Study on the comprehensive countermeasures for coastal erosion of Kujukuri Beach

摘要

Beach erosion mechanisms were investigated for Kujukuri Beach, a 60-km-long sandy beach facing the Pacific Ocean, located in Chiba Prefecture, Japan. The longshore sediment transport rate was estimated using aerial photograph analysis, physical property measurements of beach sediment samples, and land subsidence estimation owing to paleo-groundwater pumping. Alongshore distribution of sediment grain size and thermoluminescence intensity revealed that the longshore sand transport from both ends would meet at x = 40 km, where x denotes the distance from the north end of the beach. Sediment loss caused by the land subsidence was estimated at 130,000 m~3/year in the south, which was observed to play an essential role in the beach erosion, as compared with the relatively small amount of sediment transport in the south. An effective and comprehensive countermeasure was proposed in which the land subsidence could be mitigated through the restoration of pumped water in the coastal zone. The countermeasure was found effective in decreasing the sediment loss by 85,000 m~3/year, thus changing the sediment budget of the whole Kujukuri Beach positive.