DOI: 10.5061/dryad.n5qb383/14
title: Imperial Beach Nearshore Waves

description: Nearshore waves at Imperial Beach. To facilitate modeling of beach profile change, wave characteristics in 10m depth spaced 100m alongshore were extracted from the Scripps Institution of Oceanography’s wave Monitoring and Prediction (MOP) system for the California coastline (http://cdip.ucsd.edu/). Wave estimates along the coastline are produced using a linear spectral refraction model initialized with 2-D spectral estimates from multiple Datawell directional buoys. For swell waves (0.04–0.08 Hz) the model is initialized with deep water buoys located seaward of the Channel Islands. For sea waves (0.09-0.5 Hz) the model is initialized with buoys located inside the islands along the mainland shelf break. Each MOP point in 10m depth has a corresponding backbeach point, defining a MOP line. MOP line orientations are chosen to minimize the distance from the backbeach to the 10m contour. Hindcast time series of wave height (H_s), peak (T_p) and average (T_a) wave period, peak (D_p) and mean (D_m) wave direction, and radiation stress estimates (onshore S_{xx} and alongshore S_{xy}) are provided at each transect line relative to the MOP estimated shore normal (orientation also provided). Additionally, time series of wave energy, E, and low-order moment directional Fourier coefficients (a_1, b_1, a_2, b_2, in true compass “from” coordinates), as a function of wave frequency, are provided at the seaward end of each MOP line on the 10m depth contour. The 10m depth wave model output mirrors the information provided by directional wave buoys or a pressure-velocity meter (PUV), and can be treated in the same way as the spectral data from these instruments when defining boundary conditions for sediment transport models. Occasionally, wave model output is degraded due to buoy malfunctions and is flagged using the "waveFlagPrimary" variable. (Good model output has waveFlagPrimary = 1.) WHEN USING THE MODEL OUTPUT IT IS IMPERATIVE THAT THE WAVEFLAGPRIMARY IS CONSIDERED IN CONJUNCTION WITH THE WAVE ESTIMATES. The nearshore wave hindcasts were validated using shallow water wave buoys (20m depth). The hourly buoy-driven wave hindcasts show significant skill at most validation sites, but prediction errors for individual swell or sea events can be large. Model skill is fair at Imperial Beach owing to a combination of swell energy sensitivity to shadowing by the offshore islands and poorly resolved model bathymetry south of the U.S-Mexico border. Overall, the buoy-driven model hindcasts have relatively low bias such that averaging over space or time is useful for minimizing noise. Best practices for using the 100m spaced, 10m depth MOP wave hindcasts, as boundary conditions for beach change models, are not well established. It is not known if alongshore averaging or smoothing of the 100m-spaced MOP hindcasts (eg. on typical sea, swell or infragravity wavelength scales) is beneficial for beach change model stability. Space-time wave averaging questions must be explored by investigators based on their specific modeling needs and goals. Using the fixed shore normal S_{xy} estimates with 2D beach change models that predict changes in shoreline orientation is internally inconsistent, so additional second-order rotations of the S_{xy} values (or direct recalculation of S_{xy} using the a_2 and b_2 Fourier coefficients in compass coordinates) based on modeled shore normal changes, will be required.