During the 2011 Tohoku Earthquake Tsunami, instances of a “black” Tsunami were observed in certain areas. These phenomena are characterized by Tsunamis laden with sand or silt from the seabed, leading to a increase in the density of the wave. Moreover, it has been hypothesized that such Tsunamis exert a disproportionately greater force on seawalls than would be expected from the mere increase in density alone, a factor not currently accounted for in the design guidelines for coastal structures. Although previous research has touched upon this subject, the underlying mechanisms driving the variation in force exertion remain largely unexplored. 

  This paper employs numerical simulation techniques to investigate the dynamics of density-enhanced Tsunamis, specifically focusing on their increased density and viscosity. The analysis utilizes the nonlinear shallow water equations for two-dimensional domains and the Reynolds-Averaged Navier-Stokes(RANS) equations for three-dimensional domains.

  Initially, the accuracy of our model is validated through comparisons between simulated wave behavior and force-time histories with those observed in experimental settings. This comparison also demonstrates a correlation in how the depth of the wave prior to impacting a vertical wall correlates with the magnitude of force exerted. Subsequent case studies involve varying the density and viscosity of the Tsunami waves to ascertain their respective impacts on the force applied to vertical walls. The findings indicate that an increase in wave density results in a force on the seawall that is slightly more than proportional to the density increase, warranting further discussion within this study. Notably, certain scenarios exhibited exceptionally high forces in response to density increases. Conversely, the influence of viscosity on force exertion was found to be minimal.

  Through this research, we aim to shed light on the significant yet under-appreciated effects of density and viscosity variations in Tsunami waves on coastal defense structures. The insights gained from this study could have profound implications for the design and construction of future seawalls and other coastal protective measures.