Malcolm E. Scully

Effects of ambient currents and waves on gravity-driven sediment transport on continental shelves

Abstract Observations from several shelf environments show that down-slope gravity-driven transport may constitute an important mode of suspended sediment dispersal across shelves and highlight the influence of ambient waves and currents on gravity-induced sediment flux. The phenomena discussed here involve high concentrations of suspended sediment mixed with seawater and thus differ in genesis from hyperpycnal plumes released directly from sediment-laden rivers. The field sites examined are the Gulf of Bohai off the mouth of the Yellow River (Huanghe), the northern California shelf off the mouth of the Eel River, and the Louisiana shelf west of the mouths of the Mississippi River. Off the Yellow River, rapid down-slope transport over distances of a few km occurred when frictional resistance, induced by strong along shelf currents, was temporarily relaxed. More prolonged down-slope motion over longer distances occurred following floods of the Eel River, when wave and current agitation provided turbulence to sustain gravity-driven transport of fluid mud. On the Louisiana inner shelf, the down-slope gravity force was much weaker, but observations suggest that thin gravity flows may still have occurred in the presence of waves. A simple analytical theory is developed that incorporates the influence of ambient shelf currents on gravity-driven transport of suspended sediment. This theory is quantitatively consistent with the observations from the three sites. If the supply of easily suspended sediment is less than the capacity of ambient currents (including waves) to carry sediment, then intense turbulence limits gravity-induced sediment transport by increasing the drag at the base of the flow. When ambient currents abruptly cease, rapid down-slope transport can then occur over short distances until the sediment settles. Such flows do not remain intensely turbulent because the slope of the continental shelf is too gentle to induce shear instability within the gravity flow. The maximum sustained rate of gravity-induced sediment transport occurs when ambient currents are strong, but the supply of easily suspended sediment exceeds the resuspension capacity of the ambient currents. Feedback then leads to values of the gradient Richardson number (Ri) within the flow that are near the critical value of 1/4. This partially damps bottom drag, but still allows the generation of sufficient turbulence to maintain sediment in suspension. Observations also indicate systematic relationships among Ri, the supply of easily suspended sediment and the bottom drag coefficient acting on the gravity flow.

Control of Estuarine Stratification and Mixing by Wind-induced Straining of the Estuarine Density Field

Observations from the York River Estuary, Virginia, demonstrate that the along-channel wind plays a dominant role in governing the estuarine exchange flow and the corresponding increase or decrease in vertical density stratification. Contrary to previous findings that suggest wind stress acts predominantly as a source of energy to mix away estuarine stratification, our results demonstrate that the wind can play a more important role in straining the along-channel estuarine density gradient. Down-estuary winds enhance the tidally-averaged vertical shear, which interacts with the along-channel density gradient to increase vertical stratification. Up-estuary winds tend to reduce, or even reverse the vertical shear, reducing vertical stratification. In two experiments each lasting approximately a month, the estuarine exchange flow was highly correlated with the along-channel component of the wind. The changes in stratification caused by the exchange flow appear to control the amount of vertical mixing as parameterized by the vertical eddy viscosity. The degree of stratification induced by wind straining also appears to play an important role in controlling the effectiveness of wind and tidal mixing.

The influence of lateral advection on the residual estuarine circulation : a numerical modeling study of the Hudson River Estuary

Abstract In most estuarine systems it is assumed that the dominant along-channel momentum balance is between the integrated pressure gradient and bed stress. Scaling the amplitude of the estuarine circulation based on this balance has been shown to have predictive skill. However, a number of authors recently highlighted important nonlinear processes that contribute to the subtidal dynamics at leading order. In this study, a previously validated numerical model of the Hudson River estuary is used to examine the forces driving the residual estuarine circulation and to test the predictive skill of two linear scaling relationships. Results demonstrate that the nonlinear advective acceleration terms contribute to the subtidal along-channel momentum balance at leading order. The contribution of these nonlinear terms is driven largely by secondary lateral flows. Under a range of forcing conditions in the model runs, the advective acceleration terms nearly always act in concert with the baroclinic pressure gradie...

The Importance of Climate Variability to Wind-Driven Modulation of Hypoxia in Chesapeake Bay

Abstract Extensive hypoxia remains a problem in Chesapeake Bay, despite some reductions in estimated nutrient inputs. An analysis of a 58-yr time series of summer hypoxia reveals that a significant fraction of the interannual variability observed in Chesapeake Bay is correlated to changes in summertime wind direction that are the result of large-scale climate variability. Beginning around 1980, the surface pressure associated with the summer Bermuda high has weakened, favoring winds from a more westerly direction, the direction most correlated with observed hypoxia. Regression analysis suggests that the long-term increase in hypoxic volume observed in this dataset is only accounted for when both changes in wind direction and nitrogen loading are considered.

