Sedimentary response of the Delaware Estuary to tropical cyclones Irene and Lee in 2011

Duval, Daniel
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University of Delaware
Tropical cyclones have a major influence on the hydrodynamics of MidAtlantic rivers and estuaries, and by extension on processes and rates of sediment transport. In 2011 the passage of Hurricane Irene and Tropical Storm Lee through the Delaware River Basin was recorded by sensors deployed throughout the region, and the resulting observations provided unique insights into the coupled hydrodynamic and sedimentary response of the Delaware Estuary. The combined suspended sediment input of the two main tributaries, the Delaware and Schuylkill rivers, was over 1,300,000 metric tons during the two-week storm period. Time series records of sediment transport along the axis of the estuary suggested that most of the sediment transported during the storms was bed material resuspended within the estuary, not new material delivered from above the head of tide. Significantly, the resuspended sediment flux was equal to or greater than the river influx. Despite the large volume of freshwater delivered to the estuary, salt was not completely flushed from the mouth of Delaware Bay, and the resident suspended sediment inventory was retained. The storm-produced sediment pulse was attenuated and assimilated during passage from the upper to lower estuary, and near the head of Delaware Bay was strongly buffered by mixing associated with the large intertidal volume of the bay. Salinity and velocity measurements indicated that, although the salt intrusion was pushed ~55 km downestuary during Tropical Storm Lee, the two-layer estuarine circulation remained sufficiently vigorous to trap suspended sediment supplied from the upper estuary. A shipboard survey conducted shortly after the storms suggested that much of the suspended sediment was transported laterally from the axial channel to the adjacent subtidal flats, thereby increasing the potential for permanent deposition.