Observations of tidal and springtime sediment transport in the upper Delaware Estuary

Abstract

Suspended-sediment transport in the upper Delaware Estuary was investigated in Spring 2003 to examine mechanisms of material exchange from the tidal river to the turbidity maximum zone in the lower estuary. Timeseries records of currents and suspended-sediment concentration (SSC) were obtained between 18 March and 10 June 2003 using moored instrumentation deployed at two locations spanning the tidal river−estuary transition: (1) Tinicum Island, 90 km up-estuary of the Delaware Bay mouth in tidal freshwater; and (2) New Castle Flats, 40 km down-estuary of Tinicum in oligohaline waters. Environmental conditions during the observational period were typical for the spring season and included two river peakflows (1000–2000 m 3 s −1) and several moderate remote-wind events. Results indicate that SSC and tidal sediment flux vary spatially in the estuary with local current magnitude and the proximity of patches of easily resuspendable sediment. Landward of the turbidity maximum zone, SSC was not correlated with current velocity due to depletion of bed sediment sources early in the tidal cycle. In contrast, SSC and velocity within the turbidity maximum zone were well correlated due to an abundance of fine sediment generated by resuspension and advection. At both observational sites the depth-averaged residual current (Eulerian mean) and net sediment flux were seaward, and the flux magnitude increased 3–4 fold during river peakflow events on account of elevated ebb flow and bottom scour. The seaward residual current, mostly compensatory flow for Stokes Drift on flood tide, is an important mechanism of sediment transport to the estuarine turbidity maximum zone. Averaged over the 80-day study period, the cross-sectionally averaged sediment flux past New Castle (11 ± 4 × 10 8 kg) was significantly larger than that at Tinicum (4 ± 1 × 10 8 kg), and twofold larger than the estimated influx from river tributaries (5 ± 1 × 10 8 kg). The mass imbalance (∼7 × 10 8 kg) suggests that eroded bed sediment, previously deposited and stored in the upper estuary, was a major source of material to the turbidity maximum zone in Spring 2003.