Abstract
Topography of beach cusps and grain size sorting was observed at four coasts, as well as the observation of a backwash flow in a bay of a cusp. A movement of sea water flowing obliquely from the apex to the bay was observed, resulting in the generation of a backwash flow. On the basis of this observation, two pairs of sink and source were assumed and three-dimensional cusp topography was predicted, using the contour-line-change model considering the effect of grain size change by Kumada et al.(2005). The formation of a cusp and associated accumulation of coarse materials around the apex were well predicted.
