No description available.

No description available.

Detailed field observations from the York River estuary, Virginia are used to examine the processes governing vertical density stratification and to evaluate the importance of spatial and temporal variations in turbulent mixing on estuarine dynamics and sediment transport. Contrary to previous findings that suggest wind stress acts predominantly as a source of energy to mix away stratification, this study demonstrates that the wind can play a more important role in "straining" the along-channel estuarine density gradient. as a result, down-estuary winds enhance the tidally-averaged vertical shear, which interacts with the along-channel density gradient to increase stratification. Conversely, up-estuary winds tend to reduce, or even reverse the vertical shear, reducing stratification. While wind straining can play a dominant role in governing the overall degree of turbulent mixing at sub-tidal time scales, tidal straining of the along-channel density gradient can result in asymmetries in turbulent mixing at the tidal time scale. In estuarine systems with channel-shoal morphologies, tidal straining can lead to asymmetries in turbulent mixing near the deeper channel while the neighboring shoals remain relatively well-mixed. These temporal and spatial variations in turbulent mixing result in a barotropically-induced estuarine residual flow that favors inflow over the shoal regions and outflow over the channel. This pattern of residual circulation can offset, or even reverse, the pattern of residual circulation typically associated with baroclinic estuarine circulation. These tidal asymmetries in mixing have the opposite influence on the patterns of sediment flux. The higher values of eddy viscosity that occur during the less-stratified flood tide resuspend sediment higher in the water column, favoring up-estuary pumping. The presence of strong density stratification significantly damps turbulence in the upper water column, and the lateral dynamical balance is largely geostrophic at tidal time scales. Even though friction does not contribute at lowest order to the lateral balance, the lateral circulation is frictionally-driven by Ekman transport in the bottom boundary layer. The interaction of the lateral circulation and the stratification acts to limit the strength of the lateral circulation and as a result, significantly stronger lateral circulation occurs during less stratified conditions.

No description available.

An experiment conducted over the shoreface at Duck, North Carolina in 1985 embraced both fair-weather low energy and storm-related high energy conditions. to differentiate the diffusion and advection processes of suspended sediments under the high energy conditions from those under the low energy conditions, numerical modeling and the analysis of field data are exercised. A simple two-layer eddy viscosity wave-current combined boundary layer model is developed. The modeled characteristics of the boundary layer are incorporated with a diffusion equation to give suspended sediment concentration profiles. A velocity scale related to factors other than turbulent diffusion is formulated, representing the diffusion under varying energy conditions. With increasing bed friction, the vertical diffusion of sediment is reduced due to stratification, thus reducing velocity. From the measured suspended sediment concentration profiles, the resuspension coefficient, &\gamma&, shows a tendency to decrease with increased flow intensity, suggesting the role of the armoring effect. The coefficient, &\gamma&, varies between 0.0003 for high-energy conditions and 0.002 for low-energy conditions. The energetics approach to predicted sediment transport overestimates the role of wave transport for the low energy conditions. Cross-correlations between cross-shore velocity and sediment concentration show that the role of wave for the transport under low energy conditions is not substantial. The direction of transport under low energy conditions is governed by the mean current. Under high energy conditions, transport by waves is onshore but superseded by offshore transport by the mean current, resulting in net offshore transport. The energetics model based on the surfzone dynamics underestimates the transport rate by an order of magnitude compared to the depth integration of the average product of mean cross-shore velocity and mean concentration. This indicates that the calibration of the efficiency factors &\epsilon\sb{lcub}\rm s{rcub}& and &\epsilon\sb{lcub}\rm b{rcub}& in an energetics model is essential.

