Erosional Hot Spot Prediction through Wave Analysis

Construction of beach nourishment projects requires large volumes of sediment, which are usually obtained by the removal from borrow pits located in reasonably shallow water. These modifications of the nearshore bathymetry have the potential to affect wave transformation processes and thus to alter the equilibrium shoreline planforms landward of the borrow pits. Recent cases of erosional hot spots (EHS's) associated with beach nourishment projects have increased interest in the prediction of the mechanisms of borrow pit alteration of the local wave field. A better understanding of the interaction of borrow pit characteristics such as size and depth with the incident wave field is needed to determine the effect of the pits on the nourished beach and to anticipate the effects of various designs. The present study employs numerical and theoretical approaches. The numerical analysis uses a boundary element approach for a pit of finite dimensions in a uniform shallow water depth as in Williams. An analytical solution to the shallow water pit problem is obtained using the general method of MacCamy and Fuchs with a circular pit instead of a solid cylinder and allowing a solution inside the pit with appropriate boundary conditions. For both the numerical and analytical solutions, the velocity potential can be determined anywhere in a fluid domain containing a pit, or a shoal, of uniform depth. This allows for the calculation of the free surface elevation as well as other wave related quantities such as wave direction, wave energy flux and wave-induced shoreline change.