Comparison of the Effect of Three Riprap Placement Levels on Its Stability at Bridge Abutment in a 90o Bend Based on Incipient Motion of Riprap Particles

Scouring is a type of local erosion that usually occurs downstream of most of hydraulic structures and around any obstacles in rivers. This phenomenon can be intensified at the river bend due to occurrence of secondary flow in comparison with straight channels، so attention to the erosion and its control and mitigation at this part of river is very important. Using riprap is one of the usual methods to protect river banks and bed against erosion. In this research، the riprap placement level at a bridge abutment site located at a 90 degree river bend was investigated. This research was carried out to observe and analyze the riprap behavior with various specific gravities and sizes and three different riprap placement levels namely، the same، upper and lower than river bed sediments level. Three different types of riprap with specific gravities of 1. 51، 2. 05 and 2. 65 and four different sizes of 4. 75، 9. 52، 12. 7 and 19. 05 millimeters were also utilized. The results show that the riprap placed at the bed level has the most stability، and the riprap placed above the bed has the lowest stability.