Thesis advisor: Gail Kineke / The Penobscot River Estuary is heavily contaminated with mercury; previous studies indicate maximum mercury concentrations of 4.6 ppm within the Frankfort Flats reach. The transport and trapping of this contaminant is linked to the transport and trapping of fine sediment within the estuary. Hydrographic and flow measurements, coupled with a spatial and temporal characterization of the bottom sediments, were performed during and following the freshet in 2010 to determine the mechanisms driving sediment transport and trapping within the estuary. The Penobscot River likely has a turbidity maximum associated with the landward extent of the salinity intrusion that is positioned over the Frankfort Flats reach during average discharge and tidal conditions. This turbidity maximum may be responsible for a patch of fine sediments in the Frankfort Flats reach in an otherwise coarse-grained bed. Additional transport and trapping of fine sediments within this reach is the result of secondary circulation driven by centripetal acceleration around meanders in the channel. Close proximity of meanders at Frankfort Flats, within ~5 km, creates opposite secondary circulation of magnitude ~0.2 m/s during flood and ebb conditions. / Thesis (BS) — Boston College, 2011. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Geology & Geophysics Honors Program. / Discipline: Earth and Environmental Sciences.

Thesis advisor: Gail C. Kineke / An observational study was conducted from 2013 to 2016 to investigate suspended-sediment transport processes in the stratified Connecticut River estuary. Time-series measurements of velocity and suspended-sediment concentration from the upper estuary were analyzed to determine the relative importance of different processes driving sediment flux under highly-variable river discharge. Results indicate that under high discharge the salt intrusion is forced towards the mouth causing large seaward sediment fluxes throughout the water column. Seaward fluxes are dominated by mean advection, with some contribution due to tidal pumping. Under low discharge the salt intrusion extends to the upper estuary, advancing as a bottom salinity front during each flood tide. Stratification and strong velocity shear during the ebb tide cause the upper and lower water column to become dynamically decoupled. Sediment flux near the bed is landward throughout the tidal cycle despite the net seaward depth-integrated flux, and is almost fully attributed to the mean estuarine circulation. River discharge is the primary factor affecting the magnitude and direction of sediment flux because of its high variability and direct connection to the salt-wedge dynamics. A generalized three-phase conceptual model describes suspended-sediment transport in shallow, stratified estuaries with low trapping efficiencies. First, fine sediment bypasses the estuary during high river flows and exports to the coastal ocean where a portion of this sediment is temporarily deposited outside the mouth. Second, during low discharge offshore mud deposits are reworked by wave- and tidally-driven currents and some sediment is advected back into the estuary with the advancing salt intrusion that transports sediment landward. Third, spatial salinity gradients facilitate sediment transport from the main channel to channel margins, marshes and off-river coves where it is retained and deposited long-term, as demonstrated in prior studies. This re-introduction and trapping of recycled sediment under low-discharge conditions can have important implications for pollutant transport, shoaling of navigation channels and harbors, and salt marsh accretion in the face of rising sea levels. / Thesis (MS) — Boston College, 2018. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

The stratified flow of the lower Mississippi River due to density gradients is a well documented phenomenon. This stratification of fresh and saline water manifests itself as a heavier wedge of saline water that extends upriver and a buoyant fresh water plume extending into the Gulf of Mexico past the Southwest Pass jetties. The maximum absolute distance of saltwater intrusion observed anywhere in the world occurred on the Mississippi River in 1939 and 1940 when saltwater was observed approximately 225 km upstream from the mouth of Southwest Pass. The U. S. Army Corps of Engineers now prevents the wedge from migrating upstream by constructing a subaqueous barrier in the river channel. A curvilinear grid was constructed representative of the modern Mississippi River delta. Boundary conditions were developed for the drought year of 2012 and the grid was tested in order to evaluate the salinity intrusion and sediment transport abilities of the Cartesian Z-coordinate Delft3D code. The Z-model proved to have the ability to propagate the saline density current as observed in the prototype. The effect of salinity on fine sediment transport is evaluated by manipulation of the settling velocity through a cosine function provided in the model code. Manipulation of the fine sediment fall velocity through the cosine function was an effective means to simulate the re-circulation of flocculated sediments in the saline wedge turbidity maxima. In addition, the Z-model capably reproduced the fine sediment concentration profiles in a fully turbulent shear flow environment. With the ability to reproduce the seasonal saline density current and its effect on sedimentation within the turbidity maxima as well as sedimentation characteristics in a fully turbulent shear flow, a model capable of analyzing all of the major processes affecting fine sediment transport within the Mississippi River salt wedge estuary has been developed.