Wave-induced shallow-water monopolar vortex: large-scale experiments

Numerous field observations of tsunami-induced eddies in ports and harbours have been reported for recent tsunami events. We examine the evolution of a turbulent shallow-water monopolar vortex generated by a long wave through a series of large-scale experiments in a rectangular wave basin. A leading-elevation asymmetric wave is guided through a narrow channel to form a flow separation region on the lee side of a straight vertical breakwater, which coupled with the transient flow leads to the formation of a monopolar turbulent coherent structure (TCS). The vortex flow after detachment from the trailing jet is fully turbulent (Re-h similar to O(10(4)-10(5))) for the remainder of the experimental duration. The free surface velocity field was extracted through particle tracking velocimetry over several experimental trials. The first-order model proposed by Seol& Jirka (J. FluidMech., vol. 665, 2010, pp. 274-299) to predict the decay and spatial growth of shallow-water vortices fits the experimental data well. Bottom friction is predicted to induce a t(-1) azimuthal velocity decay and turbulent viscous diffusion results in a v t bulk vortex radial growth, where t represents time. The azimuthal velocity, vorticity and free surface elevation profiles are well described through an idealised geophysical vortex. Kinematic free surface boundary conditions predict weak upwelling in the TCS-centre, followed by a zone of downwelling in a recirculation pattern along the water column. The vertical confinement of the flow is quantified through the ratio of kinetic energy contained in the secondary and primary surface velocity fields and a transition point towards a quasi-two-dimensional flow is identified.

Automated summary

Field observations have reported many tsunami-induced eddies in ports and harbors. This study examines the evolution of a turbulent shallow-water monopolar vortex through large-scale experiments and validates a first-order model for predicting the decay and spatial growth of shallow-water vortices. The experiments show the effects of bottom friction and turbulent viscous diffusion on the velocity and radial growth of the vortex flow.