BEACH EROSION AROUND A SAND SPIT - AND EXAMPLE OF MIHONO-MATSUBARA SAND SPIT

摘要

Topographic changes around Mihono-Matsubara Sand Spit were studied based on bottom sounding data and aerial photographs. On the Shizuoka and Shimizu coasts located upcoast of the sand spit, erosion waves are generated due to interruption of longshore sand transport by the construction of coastal structures. The propagating velocity of these erosion waves was found to range from 0.5 to 0.8km/yr. This agrees considerably well with the theoretically estimated value of 0.47km/yr. Comparison of beach profiles showed that the dominant beach changes take place at depths shallower than -7m, and this provides the closure depth for profile changes on this coast. The longshore sand transport rate was evaluated to be 1.3 x 105m3/yr at specific locations on the Shizuoka and Shimizu coasts, and it was found that this transported sand discharges into submarine canyons, forming a steep slope of around 1 /2, thus resulting in the net loss of sand from this coast.