Effect of Debris on Piers Group Scour: An Experimental Study

Debris accumulation upstream of bridge piers is a destructive agent against the piers stability. In this study, the effect of debris geometrical characteristics on the local scour at piers group was investigated. A large set of experimental tests were conducted to investigate the effect of piers configurations, as well as, the shape, thickness, length, and position of debris on the dimensions of scour hole. The results showed that the debris with rectangular shape caused the most scour depth. In addition, among the different configurations of bridge piers, the group piers (2 × 2) demonstrated the largest scour hole. In this configuration, high complex interactions occurred among the flow, sediment and piers which generated strong horseshoe and wake vortices around the piers. The observations showed that the maximum depth of scour hole increases as the debris thickness increases. In addition, more the debris effective length, more the depth of scour hole. In the case of debris with rectangular shape, as the relative thickness of debris increased from 0.5 to 2.67, the depth of scour hole increased 67%, 80%, 84% and 104%, in single, side by side, tandem, and group piers (2 × 2), respectively. By increasing the distance of debris from the water surface, the depth of scour hole increased at the first, and then decreased when the relative submergence depth of debris became 0.46. In this condition, the debris acted as a collar, prevented the bed from scour. The measured scour depths were compared with common empirical formula, the formula were modified by considering the experimental results of this study.