OBSERVATIONS OF SEDIMENT TRANSPORT IN THE DELAWARE ESTUARY DURING SPRING RUNOFF CONDITIONS

Suspended-sediment dynamics are relevant to a number of environmental and engineering concerns in the Delaware Estuary. This study was designed to advance our understanding of sediment-transport variability in the upper Estuary, and to identify the sedimentary response to a spring freshet event (during which much of the annual sediment load is typically delivered). Time-series data of current velocity and suspended-sediment concentration were collected using moored instrumentation at two locations: (1) near Philadelphia International Airport (the approximate landward limit of salinity intrusion), and (2) 40 km down-estuary near New Castle, Delaware (oligohaline waters). During an 80-day deployment extending from mid March to early June 2003, daily mean discharge of the Delaware River at Trenton, New Jersey, exceeded 2,200 m³/s, consistent with a 1-2 year peak-flow recurrence interval. Current velocity at the landward mooring site was strongly influenced by changes in river discharge, with increased discharge leading to stronger ebb and weaker flood currents and an increased down-estuary sediment flux. At the New Castle site, low suspended-sediment concentrations were observed during periods of high river discharge, suggesting that increased freshwater input shifted the estuarine turbidity maximum seaward. Tide-filtered current velocities and suspended-sediment concentrations indicate a net down-estuary flux of suspended sediment past both sites during the study period. However, the cumulative flux at New Castle was 2 to 3 times greater than that of the Airport site, suggesting the presence of an erosional source of sediment and/or tributary influx within the study reach. Continued analysis of the time-series data will help distinguish between sediment sources and thereby further our understanding of suspended-sediment transport in the Estuary during spring runoff conditions.