Variability of turbulence in a tidal intrusion front
Joseph T Jurisa, University of Maryland Center for Environmental Science Horn Point Laboratory, Cambridge, United States, W Rockwell Geyer, Woods Hole Oceanographic Institution, Applied Ocean Physics and Engineering, Woods Hole, MA, United States, David K Ralston, WHOI, Department of Applied Ocean Physics & Engineering, Woods Hole, United States, Andone C Lavery, Woods Hole Oceanographic Institution, Woods Hole, United States, Christopher Bassett, Applied Physics Laboratory, University of Washington, Seattle, WA, United States and David Honegger, US Army Corps of Engineers, Fort Wainwright, United States
Abstract:
A tidal intrusion front is a region of strong convergence and density gradients that forms during flood when dense water enters the estuary, colliding and subducting under the buoyant estuarine water, driving intense turbulent mixing. An extensive set of observations of the James River Estuary tidal intrusion front were collected as part of the Under-Sea Remote Sensing (USRS) project. Turbulence measurements were collected on along and cross-channel transects through the intrusion front region with a rapid profiling conductivity microstructure/CTD package. Over the course of 5 flood tides, 2,500 profiles were conducted. A vast array of supporting in situ data sets include broadband acoustic instruments, ADCP’s, CTD’s mounted on the vessel, towed-body, and unmanned underwater vehicle (UUV), in addition to a shore-based X-band radar. The extremely high-resolution observations allow for a detailed description of spatial and temporal variability of turbulence and mixing in the tidal intrusion front.
The observations reveal the development of a large undular bolus of low-salinity water that becomes trapped at a lateral constriction and steep bathymetric depression when the flow becomes critical. The trapped bolus is highly energetic with large vertical velocities and becomes isolated from the retreating main low-salinity front up stream. Large isopycnal displacements are observed in the acoustic backscatter signals from the shipboard and towed-body instrument packages. The rms Thorpe displacements (LT) calculated from the profiler are on the order of 1-2m. Initial estimates of the dissipation rate using LT are 10-5-10-4 W/kg in the core of the trapped bolus. The intense mixing eventually leads the feature to dissipate after maximum flood. The Thorpe dissipation estimates are compared to the dissipation estimates from the conductivity microstructure profiler and dissipation parameterizations based on the fine-scale shear and stratification. The significant variability associated with the lateral fronts will be discussed as well.
