The research presented in this study is motivated by the need to improve prediction of sediment transport in estuaries. A novel application of the Acoustic Doppler Velocimeter (ADV) in the lower Chesapeake Bay is shown to estimate in-situ particle fall velocity at a single point without affecting the ambient turbulence. Acoustic backscatter from the ADV proved to be the best estimator of mass concentrations due to its apparent insensitivity to the size or density of muddy aggregates. Fall velocities are estimated analytically from a balance of settling and diffusive flux gradients using two methods, one employing Reynolds concentration flux, and the other estimating eddy diffusivity using the von-Karman Prandtl equation. Single elevation estimates of fall velocity using the ADV to estimate Reynolds concentration flux produced the best estimates of fall velocity, which are on the order of 1 mm/s. A novel method is presented to measure TKE production using a profiling ADV instrument that has been contaminated by boat motion. The relative importance of physical processes that determine particle size distributions differs in three mid-Atlantic U.S.A. estuaries (York R., Elizabeth R., Chesapeake Bay) with different hydrodynamics and benthic characteristics as well as in different depth regimes within each estuary. Surface particle size dynamics in all of the estuaries are affected by irregular advection events. Middepth regions in the energetic estuaries are controlled tidally by the combined processes of TKE production decreasing particle size and differential settling increasing particle size. Middepth regions in the low energy estuary are controlled by irregular resuspension and trapping at the pycnocline of large low density particles. Bottom regions in all estuaries are most strongly influenced by resuspension, tidally in the energetic estuaries and irregularly in the low energy estuary. The interrelationships between metal concentrations, particle size, percent fixed solids (PFS), chlorophyll a, and molar Carbon to Nitrogen (C/N) ratios of suspended sediment are investigated in a heavily industrialized and polluted estuary, the Elizabeth R., VA. The relationship between PFS, C/N and aggregate size are also investigated in a relatively energetic, high concentration, and undisturbed estuary, the York. R., VA. Standard paradigms of contaminant concentration relationships with particle size and particle constituents were not supported in the low energy, low concentration suspended sediments of the Elizabeth R. (Abstract shortened by UMI.).

The Northern North Sea and the Patagonian Shelf have similar characteristics, and their circulation is determined by strong tidal regimes, significant wind-driven flow, and by density gradients to a lesser degree. The influence of oceanic currents is notable in the Patagonian Shelf, where the Falkland Current flows along the shelf edge, but less important in the North Sea. While the North Sea is perhaps the most studied body of water in the world, the circulation of the Patagonian Shelf have remained fairly unknown. This study presents evidence of near-inertial oscillations in the Northern North Sea, not reported previously, derived from measurements with high vertical resolution in the presence of the seasonal thermocline. Data mainly from ADCPs and thermistor strings were processed and analysed, leading to the conclusion that the structure of these oscillations corresponds to the first baroclinic mode. Computations of the Richardson number suggest that mixing occurred during storm events, which caused the oscillations and hence an enhanced shear at the thermocline. Given the lack of direct observations on the Patagonian Shelf, its circulation was studied by means of numerical modeling. A three-dimensional (3D) model, originally developed for the North Sea, was adapted and implemented to the Patagonian Shelf. The model is barotropic and is based on a two-dimensional (2D) vertically integrated tide/surge model, with the expansion in the vertical following a functional approach with an eddy viscosity closure related to the flow. The model can be run as 2D or 3D, and for most cases here it was run as 2D, except when it was used to describe the tidal flow near headlands. Semidiurnal tides with standing wave resonance were found to dominate, while diurnal tides exhibited topographically enhanced currents. The simulation of the Falkand Current showed large influence on the shelf circulation, and the response of the shelf to the passage of low pressure centers generated surges and substantial wind-driven currents with clearly defined patterns.

This dissertation aims at utilizing the acoustic approach to measure cohesive sediment behaviors including (1) suspension, (2) settling, (3) deposition and (4) consolidation. The first two processes were attempted to interpret by means of backscattered signal analysis, while the last two processes were done by echo signal analysis. The acoustic instruments used in this study include Acoustic Doppler Velocimeter (ADV), Pulse Coherent Acoustic Doppler Profiler (PC-ADP) and Micro-Chirp system. Used sediments are pure kaolinite and in-situ sediments collected from Mai Po and Clay Bank. 5-MHz ADV was used to estimate the suspended sediment concentration (SSC) and settling velocity (ws). For a limited range of SSC, the time-averaged backscatter wave strength can be well correlated with the SSC. Backscattered signals would be sometimes too noisy due to high amplification ratio, high sampling rate and small sampling volume, and thus, a moving average was used to yield the instantaneous changes of SSC. The measurement of ws with Clay Bank sediment showed that turbulence can increase ws, up to one order larger than that for calm water. When turbulence is stronger than a limit, however, it contributes to the decrease in ws. For the measurement of SSC profile, the performance of 1.5 MHz PC-ADP was evaluated. Clay Bank sediment showed a higher correlation coefficient between range-corrected volume scattering (SSC) and backscattered signal within a limited SSC range (ca. < 10 g/L). On the other hand, kaolinite showed a much smaller range of SSC for linear correlation. This different response might be attributed to the fact that the acoustic response is primarily controlled by the SSC and particle size in suspension at a given frequency. This study suggests that PC-ADP is a potential instrument to reveal the high-resolution (about 1.6 cm) SSC profiles near the bed, if the sediment is sufficiently large. Annular flume experiments with Mai Po sediment were conducted to address a debatable issue regarding the critical shear stress for deposition (tau cd). The direct observation from the flume bottom suggests that tau cd does exist, and that the deposition only occurs when the local bed shear stress (taub) is less than taucd. The changes of deposit length and SSC under the simulated tidal cycles demonstrate that deposition can happen only at tidal decelerating phases with a recognizable tau cd. This study further proves that both taub (a hydrodynamic parameter) and taucd (a sediment parameter) are the main controlling parameters for determining cohesive sediment deposition. A non-intrusive acoustic technique and a signal-processing protocol were developed to estimate the bulk density at consolidating sediment interface. Using high-frequency (300-700 KHz) Chirp acoustic waves, laboratory measurements were carried out in a consolidation tank filled with clay-water mixtures. Because the acoustic echo strength is proportional to the difference in acoustic impedance, and the sound speed in water is close to that in clay, the approximation of bulk density could be successfully presented. The acoustic wave reflectivity increased with increasing the bulk density at the water-sediment interface, which are well correlated with the consolidation status.