The Sensor Insertion System (SIS) located at the US Army Corps Field Research Facility (FRF) in Duck, NC, allows closely spaced measurements of along-shore velocity and suspended sediment concentration to be collected across the entire surf zone during storms. The SIS infers suspended sand concentration from backscatter off of suspended material using optical backscatter (OBS) sensors. Unfortunately, OBS response is highly sensitive to grain size, including both differences between sand and mud and variations in size within the sand range. As part of this study, ground-truthing measurements were added to the SIS during a storm in October 1997, including measurements of suspended sand concentration via pump sampling, by laser in situ scattering and transmissometery (LISST) and by acoustic backscatter (ABS). The major objectives of this study were to use the resulting data to address the OBS's sensitivity to grain size and suggest corrective measures. The first objective was to test the "cutoff" method for the removal of background turbidity due to fine sediment. The study found the cutoff value, defined by some lowest percentile of the OBS response during a given burst, to be proportional to the pumped concentration of suspended particles smaller than 63 microns in diameter. The best choice of cutoff value (1% to 5%) was relatively insensitive to the precise cutoff percentage, indicating the 5% cutoff value currently used by the FRF works well. Addressing the second objective, OBS response after the removal of fines was found to be consistent with pumped sand concentration as long as corrections were made for (i) the varying size of suspended sand, (ii) the precise time of pump sampling and (iii) for apparent noise in the OBS records. Correction for the smaller size of suspended sand (average size 120 microns) relative to that used during calibration (average size 230 microns) decreased OBS estimates of sand concentration by about 42%. Accounting for noise decreased OBS estimates of sand concentration by as much as 80%. Addressing the third objective, LISST and ABS measurements of suspended sand were compared with standard laboratory rapid sand analysis (RSA). Burst-Averaged D50 sand grain size estimated with the LISST was found to have a reasonable correlation with RSA D50 sand grain size estimations. Suspended sand concentrations estimated by the LISST, by the ABS 2.5 MHz transponder and by weighing of pumped samples agreed well. The fourth objective was development of a model for determining sand size from OBS and current meters without relying on ABS, LISST or pump sampling. According to established theory, the ratio of the sand settling velocity (w) to the hydrodynamic shear velocity (u*) can be estimated from the slope of a log-log plot of the burst-averaged concentration profile versus height above the bed. Independent estimation of u* should then give w from the OBS profile and therefore the suspended sand grain size. This relationship was modified for application to OBS data by accounting for (i) a normal distribution of sand grain size and (ii) the OBS's inverse sensitivity to grain size. Theoretical models for u* due to waves and currents were then applied but did not adequately predict the measured sand grain size. A best-fit u* was derived from the observed OBS profiles by combining the observed ratio of w/u* with the directly measured grain size. The best-fit u* was then plotted against current velocity (uc) and showed a change in behavior of u* for uc > 63 cm/sec. Two multiple regressions were performed between the best-fit u* and various parameters routinely measured by the FRF. The first regression, for stations with uc > 63 cm/sec, found uc, water depth and wave height to be significant predictors of u*. For uc < 63 cm/sec, wave orbital velocity, current velocity and bottom roughness were significant. Finally, addressing the last objective, a step-by-step method to correct OBS records for estimating sand concentration was presented which included (1) removal of the effects of background turbidity, (2) removal of instrument noise, (3) estimation of sand size from the shape of the OBS profile, and (4) adjustment of measured OBS concentration based on the estimated sand grain size.

A quasi-3D storm surge modeling system was developed for forecasting the storm surge and inundation in the Chesapeake Bay. The system was constructed with one large unstructured grid covering the Atlantic Coast from Nova Scotia to Florida and a smaller, limited domain unstructured grid covering the Chesapeake Bay, Virginia Beach, Hampton Roads and the adjacent continental shelf regions. It was demonstrated that, with the large domain grid, the model could simulate the hurricane induced storm surge reasonably well using astronomical tide at the open boundary condition and in turn, provide boundary condition for the limited domain model. Since the difficulty of specifying the open boundary condition in the limited domain was solved using this dual-grid methodology, the model achieved the predictive capability for forecasting storm surge as long as accurate atmospheric wind is provided. The high resolution, limited domain grids could, at the same time, be used to map inundation features in details inside the Chesapeake Bay.. The storm surge modeling system has been linked with different atmospheric models. The simulations with different winds proved that the storm surge simulation is highly dependent on the quality of the winds generated by atmospheric models and ensemble technology is necessary for the forecasting purpose and the uncertainty associate with it. Procedures of ensemble simulations were developed in this study so that the modeling system could be used for forecasting storm surge with ensemble winds. Realistic simulations conducted in this study demonstrated that the high-resolution unstructured grid, which is able to resolve complex bathymetry, topography, and coastline structure, could generate accurate storm surge and inundation results if LiDAR data are incorporated. The high-resolution grid and accurate LiDAR topographic data are essential for generating accurate inundation maps. One of the significant findings in this study is that the coastal Ekman dynamics does play an important role in water exchanges between continetial shelf and the Chesapeake Bay. In this context, it is, thereofer, necessary to use a quasi-3D model, rather than 2D model, in order to resolve vertical varying Ekman transport, which significantly improve the storm surge prediction.

