David Ralston

Estuarine dynamics and transport processes

Estuarine dynamics

Estuarine dynamics

The physics of fluids in estuaries is at the core of our research, from the turbulent fluctuations at scales of centimeters and periods of seconds to the tidally averaged exchange flow between the river and ocean. The dominant physical processes in an estuary depends on the forcing conditions --  river flow and tidal amplitude -- and on the bathymetry. Estuarine fluid dynamics directly affect the salinity distribution and transport of material (sediment, nutrients, contaminants, organisms) between the river and the ocean. Our research has examined dominant physical processes driving exchange flow and mixing over a broad of estuarine parameter space, from large, partially mixed estuaries like the Hudson River estuary and San Francisco Bay, to shorter, shallower, energetic salt wedge estuaries like the Merrimack and Connecticut River estuaries, to extremely shallow but dynamic intertidal flats in Puget Sound and San Francisco Bay.

Selected publications

Salinity dynamics in the Hudson River estuary

Ralston, DK, WR Geyer, and JA Lerczak, 2008. Subtidal salinity and velocity in the Hudson River estuary: observations and modeling.  J. Physical Oceanogr., 38(4), 753-770.

We use observations and a simplified model of the estuarine circulation and salinity distribution to show how the Hudson River estuary responds to changes in river discharge and tidal amplitude. With modifications to account for local and remote wind forcing and the reduction in mixing due to stratification, the simplfied model could match observations with skills similar to a detailed, 3-d circulation model. Application of the model to the Hudson and to San Francisco Bay demonstrated the sensitivity of the salinity distribution in each estuary to the along-channel bathymetry because of the strong dependence on the estuarine circulation on water depth.

Salt flux in the Merrimack River

Ralston, DK, WR Geyer, and JA Lerczak, 2010. Structure, variability, and salt flux in a strongly forced salt wedge estuary.  J. Geophys. Res., 115, C06005, doi:10.1029/2009JC005806.

In the Merrimack River estuary, a tidal salt wedge, we use observations and a model to show that salt flux is predominantly due to tidal proceses rather than steady exchange (and in sharp contrast with the Hudson). The salinity distribution is sensitive to river discharge -- at moderate to high discharge the estuary is short and highly stratified, while at lower discharges it shifts to a longer, more weakly stratified system; this transition occurs when the length of the salinity intrusion is similar to the tidal excursion. Good model performance required having well‐resolved grid bathymetry and low background mixing rates.

Mixing in the Merrimack River

Ralston, DK, WR Geyer, JA Lerczak, and M Scully, 2010. Turbulent mixing in a strongly forced salt wedge estuaryJ. Geophys. Res., 115, C12024, doi:10.1029/2009JC006061.

We calculate the distribution of stress and mixing of salt in the Merrimack, a tidal salt wedge estuary. During floods, stresses are strong but mixing is limited because bottom boundary layer turbulence is separated from the pycnocline. During ebbs, fronts form at bathymetric expansions. Internal shear layers generated by opposing baroclinic and barotropic pressure gradients provide the dominant source of mixing during the early ebb (~50% of total), but late in the ebb, stratification weakens and boundary layer turbulence dominates. Mixing efficiency in the shear layers was high, and the overall mixing efficiency increased with river discharge.


Last updated: November 12, 2014