Model for the time rate of local sediment scour at a cylindrical structure

We developed a semi-empirical mathematical model for the time rate of local scour at a circular cylinder located in an erodible cohesionless sediment and subjected to an unsteady water flow. The model can be used for both clear water and live bed scour conditions. A knowledge of the structure, flow and sediment conditions and the corresponding equilibrium scour depths is required as input to the model. The scour hole is assumed to have the idealized geometry of an inverted frustum of a right circular cone that maintains a constant shape throughout the scour process. The slope of the sides of the scour hole is uniform and equal to the submerged angle of repose for the sediment. Removal of sediment from the scour hole is limited to a narrow band adjacent to the cylinder where the effective shear stress is greatest. The sediment transport function used in the model is based on commonly used functions for transport on a flat bed.; The effective shear stress in the scour hole, used in the sediment transport equation, is a function of the normalized scour depth (scour depth/equilibrium scour depth) and the structure, flow and sediment parameters. The function for the shape of the effective shear stress versus normalized scour depth and its dependency on structure, flow and sediment parameters was determined empirically using data from a number of clear water and live bed scour experiments conducted by University of Florida and University of Auckland (New Zealand) researchers. These experiments cover a wide range of structure, flow, and sediments conditions.; Once verified with prototype scale data, the model can be used to estimate such things as the depth of scour that will occur during a time-varying flow event, such as that produced by a hurricane storm surge in coastal waters.