Numerical and Field Study of Tidal and Subtidal Dynamics in a Bar-Built Estuary: Barataria Bay, Gulf of Mexico

Abstract

This study investigated tidal and subtidal dynamics of water level, currents, and suspended sediment concentration (SSC) in Barataria Bay, a shallow bar-built estuary of the Northern Gulf of Mexico. First, the local and remote wind forcing contribution on subtidal water level and current variability were examined using three different methods: (i) statistical analysis of observed data, (ii) an analytical model and (iii) a 2-D barotropic numerical model. Results suggested that the remote and local wind effects were equally important at the bay mouth, however local winds were the dominant forcing driver inside the bay. The amplitudes of subtidal fluctuations induced by local winds were twice as large as the one caused by remote winds. This finding differs from those found in the existing literature, notably for Breton Sound and Lake Pontchartrain, where remote wind effect has been reported to be dominant. These differences are attributed to the different geomorphological features of the estuaries. Furthermore, the seasonality of the SSC in Barataria Pass was explored as the offshore environment transitioned from a period of high cold front activity and low river discharge to a period of low cold front activity and high river discharge. The SSC dynamics during the winter was mainly forced by resuspension in response to the cold front winds. During the spring, the average SSC (0.23 g l-1) was significantly higher than winter (0.15 g l-1) because of the strong offshore influence of the Mississippi River plume. Finally, tidal response to the relative sea level rise and marsh accretion was investigated. Contrary to previous modeling analyses in other estuaries suggesting that flooding of the low-lying land with sea level rise would increase frictional effects and thus reduce tidal range, this study suggested that tidal range in a choked tidal system like Barataria Bay increases even when accompanied by extensive land inundation. This occurs because in Barataria Bay the channel conveyance effects are larger than the frictional effects of the low-lying areas. In the lower and the middle bay, the largest increase in tidal range occurred when the marsh area was assumed to keep pace with relative sea Level rise. However, in the upper bay the largest increase in tidal range occurred when no accretion was assumed. In addition, relative sea level rise caused tidal amplification at the head of the estuary. A detailed momentum balance analysis indicated that sea level rise shifts tidal wave from a dissipative regime