Large-Scale Physical Modelling of Complex Tsunami-Generated Currents

For tsunamis passing through sharp bathymetric variability, such as a shoal or a harbor entrance channel, z-axis vortical motions are created. These structures are often characterized by a horizontal length scale that is much greater than the local depth and are herein called shallow turbulent coherent structures (TCS). These shallow TCS can greatly increase the drag force on affected infrastructure and the ability of the flow to transport debris and floating objects. Shallow TCS typically manifest as large “whirlpools” during tsunamis, very commonly in ports and harbors. Such structures have been observed numerous times in the tsunamis over the past decade, and are postulated as the cause of large vessels parting their mooring lines due to yaw induced by the rotational eddy.

Through the NSF NEES program, a laboratory study to examine a shallow TCS was performed during the summer of 2014. To generate this phenomenon, a 60 second period long wave was created and then interacted with a breakwater in the basin, forcing the generation of a large and stable TCS. The model scale is 1:30, equating to a 5.5 minute period and 0.5 m amplitude in the prototype scale. Surface tracers, dye studies, AVD’s, wave gages, and bottom pressure sensors are used to characterize the flow. Complex patterns of surface convergence and divergence are easily seen in the data, indicating three-dimensional flow patterns. Dye studies show areas of relatively high and low spatial mixing. Model vessels are placed in the basin such that ship motion in the presence of these rapidly varying currents might be captured. The data obtained from this laboratory study should permit a better physical understanding of the nearshore currents that tsunamis are known to generate, as well as provide a benchmark for numerical modelers who wish to simulate currents.