Abstract
On the Moriya coast located in south part of Boso Peninsula, a small river flows into the sea, and the mouth of this river is stably fixed at a location behind the rocks located at the central part of this pocket beach. Because the wave-sheltering effect of the rocks were seemed to be the cause that the river mouth was fixed behind the rocks, field investigations were carried out at the river mouth to measure the topographies of the beach and river mouth, and the wave-sheltering effect was calculated by using the angular spreading method. The primary cause of the formation of a river mouth behind the rocks is found to be due to the decrease in the wave diffraction coefficient behind the rocks.
