Q. Local Scour at obstructions in rivers?

When the water flow in river is deflected by obstructions such as bridge piers, scouring would occur arising from formation of vortexes. The mechanism of formation of vortices is as below: the flow hits the bridge piers and tends to move downwards. When flow reaches the seabed, it will move in a direction opposite to its original flow direction before hitting bridge piers. Therefore this movement of flow before bridge piers results in the formation of a vortex. Owing to formation of this vertical vortex, seabed material is continually removed so that holes are formed at the seabed and this result in local scour at bridge piers. As the shape of vortices looks such as horseshoes, it's sometimes known as 'horseshoe vortex'.