Mechanical and Civil Engineering Seminar

In this talk, the well-established approaches of coupling tsunami generation to seismic seafloor motion and the following trans-oceanic wave propagation will be briefly introduction. The focus of the presentation will be on the complex transformation of the tsunami as it approaches very shallow waters, and the proper approximate theories that are applicable for this evolution. Two examples of coastal impact will be discussed and used to frame the employed theoretical approach. First, a simulation of a tsunami flooding an urban coastal town will be shown, and we the creation and behavior of the many turbulent wakes that form behind each structure will be discussed. A conclusion of this example is that it is possible to achieve very high fluid forces away from the immediate shoreline; having a row of structures seaward of a given location does not necessarily protect against the flow. The second example will look at tsunami-induced currents in ports and harbors. Tsunamis, or "harbor waves" in Japanese, are so-named due to the common observation of enhanced damage in harbors and ports. However, the dynamic currents induced by these waves, while regularly observed and known to cause significant damage, are poorly understood. We will show that the strongest currents in a port are governed by horizontally sheared and rotational shallow flow with imbedded turbulent coherent structures. Without proper representation of the physics associated with these phenomena (see Figure below for an example simulation of a whirlpool generated by the 2011 Japan tsunami), predictive models may provide drag force estimates that are an order of magnitude or more in error. Such an error can mean the difference between an unaffected port and one in which vessels 300 meters in length drift and spin chaotically through billions of dollars of infrastructure.

Numerical simulation results of the 2011 Japan tsunami, with a focus on the predictions in the Port of Oarai. Snapshots are from 188 minutes after the earthquake, with the top plot showing fluid speed, the middle showing vertical vorticity at mid-depth, and the lower plot is the captured whirlpool from helicopter video at approximately the same time.