Soft Beach Change Prediction in Japan Sea Coast

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Japan's sandy beaches facing Japan Sea have serious erosion problems which were mainly caused by steep wind waves in winter monsoon. Off-shore-going currents, so-called 'undertow' was generated in the surf zone due to wave breaking resulting in sediment transport towards off-shore to form a long-shore bar near breaker line. Sediment going off-shore was trapped with a long-shore bar. This is a circulation cycle of cross-shore sediment in winter. When large-scale coastal constructions are built to stop longshore sediment transport, a circulation cycle of cross-shore sediment is destroyed. This is a mechanism of beach erosion of sandy coasts facing Japan SeaHead-land defense works have been developed by the head researcher's group to control beach erosion mentioned above. Beach change prediction method has also been developed and pointed out that the following themes should be investigated for establishing soft beach change prediction of Japan Sea coasts.(1) Depth of closure : coastal currents induced by monsoon wind,(2) Simulation of stable beach formation,(3) Beach nourishment methods : mutual evaluation of beach preservation methods in Japan, EU and USA.This research project elaborated on prediction of coastal currents induced by winter monsoon in Japan Sea and made clear the three-dimensional structure of coastal currents and offshore sediment transport. These results will be applied to a simulation of pocket beach formation in head-land defense works.