Hydrodynamic and Sediment Transport Model Application for OSAT3 Guidance: Locations of decelerations in the direction of flow in the maximum alongshore current

The D-Flow and D-Waves components of the Deltares Delft3D numerical model suite (version 4.00.01) were used to estimate east and north components of wind and wave-driven velocity for the offshore wave conditions corresponding this scenario Hh_Dd (characteristics of which may be found in the included wave_scenarios.txt file) in each grid cell in the model domain. The wave model D-Waves, based on the Simulating WAves Nearshore (SWAN) model, is a 3rd generation phase-averaged numerical wave model which conserves wave energy subject to generation, dissipation, and transformation processes and resolves spectral energy density over a range of user-specified frequencies and directions. D-Wave was used in stationary mode. D-Flow solves the shallow water Navier Stokes equations and is run in 2-D depth-averaged mode, with linkage to D-Waves allowing the generation of wave-driven currents via wave radiation stress forcing. Default values for model parameters governing horizontal viscosity, bottom roughness, and wind drag were used. Neumann boundary conditions were used along the east, west, and south model boundaries with harmonic forcing set to zero. Model bathymetry was provided by the NOAA National Geophysical Data Center Northern Gulf Coast digital elevation map, referenced to NAVD88.
Significant wave height, dominant wave period, and wave direction were prescribed as D-Wave TPAR format files every 30 grid cells along the model boundary using results from the NOAA Wavewatch III 4' multi-grid model for a representative moment in time corresponding to the offshore wave conditions of the scenario, the specific time of which may be found in the included wave_scenarios.txt file. A JONSWAP (JOint NOrth Sea WAve Project) spectral shape was assumed at these boundary points. Wind forcing was provided using the archived WavewatchIII 4' winds, extracted from the NOAA GFS wind model, for this time. The D-Wave directional space covers a full circle with a resolution was 5 degrees (72 bins). The frequency range was specified as 0.05-1 Hz with logarithmic spacing. Bottom friction calculations used the JONSWAP formulation with a uniform roughness coefficient of 0.067 m2/s3. 3rd-generation physics are activated which accounts for wind wave generation, triad wave interactions and whitecapping (via the Komen et al parameterization). Depth-induced wave breaking dissipation is included using the method of Battjes and Janssen with default values for alpha (1) and gamma (0.73). Current model outputs of east and north current velocity component were extracted and interpolated to the wave model grid (staggered points in relation to the current model grid).
NDBC observations from station 42012 for the representative scenario time periods were used to validate the wave model results.

Identify the spatial extent of the surf zone at each alongshore location using Mathworks MATLAB software (v2012A). The shoreline is identified from the gridded bathymetry as the cross-shore grid cell of minimum water depth at each alongshore transect. In areas where lagoons separate barrier islands and the mainland coast, the shoreline is considered along the seaward side of the barrier island only. The shoreline is not continuous due to interruptions at inlets.
For each alongshore transect, the surf zone is defined as the area between the shoreline and the location of maximum wave height (found in a search area extending from the shoreline to the most offshore point of modeled depth-induced wave breaking dissipation). The extent of the surf zone (as indices into the grid at each alongshore location of the cross-shore position of the shoreline and seaward edge of the surf zone) was saved for all of the scenarios into a MATLAB .mat structure.

The maximum (in terms of magnitude) alongshore current within the surf zone at each alongshore location was computed in Mathworks MATLAB v2012A using modeled flow velocities. The alongshore variant maximum velocity vector was then smoothed with a 2-km Hanning window filter to remove small-scale variations likely caused by model noise.

Spatial decelerations in the direction of flow (e.g., the flow velocity decreases from one grid cell to the adjacent cell in the direction of flow) are defined using an advective acceleration term defined as u*du/dy where u is the alongshore current and y is the alongshore grid coordinate. A derivative of the alongshore velocity (u) is computed and used to identify locations where the flow is decelerating (du/dy < 0). We save locations corresponding to peak deceleration (convex points in du/dy) and the magnitude of u*du/dy at the identified points is also recorded. Note that for eastward flow (u > 0) the u*du/dy deceleration term is negative whereas for westward flow (u < 0) the u*du/dy deceleration term is positive.

Export the values for each alongshore location from MATLAB format into an ArcGIS point shapefile using the Mathworks MATLAB Mapping Toolbox (v2012A). The latitude and longitude coordinates correspond to the identified shoreline location for each alongshore location. A single "converge" attribute is given a value of one; all latitute and longitude points in the file are convergence locations. The shapefile is written with the "shapewrite" command. Because MATLAB does not assign a projection, the projection corresponding to the projection associated with the bathymetry used in the numerical models is added in ArcCatalog 9.3. The file was then quality checked in ArcMap to insure values were properly exported to the shapefile from MATLAB.

Keywords section of metadata optimized for discovery in USGS Coastal and Marine Geology Data Catalog.

Keywords section of metadata optimized by correcting variations of theme keyword thesauri and updating/adding keywords.

Added keywords section with USGS persistent identifier as theme keyword.