ABSTRACT This paper presents a detailed investigation on unsteady effects in bottom boundary layer beneath a tsunami. In numerical simulation of tsunami, bottom shear stress has commonly been evaluated by using steady flow friction laws such as Manning equation, simply assuming that long-period wave motion satisfies quasi-steady flow conditions. However, the present study found that the behavior of tsunami-induced bottom boundary layer has an unsteady characteristics and acts similar to that induced by wind-generated waves even under long-period wave motion. As a result, bottom shear stress under tsunami is much larger than the estimation using steady friction coefficient due to a steep velocity gradient in the bottom boundary layer. Surprisingly, the steady flow friction law is not valid in almost the entire computational domain, from the source area to shallow region. This result highly coincides with the field measurement data obtained by Lacy group during the 2010 Chilean Tsunami at the Monterey Bay mouth in U.S. A correction coefficient is proposed to take into account unsteady effects in the conventional estimation method using a steady friction factor; this approach is validated by comparing with numerical simulation results using k-ω turbulence model.

中文翻译：

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海啸下边界层发育与底部剪应力研究

摘要 本文详细研究了海啸下底部边界层的非定常效应。在海啸数值模拟中，通常采用曼宁方程等定常流动摩擦定律来评估底部剪应力，简单地假设长周期波浪运动满足准定常流动条件。然而，目前的研究发现，即使在长周期波浪运动下，海啸诱发的底部边界层的行为也具有不稳定的特征，其行为类似于风产生的波浪诱发的行为。结果，由于底部边界层中的陡峭速度梯度，海啸下的底部剪应力比使用稳定摩擦系数的估计大得多。令人惊讶的是，稳流摩擦定律几乎在整个计算域中都无效，从源区到浅层区。该结果与Lacy group在2010年智利海啸美国蒙特雷湾口获得的现场测量数据高度吻合。提出修正系数以考虑使用稳态摩擦系数的常规估计方法中的非稳态影响；通过与使用 k-ω 湍流模型的数值模拟结果进行比较，验证了该方法。