Topographic Response to High Waves and Subsequent Beach Recovery on Chigasaki Coast

Abstract

Beach topography quickly responds to the action of storm waves, resulting in foreshore erosion and accretion under calm wave conditions after a storm. Field observations were carried out on the Chigasaki coast to investigate these beach changes. It was found that the seabed shallower than 3 m depth was rapidly eroded by offshore sand transport during a storm event with the deposition of sand in a zone at depths between 3 and 5 m, and then the beach recovered within 1–2 years after the storm. Topographic changes immediately after the storm and subsequent recovery under calm wave conditions were calculated using the BG model (a model for predicting three-dimensional beach changes based on Bagnold’s concept). Given the equilibrium slope of fine sand d1 and medium-size sand d2 to be 1/120 during high waves, the erosion of the foreshore zone was numerically reproduced. Moreover, the recovery of the beach topography to a gentle slope under calm wave conditions after the storm was successfully reproduced.