Scour of sand beaches in front of seawalls.

Many previous studies were confined to the problem of beach erosion due to waves breaking on the structure. The investigation reported here involved regular non-breaking, shallow water waves progressing toward a seawall. An analytical solution was developed and compared with laboratory-scale experiments. The shallow-water wave theory and boundary layer equations were used in theoretical development, which resulted in a mathematical model for the ultimate scour depth in front of a seawall. The theoretical equation for scour is as follows: S=(D-1/2A) [(1-Cr) u* (3/4 Cd Rho(cot Theta - d(Gamma s - Gamma)))to the 1/2 power - 1] where D = still water depth, A = Hi+Hr, Hi = incident wave height, Hr = reflected wave height, and Cr = Hr/Hi = coefficient of reflection, also u* = horizontal velocity within boundary layer, Cd = coefficient of drag, Rho = fluid density, Theta = seawall slope angle, d = effective sand diameter, 50% finer, Gamma s = specific weight of sand, and Gamma = specific weight of water. The comparison between theoretically calculated values and experimental results indicates fairly good agreement. The model experiments also indicate that depth of scour depends to a large extent on wave characteristics and that scour length (distance between scour trough or crests) is independent of time, but is a function of wave length.