SEA-LEVEL EFFECT ON ANNUAL CYCLIC BEACH MORPHOLOGY IN SWASH ZONE AT HASAKI COAST

Abstract

depend on the accurate knowledge of the beach response to sea level regime on multi-time scale. For the long-term beach response, Bruun (1962) suggested that the equilibrium beach profile would move to new equilibrium profile in response to a rising sea level. In this concept called as Bruun rule, the upper part of the beach profile is eroded due to the sea level rise, resulting in the shoreline retreat. It is widely used for the future shoreline prediction. However, the Bruun rule predicts just only the final beach state with a constant wave impinging for an infinite period after sea level rise. On the other hand, simultaneous function of wave and sea level is more important on interannual to decadal-scale beach response. El niño in 2015 and 2016 increased wave energy and sea level, corresponding to large beach erosion across the US west coast (Barnard et al., 2017). Sea level influences the response sensitivity to the wave forcing as a subordinate factor on the morphological change. High water level anomalies made the beach more eroded even if the wave condition was equal. Beach morphology in the swash zone often changes on a 1-year cycle due to seasonal wave conditions. The effect of sea level on the annual cyclic beach morphology in swash zone is still unclear because long-term beach observation data required for the analysis are difficult to obtain. In this study, we investigated the simultaneous effects of the wave and sea level on annual cyclic beach morphology in the swash zone with spectrum analysis for 25-year Hasaki beach observation data.