Local Scour Around Riprap-Protected Bridge Piers with Debris Accumulation

Placing a riprap cover around the pier is a simple and cost-effective design to protect the bed against scour. One of the factors that can potentially endanger the effectiveness of riprap as a scour countermeasure is the accumulation of debris around the bridge pier. In the present study, using a laboratory flume, the effects of shape and position of the accumulated debris on scouring around a square bridge pier model with and without riprap under different hydraulic conditions have been investigated. The results showed that debris accumulation has no considerable impact on the performance of standard size riprap. However, in the presence of debris, the effectiveness of riprap against scouring reduces by 10% with a 25% reduction in the riprap stone median size compared to the standard size. For the no-riprap conditions, debris position considerably affect the maximum scour depth and the volume of the scour hole. However, in the presence of riprap, the maximum scour depth was not influenced considerably by debris position but the volume of the scour hole was greater in cases where debris is located near the bed than the corresponding test with debris located near the water surface.