Impact of vegetation cover on the hydraulic characteristics of rectangular and rounded broad-crested weirs

The hydraulic characteristics of rectangular and rounded broad-crested weirs covered by flexible vegetation were studied in a laboratory flume. Free-surface and velocity profiles were measured for a range of upstream head relative to weir crest lengths 0.075 < H / L < 1.3 and equivalent vegetation roughness heights 18 mm < k s < 318 mm. The results demonstrated significant differences in hydraulic performance between the smooth- and vegetated-crested weirs. The effect of vegetation roughness on discharge reduction was reflected by the boundary shear stress on the weir crest. Analysis of the discharge coefficient data suggested that flow over vegetated-crested weirs with different vegetation cover densities can be classified into broad-crested weir flow for 0.1 < H / L < 0.4 and narrow-crested weir flow for 0.4 < H / L < 1.3. The flow discharge coefficient increased with increasing H / L , H /( H + P ), and H / k s . Two formulae were proposed to predict the flow discharge coefficient correction over vegetated-crested weirs with different crest lengths and heights, for both rectangular and rounded crested weirs . • Different discharge capacities are shown for smooth and vegetated-crested weirs. • Weir discharge coefficient C D increases with increasing H / L , H /( H + P ), and H / k s . • Formulae are proposed to predict C D for rectangular and rounded-edged weirs.