Evolution of Beach Morphodynamics in light of Sediment Budget Assessment with the Coast of Joetsu, Niigata, Japan

摘要

Beach erosion has been a serious issue worldwide, owing to evolving fluvial supply and complex littoral dispersal. ThernJapan’s archipelago is no exception for this. It has undergone the loss of aerial beach area at a rate of approximatelyrn160ha per year (Tanaka et al., 1993), eventually having extensive lengths of coastlines protected with seawalls andrnarmour units. In order to promote integrated beach management in harmony with regional environments, it is of utmostrnimportance to address multi-scaled nature of the coastal sediment transport and associated morphological changes. Inrnthis study, the coastal erosion on the Coast of Joetsu encompassing Ogata Beach is discussed from two perspectives.rnFirst, the long-term shoreline retreat on Ogata Beach is demonstrated based on geomorphological maps available; inrnfact, the shoreline retreated 80m on average in the 70 years since 1914 and thereafter has stayed stabilized. Thisrnstatement, by no means, implies cessation of the coastal erosion. Indeed, our recent field studies have revealed thernseabed topographical evolution, through the high-resolution side scan imaging performed in August 2008. Notably,rnsignificant morphological changes were found to have occurred even in the offshore area beyond a depth of closurern(hitherto estimated at 8m). Also, the field survey identified significantly scoured areas around a series of submerged,rnoffshore breakwaters that were situated in front of the vertically-seawalled shoreline. The sediment budget analysisrnover an area of 2.1km by 0.45km showed that the loss of volume of sediment amounted to half-million cubic meters inrnthe recent past ten years since 1998. The persistent significant erosion on the seabed emphasizes the importance ofrnreexamining the performance of the coastal protection works that actually operate as a part of the landward (reflective)rnboundary to the littoral sediment transport concerned.