Shear Stress around Vertical Wall Abutments

Shear stress distribution around vertical wall abutments is studied experimentally, and maximum shear stresses that occur at the upstream corner of vertical wall abutments are determined. Shear stress distributions are presented for Froude numbers ranging from 0.30 to 0.90 and for protrusion ratios (ratio of protrusion length perpendicular to direction of flow to total channel width) of 0.1, 0.2, and 0.3. Shear stress at the nose region of an abutment can be expressed as the sum of stresses due to contraction and the abutment structure alone. An expression for the nose shear amplification factor, Λ_nose, defined as the ratio of the bottom shear stress at the nose region to the approach flow shear stress, is derived. This amplification is due to the contraction and to the presence of a protrusion. In the computation of total shear stresses due to vertical wall abutments, expressions for the shear stress amplification at short contracted reaches, Λ_cont, and amplification due to abutment structure alone, ${\Lambda }_{\mathrm{nose}}^{\prime }$, are presented. Shear stresses around vertical wall abutments are amplified up to a factor of 10 and velocities are increased by up to 50% depending upon flow conditions and abutment protrusion ratios. Results are compared with previous experimental and numerical studies for shear stress distribution around groins and spur dikes. Local scour is mainly caused by increased flow velocity and shear intensity. By quantifying the shear amplification and by identifying regions of shear concentration, the method developed in this paper can be used to determine sediment sizes needed to protect structures against local scour.