Destratification in the York River, USA, during high spring tides is the result of the interruption of normal two-layer estuarine flow by advection of relatively fresh water in to the River mouth from the Chesapeake Bay. This is due to the presence of a longitudinal salinity gradient in the Bay and a difference of tidal current phase between the River and the Bay. Similar behavior is seen in other subestuaries of the Chesapeake Bay and may be common in subestuary-estuary interactions. Correlation and regression analysis are used to examine relationships between stratification variation in the lower York River and a variety of tidal and environmental parameters. A gross measure of stratification was derived from near surface and deep salinity samples. One hundred fifty six observations were made over a 434 day period from February 1982 to April 1983. The environmental and tidal factors evaluated were assessed on a daily basis and incorporated a variety of transformations. The factors included wind speed and direction, fresh water river flow from both the York and Rappahanock Rivers, water temperature, mean sea level and the following tidal parameters: observed and predicted daily mean and maximum high and low tide height, flood, ebb, and combined flood and ebb tidal ranges for Gloucester Point and for Hampton Roads. The results indicate that: (1) almost all of the tidal range or high tide height factors tested are equally strongly correlated with salinity difference, being associated with as much as 48% of the variation in that value; (2) that a combination of functions of tidal range and mean sea level at Gloucester Point are associated with more than 70% of the variation; and (3) that with the addition of wind stress terms as much as 80% of the variation can be included in the model. Over a range of observed salinity differences from 0.01 to 11.06 per mille the 25 term model predicts a range of -1.01 to 11.09 per mile with a root mean squared error of 0.99 per mile. A model predicting variation in surface mixed layer depth from salinity difference is also presented.

No description available.

A series of observations of the density distribution across the York River estuary documents distinct lateral differences in density and degree of vertical mixing. The magnitude of the density differences varies throughout the tidal cycle; maximum lateral gradients occur at times of minimum current. When the density distribution is sufficiently inhomogeneous, longitudinal estuarine fronts are generated. These fronts are axially aligned, up to several miles in length, and are apparent for less than 2 hours at any given location. Although the density difference across the frontal boundary is often small, horizontal pressure gradients acting over a broad frontal region generate the convergent circulations necessary to maintain these fronts. Measurements of the longitudinal velocities across the same section reveals negligible phase difference but a significant amplitude difference between the currents in the channel and those over the shoals. Differential advection across the estuary due to this velocity shear is the process by which the observed density distribution is generated.

No description available.

The characteristics and effects of intrusions of estuarine outflow over the inner shelf were examined, based on hydrographic and meteorological observations obtained during the "Coastal Ocean Processes" (CoOP'94) field experiment located off the Outer Banks at Duck, North Carolina. The episodic presence of distinct low salinity water masses issuing from the Chesapeake Bay created an intermittent baroclinic coastal current along the North Carolina coast. Under low wind conditions, this current occupied the upper half of the water column within 9 km of the coast. The plume was bounded by a distinct southward-propagating front, a region offshore of high horizontal salinity and velocity gradients, and a strong pycnocline underneath. The intrusion traveled along the coast at a speed comparable to the linear internal wave speed of a two-layer system. Intrusions were generally associated with southward winds (downwelling conditions); however, several observed events opposed northward wind-driven flow. The geometry and dynamics of the low salinity plume were strongly controlled by the local winds. Northward (upwelling) winds caused the plumes to widen offshore and thin vertically. Southward (downwelling) winds acted initially to speed the intrusions' alongcoast movement and cause them to narrow and deepen. Under strong downwelling winds, however, the intrusions contacted the bottom. This greatly decreased their speeds and caused diffusive widening. Propagation speeds of all plumes were seen to slow steadily through the study region. This was attributed to the observed mixing with ambient water along the path of the intrusion which increased its salinity, thereby reducing the buoyancy forcing. Under the continued influence of upwelling winds, the low salinity intrusions moved rapidly away from the coast and formed shallow lenses floating over the ambient shelf water. These generally dissipated in 1 to 2 days. The theoretical offshore transport response to wind forcing was investigated, illustrating two dynamical behaviors of the plumes, depending on whether they occupied the entire water column or were vertically segregated by stratification. The meteorological control of Bay/shelf exchange was examined to better comprehend the pulsed timing of the low salinity intrusions, which occurred every 2 to 8 days. Estimates of volume flux were derived from temporal variations of waterlevel measurements within the Chesapeake Bay. The volume flux time series exhibited strong peaks of outflow, which preceded the low salinity events off Duck, N.C. by an average of 1.1 days, a time lag consistent with the observed alongcoast propagation speeds.

No description available.

No description available.