*3.2. Scour Hole Characteristics*

Marine structures such as bridges, docks, and so on, are supported by piers embedded into the sand. The piers obstruct the flow of water, and result in a distortion of the flow pattern and local scour. Such scour can have detrimental effects on the stability of the structure, and hence it is essential to evaluate scour hole characteristics, such as the depth, length, width, area, and volume, so as to prevent the structure from the adverse effects of bed erosion. Obstruction to the flowing stream by piers develops a vortex system in the flow field around them. A horseshoe vortex, upstream of the pier and near the bed, is dominant in the vortex system, and is responsible for sweeping out the bed material around the pier, producing a local scour hole. Moreover, a wake vortices system arises behind the piers, which is shed from their sides as a result of flow separation. Because of the vertical axes, the vortices suck the sand into their cores as they move downstream, and increase the scouring action along their line of movement. The variation in the flow pattern as a result of the piers results in the erosion of the bed material around the piers in both longitudinal and transverse directions. The lateral flow through the channel boundaries, in the form of downward seepage, leads to an enhancement of mass and momentum transfer.

Hence, in the presence of downward seepage, additional forces may be exerted, and this results in enhancing the rate of sediment transport. In this section, the effect of downward seepage on scour hole characteristics is studied.
