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In this study, field surveys along the coasts of Wakayama Prefecture, Japan, were first conducted to investigate the coastal damage due to storm surges and storm-induced waves caused by the 2018 Typhoons Jebi and Trami. Special focus was placed on the characteristic behavior of nearshore waves through investigation of observed data, numerical simulations, and image analysis of video footage recorded on the coasts. The survey results indicated that inundation, wave overtopping, and drift debris caused by violent storm-induced waves were the dominant factors causing coastal damage. Results of numerical simulations showed that heights of storm-induced waves were predominantly greater than storm surge heights along the entire coast of Wakayama in both typhoons. However, computed gradual alongshore variations in wave and surge heights did not explain locally-concentrated inundation and run-up heights observed along the coasts. These results indicate that complex nearshore hydrodynamics induced by local nearshore bathymetry might have played a significant role in inducing such local wave characteristics and the associated coastal damage. Analysis of video footage recorded during Typhoon Jebi, for example, clearly showed evidence of amplified infragravity wave components, which could enhance inundation and wave run-up.

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.

An atoll is a ring-shaped coral reef left behind because of the subsidence and/or erosion of a volcanic island. The deposition of coral pieces through wave and wind action leads to the formation of a series of small islands on the atoll, which usually have elevations that are less than a few meters above the mean sea level. Inhabited atolls in the Pacific and Indian Oceans are considered to be the most vulnerable to anticipated sea-level rise (Nicholls, 2004). The Intergovernmental Panel on Climate Change (IPCC) AR6 reported that the global mean sea level would continue to rise during the 21st century: 0.28-0.55 m under the very low emissions scenario and 0.66-1.33 m under the very high emissions scenario by 2100 with respect to the 1995-2014 average (IPCC, 2021). This sea-level rise will exacerbate coastal flooding, coastal erosion, and saltwater intrusion into small freshwater lenses on atolls. Furthermore, changes in the wave climate resulting from sea-surface warming may increase the flood risk to low-lying coastal communities (Morim et al., 2019;Reguero et al., 2019). A recent study projected that most atolls would be uninhabitable in the future because more frequent wave-driven flooding will damage infrastructure and freshwater resources (Storlazzi et al., 2018). However, the time to reach this devastating

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