Natalie Laudier Operational Oceanography 13Feb2009 Comparison of Wave Spectra Analyses from Waverider Buoy Data and CDIP Swell Model Predictions Natalie Laudier Operational Oceanography 13Feb2009

Outline Introduction Methods Results Wave Spectra Sediment Samples Discussion

Carmel River Beach, CA Purpose: to determine if the CDIP swell model accurately predicts the nearshore wave environment

Carmel River Beach, CA Buoy deployed 1927UTC 35.52N, 12.96W Model 121.95W 36.53N Buoy deployed 1927UTC 35.52N, 12.96W Recovered at 2105UTC at 35.53N, 121.95W ~300m water depth

Methods DWR-G Datawell Waverider buoy GPS three dimensional wave motion (pitch and roll) Wave periods of 1.6 to 100 seconds Accuracy of 1cm Directional: WGS84 reference and has a 1.5deg http://www.datawell.nl/inhoud.php?id=1

phase-linear, combined band-pass and single-integrating FIR filter Waverider Buoy Heave, North, West   Range 20m to -20m Accuracy 1cm Time Period 1.6-100s Freq Range 0.01Hz-0.64Hz Direction 0-360deg 1.5deg Reference WGS84 Filter Sampling Freq 2.0Hz Digital Filtering Type phase-linear, combined band-pass and single-integrating FIR filter Data Output Rate 1.28Hz Sample rate: 1.28Hz 256 samples per segment Cutoff frequency: 0.64Hz

Methods Filtering: buoy digitally filters with an integrating high-pass filter Fast Fourier-transform (FFT) used to get the wave height spectrum in the frequency domain Hanning window used to calculate cross-spectral density Directional moments a1,a2,b1,b2 calculated from cross-spectral density Mean direction and directional spreading calculated from moments Maximum Entropy Method (MEM) to calculate directional energy spectrum

Coastal Data Information Program (CDIP) Swell Model Based on wave refraction and diffraction simulations Run by SCRIPPS Oceanographic Institute Wave frequencies > 0.04Hz No tides or input from local wave generation Spectra information provided from model Based on wave observations from an offshore Datawell CDIP buoy 157 Resolution of bathymetry 100x100m O'Reilly, W. C., & Guza, R. T. (1993). A comparison of two spectral wave models in the southern california bight. Coastal Engineering, 19(19), 263-282.

CDIP Model Model uses offshore buoy data to determine incoming offshore wave spectra. It then propagates the waves into the depths greater than 10m. http://cdip.ucsd.edu/?nav=recent&sub=nowcast&units=metric&tz=UTC&pub=public&xitem=model_request

MOP Location Site MO634 Carmel River Beach 15m MOP station normally run Location: 36 32.14 N 121 55.96 W (36.5356 -121.9326) Water depth: 15.07 m  (49 ft, 8 fm) Modeled parameters: wave energy, wave direction

Results

Buoy Data Spectra Time 1 Time 2 Time 3 Tp=15.39 Dp=292.46 Swh=2.067 Buoy data was sectioned into 3 time periods. Time 1: 1937, Time 2: 2007, Time 3: 2037 Time 1 Time 2 Time 3 Tp=15.39 Dp=292.46 Swh=2.067 Tp=15.38 Dp=291.01 Swh=2.34 Tp=14.29 Dp=288.17 Swh=2.89

Buoy Spectra vs. Model Spectra Time 1 Time 2 model over exaggerates the two peak directions. Time 3 Tp=14.29 Dp=292 Swh=2.06

Buoy Spectra Gridded to Fit Model Spectra Time 1 Time 3 . In order to compare the buoy and model wave spectra, the data needed to be equally gridded. Since the model data had less points than the buoy data, the buoy data was gridded down to have the same number of point as the model. Time 2

Differencing % Difference between data spectra and model spectra Time 1 Time 3 The directional spectra were then able to be differenced. This found the percentage of model deviation from the buoy data based on frequency and directional energy. approximately 10-15% differences in all directions. There are a few outliers with higher percentage differences but these could merely be accounted for by a small directional difference. This exaggeration of the two wave directions is also clear in the differencing where the two areas which the models predicts are outlined by 15% differences. The lower frequencies between 0.1 and 0.2 Hz seem to depict a better accuracy of the model to the buoy data. Time 2

Wave Direction vs. Energy Time 1 Time 3 In all three, the model has less energy further away from the peak direction than the buoy data. However, for both the model and buoy, they all show the two peak directions with one more dominant than the other. In time 2 and 3 the buoy data indicates a smaller peak at 50 degrees where the model does not show this at all. Also, at time 1 and 2, the buoy data shows a few more peaks near the peak direction, which the model does not capture. Time 2

Frequency vs. Energy 95% Confidence Interval using Time 1 Time 3 For the buoy data cross-spectra, the 95% confidence interval was calculated using the chi-squared method. In the frequency and energy plots the model overall proves the model is accurate. The plots indicate accurate peak frequencies with slightly less energy than the buoy data. The buoy data also shows more of fluctuation than the model but this may have been a result of using a larger number of points per segments. However, the entire model is within the 95% confidence interval of the buoy data. Time 2

MUD

Sediment Down Canyon

Discussion Wave Model describes the data well within ~10-15% error Model energy falls within 95% confidence intervals Need longer periods of data over several weeks to compare sufficiently Use a moored buoy closer to shore

References Datawell BV. (2007). Datawell waverider reference manual. The Netherlands. De Vries, J. J., Waldron, J., & Cunningham, V. (2003). Field tests of the new datawell DWR-G GPS wave buoy. Sea Technology. O'Reilly, W. C., & Guza, R. T. (1993). A comparison of two spectral wave models in the southern california bight. Coastal Engineering, 19(19), 263-282. O'Reilly, W. C., Herbers, T. H. C., Symour, R. J., & Guza, R. T. (1996). A comparison of directional buoy and fixed platform measurements of pacific swell. Journal of Atmospheric and Oceanic Technology, 13(1), 231-238.

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