This study aims to investigate the characteristics of bottom mud suspension and tsunami wave force, and sediment transport experiments with movable bed and tsunami impact experiments with different fluid density are conducted. The results showed that the bottom mud transport pattern differs depending on the relationship between the yield shear stress of the bottom mud and the pressure gradient of the flow. In addition, it was shown that the relationship of the wave force to the fluid density depends on the incident tsunami waveform.
In the tsunami generated by the 2011 Great East Japan Earthquake, so-called black tsunamis, which contained suspended sediment, were observed in many locations of Tohoku area. There are reports of building damage and health problems related to the black tsunamis, with the damage being especially severe in the coastal urban areas. (Imamura, 2022). This raises concerns about the effects of tsunami loads on coastal structures, particularly seawalls as critical structures.
In the guidelines by FEMA (Federal Emergency Management Agency, 2019) and ASCE (American Society of Civil Engineers, 2017), the tsunami wave force is calculated by assuming the minimum density of the fluid to be 1.1 times of seawater (1,128 kg/m3), containing the suspended sediment. This is assuming that the specific gravity of suspended sediment is 2.5 and the vertical mean volume concentration of sediment in seawater is 7 %. However, these guidelines do not provide the basis, and the mechanism of tsunami wave forces including sediments has not yet been clarified. For appropriate design against tsunami wave force, it is necessary to quantitatively evaluate the effects of density and viscosity changes due to sediment contamination on the tsunami wave force.
Many hydraulic experiments have been conducted in the past on the transport of bottom mud in steady currents such as rivers and in usual waves. For example, Otsubo and Muraoka (1988) conducted bottom mud transport experiments under one-directional flow. As a result, they formulated critical shear stresses for two groups of muds classified according to the characteristics related to the transport pattern of muds such as kaolin, bentonite, and other materials. Thimakorn (1980) experimentally investigated the physical behavior of waves due to the suspension of fine cohesive muds. The results indicated that the ratio between the mean bottom concentration and the vertical average concentration is constant, and that the bottom concentration at equilibrium state is linearly proportional to the maximum bottom velocity of waves.
