FORMATION OF DYNAMICALLY STABLE SANDY BEACHES ON THE AMANOHASHIDATE COAST BY SAND BYPASSING

Abstract

Due to the construction of fishery harbors up coast, severe beach erosion has taken place on the Amanohashidate Coast of Japan, a typical sand spit, resulting in such a reduction of the sand spit that countermeasures have had to be taken. The sand bypass method recently has been used to stabilize the sandy beaches on this spit. Annual change in the configuration of a sandy beach formed between groins first is considered in relation to erosion countermeasures and the sand volume bypassed. We found that the sandy beaches along the Amanohashidate Coast are being formed with little fluctuation at what approaches the stable condition. On the basis of the theoretical background of the formation of stable sandy beaches (Tsuchiya et al.,1993; Tsuchiya, 1994), a satisfactory comparison of the actual and theoretical shoreline configurations was made. Next, alongshore change in the longshore sediment transport rate was investigated in relation to the sand volume nourished and bypassed. We found that when the sandy beaches formed between groins have nearly reached the stable condition, the relation between the longshore sediment transport rate and wave energy power results in an estimated necessary sand volume to be bypassed of 5,000m3/yr. We conclude that in the case of the Amanohashidate Coast, the use of sand bypassing could well form a series of dynamically stable sandy beaches between groins, the sand volume required for bypassing being only 5,000m3/yr.