Potential impact of climate change at five Japanese beaches

Coastal erosion caused by sea level rise is a serious problem over the world. Future sea level rise will almost certainly accelerate through the 21st century. Although its extent remains uncertain, some predictions expect the increase to be between up to 30 and 180 cm by 2100 (Nicholls and Cazenave, 2010). Several studies show that sea level rise is responsible for long-term beach erosion. Moreover, change of wave height and non-climate-related processes such as ground subsidence amplify coastal vulnerability associated with climate change. It is a pressing issue to predict shoreline change considering the fact that 10% of the global population lives in the coastal regions within 10 m elevation. This study estimates the potential impact of climate change in terms of the effects of: sea level rise; wave height variation; and land subsidence by comparing the past coastline evolutions at five beaches in Japan. The past long-term shoreline change was found to be related to coastal protection measures according to Japan's national policy (Coast Act). Notable erosion was resulted from the construction of coastal facilities that blocked coastal sand drift and the decrease of sediment supply from rivers caused by dam building from 1950 to 1990. Since 1990, shoreline has relatively unchanged because some measures, such as beach nourishment, groins and detached breakwaters, have worked to conserve sand beaches. The estimation results showed that shoreline would retreat over 15 m due to sea level rise and up to 5 m due to wave height variation at the five beaches by 2100. Projection of ground subsidence is uncertain because its rate varies depending on natural forces and human activities. This study suggests that sea level rise would have much greater impact on beach erosion compared with wave height variation and ground subsidence.