Bed sill effect on bridge pier scour with debris obstruction: an experimental investigation

Flooding and scouring are two important physical phenomena in river environments. Evaluating the bridge pier scour after floods is essential owing to the woody debris accumulation upstream of these facilities. The present study considered three models of experimental tests a cylindrical pier with a diameter of D (model A), a cylindrical pier with rectangular debris (model B), and a cylindrical pier with rectangular debris protected by bed sill (model C) to investigate the pier scour values under different densimetric Froude numbers (Fr_d) and the longitudinal gap between sill and downstream pier’s face (L_b). Results indicated that maximum scour depth (d_s) increases with debris accumulation. Therefore, it seems necessary to conduct the present research, the bed sill with five ratios of L_b/D was considered for three flow conditions. When the bed sill was utilized between the pier and debris in the sediment bed, d_s/D was reduced compared to the case without sill, and the best efficiency of bed sill was about 40% for L_b/D = 0. The efficacy of the sill was reduced, by increasing Fr_d, and at Fr_d = 2.4, only the sill attached to the pier was effective for scour reduction, and L_b/D greater than 1 even increased d_s/D to a greater value than the condition of the bridge pier without debris. Finally, an equation based on the effective parameters was suggested with the observed data to predict d_s/D in the conditions of using the bed sill with debris accumulation with RMSE = 0.046 and R² = 0.998. It was observed that scour depth increases in the higher densimetric Froude number and becomes larger in a farther sill case.