Experimental study of scour of cohesive bed material subject to side weir flow

Side weirs are hydraulic structures widely used in irrigation and drainage systems, urban sewage systems, water treatment plants and also intake structures for flow diversion or intake purposes. Local scour around hydraulic structures in non-cohesive sediment beds has been extensively studied before. But hydraulic structures can be existed in either purely cohesive sediment or sand-clay mixed bed. The scour process in cohesive sediment is different from that in non-cohesive. The scour process in cohesive sediment is not well understood, because of resistance of the material and also the complicated behavior of cohesive material. In this study, scour process and dimensionless equilibrium scour depths around rectangular side weir located on the rectangular channel is investigated. Three different dimensions of side weir with a length of L=25, 40 and 50 cm and height of p=7, 12 and 17 cm are considered. All experiments are conducted in clay-sand mixed bed for subcritical flow conditions (Fr=0.33-0.81). Dimensional analysis showed that the dimensionless equilibrium scour depth (Hde/h1) depends on the dimensionless parameters of Fr, p/h1, L/b, C, Wc and s/(.g.h1). For purely kaolin and in low sand ratio (5, 7 and 10%) bed material, scour did not occur. The dimensionless bed shear strength of cohesive sediment increases with consolidation time. Therefore, the scour depths decrease with consolidation time. In sand-clay mixed beds the scour depth increased rapidly with time and then asymptotically approached constant values in all experiments for all side weir dimensions as in non-cohesive sediment. Even though Froude Numbers in cohesive sediment are larger than that in non-cohesive, the scour depths are smaller with respect to non-cohesive sediment. For all side weir dimensions, dimensionless scour depths increase with increasing upstream Froude number. As the side-weir crest height increases, equilibrium scour depths decrease. The data indicates that 10-70% of the equilibrium scour depth is attained during a time period 10% of equilibrium time. 50-95% of the equilibrium depth of scour is attained during a time period 50% of equilibrium time, depending on the flow intensity and side weir dimensions. An equation is obtained for dimensionless equilibrium scour depths depending on dimensionless parameters. Regression analysis showed that the effect of the side weir lenght is quite small for these experiment parameters range. Side weir crest height is more effective parameter than the side weir length. Another important parameter is initial water content of bed material. Even small changes of this parameter ( ) cause significiant effect in scour depths.