Sediment transport dynamics in Delaware Estuary
Creators: name: McSweeney, Jacqueline
Publisher: name: No Publisher Supplied
Year: 2017

description: Estuaries are dynamically complex systems that connect riverine sources to the coastal ocean. The circulation within an estuary is fundamentally 3-dimensional, and lateral processes contribute significantly to material transport. Estuaries are known to trap sediment through convergent processes, and the estuarine turbidity maximum (ETM) zone, an area of elevated suspended sediment concentrations, is a particularly important region of sediment trapping, resuspension, and deposition. Though it is understood that sediment transport processes vary spatially and temporally within the ETM, the details of this variability and their impact on transport pathways and trapping efficiency are unknown. This dissertation characterizes the spatiotemporal variability of sediment processes in the Delaware Estuary, focusing on the contribution of lateral processes, pathways of sediment export, and sediment impacts on primary productivity. Sediment and velocity observations were acquired during 6 months in 2011 from seven moorings deployed across-channel in the known vicinity of the ETM. The data included upward-looking ADCPS calibrated to in situ sediment concentrations from which high-resolution sediment flux estimates were derived to evaluate the relative contributions of tidal and non-tidal processes and how they varied laterally. Tidal pumping contributed to roughly 30% of the along-channel sediment transport, facilitating export on the flank on the Delaware (DE) side but mostly import in the channel. In contrast, tidal pumping contributed very little to across-channel transport. The subtidal (or residual) sediment transport varied both spatially and temporally, driving export at all locations during high river discharge but export on the flank and import in the channel during lower river flows. Residual fluxes dominated the across-channel transport, and an area of divergence on the DE flank was identified that could facilitate sediment delivery to edging marshes. These observations highlight that sediment transport processes are spatially variable and directly impact sediment trapping in an estuary A coupled hydrodynamic and sediment transport Regional Ocean Modeling System (ROMS) model was used to evaluate the spatial variability of residual sediment fluxes over spring-neap, seasonal, and annual timescales. Since sediment transport observations are sparse and spatially limited, the model can help clarify the 3-dimensional structure of the ETM and how lateral variability may differ along-channel. This analysis provided a more comprehensive picture of how sediment may be reworked in the ETM or exported either out of the estuary or to the fringing marshes. The Delaware Estuary is turbid, and primary production within the ETM can become light limited due to high sediment concentrations. In 2010 and 2011, measurements of suspended sediment, light levels, oxygen and nitrate concentrations, and chlorophyll were collected along the main axis of the estuary in March, June, September, and December. These observations were used along with an idealized ROMS model to evaluate stratification and other processes that control sediment resuspension, which in turn impact light availability and primary productivity. This study emphasized that estuarine sediment dynamics have importance implications beyond material transport.