Beach Morphodynamic Time Scales Analysis at Hasaki Beach, Japan

Abstract Understanding beach morphodynamic behavior across different time scales is crucial for developing reliable shoreline models, yet comprehensive analyses are limited by the scarcity of high-frequency, long-term observational data. This study analyzes a 34-year (1986-2019) dataset of daily beach profile measurements from Hasaki Beach, Japan, alongside wave data from the JRA-55 atmospheric reanalysis model. We examined the short-term shoreline response during storm arrival and post-storm conditions, analyzing two-day response patterns under different wave conditions. For decadal-scale analysis, we employed Discrete Wavelet Transform to decompose shoreline variations into different temporal scales. Storm response analysis revealed that erosion continues beyond the initial storm impact despite wave height stabilization, while post-storm recovery shows no significant deviation from stable wave conditions. The wavelet analysis of monthly averaged shoreline positions identified dominant energy concentrations at time scales of 4–8 and 8–16 months, corresponding to seasonal variations. Components exceeding 5-year time scales were found to contribute substantially to overall shoreline variance. After removing this long-term trend, the analysis revealed a marked decrease in shoreline variability after July 2007, with variance reducing from 108.74 m 2 to 34.95 m 2 .