Abstract

From 09 February 2015 to 01 May 2015, a state-of-the-art Sentinel V ADCP was deployed at a site off the west coast of Ireland, with the aim of gathering accurate wave measurements in extreme conditions. The Sentinel V is equipped with 5 acoustic sensors (1 vertical and 4 slant beams) and records intensity and velocity values along each beam. The recovered dataset contains on the order of 2.5 x 109 data points. A quality control procedure, described here, has been implemented. The resultant high quality collection of 750,000 waves is analysed, with extreme and rogue waves identified for comparison.

Introduction

In winter, ocean conditions off the west coast of Ireland are particularly energetic - wave heights of 20 m or more are not uncommon. From 09 February 2015 to 01 May 2015 an experiment was conducted by our group, in collaboration with Teledyne RD Instruments and TechWorks Marine, with the aim of gathering accurate wave measurements in extreme conditions.

A state-of-the-art Sentinel V ADCP was deployed at a site, with mean water depth 36.5m, in the Killard Point area off the west coast of Ireland (Fig. 1). The Sentinel V is equipped with 5 acoustic sensors (1 vertical and 4 slant beams) and records intensity and velocity values along each beam. A sampling frequency of 2 Hz was chosen, along with 34 bins of width 1.2m along each beam leading to a recovered dataset that contains on the order of 2.5 × 109 data points.

Intensity values along the vertical beam can be utilised to calculate surface displacement. However, an inherent difficulty is the disruption of intensity profiles, possibly due to passing large-bodied scatterers and/or bubbles, which in turn lead to "spikes" in the time series of surface elevation. An initial sweep of the dataset to remove such anomalies has been performed. The dataset has been partitioned into a consecutive series of 20-minute samples with statistics (significant wave height, peak period, mean water level etc.) derived for each, utilising spectral analysis and the zero-crossings method where appropriate. An automated quality control algorithm that relies on a series of flag checks to eliminate potential error samples has been implemented. Significant wave heights are compared to those of a WaveWatch III hindcast for the relevant period and are found to be in excellent agreement, giving confidence in the outlined methodology. The resultant high quality collection of 750,000 waves is analysed, with extreme and potential rogue waves identified for comparison. Raw velocity data are beset by the difficulty raised by the so-called "Ambiguity Velocity" – 1750 mm/s in this case. An algorithm that overcomes this problem is outlined and an example velocity field, both before and after resolution, is presented.

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