THE PREVAILING DIRECTION AND MECHANICS OF BEACH DRIFT INFERRED FROM VARIATION SERIES OF BEACH SEDIMENTS ALONG THE SAGAMI BAY COAST, JAPAN

Abstract

A petrographic study of beach sediments can be expected to contribute much to the understanding of their source, transportation and deposition. Therefore, we examined the gravel and mineral content of sediments samples taken along the shoreline in an effort to determine the prevailing direction of beach drift and to make clear the mechanics of littoral transport. Field work was done in the Sagami Bay area, on the Pacific coast of Japan. The shoreline along the coast of the Bay is marked by some 40 miles continuous beach which is exposed to the open sea. At the eastern end of beach, there lies Miura Peninsula and at the western end, Izu Peninsula. 38 sampling stations were selected at one mile intervals along this stretch of beach. Then, those samples collected were used as basic data for petrographic analyses. The major conclusions of this investigation are as below : 1) The lithological composition of beach deposits in the coast of major river mouths and their neigh borhood are similar to that of fluvial deposits of those rivers. This similarity led to the conclusion that beach deposits have a close relationship to river-transported materials. In addition, the beach sediments deposited in this coastal region are in a great quantity derived from the rivers of Sagami, Sahawa and Maya. 2) Based on the areal distribution and systematic variation series, it is believed that littoral brift in this Bay prevailingly moves eastwards by longshore currents. 3) There is decrease in grain size of the beach gravels with increase in distance away from the source areas. This decrease in size is probably due to the result of abrasion of gravels rather than the sorting action of waves which transports gravels selectively. Moreover, beach gravels seem to move intermittently along the shoreline and once these gravels deposit on the beach, they should be abraded and deformed to various shapes in accordance with the strength of local littoral force. 4) In some places of this stretch of beach, where are islands or reefs in the offshore, numerous wellrounded, extremly flat gravels are present. In addition, both the amount of heavy mineral contents and the ratio of quartz to feldspar increase materially. Those phenomena might be explained by the fact that the beach sediments once deposited remain for a long time under the effect of the refracted offshore waves caused by the hindrance of islands or reefs.