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Estimating Directional Wave Spectra Properties in Non-Breaking Waves from a UAS-Mounted Multi-beam Lidar

This paper was written by F. Feddersen, O. B. Marques, J. H. MacMahan, and R. L. Grenzeback.

Abstract

Wave spectra and directional moment measurements are of scientific and engineering interest and are routinely measured with wave buoys. Recently both fixed and UAS-mounted lidar remote sensing have measured surf zone wave spectra. However, wave statistics seaward of the surf zone have not been measured with a lidar due to a lower number of returns and directional moments have not been measured at all. We use a multi-beam scanning lidar mounted on a gasoline-powered UAS to estimate wave spectra, slope spectra, and directional moments on the inner shelf in ≈ 10 m water depth from an 11-min hover and compare to a co-located wave buoy.Lidar returns within circular sampling regions with varying radius 𝑅 are fit to a plane and a 2D parabola, providing sea-surface and slope time series. Wave spectra across the sea-swell (0.04–0.4 Hz) are robustly estimated for 𝑅 ≥ 0.8 m. Estimating slope spectra is more challenging.Large 𝑅 works well in the swell band and smaller 𝑅 work well at higher frequencies, comparing well with a wave buoy inferred slope spectra. Directional Fourier coefficients are estimated from wave and slope spectra and cross-spectra and are compared to a wave buoy in the sea-swell band.Larger 𝑅 and the 2D parabola-fit yield better comparison to the wave buoy. Mean wave angles and directional spreads, functions of the directional Fourier coefficients, are well reproduced at𝑅 = 2.4 m and the 2D-parabola fit, within the uncertainties of the wave buoy. This UAS-mounted multi-beam scanning lidar and this methodology can be used in regions where wave buoys are not easily deployable, e.g., near rocky coasts or cliffs.

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