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Comparison of Primary Directional Wave Parameters

TRIAXYS Wave Buoy and RD Instruments 1200 kHz ADCP:
Comparison of Primary Directional Wave Parameters

P.A. Work, Ph.D., P.E.
Elza Bystrom
Civil and Environmental Engineering, Georgia Tech

One of the goals of the SEACOOS program (Southeast Atlantic Coastal Ocean Observing System, see (www.seacoos.org), sponsored by the U.S. Office of Naval Research, is to improve the density and availability of observations of physical oceanographic parameters within the coastal ocean, including wind waves. In addition to providing new, or more, observations, the development and testing of new observation techniques also fall within the SEACOOS effort.

Since July, 2004, the first author has been routinely measuring waves ten km offshore of Savannah, Georgia, at a site with mean water depth of 13 m, using a Triaxys wave buoy (see wavebuoy.gtrep.gatech.edu). Although buoys have been used for wave observations for decades (e.g. Longuet-Higgins et al. 1963), this particular design is a newer tool for wave measurements, and one with which the authors had no prior experience. Thus it was desirable to do some comparisons against another wave measurement technique. The primary purpose of this report is to briefly provide and discuss such a comparison. A publication describing a more extensive comparison is planned for a later date.

The author deployed a 1200 kHz RD Instruments acoustic Doppler current profiler (ADCP), equipped with a pressure transducer and waves firmware, from the Skidaway Institute of Oceanography’s R/V Savannah on November 5, 2004. This instrument was deployed beside the Triaxys wave buoy for nearly four months. Ultimately, 72 days of simultaneous hourly measurements of directional wave energy spectra and related
parameters from the two systems are available for comparison.

The goal of this report is to provide some results for objective comparisons of the two instrument systems, as the author routinely uses both instruments for measurement of waves. No instrument exists which can report the true wave energy spectrum – all instruments report an estimate. So the goal here is to compare these estimates. The focus here will be on parameters derived from the directional spectra; higher-order statistics will be addressed in a later publication.