We study the compound flooding processes that occurred in Hurricane Florence (2018), which was accompanied by heavy precipitation, using a 3D creek-to-ocean hydrodynamic model. We examine the important role played by barrier islands in the observed compound surges in the coastal watershed. Locally very high resolution is used in some watershed areas in order to resolve small features that turn out to be critical for capturing the observed high water marks locally. The wave effects are found to be significant near barrier islands and have contributed to some observed over-toppings and breaches. Results from sensitivity tests applying each of the three major forcing factors (oceanic, fluvial, and pluvial) separately are succinctly summarized in a “dominance map” that highlights significant compound effects in most of the affected coastal watersheds, estuaries, and back bays behind the barrier islands. Operational forecasts based on the current model are being set up at NOAA to help coastal resource and emergency managers with disaster planning and mitigation efforts.

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.

Lake Pontchartrain is a brackish estuary and restricted lagoon in the microtidal northern Gulf of Mexico. Using a validated Finite Volume Community Ocean Model (FVCOM), we study the flow regimes and wind‐driven adjustment of circulation in this system. It is found that tidal currents are only significant in the eastern end near the open boundary. Local wind inside the estuary is important in driving the circulation including coastal currents. Numerical experiments confirm that the subtidal wind‐driven flow is in a quasi‐steady balance between wind stress and surface slope. Our results show that the adjustment of surface level inside the system to a sudden change in wind is a transient seiche, which dissipates within two to three cycles, each lasting ~3 hr. After the oscillations, the system reaches a new equilibrium. We show in theory that this is a damped oscillation and as wind changes, an adjustment to a new equilibrium goes much faster than a complete tidal period. This leads to the quasi steady state balance of subtidal wind‐driven flows with essentially no phase lag with wind even though wind changes with time continuously. Wind‐induced circulation behaves differently from the Csanady model with Earth rotation but similar to the prismatic lake model of Engelund without Coriolis: The coastal and shallow regions tend to have flows in the direction of wind, while the interior and deeper waters have return flows against the wind especially in bottom layer. The surface flow may not be in the direction of wind, while the bottom usually has broad and uniform counter wind flows.

Atmospheric cold fronts provide recurring forcing for circulations and long-term transport in estuaries with microtides. Multiple horizontal ADCPs were used to obtain time series data from three inlets in Barataria Bay. The data cover a period of 51 atmospheric cold fronts between 2013 and 2015. The weather and subtidal ocean response are highly correlated in the “weather band” (3–7 days). The cold front–associated winds produce alternating flows into, out of, and then back into the bay, forming an asymmetric “M” for low-pass filtered flows. Results show that cold front–induced flows are the most important component in this region, and the flows can be predicted based on wind vector time series. Numerical simulations using a validated Finite-Volume Coastal Ocean Model (FVCOM) demonstrate that the wind-driven oscillations within the bay are consistent with the quasi-steady state with little influence of the Coriolis effect for cold front–related wind-driven flows. The four major inlets (from the southwest to the northeast) consistently carry 10%, 57%, 21%, and 12% of the tidal exchange of the bay, respectively. The subtidal exchange rates through them however fluctuate greatly with averages of 18% ± 13%, 35% ± 18%, 31% ± 16%, and 16% ± 9%, respectively. Several modes of exchange flows through the multiple inlets are found, consisting of the all-in and all-out mode (45% occurrence) under strong winds perpendicular to the coastline; the shallow-downwind, deep-upwind mode (41%), particularly during wind-relaxation periods; and the upwind-in and downwind-out mode (13%) under northerly or southerly winds. These modes are discussed with the low-pass filtered model results and verified by a forcing–response joint EOF analysis.