Major Research Development of the estimation method for local scour around coastal structures due to tsunami

Local scours behind breakwaters and around breakwater heads are critical to the structural stability against a tsunami, but the amount of the scour remains difficult to estimate.

Therefore, in this study, we will elucidate the mechanism of local scour by conducting hydraulic experiments, and establish a model to estimate the amount of the scour by applying a particle-based numerical simulation, etc.

In FY 2019, we conducted a hydraulic experiment and numerical simulations to elucidate the phenomenon of local scour that will occur around the head of the bay-mouth breakwater which is planned to be built in Urado Bay, Kochi Prefecture.

In this experiment, we installed a 1/40 scale model breakwater in our large experimental wave basin and used a pump to reproduce tsunami. We then measured the currents around the breakwater and the wave forces acting on the caisson. A fixed floor was used in this experiment.

Meanwhile, we used the tsunami and storm surge simulator (STOC) to run simulations covering the large area extending from the tsunami wave source area to Urado Bay, and calculated the current velocity distribution at the mouth of the bay, which would be the boundary condition of the experiment. In addition, we found that the conventional calculation method, which approximated the breakwater as the demarcation line, tends to underestimate the current velocity and the amount of scour.

Furthermore, to elucidate the characteristics of large eddies that occur around breakwaters, we performed 3-D calculations by applying the particle-based numerical simulation method, PARISPHERE. The results showed that the eddies around the breakwater head flowed to a diagonally downward direction, from upstream to downstream.

In addition, we conducted experiments to study the scour that occurs behind a breakwater by a tsunami overflowing current, varying the rubble mound shape and the water level behind the breakwater caisson. The experiments revealed that the velocity of the overflow current increases and bends toward the caisson. The bend current hit not the sandbed but the rubble mound resulting in the suppression of the sandbed scour.