Cold fronts control multiscale spatiotemporal hydroperiod patterns in a man-made subtropical coastal delta (Wax Lake Region, Louisiana USA)

Abstract

Atmospheric cold fronts quasi-periodically produce storm surges and generate significant subtidal oscillations of water levels and water transport in coastal environments. Yet, it is unclear how these weather events—at regional scales —control the hydrodynamics in delta-dominated coasts. Here, we used a numerical model (SCHISM) to simulate the inundation/drying and water circulations generated by varying winds associated with cold front passages across the Atchafalaya Bay in the northern Gulf of Mexico, specifically the Wax Lake Delta (WLD) region. Water transport induced by winds through major channels of the WLD and that between the adjacent Vermilion Bay and inner shelf were quantified to evaluate the impact of cold fronts. Results show that significant wetting/drying conditions are highly correlated with wind direction and strength. Southerly/easterly winds tend to cause water set-up, thus inundating the delta region, while northerly and westerly winds cause water set-down, draining the bay into the continental shelf. As a result, up to 60% of the delta area (~ 50 km²) can become exposed land under northerly winds. The interconnectivity of the delta channel system is also highly dependent on the wind direction and magnitude: up to 37% of the total transport is through the shallow waters outside of the channels during cold fronts. In contrast, the water levels and velocity variations in the delta region were negatively correlated with the alongshore wind, a result of Ekman transport. At the delta head, where freshwater flows like a strong jet into the region, the velocity is marginally correlated with the wind but mostly correlated with seasonal river discharge variability of river discharge. At the transitional zone between the bay and coastal ocean, water level and surface flows are dominated by tidal forcing in contrast to the delta lobes area where wind regulates the flooding area extension. The bottom velocity and water levels at sites along the Atchafalaya Bay mouth negatively correlate with onshore wind, indicating bottom return flows against the wind. Our study offers a glimpse of how a combination of atmospheric and hydrodynamic forces operate in a young delta, where the coast is experiencing the highest sea level rise in North America.