BEACH CHANGE AROUND DETACHED BREAKWATERS DUE TO ARTIFICIAL NOURISHMENT OF BYPASSED SAND

Abstract

Field experiment of sand bypassing was conducted on the Shimoni-ikawa Coast in Toyama Bay. The materials of 5900 m3 dredged on the updrift coast of the harbor were thrown into behind the detached breakwater. After the nourishment detailed surveys of the bottom topography around the breakwaters, wave observations and tracer tests using colored concrete blocks were carried out in order to investigate the movement of the nourished sand. Temporal and spatial changes of the shoreline positions and the sectional area of the beach in the shoreward zone of the detached breakwater are examined. It is found through the field experiment that the materials nourished behind the detached breakwater were carried slowly in the longshore direction by the westward littoral drift, dominating on the coast, without the outflow of sand through the openings of the breakwaters. It is concluded that the detached breakwater is useful to control the on-offshore sand movement and to retain the sand behind the detached breakwater.