BEACH TRENCH SURVEY FOR 3-D STRUCTURE OF SAND GRAVEL BEACH PROTECTED BY DETACHED BREAKWATERS

Abstract

 Beach trench surveys were conducted on a sand gravel beach protected by detached breakwaters in Kochi Coast. Alongshore and cross-shore trenches were excavated with a total length of 150m. The trench faces were recorded by a RTK-UAV and compared with topography data. A clear boundary between sand and gravel layers was observed at a level of 2m above sea level. The bottom gravel layers were considered to be developed by swash waves at the construction of detached breakwaters. The beach was found to be eroded by several high waves in last 15 years. The erosion level, however, was found to be up to the level of sand-gravel boundary, which implied that the sand gravel beach protected by detached breakwaters was resistant to erosion owing to the presence of buried gravel layers.