Numerical simulation of sand spit erosion and recovery under a flood event: Case study at Arakawa River mouth, Niigata, Japan

Sand spits at river mouths change dynamically under various external forces, such as river flow and high waves. This study included a sensitivity study of the parameters of the sediment transport model and analysis of the morphological mechanism in sand spits using the case of Arakawa River mouth in Niigata, Japan, during the 2018 flood event. Numerical simulations using the XBeach model reproduced the morphological changes of the sand spit during the flood and indicated that high river discharge was the main cause for morphological changes. The influences of the XBeach parameters were determined using calibration by morfac and by a sensitivity analysis with two parameters ( facua and wetslp ). Morphological mechanism analysis suggested that non-tidal residual and waves had an influence on the morphological changes around the sand spit during flood. Further, the sand spit was eroded because of the rapid increase in bed shear stress caused by overflow from the river to the coast. After the flood, the simulated net sediment volume in the analyzed area around the sand spit recovered to near its pre-flood level within approximately one day under the subsequent high wave conditions. The simulations suggest that wave-driven sediment redistribution in the shallow offshore area contributed to this volumetric recovery following the flood-induced erosion. Here, ‘recovery’ primarily refers to the recovery of sediment volume within the analyzed area and does not imply complete restoration of the pre-flood sand spit geometry. In particular, the shoreline position, crest elevation, and spatial sediment distribution did not necessarily return to their pre-event configurations. These results indicate a volumetric recovery through post-flood sediment redistribution rather than complete restoration of the pre-event morphology.