The wave propagation process caused by the penetration of long-period waves into the mouth was investigated through field investigation, taking the Segami River flowing into Hitachi Port in Ibaraki Prefecture as an example. Five wave gauges and current meters were installed around the river mouth and in the river in September, 2002. Long-period waves were measured during two typhoons and one storm. Long-period waves of the period 100-120s selectively invaded the river channel and wave amplitude increased two times that of the river mouth. Numerical simulation was carried out in order to reproduce the long-period waves and to examine measures against long-period waves. As a result, the effectiveness of the pond damping the long-period waves was confirmed.
Beach erosion induced by construction of detached breakwaters in a pocket beach of a closed littoral cell was investigated taking the Ariake-Takahama coast facing the Pacific Ocean in Ibaraki Prefecture as the example. In order to protect houses against wave overtopping 8 detached breakwaters were built on this coast. This caused longshore sand movement toward the wave calm zones behind the detached breakwaters, whereas south beach was severely eroded by this sand movement. Advance of coastal forest and construction of sea dike to protect this forest further accelerated narrowing the sandy beach. Four measures to prevent this phenomenon are proposed and compared.
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