糸魚川海岸の侵食制御について

Abstract

In Itoigawa Coast, Niigata, facing the Japan Sea, of which the main sediment source isfrom Himekawa River, recently beach erosion has remarkablly advanced. Long-term data of wavesand sea bed topography were systematically analyzed to find the main causes of the beach erosion.They are conceived as (I) cut of sediment source by construction of the west breakwater ofHimekawa Harbour which is located eastside the river mouth, (ii) increase in longshore sedimenttransport by change of the coastline by construction of bypass-road, and (iii) direct reflection ofwaves by seadikes with concrete units.The predominant waves governing shoreline changes are estimated with relationship betweenthe rate of longshore sediment transport and annual wave energy flux calculated by averagedduration of high waves. The estimated annual rate of longshore sediment transport is about15000 m3/yr to the eastward. These informations make prediction of shoreline change possible.The predicted shoreline change agrees well with the observed one.Based on the results obtained by fundamental investigation a methodology of erosion controlis discussed. A headland control with man-made reefs is proposed to reform sandy beachesalong the coast.