SETUP INSTABILITIES IN THE NEARSHORE HYDRODYNAMICS

The incoming wave field on a beach produces a setup on the mean water level elevation. In earlies studies, this setup had been hypothesized as an instability source in nearshore hydrodynamics capable of driving processes such as rip currents, horizontal eddies and rhythmic topogr aphy. An analytical study is presented in order to investigate if such a setup in isolation, that is, without longshore current and without wave-refraction, can cause the growth of infinitesimal disturbances ca pable of driving such processes. It is shown that the setup can not be considered as an instability source since the equation governing the disturbances is found to be a wave-type equation. However, the phase s peed differs slightly from the classical value root gD, and becomes an isotropic. This can be interpreted as an effect of the radiation stres ses of short incoming waves on long wave propagation.