TL;DR: In this paper, the capabilities and limitations of two different Doppler current profilers for directional wave measurements in shallow coastal waters of 0-25 m water depth were compared with bottom mounted PUV (pressure-velocity) sensors sampling at wave frequencies and wave buoys.
Abstract: 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
TL;DR: In this paper, the authors derived a nonlinear weakly dispersive formula to reconstruct the surface elevation of nonlinear waves propagating in shallow water, which is easy to use as it is local in time and only involves first and second order time derivatives of the measured pressure.
31 citations
Cites background or methods from "The capabilities of Doppler current..."
...the surface elevation, such as acoustic surface tracking (Birch et al., 2004) or LiDAR scanning (Martins et al....
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...Even if some methods are now available for a direct measurement of
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the surface elevation, such as acoustic surface tracking (Birch et al., 2004) or LiDAR scanning (Martins et al., 2016), pressures sensors remain a very useful tool for coastal wave applications....
TL;DR: It is concluded that neural networks can make an optimal use of the data produced by wave monitoring instrumentation and are useful to characterize the wave energy resource of a coastal site.
TL;DR: In this paper, the surface elevation of irregular waves propagating outside the surf zone from pressure measurements at the bottom was compared with the traditional transfer function method, based on the linear wave theory, which predicts reasonably well the significant wave height but cannot describe the highest frequencies of the wave spectrum.
17 citations
Cites background from "The capabilities of Doppler current..."
..., 2004 [32]), but the present study focuses on waves propagating outside the surf zone (see Fig....
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...Hence, the AST ability to provide reliable measurements under wave breaking can be questioned (Pedersen et al., 2002 [31]; Birch et al., 2004 [32]), but the present study focuses on waves propagating outside the surf zone (see Fig....
TL;DR: In this paper, the authors used subsurface pressure and lidar data to study the non-linear and non-hydrostatic character of surf zone waves and found that the nonlinear 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.
Abstract: 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.
17 citations
Cites background from "The capabilities of Doppler current..."
...The presence of air bubbles associated with wave breaking processes prevents sound waves to reach the surface, hence making this method inappropriate for surf zone applications (Birch et al., 2004)....
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...These can be deployed at the bottom of the water column (Pedersen et al., 2002; Birch et al., 2004; Mouragues et al., 2019) or above the surface (Turner et al., 2008), although the latter study focuses on the swash zone....
TL;DR: In this article, the performance of an improved 5-beam ADCP, with a vertical beam, when deployed to measure non-directional waves in waters of 40m depth is compared with four co-located directional wave buoys.
TL;DR: In this article, it is shown that it is possible in shallow water to estimate both wave height and direction from a conventional bottom-mounted, upward-looking acoustic Doppler current profiler.
Abstract: Routine monitoring of waves and currents in the nearshore region is of great interest both scientifically and to the general public because of their role in coastline erosion and their impact on recreational activities Historically, the technology for measuring these quantities has been distinct, requiring separate instrumentation for each In this contribution the authors show that it is possible in shallow water to estimate both wave height and direction from a conventional bottom-mounted, upward-looking acoustic Doppler current profiler Height and direction spectra compare well with a co-located array of pressure gages
67 citations
"The capabilities of Doppler current..." refers methods in this paper
...RDI uses a wave array algorithm [1] based on twelve (three or more from each beam) independent measurements of velocity that increases the resolution of the directional wave spectrum estimates....
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...Acoustic Doppler Current Profiler (ADCP) instruments, which were first developed for remote measurements of current profiles in the 1980 s, have been adapted to the measurement of directional waves over the past several years [1,2,3] with initial tests conducted in comparatively shallow waters of 7 8 m depth....
TL;DR: In this article, a bottom-mounted upward looking ADCP was used to determine wave height and direction in coastal-depth waters, and the ADCP yielded three independent estimates of the non-directional wave height spectrum and hence provided an internal consistency check on the performance of the instrument.
