Numerical Study on the Turbulent Structure of Tsunami Bottom Boundary Layer Using the 2011 Tohoku Tsunami Waveform

In this study, the tsunami-induced bottom boundary layer was investigated based on actual waveforms obtained by the GPS buoys along the coast of the Tohoku region during the 2011 Great East Japan Earthquake tsunami. The k-omega model was utilized for the numerical analysis in this study. As a result, the tsunami boundary layer thickness was found to be extremely thin compared to the water depth. The velocity distribution was similar to that of the bottom boundary layer under wind-generated waves. The flow regime is located in the transition from smooth turbulence to rough turbulence. Because of this, the gradient of the flow across the layer is much greater than the gradients in the steady flow direction. Therefore, the bottom friction is underestimated if the steady friction factor, such as in the Manning formula, is used. This study proposes a new simple method for calculating the bottom shear stress due to an irregular tsunami based on the wave friction law, and the k-omega model results are used to validate the proposed methods.

Automated summary

This study investigated the bottom boundary layer induced by tsunamis, based on the actual waveforms obtained during the 2011 Great East Japan Earthquake tsunami. The results showed that the thickness of the tsunami boundary layer is extremely thin compared to the water depth, and the velocity distribution is similar to that of the bottom boundary layer under wind-generated waves. The study also found that the flow regime transitions from smooth turbulence to rough turbulence, leading to a greater gradient of flow across the layer than in the steady flow direction.