Investigation of tidal exchange and the formation of tidal vortices at Aransas Pass, Texas, USA.

摘要

Laboratory and field measurements are presented as part of a study of tidal exchange through Aransas Pass, Texas. At the mouth of Aransas Pass, the input of circulation by the ebb tide forces the formation of a starting-jet dipole vortex. These vortices are believed to play an important role in the flushing of coastal regions, and affect the transport of passive tracers, such as nutrients and sediment, from the estuary to the ocean and vice versa. Tidal vortex formation was first measured in the laboratory to gain knowledge of the vortex structure and movement. This information was subsequently used to design and conduct a field campaign to measure these large-scale vortices. A combination of measurements from a towed acoustic Doppler current profiler (ADCP), CTD (conductivity, temperature, depth) and Lagrangian surface drifters were implemented for field data acquisition during ebb and flood tide. Drifter trajectories were used to estimate the size of each observed vortex as well as the statistics of relative diffusion offshore of Aransas Pass. The size of the rotational core of the vortex was shown to be approximated physically by the inlet width or by 0.02UT, where U is the maximum velocity through the inlet channel and T is the tidal period, and confirms results found in previous laboratory experiments. Additionally, the scale of diffusion was approximately 1--15 km and the apparent diffusivity was between 2--130 m2/s following Richardsons law. During flood tide, tidal vortices do not form due to the bay configuration. Instead, flow is distributed into three bay channels. Through the CTD vertical profiles, the data indicate that the system is generally well-mixed over the course of diurnal flood tide. For measurements taken during a semi-diurnal tide, a freshwater event was detected in the profile and confirmed with USGS gauge data. For currents during flood, the Lagrangian drifter data suggest that there is a narrow region to the north of the inlet by which passive tracers are transported through the inlet from offshore. Generally, the majority of the flow from the inlet continues through the Corpus Christi Ship Channel (50--80%) followed by the Lydia Ann Channel ( 20--40%) and the remainder flows through Aransas Channel.