Full-scale CFD simulation of tsunamis. Part 2: Boundary layers and bed shear stresses

Abstract

This paper presents results from numerical simulations of the propagation and run-up of full scale tsunamis, using a Reynolds-Averaged Navier-Stokes model, with emphasis on the resulting boundary layers and bed shear stresses. Spatial distributions of the Shields and Rouse parameters during run-up and draw-down show that for the tsunamis considered, with a sediment grain size corresponding to medium sand, considerable sediment transport (Shields parameter O(10)O(100)) can be expected during run-up and that the sediment transport can be expected to be dominated by suspended load. The results likewise show that the expected sediment transport during draw-down will similarly be considerable and dominated by suspension. The tsunami-induced boundary layers are monitored in time, and the observed boundary layer thickness ranges from spanning only a small fraction of the water depth to spanning the entire depth. The velocity profiles beneath the tsunamis are shown to have good correspondence with a logarithmic profile within the boundary layer. Similarly, the bed shear stresses beneath the tsunamis are investigated and a new and simple engineering model is developed for predicting the temporal variation of the bed shear stress based only on a free-stream velocity signal. The new engineering model is shown give better predictions for the bed shear stress than a standard Manning approach, and is likewise shown (in the Appendix) to produce reasonable predictions for shorter (wind scale) waves. It is finally also shown how the temporal evolution of the boundary layer thickness can be predicted based on the free-stream velocity signal alone. The results presented here are Part 2 of a larger study, where Part 1 presents the model validation and detailed descriptions of the run-up process.