Tsunami-induced bottom shear stress has commonly been evaluatedusing a friction coefficient for steady flow, such as Manning’s roughnesscoefficient, Chézy formula and Darcy-Weisbach equation, assumingdevelopment of bottom boundary layer up to the water surface undertsunami. However, applicability of steady friction law to tsunami wavehas never been scientifically investigated, and it has simply beenassumed that long-period wave motion satisfies quasi-steady flowcondition. In the present study, the κ-ω model is used to calculatevelocity profile and resultant sea bottom shear stress under shoalinghypothetical tsunami propagating from tsunami source area to nearshoreregion. It is found that steady friction is not valid in the entirecomputational region from the source area to shallow region due toextremely thin boundary layer thickness beneath tsunami.
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