海岸構造物による海岸侵食とその制御(1)-海岸侵食の機構-

Abstract

In the first report, the main causes for beach erosion by large a coastal sructure are first classified into 1) interuption of longshore sediment transport by the structure and 2) beach change due to the formation of nearshore currents by deformation of incident wave by the structure, and 3) additionally change in beach profile due to waves reflected by sea dikes and walls when the sandy shore in front of the sea dikes and wails has reduced. Secondly, the mechanism of beach erosion due to the erosion causes mentioned is explained both in the cases of single predominant wave and two predominant waves respectively. Thirdly numerical examples of shoreline change by a structure are shown. It is demonstrated that, when a couple of the suitable boundary conditions for shoreline change due to the first erosion cause are given, the shoreline change tends to an equilibrium state that shows a formation process of stable sandy beach downstream the structure. An example of numerical simulation results of nearshore currents behind an offshore structure is also presented to show a nearshore circulation pattern that may cause beach erosion by the second erosion cause. It is conduded from numerical simulation that when a small groin is constructed at a special position the beach change can be well controlled. Forthly a practical example of such beach erosion problems is shown in the case of Naoetsu Harbour to demonstrate the mechanism of beach erosion proposed.