Comparison of Flows through a Tidal Inlet in Late Spring and after the Passage of an Atmospheric Cold Front in Winter Using Acoustic Doppler Profilers and Vessel-Based Observations

Abstract

This paper discusses the application of acoustic Doppler current profilers (ADCP) for the quantification of transport of water and the underlining physical mechanism. The transport of water through estuaries and tidal inlets is affected by tide, river flow, and wind. It is often assumed that wind effects in such systems are negligible unless under severe weather conditions. This study compares the ADCP-measured flows across a tidal inlet under weak wind conditions in late spring and those after the passage of an atmospheric cold front in winter. The Barataria Pass is a major inlet connecting Barataria Bay and northern Gulf of Mexico. The water exchange between the bay and coastal ocean is influenced by wind, especially in winter, because tide in the region is small (microtidal). The winter weather and late spring–summer weather are different. This difference results in different estuarine circulations. To examine this, two surveys were carried out with ship-mounted ADCPs—one in winter (19 December 2014) shortly after the passage of a cold front from the northwest, and the other in late spring (4 May 2015) with weak southeasterly winds. Distinctly different features of mean transport through the inlet were observed between the two surveys. The results from the first survey in winter showed that the total water transport was from the bay to the coastal ocean under northerly winds with intense outflows in shallow water, which is a typical signature of wind effects. The net flow was outward when the water level dropped. Data from the second survey in spring showed that the mid-channel water flew out of the bay (against the wind), whilst inflow appeared at both ends across the inlet, which was also a response to the weak wind stress and outward pressure gradient force set by the estuarine flow. The inflow at the eastern end (exceeding 0.1 m/s) is consistent with the idea that the coastal current resulted from the Mississippi River outflow enters the bay from the eastern end. The influence of tidal oscillations on water exchange appeared to be higher in the late spring data. The hydrographic observations in spring showed typical tidal straining features of an inverse estuary during the ebb–flood cycle, while salinity in the eastern shallow water generally varied with time, indicating the inflow of fresher water into the bay, confirming previous observations from summer 2008.