Simulated coastal fine-grained sediment plumes from beach nourishment near Santa Barbara, California

The hydrodynamic and sediment transport model was applied using the Delft3D modeling system (Deltares, 2024) to simulate fine-grained coastal sediment plumes from beach nourishment projects along the southern California coastline near Santa Barbara. The computational model domain consisted of a structured, orthogonal curvilinear grid with an alongshore extent of 22 km and cross-shore extent of 7 km that was incorporated into the project area. The resolution of the grid varied with maximum resolution of 20 m in the nearshore area near the project site to about 180 m at the seaward extent of the model domain. The bathymetry of the model was based on a high resolution (1 m) bathymetric and topographic digital elevation model developed by the USGS Coastal National Elevation Database (CoNED) project (Tyler and Danielson, 2018). The offshore oceanic boundary was forced with astronomic tidal constituents derived from the TPXO 7.2 global tide model (Egbert and Erofeeva, 2002) and lateral open boundaries were defined as zero-velocity Neumann boundaries, which result in no alongshore water level gradients. A static vertical offset of 0.81 m was added to the water levels on the model boundaries to account for the local difference between the vertical datum of the elevation data (North American Vertical Datum of 1988) and local mean sea level based on published tidal datums at NOAA site 9411340 (Santa Barbara). A uniform water density of 1025 kg/m3 was used throughout the model domain and the effects of temperature and salinity variations on circulation were neglected in the present model application. The model included 10 equidistant vertical sigma layers to account for 3D effects, including variations in sediment concentration through the water column. A uniform Chezy roughness of 65 m^(1/2)/s was applied throughout the model domain. The governing equations for the model were solved at 7.5 s intervals and output was requested at hourly intervals for every computational grid point.

The hydrodynamic and sediment transport model was 2-way coupled to the spectral wave model, SWAN (Booij, Ris, and Holthuijsen, 1999) to simulate wave transformation between the Santa Barbara channel and shoreline using the same curvilinear grid as the flow model. Wave energy was discretized into 24 frequency bins between 0.05 and 1 Hz and 36 directional bins between 0 and 360 degrees. Open boundaries of the wave model were forced with spatially uniform wave parameters fitted with a Jonswap distribution. For hindcast simulations, time-varying boundary conditions were derived from wave observations at NDBC buoy 46053. Wind growth and white capping were based on parameterizations of the energy transfer equations (Komen, Hasselmann, and Hasselmann 1984). Spatially uniform, temporally varying wind forcing based on observations made at NOAA Station 9411340 were applied to the free surface in the hindcast simulations. The JONSWAP bottom friction model with a coefficient of 0.067 m2s-3 and wave breaking based on Battjes and Janssen (1978) with default settings (alpha = 1, gamma =0.73) were used. Convergence criteria were set to 98 percent of cells and a maximum of 15 iterations during the stationary wave simulations. The wave- and flow-models were coupled at hourly intervals.

Sediment nourishments were added to the model domain within a polygon that was approximately 250 m alongshore and 65 m across shore at elevations between -1.9 and -3.6 m. Two sediment fractions were included in all simulations: (i) a non-cohesive sand with a median diameter of 0.2 mm to represent native beach, shallow nearshore areas, and the coarse-grained fraction of the nourishment sediment, and (ii) a cohesive sediment to represent the fine-grained sediment of the nourishment sediment. All nourishment sediment added in the simulations was composed of 90 percent non-cohesive sand identical to the native sediment and 10 percent fine-grained, cohesive sediment. Transport of the non-cohesive sediment fraction was determined using the Van Rijn and others (1993) transport formula using default settings. Transport of the cohesive sediment was computed with the advection-diffusion solver, and the linear erosion model of Partheniades (1965) was used simulate erosion of cohesive sediment from the seabed. The properties of the cohesive sediment included a settling velocity of 0.25 mm/s, erosion rate coefficient of 0.0025 kg/m2/s, and critical shear stress for erosion of 0.1 Pa. Placement of sediment nourishments for both hydraulic dredging and drag and haul projects were based on real-world project conditions that control sediment placement rates. For both placement types, the rate of sediment delivery was similar with delivery rates of 1840 m3/d and 2070 m3/d for hydraulic dredge and drag and haul placement, respectively. The main difference between the two sediment placement types was that drag and haul was only active for 8 hr/d, while hydraulic placement operations were held constant day and night. This resulted in shorter overall project durations from hydraulic dredge projects for the same volume of sediment. The hindcasts provided in this data release simulated placement and dispersal of a moderate-sized nourishment project, 38,000 m3 (50,000 cubic yards), between January 1 and March 26, 2014, to characterize a typical nourishment project that would result in several weeks of placement operations and experience a range of environmental conditions. Sensitivity models simulate a 3-day period and include nourishments of approximately 1,400 m3 (1,830 cubic yards). The drag and haul scenario delivers the nourishment sediment between 0800 and 1600 hours for the first two days of the simulation while the hydraulic dredging scenario introduces the nourishment over 18 consecutive hours starting 8 hours after the start of the simulation.

Files needed to run the hydrodynamic and sediment transport models using the Delft3D modeling system were compiled into compressed archives for distribution.