BEACH EROSION INDUCED BY CONSTRUCTION OF DETACHED BREAKWATERS IN A POCKET BEACH OF A CLOSED LITTORAL CELL AND ITS PREVENTIVE MEASURES

Abstract

Beach erosion induced by construction of detached breakwaters in a pocket beach of a closed littoral cell was investigated taking the Ariake-Takahama coast facing the Pacific Ocean in Ibaraki Prefecture as the example. In order to protect houses against wave overtopping 8 detached breakwaters were built on this coast. This caused longshore sand movement toward the wave calm zones behind the detached breakwaters, whereas south beach was severely eroded by this sand movement. Advance of coastal forest and construction of sea dike to protect this forest further accelerated narrowing the sandy beach. Four measures to prevent this phenomenon are proposed and compared.