QUANTIFICATION OF PROCESSES OF MORPHOLOGICAL CHANGES BY NUMERICAL SIMULATION AND ELUCIDATION OF FLUSHING MECHANISM AT ARAKAWA RIVER MOUTH SANDBAR IN NIIGATA

In the present study, numerical simulations on morphological changes at river mouth sandbar were performed at the Arakawa estuary in Niigata during Spring season of 2018. The results of numerical simulations by using XBeach were in qualitatively agreement with the aerial pictures. Furthermore, in order to indicate the flushing mechanism of the river mouth sandbar, the numerical simulations of morphological changes were quantitatively assessed. The Brier Skill Score (BSS) were employed as a quantitative index for the evaluation of lateral lines for the result of numerical simulations. As a result, BSS exceeds 0.5 in lateral lines closest to rivers and in trench excavation, indicating that the morphological changes are calculated with high precision. In addition, the numerical reproducibility of morphological changes near sea are highly dependent on the accuracy of simulated wave height. Finally, from the result of quantitatively assessment, it can be said that the opening mechanism of river mouth sandbar were associated with rapid changes in bed shear stress caused by a rapid increase in flow rate.