Wave reflexion from beaches

Abstract

The usual way of posing the problem for the reflexion of wave trains from beaches seems inevitably to imply perfect reflexion. Energy considerations show that wave absorption must be associated with the degradation of mechanical energy either through wave breaking or viscous effects. Some experiments reported here showed substantial wave absorption in the absence of any breaking.We describe some theoretical and experimental work aimed at assessing the role played by friction at the bottom in determining the reflexion coefficient of a beach. The results suggest that, if the parameter (νω3)½½gα2 is not too small, bottom friction can be a significant factor in the absorption process for waves on beaches. Here ν represents the kinematic viscosity (or perhaps an ‘eddy’ viscosity) of the fluid, ω is the frequency of the motions, α is the slope of the beach and g is the acceleration due to gravity.