Wind surge and saltwater intrusion in Atchafalaya Bay during onshore winds prior to cold front passage

Cold front passages are largely responsible for accretion along the Chenier Plain, west of the Atchafalaya River as well as many processes impacting the overall health and functioning of the coastal bays and wetlands. The associated water setup and set down during a frontal passage, when wind quickly switches from generally south to north, has significant implications for Louisiana’s bays, coastlines, larval transport, fishery, and oyster resources. The Atchafalaya River discharges up to 30% of the freshwater from the Mississippi River which results in an almost entirely fresh Atchafalaya Bay in spring. A one-month deployment of two tripods equipped with multiple sensors was made in the central Atchafalaya Bay near the Wax Lake Delta for the study of the impact of cold front passages on saltwater flux into the bay between March 20 and April 19, 2006. It was found that two episodes of saltwater intrusion occurred during the 1-month deployment. These events had rapid and transient increases of salinity of approximately 2 PSU. These saltwater intrusion events occurred after a high water slack and lasted for 0.5 to 2 h, respectively. High tide appears to be a necessary condition for the saltwater intrusion. This “high tide”, however, can be a combination of the astronomical tides and wind-induced surge. Strong southerly wind prior to a cold front passage can be an important additional forcing to initiate and/or strengthen saltwater intrusion. It is estimated that roughly 50% of the observed setup is due to local wind stress, 25% due to wave setup, and 25% due to low atmospheric pressure during a cold front passage. The Coriolis-induced setup is found to be negligible in the current study.