AbstractThe mechanism by which the mouth of a small river is stably fixed in the wave-shelter zone behind an offshore reef composed of natural rocks was studied, taking a small river flowing into the Moriya coast together with two other small rivers as examples. The beach topography around the river mouth and the shape of the stream behind the reef were measured on this coast, and the wave height distribution around the reef was calculated using the angular spreading method, and the reason why the river mouth is stably fixed at this location was considered. Furthermore, the numerical simulation of beach changes using the BG model when an offshore breakwater was installed as a model of a natural reef was carried out to study the longshore change in the berm height. The primary cause for the fixation of a river mouth behind a reef was found to be the decrease in wave height behind the reef, which in turn decreases in the berm height.
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