Scouring in the river is a phenomena which is the result of interaction of flowing water and sediments which change the topology of the rivers bed and coastal area. Every year, many bridges fail not for structural reasons, but due to the scouring at their piers and abutments (Fig. 1). This happens particularly during floods, i.e., when bridges are mostly needed (Fig. 2). Bridge failure during the flood leads to disorder in transportation systems and may cause loss of life and properties.
The most common cause for bridge failures reported in the literatures was due to pier scour. Flow pattern around bridge piers is very complex, particularly at the vicinity of the scour hole (Fig. 3). The key factor in development of the scour hole is the presence of complex vortex system formed around the bridge piers. Upon the impact of the flow on the pier nose, a down-flow is formed in front of the pier. The down flow impinges the stream bed, initiating the scour hole in front of the pier, and rolls up to form a complex vortex system. Owing to its similarity to a horseshoe, this vortex is called horse-shoe vortex. Furthermore, separation of the flow downstream of the pier forms the wake vortices. These vortices act as little tornados lifting the bed material and generate an independent scour hole downstream of the pier.
Many different types of countermeasures for local scour at bridge piers are introduced in the literature. Using riprap is one of the common countermeasures to control the pier scour (Fig. 4). Placing a riprap layer locally around the pier increases the resistance, and scouring around the pier can be prevented.
Collar is another type of countermeasures to control scouring around the piers by diverting the down-flow (Fig. 5). Collar acts as an obstacle for the down-flow and reduces the strength of the horse-shoe vortex. The efficiency of collars depends on the collar size and the elevation.
Recent researches are focused in the pressure flow where the bridge deck is submerged (Fig. 6), scour of wide pier with raft footings (Fig. 7), the effect of scour hole on the capacity of foundation pile (Fig. 8), etc.
Author Papers on Pier Scour
Zarrati, A.R., Chamani, M.R., Shafaie, A., and Latifi, M. (2010), “Scour countermeasures for cylindrical piers using riprap and combination of collar and riprap”, International Journal of Sediment Research, 25(3), 313-321. More details...
Aarabi, M.J., Chamani, M.R., Dehghani, A.A., and Asghari, K. (2011), “Numerical Simulation of Temporal Evolution of Local Scour in Bridge Pier with Nonuniform Sediment”, International Conference on Civil Engineering, Architecture and Building Materials(CEABM 2011), Haikou, China, June 18-20, 3610-3614, More details...