Investigation of Different Modes of Riprap Failure for Submerged Spur Dike

Spur dikes are the most common structures for river bank protection. Spur dikes are used in the submerged and non-submerged forms. The submerged type is used when navigation is dominant in a river and in other cases the non-submerged ones are usual. Since construction of this structure causes higher flow velocities around it, this structure is exposed to erosion. Riprap around the dike structure nose is one of the most common and economic ways to protect spur dike. In this study the riprap stability and different modes of riprap failure in a mild 90-degree bend have been investigated. Experiments were conducted in a laboratory flume with a 90-degree bend. After specifying the critical spur dike along the bend, this one and the other two spurs setted before and after the critical one were protected by riprap. The variables were the length of the structure, spur spacing and height, riprap size and different Froude numbers, with a total of 200 experiments being carried out in this flume. Three modes of riprap failure including rolling, sliding and undermining as had been detected for straight channels and other hydraulic structures in the literature, were verified and finally an equation was developed to determine the mode of failure, which would be probably useful for designing riprap in real river engineering projects.