Cold Front Driven Flows Through Multiple Inlets of Lake Pontchartrain Estuary

Abstract

With in situ observations using acoustic Doppler current profilers (ADCPs) and numerical experiments using the Finite Volume Coastal Ocean Model (FVCOM), this study investigates atmospheric cold front induced exchange of water between Lake Pontchartrain Estuary and coastal ocean through multiple inlets. Results show that the subtidal hydrodynamic response is highly correlated with meteorological parameters. Northerly and westerly winds tend to push water out of Lake Pontchartrain, while south and east winds tend to produce currents flowing into it. For most cases, the subtidal water level is inversely correlated with the east wind, with the correlation coefficient being ∼0.8. The most important finding of this work is that, contrary to intuition, the cold front induced remote wind effect has the greatest contribution to the overall water level variation, while the local wind stress determines the surface slope inside the estuary. It is found that wind driven flow is roughly quasi steady state: the surface slope in the north‐south direction is determined by the north‐south wind stress, explaining ∼83% of the variability but less so in the east‐west direction (∼43%). In other words, the north‐south local wind stress determines the water level gradient in that direction in the estuary while the overall water level change is pretty much controlled by the open boundary which is the “remote wind effect,” a regional response that can be illustrated only by a numerical model for a much larger area encompassing the estuary.

Flow Regimes and Adjustment to Wind‐Driven Motions in Lake Pontchartrain Estuary: A Modeling Experiment Using FVCOM

“…We have seen remarkable quasi steady state balance between the wind and surface slopes, even when the wind is changing with time (Experiments 2–4). The steady state balance has been shown in Huang and Li () in the LPE system and further confirmed here with more simulations and analysis. These give us a justification of conducting Experiments 5–6, which only include constant wind from fixed directions.…”

“…The 40‐hr low‐pass‐filtered results show that the water level difference from the FVCOM results can be reproduced by remarkably well. This is true for the simulation for 2005 period (Experiment 1), the simulation for the 2008 period (Huang & Li, ), and the simulation for the 2016 period (Figure ). Figure a shows the comparison of water level differences from FVCOM and that from for the local wind scenario calculated from values from the north and south stations (H1 and H2 in Figure ).…”

“…Experiments 2–4 cover the period between 12 February and 24 March 2016. The computations with scenarios of local (Experiment 2), remote (Experiment 3), and combined local and remote winds (Experiment 4) allow us to show in this system that the remote wind effect (water level perturbation propagated through the open boundary) controls the overall water level variation inside the LPE, while the local wind stress causes the water level slope inside the system (Huang & Li, ). It is found that the low‐pass‐filtered water response to local wind variation is very well approximated by a simple quasi steady state balance between the wind stress and the surface slope‐induced pressure gradient in both east‐west and north‐south directions, in which x , y , ζ , τ ax , τ ay , ρ , h , and g are the east and north distance (coordinate), water elevation from the mean sea level, wind stress in x and y , water density, and water depth at equilibrium when there is no force, respectively.…”

“…The bathymetry used in the model is a combination of bathymetry from previous models (Huang & Li, ; Li et al, ) and our own observations prior to deployment of instruments and vessel‐based surveys (Li et al, , ). The model grid has 10,159 triangles covering the entire domain with a total of 5,809 node points (Figure ).…”

“…The FVCOM for LPE has been well established with validations using observations made in several deployments throughout the years (between 2007 and 2016). More specifically, we deployed acoustic Doppler current profilers (ADCPs) and water pressure sensors for measurements of hurricane storm surges, water transport, and saltwater flux (Li et al, , ) and for cold front‐induced storm surges and water transport (Huang & Li, ). In Huang and Li () skill assessment is done to show the reliability of the FVCOM's flow calculations at the inlets.…”