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Abstract: A methodology for assessment of the potential impacts of extraction of energy associated with astronomical tides is described and applied to a site on the Beaufort River in coastal South Carolina, USA. Despite its name, the site features negligible freshwater inputs; like many in the region, it is a tidal estuary that resembles a river. A three-dimensional, numerical, hydrodynamic model was applied for a period exceeding a lunar month, allowing quantification of harmonic constituents of water level and velocity, and comparison to values derived from measurements, recorded at a location within the model domain. The measurement campaign included surveys of bathymetry and velocity fields during ebb and flood portions of a tidal cycle for model validation. Potential far-field impacts of a generic tidal energy conversion device were simulated by introducing an additional drag force in the model to enhance dissipation, resulting in 10–60% dissipation of the pre-existing kinetic power within a flow cross-section. The model reveals effects of the dissipation on water levels and velocities in adjacent areas, which are relatively small even at the 60% dissipation level. A method is presented to estimate the optimal vertical location for the energy conversion device and the potential power sacrificed by moving to a different altitude. [Copyright &y& Elsevier]

Abstract: The tidal stream power potential along the coast of the state of Georgia is evaluated based on numerical modeling and validated with the available data. The Georgia coast consists of a complex network of tidal rivers and inlets between barrier islands that funnel and locally amplify the strength of the ambient tidal currents in the region. The number of existing tidal current prediction locations is not sufficient to resolve the temporal and spatial changes in the current speeds and patterns. Therefore, the currents are modeled with the Regional Ocean Modeling System (ROMS) to determine the locations with high tidal stream power potential and the results are validated against measurements. The wetlands and the topographical features are integrated in the computational model with wetting and drying of computational cells. The locations with the largest mean tidal stream power density are identified and their characteristics are provided. The effect of power extraction on estuarine hydrodynamics is simulated by implementing an additional retarding force in the governing momentum equations in ROMS. Two different power extraction schemes are simulated at the Canoochee River. The first scheme involves extracting 20% of the original kinetic power across the entire cross-section of the river, and is found to have substantially lower impact on the original flow than the second scheme with 45% extraction. The summation of removed and residual kinetic powers is found to be larger than the original kinetic power in the cross-section, which is attributed to the recovery in the flow momentum through reorganization of stream flow energy. In both of the cases the major impact on the currents is limited to a partial reach of the river. The change in the maximum and minimum water levels is observed to be on the order of centimeters. [Copyright &y& Elsevier]

A process-based numerical model has been used to study nearshore hydrodynamics on barred beaches. A laboratory experiment with an offshore bar migration case followed by an onshore bar migration is simulated. A new mechanism is incorporated in the model accounting for the breaking-wave persistence improving model performance for wave properties, particularly in the bar-trough region. A cross-shore variation of the breaking-wave parameter is introduced to the dissipation model. The effects of the surface shape parameter and a roller lag on radiation stresses and mean water-level predictions are investigated and found to improve the mean water-level predictions. The cross-shore variation of the momentum balance is presented to illustrate the variation of forcing for the undertow current. The persistence length and the roller lag mechanisms are shown to be important for predictions of the undertow currents on barred beaches and the current predictions are improved once both methods are used. [ABSTRACT FROM AUTHOR]

Abstract: Predictions of nearshore and surf zone processes are important for determining coastal circulation, impacts of storms, navigation, and recreational safety. Numerical modeling of these systems facilitates advancements in our understanding of coastal changes and can provide predictive capabilities for resource managers. There exists many nearshore coastal circulation models, however they are mostly limited or typically only applied as depth integrated models. SHORECIRC is an established surf zone circulation model that is quasi-3D to allow the effect of the variability in the vertical structure of the currents while maintaining the computational advantage of a 2DH model. Here we compare SHORECIRC to ROMS, a fully 3D ocean circulation model which now includes a three dimensional formulation for the wave-driven flows. We compare the models with three different test applications for: (i) spectral waves approaching a plane beach with an oblique angle of incidence; (ii) monochromatic waves driving longshore currents in a laboratory basin; and (iii) monochromatic waves on a barred beach with rip channels in a laboratory basin. Results identify that the models are very similar for the depth integrated flows and qualitatively consistent for the vertically varying components. The differences are primarily the result of the vertically varying radiation stress utilized by ROMS and the utilization of long wave theory for the radiation stress formulation in vertical varying momentum balance by SHORECIRC. The quasi-3D model is faster, however the applicability of the fully 3D model allows it to extend over a broader range of processes, temporal, and spatial scales. [Copyright &y& Elsevier]

