Characteristics and mechanism of local scour reduction around spur dike using the collar in clear water

To reduce the local scour around the spur dike, the U-shaped collar is proposed in this study. The influence of the collar's length, width, and porosity on the local scour reduction in clear water is studied by model tests and numerical simulations. Experimental studies show that the collar has a significant effect on reducing the local scour. The location of the maximum scour depth of the spur dike moves downstream. The width of the collar has the greatest impact on the reduction effect among the three selected factors, followed by the porosity and the length. Local scour reduction efficiency of the collar can reach 56.9%. Based on the regression analysis of the range and variety, a formula for predicting the reduction effect around the spur dike is put forward, and the deviation between the values by formula and that in experiments are within ± 4%. The characteristics of the flow field around the spur dike under constant conditions with a collar are studied via numerical simulation. The numerical simulation results show that compared to the case without collar, the flow velocities around the spur dike in cases with permeable collar and solid collar reduced by 45% and 25%, respectively, and the shear stresses reduced by 20% and 28.6%, respectively. The results of this study can provide a reference for local scour reduction using the solid collar or collar made of permeable materials such as gabions.