Effect of Non-submerged Rigid Vegetation in Floodplain on Bed Shear Stress around Bridge Abutment

Most of bridge abutments are located in floodplains and locating them in the main channels are less common. One of common cases in floodplains is the existence of vegetation cover. Scope of this study is investigation on the effect of non-submerged rigid vegetation in floodplain on bed shear stress around bridge abutment in compound channels. Experiments were carried out for different vegetation densities. Increasing vegetation density was 2 times at each step. Reynolds stresses of، and were used to calculate bed shear stress. Flow velocity was measured by electromagnetic 3D velocimeter at different depths. In this study، because of low flow depth in floodplain، electromagnetic velocimeter cannot measure fluctuations of velocity near the bed. Therefore، Reynolds stresses was calculated at different depths and extrapolation of them to the bed were used to estimate bed Reynolds stresses of، and. Results show that maximum bed shear stress occurred at the upstream corner of the abutment. Bed shear stress development at the upstream corner of the abutment was seen at about a 45 degree angle. In cases that vegetation cover was used، shear stress at the abutment nose decreased and shear stress at the main section of the compound channel increased. Also، with increase in vegetation density، shear stress at the abutment nose decreased. Shear stress at the abutment nose، 3. 84 N/m2 for the case with no vegetation، 3. 41 N/m2 for the case with s=16، 3. 04 N/m2 for the case with s=12 and 2. 61 N/m2 for the case with s=8، were calculated. The parameter S is the distance between cylinders in centimeter.