The coastal erosion is a serious problem in Japan, especially along a coast of the Miho Peninsula, Shizuoka. Since 1980's, numerous coastal structures have been built and nourishments have been performed for protection and preservation of the coastline, however, it has not become a fundamental solution so far. The mechanisms of clastic sediment transport, timing of the coastal erosion, and an influence of the erosional debris to a seafloor are not clearly understood. It is important to examine the problem of the coastal erosion from the beach to the continental shelf comprehensively. This paper describes topographic changes from the beach to the inner shelf to see seasonal variation and examines the relation of the coastal erosion and clastic sediment transport. The following results are discussed; 1) A timing and amount of the coastal erosion are examined in wave-dominated zone. The coastal erosion occurs dominantly at summer and autumn seasons when several typhoons hit the coast, and the coastal geographical feature change in winter and spring seasons is not significant. The seafloor becomes depositional zone when the coastal erosion occurs. 2) Several bulges of the sea floor (Coastal Swells) are found just seaward side of the coastal structures. Numerous linear and positive topographic features (Sand Ridges) are found off these Coastal Swells. The topographic characteristics determined by narrow multi-beam system and grain size analysis on dredged samples together suggest bottom materials of these positive topographic features are results from masstransportation from the coast to shelf. 3) Configurations of subaqueous dunes distributed at the northern part (the Hagoromo Spur) of the study area were clearly changed for about half a year. These changes reflect the influence of the storm, typhoons during summer and autumn seasons. 4) Sedimentary environment is greatly different between the northern and southern parts of the study area. In the northern part (the Hagoromo Spur), clastic sediments are chiefly transported by the strong bottom current. In the southern part (off Komagoe), clastic sediments appear to be migrated by mass-transportation.
Gravity measurements on the sea bottom using an ocean bottom gravimeter(OBG) and a small survey vessel of 8.5 tons were performed at Harima-nada, in the Seto Inland Sea, Japan. Measurements at one bottom station were completed in about 30 minutes including 2 mile transit from the previous station, and 23 new data were obtained during 4 days. The measurement noise on the shallow sea-bottom was reduced considerably by attaching an anchor to the rope between the deployed ocean bottom gravimeter and the ship, and by keeping the ship almost fixed to the deployed anchor. The measurement accuracy is better than 0.005 mgal at the base station and is better than 0.05 mgal for the sea bottom measurements with the anchor. The new measurements combined with old data revealed the presence of high gravity anomaly zone running in Harima-nada sub-parallel to the Median Tectonic Line.