Scouring of riverbed is caused by natural phenomena as well as by imposed structure like bridge piers. Scouring due to natural reasons is called general scour and takes place on large space-time scales. The principal reason for general scour in large river systems is geomorphology of the catchment areas. A second reason is varying width of a river. A bridge is often located at a narrow cross section of a river, but during flooding increased discharge at such sections causes sediment dislodging and consequent scour. Thus bridge piers at narrow sections may become susceptible to general scour depending on geomorphology of the bed.
Hydraulic structure such as bridge piers can cause scouring of the bed, such scour is called local scour. A flowing stream rapidly decelerates as it approaches the pier due to obstruction of flow. Consequently near the pier nose at narrow azimuthal angle it spreads in all four directions. Near the water surface a small volume moves up and rolls back forming the bow wave. The bulk of the fluid moves down and to continue the motion moves back along the bed and again returns to the main flow forming a complete circulation. At increased azimuthal angles the forward flow continues in degenerate spiral motion. In this way the circulatory motion along the base of the pier is formed in the shape of a horseshoe around the pier and is called the horseshoe vortex. The strong downward current (downflow) of the horseshoe vortex has a digging action around the base of the pier initiating scour process which grows in time creating a circular hole at the base. On the lee side of the pier, the flow is detached forming a wake consisting of vertical vortices that act as vacuum cleaners for the detached sediment particles that are drifted away .
