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

The local scouring behind breakwaters and in a sandy ground around breakwater heads is critical to structural stability for a tsunami, but the amount of scouring remains difficult to estimate.
 

Therefore, in this study, we will elucidate the mechanism of local scouring by conducting hydraulic experiments, and establish a model for estimating the amount of scouring by applying a particle-based method, etc.
 

During FY 2018, we continuously worked on the project from the previous fiscal year to develop a model for the scouring process in the ground behind breakwaters as tsunami overtops them. This was done by applying a particle-based numerical wave flume (PARISPHERE). While it had achieved the reproduction of the detaching process of sand particles from the seabed under tsunami overflows, issues remained: the sand particles would become stationary in an unnatural manner after floating, and they would not become attached to the seabed upon reaching it.
 

In order to resolve such issues, we first performed solid-liquid multiphase flow simulations of the sedimentation process of high specific-gravity particles under water and discovered that, when the standard ISPH model was applied, an unnatural decrease in the fluidity of fluids would occur along with the unphysical phenomenon whereby high specific-gravity particles became stationary. In addition, we improved the fluid pressure gradient term model and made it possible to reproduce the physical fluidity of fluids with suppressing the pressure noises.
 

We then developed a model with sand particles reattached after their floating around, and confirmed that sedimentation of the floating sand helped better reproduction of the natural phenomena.
 

Our future plan for this research project includes conducting experiments on the local scouring that occurs near breakwater heads and establishing a method to estimate the amount of such local scouring, in addition to ongoing experiments on the scouring that occurs along breakwater trunks.