The behavior of rip currents with changing bar and gap lengths was tested in a series of laboratory experiments using video-tracked floating drifters. Qualitatively, currents showed strong differences in overall character with changing topography. As narrow gaps became wider, the circulation tended to first travel parallel to the shoreline instead of immediately offshore, although strong offshore-directed flows were found for all topographies. Quantitatively, the width of the rip neck was found to increase slowly when compared to the increasing width of the rip channel. Maximum offshore and longshore velocities showed little dependence on bar or gap lengths, and were found to scale well with the predicted rate of generation of circulation. Finally, volumetric offshore flow rates through the rip channel showed no proportionality with the bar length as predicted by the mass transport hypothesis of rip current strength. [ABSTRACT FROM AUTHOR]

Energetic tidal streams are an especially attractive source of clean energy due to the daily occurrence of high tidal flows. However, before any deployment of tidal turbine farms, it is essential to perform a resource assessment depending on the scope and scale of the project. The International Electrotechnical Commission has developed a technical specification for assessing the tidal stream resource "IEC TS 62600-201" to aid in this effort: determine a particular site's feasibility and design the project layout. In this study, we implemented and validated a three-dimensional numerical model and provided results following the specification for a 'Stage 2' project layout design in a highly energetic tidal channel: Tacoma Narrows of Puget Sound, in the State of Washington, USA. Implementation of the numerical model based on the specification has helped estimate the annual energy production with higher model accuracy. We also used the same setup to evaluate the undisturbed theoretical and technical resources, where the latter included energy extraction from the flow field arranging a hypothetical tidal energy converter array. Ultimately, this work has shown the important role of the technical specification in larger project layout design, which can simultaneously act as a benchmark for other 'Stage 2' theoretical and technical resource assessment studies worldwide. • We conducted a 'Stage 2' tidal energy resource assessment following IEC Guidelines. • The Guidelines helped better predict the AEP required for project layout design. • The model validation and result reporting serve as a benchmark for future studies. [ABSTRACT FROM AUTHOR]

Modeling studies of estuary circulation show great sensitivity to the water exchange into and out of adjacent marshes, yet there is significant uncertainty in resolving the processes governing marsh surface flow. The objective of this study is to measure the estuary channel-to-saltmarsh pressure gradient and to guide parameterization for how it affects the surface flow in the high marsh. Current meters and high-resolution pressure transducers were deployed along a transect perpendicular to the nearby Little Ogeechee River in a saltmarsh adjacent to Rose Dhu Island near Savannah, Georgia, USA. The vertical elevations of the transducers were surveyed with static GPS to yield high accuracy water surface elevation data. It is found that water level differences between the Little Ogeechee River and neighboring saltmarsh are up to 15 cm and pressure gradients are up to 0.0017 m of water surface elevation change per m of linear distance during rising and falling tides. The resulting Little-Ogeechee-River-to-saltmarsh pressure gradient substantially affects tidal velocities at all current meter locations. At the velocity measurement station located closest to the Little Ogeechee River bank, the tidal velocity is nearly perpendicular to the bank. At this location, surface flow is effectively modeled as a balance between the pressure gradient force and the drag force due to marsh vegetation and bottom stress using the Darcy–Weisbach/Lindner's equations developed for flow-through-vegetation analysis in open channel flow. The study thus provides a direct connection between the pressure gradient and surface flow velocity in the high marsh, thereby overcoming a long-standing barrier in directly relating flow-through-saltmarsh studies to flow-through-vegetation studies in the open channel flow literature. [ABSTRACT FROM AUTHOR]

A 3D numerical model is used to simulate the distortion of tidal hydrodynamics in the Ogeechee Estuary, GA. The Ogeechee, like many estuaries found in the Southeastern US, consists of shallow channel networks and extensive intertidal storage in the form of wetlands. Such features are known to induce non-linear overtide generation and significant tidal distortion, otherwise known as tidal stage asymmetry. Simulations are run with varying parameters to assess their effects on modeling tidal distortion for the Ogeechee Estuary: bottom friction coefficients, enhanced wetland friction coefficients, and tidal flat elevations. To succinctly quantify the degree of distortion across the domain, the statistical parameters of skewness and asymmetry are calculated for time series of water surface heights and channel volume fluxes. The intertidal storage causes the peak flood flux to occur later and the peak ebb flux to occur earlier, thereby resulting in positive asymmetry for the volume flux for the full estuary. However, ebb dominance is a localized feature and varies throughout the estuary. Increasing the intertidal storage by lowering wetland elevation enhances the effects on high tide and volume flux magnitudes, decreasing the ebb-dominance and volume flux asymmetry typically associated with intertidal storage thereby indicating the importance of the wetland elevation over the total storage volume. Increased channel bottom friction reduces ebb-dominance by extending the duration of the falling tide. More interestingly, increased wetland friction reduces the influence of wetland intertidal storage on tidal distortion. The model suggests an increase in wetland friction does little to dampen wave propagation at high tide but rather impedes the lateral flooding of wetlands, reducing ebb dominance. Tidal flat elevation has the largest impact on distortion for the Ogeechee Estuary whereas enhanced wetland and bottom frictional influences on distortion are small, albeit not insignificant. [ABSTRACT FROM AUTHOR]