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Georgia Tech Conducts Comparison of Directional Wave Parameters

Since July 2004 the author has been routinely measuring waves 10km offshore Savannah, Georgia, at a site with mean water depth of 13m, using a TRIAXYS ™ Wave Buoy. This report summarizes a comparison of directional wave parameters from the buoy and 1,200kHz RD Instruments Acoustic Doppler Current Profiler (ADCP) equipped with pressure transducer and waves firmware.

The ADCP was deployed beside the wave buoy, resulting in 72 days of comparative simultaneous hourly measurements of directional wave-energy spectra from the two systems. The goal of this report is to provide some results for objective comparison of the two instrument systems. No instrument exists which can reliably report the true wave-energy spectrum: all report an estimate. So the goal here is to compare these estimates with emphasis on scalar parameters derived from the directional spectra.

Deployment Site

Figure 1 shows the deployment site 10km offshore Tybee Island, Georgia. Mean water depth here is 13m and mean tidal range 2.1m. The site features diurnal tides and the mean flow magnitude, depth-averaged, for the period under consideration was 20cm/s. Instruments were programmed to collect twenty minutes of wave-burst data every hour. The ADCP samples velocities at 2Hz, and the buoy samples multiple parameters at 4Hz for determination of wave-energy spectra. The buoy was configured with an iridium satellite modem (as the first to be so equipped) which provided hourly updates of directional wave spectra and other parameters. Details regarding the deployment may be found in Table 1 (in original article).

Measure and Analysis

The two measurement approaches differ greatly. The ADCP range gates the acoustic signal returned from particles in the water along four narrow acoustic beams. It saves data defining along-beam velocity in the three top bins nearest the water surface for post-processing to determine wave parameters. This approach yields twelve different time-series for processing, four velocities in each of three layers, to determine directional wave characteristics: Terray et al (1997), RØrbæk et al (2000).

The buoy records six time-series, defining motion plus heading. The time-series come from three accelerometers plus three rate gyros. These signals are processed to determine heave, east and north velocities, from which directional wave-energy spectra are computed. The buoy is assumed to follow the water surface and thus mimic wave-induced water motion at the surface.

Data processing for each instrument was done using manufacturer-provided proprietary software. The buoy actually processes the data onboard and transmits the resulting Fourier coefficients, although it also stores both this information and raw data onboard. The WavesMon software provided by RD instruments was used to process the ADCP data after recovery. The buoy reports energy up to 0.64Hz, but a common cut-off frequency of 0.35Hz was used for all comparisons shown here. Other than sampling schedule, there are few user-specified options for the collection or processing of the wave-buoy data. Parameters for processing of the ADCP data are detailed in the report by Work and Bystrom (2005).

Results and Report

The wave-buoy estimates wave parameters based on both time-domain and frequency-domain analysis. Only the latter will be considered here since time-domain analysis, i.e. zero-crossing analysis, is not an option with the ADCP. The data analysis techniques are not exactly the same for the buoy and ADCP but both systems yield directional energy spectra for comparison. In both cases wave parameters were computed from the directional spectra output by the appropriate proprietary software. Parameters and further results are detailed in the original article below.