INTRODUCTION

In the past few years, increasing demand for directional wave information has led to the development of new instrumentation and analysis procedures. Two basically different designs of directional wave buoys are currently available on the market. The first is the traditional surface slope-following buoy including the Norwave (Bergen Ocean Data), Wadibuoy (CNE-OX and Néréides) and Wavec (Datawell) buoy. The second, exemplified by the Wave-track Type 956 (Endeco Inc.) buoy, is the wave orbital-following buoy. An understanding of the operation, reliability and applicability of these two instrument types would prove useful when designing field experiments. Two major studies have recently been conducted in Canadian waters, whose goals were in part to assess the performance of the Datawell Wavec and Endeco Type 956 Wave-track buoy. Part of this assessment included comparison of the derived information with visual observations and hindcast model results. This paper details some of their findings.

Study Background

Two studies were conducted in Canadian coastal waters employing Datawell Wavec and Endeco Type 956 Wave-track buoys. The first occurred along the northern coast of British Columbia from October 1982 to May 1984. The Wavec buoy was moored in 189 m of water at 52°21.2'N, 130°46.7'W. Although neither buoy operated continuously over the 2 years, at least 10 months of data were obtained from both.

The second study was of shorter duration, February to March 1984, but was more intensive as it was designed for specific intercomparison between these two buoys as well as against a standard Waverider (Datawell), visual observations and hindcast model results. Two Wavec buoys and one Wave-track buoy were moored in approximately 85 m of water on Newfoundland's Great Banks, off Canada's East Coast. Their positions were, respectively, 46°45.8'N, 48°48.8'W, 46°44.83'N, 48°49.75'W and 46°45.67'N, 48°45.75'W with less than 1 nautical mile separating them from each other. The visual observations were obtained from a nearby manned mobile drilling unit (the West Venture, operated by Mobil Oil Canada Ltd) and the model results supplied by F.G. Bercha Ltd of St John's, Newfoundland.

The Wavec is a slope-following buoy, approximately 2.5 m in diameter, consisting of a central instrument canister to which are strapped peripheral flotation segments. The Wave-track has an inverted-pendulum design composed of a surface sphere and subsurface shaft to which is attached the sensor package. The wave direction components are obtained not from east-west and north-south surface slopes as for the Wavec, but from the angular tilt of the shaft induced by the wave orbital velocities.

Standard time-series analyses methods were used to process the data to spectral components and the approach of Longuet-Higgins et al. (1963) was taken to calculate directional spectra. A second processing scheme, a frequency bandpass analysis, as described in Braninard (1982) can also be used for Wave-track data. However, this method proved inefficient and the results were of limited usefulness.

Buoy Operation

Over the two studies, the Wavecs were generally reliable in term of hardware, telemetry and data quality. The major concern was its ruggedness, as on two occasions during the first study.

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