Estimation of Form Drag Caused by Rigid Vegetation Based on Equivalent Roughness

Abstract

The flow structure over vegetation in ecological channel is rather complex and the resistance caused by vegetation is affected by many factors, such as discharge, flow depth, vegetation density and patch arrangement types, etc. In order to simplify vegetation characteristics, the rigid cylindrical sticks arranged in the open channel were used to simulate the stems of non-submerged vegetation. The volume of the vortex structure near the vegetation patch can be determined through velocity distribution in vegetated channel and the velocity distribution is measured by Acoustic Doppler Velocity meter. A new formula of equivalent roughness based on the vortex volume near the vegetation patch has been proposed and the form drag can be further determined by the new equivalent roughness. Based on theoretical analysis and experimental results, it shows the vortex volume plays an important role on flow resistance caused by vegetation. The theoretical and experimental results on form drag achieve a good agreement on various experimental conditions. The relation between the relative roughness and relative vortex length was also derived and verified by different experimental conductions. For the dependence of relative roughness on vegetation density, the empirical formula between relative roughness and vegetation density was further proposed and analyzed.

Keywords

• Form drag
• equivalent roughness
• vortex structure
• rigid vegetation
• open channel