Coastal erosion risk assessment of Hainan Island, China

Coastal erosion on islands is increasing due to sea level rise, frequent extreme events, and anthropogenic activities. However, studies on the multifactorial coastal erosion risk and the vulnerability of islands are limited. In this study, the Coastal Erosion Risk Assessment(CERA) method was applied for the first time to the study area in China to assess the erosion risk on the coast of Hainan Island; to explore the effects of coastal ocean dynamics,sediment movement characteristics, and anthropogenic construction; and to discuss the suitability of the method and countermeasures for coastal protection. The results show that the coast of Hainan Island shows high sensitivity, high value, low exposure, and moderate erosion. The whole island showed high vulnerability but low erosion risk, with the eastern region being more affected by erosion, particularly the eastern side of Wulong Port and Yalin Bay in Wenchang, and the shore section of Yalong Bay in Sanya, having a very high risk of coastal erosion. In addition, Monte Carlo simulation was used to check the applicability of the CERA method, and it was found that the rate of shoreline change, population density, and number of storms significantly contributed to coastal erosion, but only the short-term effects of sea level rise were considered. The effects of sea level rise and sediment grain size were primarily analyzed as influencing factors. The effects of sea level rise continue to strengthen, with coastal retreat expected to be greater than 2 m by the mid-21st century. Moreover, Hainan Island is primarily composed of the fine and medium sand types, which have little resistance to coastal erosion.Currently, the impact of sediment grain size is rarely considered in coastal erosion risk assessment studies.However, it can be incorporated into the indicator system in the future, and the spatial variation of indicators can be fully considered to strengthen the refinement study.