2021 Volume 77 Issue 2 Pages I_79-I_84
The bottom boundary layer under wind waves is extremely thin compared to the water depth, but its thickness increases as the period increases. Therefore, it is often assumed that a current-like flow is formed under a long wave such as a tsunami or a tide, and the steady flow friction coefficient is used. However, the transition process from the wave type to the steady flow type boundary layer is not clear. In this study, we investigated the transition of the wave boundary layer for smooth surface turbulence and formulated the transition conditions based on the results of previous experiments and newly conducted numerical experiments. In addition, a modified simple formula was derived to judge the transition condition by using the linear long wave theory in terms of the relationship between the wave height/water depth ratio and the water depth/wavelength ratio. From this result, it was found that there is a wide range in which the steady friction law cannot be applied even if it is classified as a long wave. Hence, it is necessary to appropriately reflect such transition of the boundary layer in a simulation of tsunami-induced sediment movement.
