https://hal.archives-ouvertes.fr/hal-01778147/document

A nonlinear weakly dispersive method for recovering the elevation of irrotational surface waves from pressure measurements

Wave resource characterization through in-situ measurement followed by artificial neural networks' modeling

Field data-based evaluation of methods for recovering surface wave elevation from pressure measurements

In the surf zone, non-hydrostatic processes are either neglected or estimated using linear wave theory. The recent development of technologies capable of directly measuring the free surface elevation, such as 2-D lidar scanners, allow for a thorough assessment of the validity of such hypotheses. In this study, we use subsurface pressure and lidar data to study the non-linear and non-hydrostatic character of surf zone waves. Non-hydrostatic effects are found important everywhere in the surf zone (from the outer to the inner surf zones). Surface elevation variance, skewness, and asymmetry estimated from the hydrostatic reconstruction are found to significantly underestimate the values obtained from the lidar data. At the wave-by-wave scale, this is explained by the underestimation of the wave crest maximal elevations, even in the inner surf zone, where the wave profile around the broken wave face is smoothed. The classic transfer function based on linear wave theory brings only marginal improvements in this regard, compared to the hydrostatic reconstruction. A recently developed non-linear weakly dispersive reconstruction is found to consistently outperform the hydrostatic or classic transfer function reconstructions over the entire surf zone, with relative errors on the surface elevation variance and skewness around 5% on average. In both the outer and inner surf zones, this method correctly reproduces the steep front of breaking and broken waves and their individual wave height to within 10%. The performance of this irrotational method supports the hypothesis that the flow under broken waves is dominated by irrotational motions. Plain Language Summary In the surf zone, waves undergo rapid changes in shape, passing from steep and skewed waves right before breaking to sawtooth-shaped asymmetric bores. Capturing and understanding these changes is crucial for coastal researchers and engineers since the breaking wave-induced hydrodynamics shape beaches at various temporal and spatial scales. In this study, we use lidar scanners and pressure sensors to study the non-hydrostatic and non-linear character of surf zone waves. We show that non-hydrostatic effects remain strong over the entire surf zone; that is, fluid accelerations are important and the hypothesis of a hydrostatic pressure field leads to large deviations of the real surface elevation. More specifically, wave crests elevation are underestimated, and the sharp-crested shape of broken waves is rounded off. A recently developed non-linear weakly dispersive method to reconstruct the free surface from subsurface pressure is found to consistently outperform the hydrostatic or classic transfer function reconstructions over the entire surf zone, with relative errors on the surface elevation variance (related to the wave energy) and skewness (related to wave shape) around 5% on average. The performance of this irrotational method supports the hypothesis that the flow under broken waves is dominated by irrotational motions.

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Non-hydrostatic, Non-linear Processes in the Surf Zone

Field measurements of surface waves using a 5-beam ADCP

The adaptation of Doppler current profilers to measure directional wave spectra has provided a new instrumentation approach to coastal and nearshore oceanographic studies Past studies have shown favorable comparisons between Doppler current profiler wave instruments with bottom mounted PUV (pressure-velocity) sensors sampling at wave frequencies and wave buoys In this paper, we examine the capabilities and limitations of two different Doppler current profilers for directional wave measurements in shallow coastal waters of 0-25 m water depth Data collection programs using Doppler current profilers for wave measurements have been conducted for one month long periods in the early spring of 2002, 2003 and 2004 on Roberts Bank in the Fraser River foreslope region of the Strait of Georgia, British Columbia, Canada In 2004, an RD Instrument ADCP along with the newly-released 1000 kHz Nortek AWAC current profiler and wave instrument were co-located in 7 m water depth at a different site on the edge of Roberts Bank Inter-comparisons between these bottom mounted instruments are used to examine the capabilities of the directional wave spectral parameters, in terms of: resolvable frequencies for directional and nondirectional wave spectra; wave directional resolution and reliability, and limitations arising from the use of linear wave theory For a preliminary assessment of the capability of Doppler wave spectra in deeper waters of 20-25 m depths, in particular for very long wave periods, some experiences derived from a long-term measurement program being conducted off the west coast of Africa are presented

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V. Lee

Papers

The capabilities of Doppler current profilers for directional wave measurements in coastal and nearshore waters