Mechanism of beach deformation elucidated by bathymetry survey and beach trench survey on miyazaki coast

Abstract

Significant beach deformation is developed on Miyazaki Coast facing the Pacific Ocean. Around the jetties located at the river mouth of the Hitotsuse River, excessive sedimentation is observed on the north side while severe erosion is developed on the south side. A field survey was conducted on the northern beach in order to understand the mechanism of the coastal deformation. Three beach trenches, 1.5 m deep, 30 to 50 m in cross-shore length, were excavated at north, central and south locations. Sediment layers were investigated by precise inspection of the stratum. Sediment samples taken from each trench were vertically sliced by every 2.5 cm. Particle size and the magnetism were analyzed for every sliced sample. Thermo-luminescence and Optically-stimulated-luminescence capable of estimating the alongshore sediment movement processes were measured. From the results, combined with analysis of topography surveys, temporal and spatial progression of coastal erosion was examined together with the short-term erosion developed by a big event such as typhoon. The trend of topographic change differed at years when there was a big typhoon. Furthermore, the erosion process at Miyazaki coast was quantitatively investigated on the basis of the bathymetry survey data for about 30 years.