Non-Intrusive Measurements of Wave-Induced Flow over Dikes by Means of a Combined Ultrasound Doppler Velocimetry and Videography

Abstract

The performance of non-intrusive instruments, such as acoustic profilers and cameras, to describe the wave-induced flow processes over maritime dike crest was investigated in experiments carried out at the University of Bologna. Direct and derived measurements from the acoustic probes deployed along the structure crest were discussed in relation to the observed backscatter rates. Image processing was implemented by means of clustering algorithm, in order to detect the free surface during overtopping events and characterize wave front propagation over the dike crest. UVP data were processed to indirectly derive flow depths and overtopping rates and compare them with the direct measurements in order to assess the measurement reliability and discuss their limits. Individual overtopping volume distribution as obtained by UVP data were estimated and compared with well-consolidated formulations, showing a good agreement. Finally, suggestions for an appropriate use of non-intrusive instruments to characterize a shallow, transient and aerated flow were provided, such as the control of the artificial seeding density, the use of a bi-static UVP configuration and adjustments to light exposure.

Statistical Analysis of the Wave Runup at Walls in a Changing Climate by Means of Image Clustering

“…The accuracy in the camera calibration is a direct indicator of the error associated to the h-values and, by extension, to all the quantities representing lengths or heights which are directly computed from h, i.e., the flow depths at WG1 and WG2, the runup heights, the height of the water columns in front of P1, P2 and P3 and the depth of the overtopping flow behind the wall. Similarly to Gaeta et al [31] and Formentin et al [28], the calibration was performed following the approach indicated by Bouguet et al [33] and the accuracy of the camera calibration error was calculated by selecting a sample frame for validation for each experiment, and by comparing the main geometrical distances (e.g., width of the berm or slope angle of the dike) of the picture to the corresponding real-world dimensions obtained after the application of the rotation matrix and translation vector. Following this approach, the relative errors characterizing the length of the berm (B) and the inclination of the dike slope (cotα d ) were calculated for each of the eight selected tests and are shown in Table 2.…”

“…The image-processing methodology referenced in the present contribution was developed by Formentin et al [28] on the basis of the video records of the experiments conducted at Unibo and recalled in Section 2. The core of such a methodology is represented by the application of an image clustering technique [31] to automatically detect significant "features" from the images of the experiments. The open-source toolbox "PRTools" (Pattern recognition Tools, http://prtools.tudelft.nl/?from=www.website80.com, [32]) developed by TU Delft was adopted for the cluster analysis and the features extraction from the videos.…”

“…The new methodology includes several novel aspects related to the elaborations of the free-surface elevation (h, hereinafter) in specific points of the framing window, which New part of the methodology Figure 3. Conceptual scheme of the whole methodology of image cluster analysis, where the new part is highlighted in the frame [28,31].…”

The Assessment of Acoustic Doppler Velocimetry Profiler from a User’s Perspective

“…However, for transient processes, such as seen in moving boundaries, e.g. over a wave induced flow field (Gaeta et al 2020), high SNR and correlation may be indicative of a reflective boundary, implying that other means need be sought after (such as to analyse particles scattering and flow velocity spatiotemporal patterns in parallel with videography), and that high correlation is not an imperative goal (especially in cases where flow turbulence may be such where the acoustic pulses get to become uncorrelated).…”

New insights in the probability distributions of wave-by-wave overtopping volumes at vertical breakwaters

“…Clearly, a reliable prediction of the Weibull b shape parameter (or Weibull b ) in a distribution of individual overtopping volumes is critical for reliable estimates of maximum individual overtopping volumes 20 . However, no formal guidance exists for determining the shape parameter in these instances and several approaches for fitting shape parameters to Weibull distributions are reported in literature – some research using the upper part of the Weibull distribution function 6 , 20 , 25 , 28 – 30 , with other research using 10% 23 and 20% 31 of the upper part of the distribution.…”