Abstract: The authors have shown that a bottom-mounted, upward looking ADCP provides a robust means of determining wave height and direction in coastal-depth waters. When equipped with a pressure sensor, the ADCP yields three independent estimates of the non-directional wave height spectrum, and hence provides an internal consistency check on the performance of the instrument. Directional spectra obtained from the ADCP tend to be sharper than those from point measurements, such as PUV triplets or directional wave buoys and, because of the greater number of degrees of freedom in the measurement, the ADCP can resolve complex multi-directional wave distributions.
TL;DR: In this paper, the authors compare PUV wave observations from two collocated velocity/pressure sensors (Aquadopp Current Profiler and Vector Velocimeter) to evaluate their performance.
Abstract: We compare PUV wave observations from two collocated velocity/pressure sensors (Aquadopp Current Profiler and Vector Velocimeter). Our objective was to use the differences in the two sensors to evaluate their performance. We evaluate limitations and uncertainties in the wave measurements, focusing particularly on the high frequency cutoff and uncertainties in direction and spreading. We model direction and spreading uncertainties with a simple Monte Carlo simulation, which compares well with our wave data. We conclude that either instrument is able to observe wave spectra and wave height with an uncertainty of a few percent and with wave direction uncertainties of a few degrees.
TL;DR: In this paper, a vertical, acoustic beam that detects the surface is introduced, which allows for directly measuring waves as opposed to interfering wave estimates from wave energy spectra, which improves the accuracy at higher frequencies.
Abstract: Nortek has improved upon its AWAC, a current and wave measurement sensor package, by introducing a vertical, acoustic beam that detects the surface. This added functionality allows for directly measuring waves as opposed to interfering wave estimates from wave energy spectra. Traditionally, wave measurements from bottom-mounted instruments, such as the combine pressure-velocity (PUV) approach, are limited in their frequency response. This is due to attenuation of the surface signal with increasing depth. Recent advances employ the alternative solution of measuring orbital velocities close to the surface and incorporating the Maximum Likelihood Method (MLM) estimate technique (Krogstad et al., 1988). This improves the accuracy at higher frequencies. However, for deployment depths of 10 metres or deeper, these methods cannot resolve waves periods that are 3 seconds or shorter. Moreover, these bottom-mounted systems do not measure the real surface time series, which makes it difficult to calculate extreme value statistics. The following paper provides an overview of the process of (1) developing the surface track algorithms, (2) comparing with a Datawell wave buoy off the coast of Carqueirance, France (3) and testing limiting conditions such as breaking waves and greater depths (35 metres).
TL;DR: In this article, a vertical, acoustic beam that detects the surface is used to measure the orbital velocities close to the surface and incorporating the Maximum Likelihood Method (MLM) estimate technique (Krogstad et al., 1988).
Abstract: Nortek has improved upon its AWAC, a current and wave measurement sensor package, by introducing a vertical, acoustic beam that detects the surface. This added functionality allows for directly measuring waves as opposed to inferring wave estimates from wave energy spectra. Traditionally, wave measurements from bottom-mounted instruments, such as the combined pressure-velocity (PUV) approach, are limited in their frequency response. This is due to attenuation of the surface signal with increasing depth. Recent advances employ the alternative solution of measuring orbital velocities close to the surface and incorporating the Maximum Likelihood Method (MLM) estimate technique (Krogstad et al., 1988). This improves the accuracy at higher frequencies. However, for deployment depths of 10 meters or deeper, these methods cannot resolve waves periods that are 3 seconds or shorter. Moreover, these bottom-mounted systems do not measure the real surface time series, which makes it difficult to calculate extreme value statistics. The following paper provides an overview of (1) the process of developing the surface track algorithms, (2) comparing with Datawell wave buoys off the coasts of Carqueiranne, France and Gabbard, UK (3) and finally we show how the same technology has been transferred from a 1 MHz to a 600 kHz AWAC to achieve similar accuracy and resolution at depths of 60 meters.