Projections of Future Beach Loss in Japan Due to Sea-Level Rise and Uncertainties in Projected Beach Loss

In this study, we projected the future beach loss in Japan's 77 coastal zones due to sea-level rise (SLR) based on representative concentration pathway (RCP) scenarios and 21 models of the Coupled Model Intercomparison Project Phase 5 (CMIP5). The beach-loss curve for SLR in Japan was constructed, and uncertainties associated with SLR projections and sediment sizes were evaluated. Beach-loss rates in the future (2081-2100) were projected to be 62% for the ensemble mean RCP2.6 scenario, 71% for RCP4.5, 73% for RCP6.0, and 83% for RCP8.5, and the rates projected by the CMIP5 models for RCP4.5 ranged widely from 61% to 87%. The effect of the spatial distribution of SLR in each CMIP5 model on beach-loss rate in Japan is insignificant, while the effects of differences in the SLR values among RCP scenario and CMIP5 models are significant. The maximum uncertainties associated with sediment sizes (0.2-0.6 mm) against the same SLR were assessed to be 38%. Despite significant uncertainties in the projected beach loss, results in the near future (2046-2065) reveal that the beach-loss rates between 18% and 79% differed by 60% in the near future and between 28% and 96% differed by 70% in the future. For the worst case scenario in the near future, the projected beach width is less than 10 m in more than half of the 77 coastal zones, which would cause serious damage to coastal structures such as seawalls and revetments in beach areas. Thus, the development of effective measures to combat beach loss is critical for Japan's coastal management.