Determination of flow resistance of vegetated channel banks and floodplains

Flow resistance due to vegetation may greatly affect the conveyance of a channel, and thus evaluating the resistance is a critical task in river engineering and restoration. Therefore, flow resistance of natural willows and sedges was studied in a laboratory flume. The aim was to investigate, how type, density and combination of vegetation, flow depth and velocity influence vegetal drag or friction losses. Friction factors, f, and vegetal drag coefficients, C'd, were determined for a selection of 170 test runs. The results showed large variations with depth of flow, velocity, Reynolds number and vegetal characteristics. E.g. the vegetal drag coefficient for the leafy willows was three to seven times that of the leafless willows. The experimental drag coefficients for the leafless willows were compared to the values predicted by four methods, which were developed based on theory and experiments on rigid cylinders. River Flow 2002, Bousmar & Zech (eds.) © 2002 Swets & Zeitlinger, Lisse, ISBN 90 5809 509 6