Tanimoto's formula, the static-pressure formula at the time of overflow, and other formulas have been examined to determine the tsunami wave force to be used in anti-tsunami design for breakwaters. These study results were published as the Guideline for Tsunami-Resistant Design of Breakwaters in FY 2013. However, there are many cases of overflow-induced scouring damages such as the case of the northern breakwater of Hachinohe Port during the 2011 Great East Japan Earthquake. These damages have not been fully investigated.

Therefore, in this study, we conduct hydraulic-model experiments using a movable-bed regarding breakwaters and their peripheral submarine topography. We vary the sanddeposit conditions including particle diameter and then compare the results. In addition, we investigate the armoring effects of crushed stones of a mound. Through these investigations, we propose a versatile equation for estimating scouring amount. Furthermore, we investigate a method for estimating scouring amount using a particle method.

Seafloor topography changes through seafloor sand/mud migration, which is caused by tsunami flow. There are various modes in the migration, including sliding, rolling, saltation, and floating. Several theoretical and experimental equations have been proposed for each of these modes. The equations for respective modes need to be combined in the simulation for estimating local scouring amount associated with sediment migration.

However, the optimal combination for a given phenomenon and the application limitation for the combination have not been fully investigated. In order to establish a model that accurately replicates phenomena that affect local scouring, including armoring effects of crushed stones of a mound, we must first confirm the existing equations and limitations for each of these combinations.

Therefore, in FY 2017, we conducted many simulations in which various equations were combined. We compared the simulations with the results of experiments in which an artificial tsunami engulfs a landform like a port and with actual tsunami cases including Hachinohe Port, where focal scouring occurred due to the 2011 Great East Japan Earthquake. Through the comparisons, we organized the geometric changes estimated by each estimation model and their combinations, and then elucidated their characteristics.

In the future, we will continue to investigate the similarity laws, and will calculate flow and seafloor changes using a particle method. After taking the results into account, we will establish an equation for estimating scouring amount, and then will reflect our achievements in technical standards and so forth.