Prediction of scour profiles downstream of grade control structures via the shear stress and sediment bed curvature model

A new semianalytical model for the prediction of local scour profiles downstream from typical grade-control structures is proposed on the basis of the variation in bed shear stress and the sediment bed curvature concept. The proposed method was applied to a continuous boundary between the flow and sediment regions to predict the scour profile downstream of submerged sharp-crested weirs. By applying the momentum equation, the nappe flow over the weir was modeled as an oblique point force on the bed surface boundary, and the eroded profile was represented by a system of differential equations. The scour length and sediment resistance strength are the two unknowns in the shear stress and sediment bed curvature (SSC) differential equations. A series of laboratory experiments were carried out under clear water conditions to evaluate the accuracy and performance of the proposed model. The scour profile was calculated via prediction equations that are based on the known maximum scour depth, dmax, which was proposed in this study and in the literature. The effects of the submergence ratio and flow intensity on the maximum scour depth and scour profile were investigated, and a model was developed to predict the equilibrium scour depth. The prediction error associated with equations based on the equilibrium scour depth, dmax, resulted in a significant error in scour length prediction. Furthermore, the deviation between the measured and predicted geometrical characteristics was also correlated with predictions of dmax and scour length, L, as functions of flow intensity and submergence.