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Mcsweeney J.M.,Rutgers UniversityNew Brunswick | Chant R.J.,Rutgers UniversityNew Brunswick | Sommerfield C.K.,Ocean and EnvironmentUniversity of DelawareLewes
Journal of Geophysical Research: Oceans | Year: 2016

Lateral processes contribute significantly to circulation and material transport in estuaries. The mechanisms controlling transport may vary spatially such that shallow and deep regions of an estuary contribute differently to the total transport. An observational study was conducted to explore the importance of lateral variability in sediment transport mechanisms in the Delaware Estuary. Seven moorings were deployed across the channel in the region of the estuarine turbidity maximum (ETM) zone from April to August 2011. Time series of along-channel sediment transport reveal a consistent pattern of sediment export across the entire estuary during periods of high river discharge, followed by a transition to import within the channel and export on the flanks during low river flow. There is a persistent divergence of across-channel sediment fluxes on the Delaware side, where sediment from the flank is transported toward both the channel and wetland coast. Decomposition of the fluxes highlight that across-channel sediment transport is driven by mean lateral circulation, whereas along-channel transport is driven primarily by mean advection, with tidal pumping contributing to about 30% of total transport. The spatial and temporal variability of mean advection and tidal pumping were generally complementary, with both contributing to the observed sediment transport pathways. Tidal pumping, linked to tidal asymmetries in stratification and sediment resuspension, was shown to drive both ebb-driven export and flood-driven import depending on the tidal variability of stratification. The spatiotemporal patterns of sediment transport highlight the three-dimensional structure of the ETM and shed light on the variability of sediment transport mechanisms. © 2015. American Geophysical Union. Source

Li M.,University of Maryland Center for Environmental SciencesCambridge | Chant R.J.,Rutgers UniversityNew Brunswick | Souza A.J.,National Oceanographic CentreLiverpool UK
Journal of Geophysical Research C: Oceans | Year: 2015

Microstructure and current velocity measurements were collected at a cross-channel transect in the James River under spring and neap tidal conditions in May 2010 to study cross-estuary variations in vertical mixing. Results showed that near-surface mixing was related to lateral circulation during the ebb phase of a tidal cycle, and that the linkage was somewhat similar from neap to spring tides. During neap tides, near-surface mixing was generated by the straining of lateral density gradients influenced by the advection of fresh, riverine water on the right side (looking seaward) of the transect. Spring tide results revealed similar findings on the right side of the cross section. However, on the left side, the straining by velocity shears acted in concert with density straining. Weak along-estuary velocities over the left shoal were connected to faster velocities in the channel via a clockwise lateral circulation (looking seaward). These results provided evidence that in the absence of direct wind forcing, near-surface vertical mixing can occur from mechanisms uncoupled from bottom friction. © 2015. American Geophysical Union. Source

Chant R.J.,Rutgers UniversityNew Brunswick
Journal of Geophysical Research C: Oceans | Year: 2015

An observational study was conducted in Delaware Bay during the summer of 2011 aiming to quantify the different mechanisms driving the salt flux in this system. Seven moorings, equipped with bottom-mounted ADCPs and CT sensors at difference depths, were deployed across a section of the estuary. The total area-averaged and tidal-averaged salt flux was decomposed in three different contributions: the advective salt flux that represents the flux caused by river input and meteorological-induced flows, the steady shear dispersion that is the salt flux driven by the estuarine exchange flow, and the tidal oscillatory salt flux that is induced by the tidal currents. The advective salt flux dominated over the steady shear dispersion and tidal oscillatory salt flux, because it was driven mainly by changes in sea surface height associated with wind-driven setup and setdown. The steady shear dispersion was always positive and presented a spring/neap variability that was consistent with a two-layer exchange flow. On the other hand, the tidal oscillatory salt flux fluctuated between positive and negative values, but increased around a strong neap tide and decreased on the following spring tide. This variability is contrary to previous parameterizations, whereby the tidal salt flux is proportional to the amplitude of the tidal currents. The observational estimate was compared to a parameterization that relates tidal salt flux as proportional to tidal current amplitude and stratification. The observational estimate agreed with this new parameterization when the river discharge was relatively constant. © 2015. American Geophysical Union. Source

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