Optimal Dimensions of Protective Spur Dike for Mitigation of Scour Depth around Main Spur Dikes within a Straight Reach for Incipient Motion for Bed Sediment Movement

Maximum scour depth around a series of spur dikes is one of the important problems in their design. In this paper, the mitigation of scour depth around a series of consecutive spur dikes within a straight reach was investigated by using a protective spur dike located upstream of the first main spur dike. Effect of different parameters of protective spur dike is experimentally investigated including 4 ratios of protective spur dike length to the first main spur dike length (Lp/Lf) equal to 0.2, 0.4, 0.6 and 0.8, three angles of protective spur dike (θ) of 45, 90 and 135 degree and 4 ratios of protective spur dike distance to the first main spur dike length (X/Lf) equal to 1, 1.5, 2 and 2.5. The experiments were conducted for the velocity of incipient motion for bed sediment movement and bed size diameter of 0.91 mm (D50) and standard deviation of 1.38. The effect of protective spur dike on the flow pattern around the spur dikes was investigated in a rigid bed too. The experiments showed that the proper design of protective spur dike can significantly mitigate maximum scour depth around the main spur dikes. In particular, the protective spur dike with Lp/Lf =0.8, X/Lf=2, θ=45° decreased an average of 76.6 percent of scour depth around the spur dikes. The maximum decrease of scour depth around the first spur dike (74.7 percent) was obtained by applying the protective spur dike with Lp/Lf=0.8, X/Lf=2.5, θ=90°.