Tidal Distortions Related to Extreme Atmospheric Forcing Over the Inner Shelf



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Inner shelf transport is driven by the combination of tides, waves, density gradients, and wind that interact with bottom bedforms. To extend understanding of shoal-swale bathymetry influences on inner-shelf hydrodynamics, we provide observations at a swale similar to 13 m deep within Cape Canaveral shoals, similar to 36 km away from Hurricane Matthew's path during October 2016. Observations document a unidirectional flow that reached 2.7 m/s at a distance similar to 15 km away from the shoreline. The flow changed direction, over the course of the storm, counterclockwise from southwestward (similar to 2.7 m/s) to northeastward (similar to 1 m/s). The unidirectional flow modified tides over the inner shelf analogously to a river discharge in a channelized tidal flow, producing overtides of the semidiurnal tidal harmonic. Our results provide measurements during extreme conditions related to shore parallel-propagating hurricanes that can inform morphodynamic and ecological models. Plain Language Summary Hurricane conditions (winds, waves, and storm surge) are conspicuous at the surface of the ocean. Beneath the surface, however, enhanced water motions strongly influence sand transport and ecological processes at nearby beaches. We present a unique data set of water motions collected over the sand banks near Cape Canaveral, on the Florida Atlantic coast, during the passage of Hurricane Matthew in 2016. At the peak of the storm, water moved with a maximum speed close to 2.7 m/s (9.7 km/hr or 6 mi/hr). These speeds were six times greater than speeds measured during typically strong winds and 10 times greater than normal. Hurricane-induced flows deformed water levels when compared to levels from predicted tides. Models of ocean bottom change and ecological processes should acknowledge these extreme water motions. The conditions documented in this study are likely to be observed whenever hurricanes propagate northward along the U.S. Atlantic coast, as in the cases of Hurricane Matthew in 2016, Hurricane Irene in 2011, and Hurricane Sandy in 2012.


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