Effects of Hurricane Irma on residual flows and saltwater intrusion in a subtropical estuary

The variability of residual flows and saltwater intrusion was evaluated for the duration of a hurricane-related freshwater pulse in a subtropical estuary, the Suwannee River. Increased rainfall affected the river’s watershed when Hurricane Irma skirted the west coast of Florida in September 2017. The river remained short of reaching flood stages, but the increased river discharge was enough to modify the residual circulation and restrict saltwater intrusion. Current velocity profiles and hydrographic variables were collected in five tidal-cycle experiments to study the lower estuary’s response to the peak river discharge and its subsequent relaxation. Data were recorded in three cross-estuary transects. In addition, continuous salinity measurements were recorded for 48 days over the shallow portion of the seaward transect. Results showed that the subtidal water-level slope inside the estuary, between the mouth and a river station at Wilcox, modified the residual circulation and restricted the intrusion of salt water. During high tide, salt water entered the estuary due to tidal pumping even though residual circulation was seaward in the cross-section. Residual seaward flows persisted for the duration of the hurricane-related freshwater pulse. After relaxation of the freshwater pulse, saltwater intrusion was observed in the time series over the shallow portion. This saltwater intrusion responded to changes in wind forcing, with largest intrusions when the wind blew into the estuary, i.e., toward the N and NW.