Seabed ground can flow markedly owing to diverse external loadings such as earthquakes and tsunamis. However, the seabed ground flow dynamics remain largely unclear, since on-site observations are formidable if not impossible. These submarine ground dynamics can greatly affect coastal structures; therefore, it is important to understand the phenomena and establish an evaluation method.

The submarine liquefied flow dynamics greatly affect coastal and marine structures as well as submarine-landslide tsunamis; on the other hand, the dynamics have previously been difficult to predict. In this study, we established new theories and numerical-analysis methods which can predict and reproduce a series of concurrent processes regarding the submarine liquefied flow dynamics, and then comprehensively validated their effectiveness. In addition, we elucidated the effects of fine contents and percolation on the development of submarine landslides and the associated gravity flows.

We systemically elucidated the dynamics of the seabed and rubble mound of caisson breakwaters under tsunami-induced seepage, regarding focal ground flow dynamics around coastal structures and its effects. Moreover, for the first time in the world, we elucidated the mechanism of destabilization of a breakwater caused by tsunami overflow-seepage coupling. Then, we established a new stability assessment method for caisson breakwaters that take into account both the dynamics of the breakwater foundation mound/seabed and countermeasures against them (embankment).

The developed analytical framework for submarine liquefied flow dynamics can be widely used for predicting coastal large-scale processes of submarine mass movements and assessing the impacts of the submarine liquefied flows on coastal and marine structures as well as submarine landslide tsunamis, and thus serve as basis of their rational evaluation and prediction. In addition, the new stability assessment method of breakwaters considering both the dynamics of breakwater foundation mound/seabed against tsunamis and their countermeasures, are comprehensively reflected in the technical standards for ports. Therefore, wider utilization is expected not only nationally but also internationally.