Study on the comprehensive countermeasures for coastal erosion of Kujukuri Beach

Abstract

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.

Detecting the effects of rapid tectonically induced subsidence on Mayotte Island since 2018 on beach and reef morphology, and implications for coastal vulnerability to marine flooding

“…This geological influence is also commonly manifested by tectonic activity, notably earthquakes and associated co-seismic and inter-seismic changes in land level (Wang et al 2012) that can generate beach uplift and/or subsidence. This is notably the case of beaches situated along active tectonic margins such as in New Zealand (Little et al 2009;Hart et al 2020), beaches of the Columbia River littoral cell in Oregon associated with the Cascadia subduction zone (Peterson et al 2010) and its northward extension in Alaska (Kelsey et al 2015), the subduction coasts of Chile (Martínez et al 2015), Sumatra and the Andaman Islands (Monecke et al 2015), and Niigata and Kujukuri in Japan (Kuriyama and Banno 2016;Shibata et al 2019). Subsidence affecting beaches has also been identified in areas subject to glacial-isostatic adjustments, as along the eastern seaboard of North America from Nova Scotia to Florida (Kemp et al 2014;Forde et al 2016;Fiaschi and Wdowinski 2020), often compounded by human activities.…”