Seasonal variations of sediment transport to a canyon and coastal erosion along the Shimizu coast, Suruga Bay, Japan

We examined seasonal variations in offshore and longshore sediment transport along the Shimizu coast, Suruga Bay, Central Japan, facing the Pacific Ocean, and considered the relationship between these variations and coastal erosion, based on coastal topographic data and detailed bathymetric data acquired by narrow multi-beam echo soundings in 2004–2007 and analyses of material collected in 2004–2005, combined with wave data in the nearshore zone. nt transport in the study area is divided into two seasonal modes. In winter–spring, a period dominated by the winter monsoon, littoral drift is recognized but offshore-directed sediment transport is minimal. Sediment is deposited around coastal structures during this period; consequently, tombolos are formed from gravelly deposits and offshore banks which represent the bulges of the seafloor (in water depths of 3–7 m) are formed seaward of the structures due to the deposition of sandy sediment. In summer–autumn, when typhoons commonly strike the coast, we observe pronounced longshore and offshore sediment transport. Consequently, the deposits that form tombolos and offshore banks are intensely eroded. The bulk of the eroded sediments are transported into the submarine canyons in the case that a canyon is located close to the coastline and the shelf is narrow. As a result of this transport, sand ridges and depressions are formed from sandy and gravelly deposits, respectively. The amount of erosion (− 34,000 m3) from the seafloor where most of the active transport occurred in the study area during summer–autumn approximately corresponds to the amount of nourishment (35,000 m3) recorded throughout the entire study area, corresponding to about 50% of the amount of the sediment carried by longshore transport (70,000 m3/year). Long-period deep-water waves and high incident waves generated by typhoons during summer–autumn are essential in transporting sediment from the coast to offshore areas; the winter monsoon makes only a minor contribution in this regard. The amount of sediment outflow generated by high-energy waves appears to increase with the accumulation of sediment around offshore breakwaters built in regions with a narrow shelf. Research on offshore-directed sediment transport, as in this study, is important for managing and preserving the nearshore environment (e.g., to examine the budget between supply and outflow), as deposits that flow out into the deep ocean never return to the beach.