Collins, Clarence; Hathaway, Kent K; Birkemeier, William A; Forte, Michael F; Dickhudt, Patrick J; The FRF Data Team: Full 8m Array of Pressure Gauges at 8 m Depth at USACE Field Research Facility 1990-2018 [dataset]. U.S. Army Corps of Engineers, PANGAEA, https://doi.pangaea.de/10.1594/PANGAEA.902961 (dataset in review),

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In: Collins, C et al. (2019): Nearshore Topographic and Bathymetric Surveys, Wave, and Wind Records from the USACE Field Research Facility in Duck NC from 1980 to 2018 [dataset publication series]. U.S. Army Corps of Engineers, PANGAEA, https://doi.org/10.1594/PANGAEA.902954

By 1986, FRF scientists and technicians designed and built a novel wave sensor. A fixed, precisely-surveyed array of pressure gauges working in sync forming a wave antennae with a directional resolution well beyond that of buoys; details can be found in [1]. From 1986 - 1990, the array consisted of 10 elements parallel to the shore, referred to as the "linear array". The linear array resolved directional spectra but with 180 degree ambiguity. Although most of the wave energy propagates towards the coast, there is some fraction of reflected wave energy (not resolved). In 1990, the linear array was expanded with an additional 6 elements in a cross-shore orientation; eliminating the directional ambiguity. The configuration, which resembles a cross, is referred to as the "8-m array" and remains in place today.

Each element in the array is mounted approximately 0.5 m off the bottom. The array was designed so that most gauge pairs have a unique spacing [2]. Minimum gauge spacing is 5 m in both the alongshore and cross-shore directions. Maximum spacing is 255 m in the alongshore direction and 120 m in the cross-shore direction. Intermediate gauge spacings are in multiples of 5 m. With 15 gauges, there are 105 possible pairs - 12 have redundant spacing, 93 are unique. The location is along the 8 m isobath about 900 m offshore and 400 m to the north of the research pier. The location of the array was chosen to satisfy three constraints: 1) Not too shallow - outside the surf zone almost all of the time. This important for satisfying linear wave theory, which is assumed for transforming a pressure signal at depth to surface displacement; 2) Not too deep - the pressure signal from a surface wave attenuates exponentially with depth and wavelength. At an 8 m depth, down to 13.5 m wavelengths (3 second wave period) are measured well; 3) The topography must be homogeneous, smooth, and stable in order to avoid related wave variability. In other words, criteria 3) ensures a homogeneous wave field over the extent of the array.

The array uses peizoelectric pressure sensors. Directional spectra are calculated using array processing techniques, specifically an iterative maximum likelihood method (IMLM) [3]. Due to the array design, the resulting directional resolution is ~5 degrees, much greater than typical, single-point-triplet wave measurements (e.g., [1, 4]).

Please see the FRF data paper, FRF website, and FRF data portal for more information.

[1] Long, C. E. and Oltman-Shay, J. M. (1991)

[2] Reiger, L. A. and Davis, R. E. (1977)

[3] Oltman-Shay, J. M. and Guza, R. T. (1984)

[4] Young, I. R. (1994)