Climate change driven shoreline change at Hasaki Beach Japan: A novel application of the Probabilistic Coastline Recession (PCR) model

Low elevation coastal zones around the world are increasingly threatened by the effects of climate change, and adaptation in these areas is becoming urgent. One of these threats, especially to sandy beaches, is the slow chronic landward movement of the coastline, known as coastal recession. The Probabilistic Coastline Recession (PCR) Model is a physics-based approach that is specifically designed to support modern day risk informed coastal zone management. Here, the PCR model is applied at the highly monitored Hasaki Beach in Japan, which presents challenges that have hitherto not been experienced in previous applications of the model elsewhere in the world (e.g. lack of a clear correlation between high wave events and beach erosion, no storm erosion of dunes but only of the beach strong seasonal variation in erosion events). Therefore, the model is here applied following a novel approach to derive projections of coastal recession (here indicated by shoreline retreat) at Hasaki, under the two end-member IPCC scenarios (RCP 2.6, RCP 8.5.) Shoreline retreat is computed in two different ways that may be useful for two different management purposes; method (a) for computing the probability of erosion reaching and damaging an infrastructure or settlement at any time, and method (b) for computing the average shoreline retreat by a certain future time horizon. Projections for the end of the 21st century obtained with PCR method (a) indicate median shoreline retreats of 17 m (RCP 2.6) and 29 m (RCP 8.5), while those obtained with PCR method (b) are −15 m (RCP 2.6) and −7 m (RCP 8.5) (positive values indicate retreat and negative values indicate progradation). Bruun rule projections, which are comparable with those obtained from PCR method (b), indicate end-century shoreline retreats of 36 m and 66 m at Hasaki, for RCP 2.6 and RCP8.5 respectively. These projections fall at around 34% and 21% exceedance probability PCR method (b) projections respectively, consistent with several previous studies which also show that Bruun rule provides larger coastal recession projections compared to the PCR model. Furthermore, the Bruun Rule appears to provide increasingly conservative (relative to the PCR model) recession projections with increasing time horizons.