The Importance of Tidal and Lateral Asymmetries in Stratification to Residual Circulation in Partially Mixed Estuaries

Abstract Measurements collected in the York River estuary, Virginia, demonstrate the important impact that tidal and lateral asymmetries in turbulent mixing have on the tidally averaged residual circulation. A reduction in turbulent mixing during the ebb phase of the tide caused by tidal straining of the axial density gradient results in increased vertical velocity shear throughout the water column during the ebb tide. In the absence of significant lateral differences in turbulent mixing, the enhanced ebb-directed transport caused by tidal straining is balanced by a reduction in the net seaward-directed barotropic pressure gradient, resulting in laterally uniform two-layer residual flow. However, the channel–shoal morphology of many drowned river valley estuaries often leads to lateral gradients in turbulent mixing. Tidal straining may then lead to tidal asymmetries in turbulent mixing near the deeper channel while the neighboring shoals remain relatively well mixed. As a result, the largest lateral asymm...

The Influence of Stratification and Nonlocal Turbulent Production on Estuarine Turbulence: An Assessment of Turbulence Closure with Field Observations

Abstract Field observations of turbulent kinetic energy (TKE), dissipation rate e, and turbulent length scale demonstrate the impact of both density stratification and nonlocal turbulent production on turbulent momentum flux. The data were collected in a highly stratified salt wedge estuary using the Mobile Array for Sensing Turbulence (MAST). Estimates of the dominant length scale of turbulent motions obtained from the vertical velocity spectra provide field confirmation of the theoretical limitation imposed by either the distance to the boundary or the Ozmidov scale, whichever is smaller. Under boundary-limited conditions, anisotropy generally increases with increasing shear and decreased distance to the boundary. Under Ozmidov-limited conditions, anisotropy increases rapidly when the gradient Richardson number exceeds 0.25. Both boundary-limited and Ozmidov-limited conditions demonstrate significant deviations from a local production–dissipation balance that are largely consistent with simple scaling r...

The role of advection, straining, and mixing on the tidal variability of estuarine stratification

AbstractData from the Hudson River estuary demonstrate that the tidal variations in vertical salinity stratification are not consistent with the patterns associated with along-channel tidal straining. These observations result from three additional processes not accounted for in the traditional tidal straining model: 1) along-channel and 2) lateral advection of horizontal gradients in the vertical salinity gradient and 3) tidal asymmetries in the strength of vertical mixing. As a result, cross-sectionally averaged values of the vertical salinity gradient are shown to increase during the flood tide and decrease during the ebb. Only over a limited portion of the cross section does the observed stratification increase during the ebb and decrease during the flood. These observations highlight the three-dimensional nature of estuarine flows and demonstrate that lateral circulation provides an alternate mechanism that allows for the exchange of materials between surface and bottom waters, even when direct turbu...

Broadband acoustic quantification of stratified turbulence

High-frequency broadband acoustic scattering techniques have enabled the remote, high-resolution imaging and quantification of highly salt-stratified turbulence in an estuary. Turbulent salinity spectra in the stratified shear layer have been measured acoustically and by in situ turbulence sensors. The acoustic frequencies used span 120-600 kHz, which, for the highly stratified and dynamic estuarine environment, correspond to wavenumbers in the viscous-convective subrange (500-2500 m(-1)). The acoustically measured spectral levels are in close agreement with spectral levels measured with closely co-located micro-conductivity probes. The acoustically measured spectral shapes allow discrimination between scattering dominated by turbulent salinity microstructure and suspended sediments or swim-bladdered fish, the two primary sources of scattering observed in the estuary in addition to turbulent salinity microstructure. The direct comparison of salinity spectra inferred acoustically and by the in situ turbulence sensors provides a test of both the acoustic scattering model and the quantitative skill of acoustical remote sensing of turbulence dissipation in a strongly sheared and salt-stratified estuary.

Observations of the Transfer of Energy and Momentum to the Oceanic Surface Boundary Layer beneath Breaking Waves

AbstractMeasurements just beneath the ocean surface demonstrate that the primary mechanism by which energy from breaking waves is transmitted into the water column is through the work done by the covariance of turbulent pressure and velocity fluctuations. The convergence in the vertical transport of turbulent kinetic energy (TKE) balances the dissipation rate of TKE at first order and is nearly an order of magnitude greater than the sum of the integrated Eulerian and Stokes shear production. The measured TKE transport is consistent with a simple conceptual model that assumes roughly half of the surface flux of TKE by wave breaking is transmitted to depths greater than the significant wave height. During conditions when breaking waves are inferred, the direction of momentum flux is more aligned with the direction of wave propagation than with the wind direction. Both the energy and momentum fluxes occur at frequencies much lower than the wave band, consistent with the time scales associated with wave break...

Characterization and modulation of Langmuir circulation in Chesapeake Bay

AbstractMeasurements made as part of a large-scale experiment to examine wind-driven circulation and mixing in Chesapeake Bay demonstrate that circulations consistent with Langmuir circulation play an important role in surface boundary layer dynamics. Under conditions when the turbulent Langmuir number Lat is low (<0.5), the surface mixed layer is characterized by 1) elevated vertical turbulent kinetic energy; 2) decreased anisotropy; 3) negative vertical velocity skewness indicative of strong/narrow downwelling and weak/broad upwelling; and 4) strong negative correlations between low-frequency vertical velocity and the velocity in the direction of wave propagation. These characteristics appear to be primarily the result of the vortex force associated with the surface wave field, but convection driven by a destabilizing heat flux is observed and appears to contribute significantly to the observed negative vertical velocity skewness.Conditions that favor convection usually also have strong Langmuir forcing...