As part of a large interdisciplinary study, particulate fluxes in the Waipaoa River sedimentary system in New Zealand have been studied from the terrestrial headlands of the catchment to the oceanic basin over timescales spanning storm events, seasons, and the Holocene. Here, we complement prior efforts by evaluating the formation and reworking of riverine deposits during episodic flood and wave events, and considering their role in accumulation patterns created over longer timescales on the Waipaoa shelf. Using a numerical hydrodynamic and sediment transport model, sediment fluxes and deposition were analyzed from January 2010 through February 2011. A version of the three dimensional ROMS-CSTMS (Regional Ocean Modeling System – Community Sediment Transport Modeling System) was used to investigate the spatial and temporal variability of sediment fluxes on the Waipaoa shelf. The model could account for river input, waves, winds, larger-scale currents, tides, multiple sediment classes and a multi-layered seabed. Sediment sources to the water column included both the river plume and resuspension from the seabed. For model stability and to prevent the reflection of the river plume at the open boundary, the Waipaoa shelf model was nested within a larger-scale New Zealand ocean model. Model inputs were based on observations and model estimates, depending on availability.

In order to better understand storm tides in the Lower Chesapeake Bay, water levels during eleven storms at eight stations were analyzed using several methods. Storm tide was separated into individual components: predicted tide, storm surge, and local anomaly. These components were quantified and then analyzed for spatial trends. Trends were verified using Principal Component Analysis (PCA). The predicted tide and the storm surge each exhibited spatial variability, while the anomaly was spatially uniform. Anomaly values varied from storm to storm, ranging from 0.01m to 0.3m. Potential water levels were determined for each storm by applying a time-shift to match the minimum or maximum predicted tide with the maximum storm surge and the anomaly. In many cases if the maximum observed level had occurred at high tide, the potential observed could have been as much as 0.5m larger than actually experienced. If the maximum observed level had occurred at low tide, the potential observed level could have been as much as 0.8m lower. Thirteen-year potential maximum results indicate that this potential maximum has not been reached at any station. Stations are between 0.3m and 0.5m away from their thirteen-year potential maximum. Maximum storm tide values were assessed relative to both mean lower low water (MLLW) and highest astronomical tide (HAT). HAT was determined to be a better metric for storm impact than MLLW. Integrated intensity, or area under the storm tide curve relative to HAT, is a metric that combines storm duration with the height above HAT. Integrated intensity values were generally higher during extratropical storms than during tropical storms due to the long duration of these storms.

Mixing and dispersion in a partially mixed estuary are driven by, among other processes, the gravitational circulation, wind-events and tides. These processes are important in the vertical and horizontal distribution and exchange of salinity, oxygen, nutrients, and organisms. The objective of this research was to examine the temporal variability of the pycnocline in the mid-Chesapeake Bay in response to forcing at subtidal and higher frequencies as observed in the spring of 1993. The largest change in the pycnocline was associated with meteorological forcing. Up-Bay wind stress and subsequent abatement of the wind stress produced a 5 meter displacement of the mean pycnocline depth. The data suggest that the change in depth of the pycnocline was the result of the geostrophic response of the across-channel pycnocline tilt to wind-induced reversal of the along-channel residual circulation. The observed 2-4 meter semi-diurnal vertical oscillations of the pycnocline were attributed to a topographically induced internal tide. A simple tow-layer model was developed to determine the amplitude and phase of the pycnocline displacements associated with the internal tide. Model results and observed displacements of the pycnocline show good agreement. Other processes, such as an internal lateral seiche of semi-diurnal tidal frequency, advection of the along-channel density gradient and geostrophic adjustment were also investigated but were found to be inadequate in describing the observed oscillations. Finally, the smallest changes of the pycnocline were associated with high-frequency internal waves, fronts and homogeneous layers which were found to occur frequently throughout the field survey. A classification system based on the characteristics of the homogeneous layers observed was implemented to investigate source mechanisms and potential for transport and mixing. Indications are that the relatively short, thin layers were the result of local mixing, and that the relatively long, thick layers were inconsistent with local mixing.