and consequently adds extra resistance. Rigid vegetation, on the other hand, generates turbulent vortices and wakes, which dissipate flow energy and create additional resistance. How would flexible and rigid vegetation compare with one another in formulating overall resistance? The answer to this question provides theoretical support for the design and construction of vegetation-related projects (e.g., vegetative filter strips) on hillslopes for controlling soil erosion and nonpoint-source pollution. The present study investigates and compares, through flume experiments, the behavior of flexible and rigid vegetation in overland flow resistance. The result shows that rigid vegetation created significant form drag via turbulent eddies and wakes. The form drag of rigid vegetation dominated flow resistance and increased with flow depth. In contrast, flexible vegetation caused much smaller flow resistance because it tilted toward the downslope direction, which significantly reduced turbulent wakes. For flows with rigid vegetation, the resistance can be well estimated using two variables: slope angle and relative roughness of vegetation width vs. flow depth.