Trench Survey Technique for Sand–Gravel Mixed Beach with Application at Hamamatsu-Shinohara Beach

Abstract

A unique beach trench survey technique was developed to understand the morphology process of a sand–gravel mixed beach. The survey technique includes trench excavation, layer dating from periodic cross-shore profile analysis, seamless ortho-photo montage, deposit layer tracing, grain size analysis, and deposit layer dating with hydraulic conditions, as well as radiocarbon dating. An advantage of the technique is the ability to collect samples at arbitrary points determined from the visual observation of the whole layer. The trench survey can investigate the formation process of a sand–gravel mixed beach, stretching from foreshore berm to backshore dune. A field study was conducted on the Hamamatsu-Shinohara coast, where sand–gravel mixed materials have been nourished. Three survey lines for the trench survey were selected on the downdrift side of the beach nourishment. The beach was protected by three detached breakwaters to control the sediment movement. Tracing the gravel layers from the berm side to the dune side made it possible to clarify the mechanisms of the gravel movement caused by swash motion of storm waves. Investigating the relation between gravel deposition and the wave and tide observation data made it possible to identify the role of storm events in the beach morphology.