Beach Changes Caused by Extension of Offshore Breakwater and Limitations of Artificial Reefs Constructed as a Measure Against Beach Erosion

Abstract

Beach changes on a pocket beach of 6.5 km length bounded by Ryugu and Unoko Points facing the Pacific Ocean in Fukushima Prefecture were investigated using the contour-line-change model. The shoreline recession and excess deposition of sand in the central and southern parts of the study area, respectively, between 1980 and the 1990s were triggered by the wave-sheltering effect due to the extension of the offshore breakwater of Hirakata fishing port south of the study area. Although artificial reefs were constructed in the eroded area of the study area as a measure against beach erosion, their protective effect was limited because the wave-sheltering effect of the offshore breakwater of the fishing port is much larger than that of the artificial reefs.