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Projects

Salinity fronts and sediment transport in the Connecticut River estuary

We are studying salinity fronts and sediment transport in the Connecticut River estuary, a tidal salt wedge estuary. Topographic variation due to the underlying geology induces frontal formation, and the resulting spatial gradients in circulation, stratification, and mixing lead to local trapping of fine grained sediment. The Connecticut has strong tidal and river velocities, and in such an energetic system the fine sediment might be expected to be carried out to sea, but the salinity fronts enhance retention in the estuary and help sustain wetlands at the estuarine margins.

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Several publications have come out of this work so far. Holleman et al. (2016) finds that mixing in shear instabilities at salinity fronts is highly efficient with flux Richardson numbers averaging 0.23, much greater than the typically reported values of 0.15-0.17.  Geyer et al. 2017 shows that despite this active mixing during ebbs the gradient Richardson number is maintained near the threshold value of Ri = 0.25, and this flow at lateral expansions can be represented with two-layer hydraulic theory including an interfacial mixing coefficient.  Ralston et al. 2017 shows that the modeling sharp salinity gradients and steep topography induces significant numerical mixing, but the numerical artifacts correspond with observed turbulent mixing and that the turbulence closure can be adjusted to compensate. Yellen et al. 2017 combine observations and the model to show that high rates of deposition in marginal coves occur not during the peak river discharge when sediment comes in from the watershed but rather during lower flow when the salinity intrusion brings sediment landward back up the estuary.

Funding for this project is provided by NSF.

Jetyak CTD cast

Sediment transport and extreme events in the Hudson

Through a combination of observations, 3d hydrodynamic models, and a simplified model of estuarine dynamics we have been investigating sediment transport processes in the Hudson River estuary, focusing on importance of bathymetric variability and intermittent discharge events. Read more

The Hudson is considered a partially mixed estuary, but strong salinity fronts can form at constrictions and create spatial gradients in stratification, bottom stress and suspended sediment that traps sediment in the channel and moves it landward (Ralston et al. 2012). In contrast, on the shoals the sediment transport is dominantly seaward, providing the primary route for export of sediment to the coastal ocean. A simplified model of sediment transport in the estuary over the past 100 years of flow in the Hudson showed that sediment accumulates in the estuary during extremely low- and high-discharge periods and is exported during moderate discharge events (Ralston and Geyer 2009). A recent study of sediment transport during tropical storms Irene and Lee also found that sediment supply far exceeded the transport capacity, and consequently about 2/3 of the 3 Mton of new sediment that entered from the watershed remained trapped in the tidal river months after the storms (Ralston et al. 2013). Observations and modeling upstream in the tidal river showed that sediment trapping occurs on shallow shoals and side embayments, and depends critically on settling velocity (Ralston et al. 2017).  Sediment moves seaward much more slowly than the river flow, such that the transport time from the watershed to the ocean takes several years to several decades, again depending primarily on settling velocity.  This work has involved collaborators at the USGS, and has been funded by the Hudson River Foundation and NSF.

Sustainability of estuaries in the anthropocene: sediment supply, dredging, and wetland loss

Estuaries are often home to human population centers due to the advantages the present for transport and natural resources, and consequently many estuaries have been heavily modified by human activities such as shoreline hardening, infilling, dredging, and pollutant discharge. This project examines feedbacks among human activities, changes in estuarine morphology, and impacts on built human systems. Read more

For example, dredging allows for commercial shipping traffic, but the increased channel depth will enhance sediment deposition, requiring more frequent dredging and reducing the supply of sediment for adjacent marshes to keep up with sea level rise; deeper channels also increase the salinity intrusion, which may threathen drinking water supplies farther up the river. This project examines both the physical links among the different parts of the estuary, but also the economic and social factors that drive the human activities, which must in turn respond to the environmental changes. The coupled system approach is designed to optimize sustainability, recognizing that human impacts are intrinsic to modern estuaries and that the costs and benefits must be balanced. This project involves collaborators from the WHOI Marine Policy Center, University of Delaware, Rutgers, and LSU, and is funded by NSF Coastal SEES.

Physical controls on an estuarine harmful algal bloom

Nauset estuary on Cape Cod has a recurring harmful algal bloom (HAB) of Alexandrium tamarense (formerly called A. fundyense) that results in nearly annual closure of the local shellfish resource. The Nauset system is representative of many coastal embayments where HABs may have been exacerbated by increased nutrient inputs, and more generally the relatively isolated population in Nauset provides an excellent natural laboratory for studying an organism that is distributed around the world.  Read more

We have shown how the estuarine morphology of deep salt ponds separated from the ocean and from each other by a network of channels through shallow marsh creates an ideal environment for bloom development, particularly with groundwater fluxes to enhance stratification and deliver nutrients (Crespo et al 2011). Water temperature appears to be a key factor in the rate of bloom development, as a relatively simple growing degree day model that accounts for the dependence of growth rate on temperature can be used to collapse significant differences in the timing of the bloom between years and among different parts of the estuary (Ralston et al. 2014).  A 3-d hydrodynamic model of Nauset with a HAB population module is consistent with multiple years of observations (and the simpler degree day model), and it demonstrates the importance of stratification in the ponds and vertical migration by Alexandrium to maintain the high rates of retention in the ponds and the extremely high growth rates (Ralston et al. 2015). This work is in collaboratoration with the Anderson Lab in the WHOI Biology Department, and has funded by NSF and NIH through the Woods Hole Center on Oceans and Human Health, and by NOAA.