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Experiments by Invited Researchers

 

Measurements of sand transport and its underlying processes under large-scale breaking waves

Project acronym: HyIV-CIEM-01
Name of Group Leader: Dr. J.S. Ribberink
User-Project Title: Measurements of sand transport and its underlying processes under large-scale breaking waves
Facility: CIEM Flume
Proceedings TA Project: MEASUREMENTS OF SAND TRANSPORT AND ITS UNDERLYING PROCESSES UNDER LARGE-SCALE BREAKING WAVES (SANDT-PRO)
Data Management Report: Report

User-Project Objectives

The primary advantage in using the CIEM facility for these experiments is its large size producing near full-scale-size waves eliminating the scale effects associated with sediment-based experiments in smaller facilities. Two types of experiment were conducted: (i) Regular breaking waves, which focused on the effects of wave breaking on near-bed sediment dynamics, especially the effects of breaking-induced turbulence and sediment stirring; (ii) Irregular non-breaking wave, bi-chromatic superposition or amplitude modulation of regular waves; to study the sequencing on the near-bed hydro- and sediment dynamics. State-of-the-art instrumentation deployed from a mobile measuring frame custom-built for the experiments was used to obtain the process measurements. The frame is a stiff construction of small-diameter, steel tubing, and was mounted to the flume’s carriage, which runs on rails located at the top of the flume. A spindle adjustment enables the instrument frame to be vertically positioned with O(mm) accuracy. Flow velocities were measured using acoustic Doppler velocimeters, a Vectrino profiler and a high-resolution acoustic concentration and velocity profiler (HR-ACVP). Sediment concentrations were measured using the HR-ACVP, 3 optical backscatter sensors, an AQUAscat acoustic backscatter system and a 7-nozzle transverse suction sampling system. Two conductivity concentration measurement tanks for sheet flow measurements were installed in the sand bed, below two additional HR-ACVPs. Water surface elevation was recorded by means of resistive wave gauges and pore pressure sensors. Net transport was calculated using mass conservation principles applied to pre- and post-test bed profiles measured with acoustic bed profilers.

Short description of the work carried out

While all instruments are important to the experiment, it is the instruments measuring close to the bed, within the wave bottom boundary layer, which are of primary interest here. Of these, the HR-ACVP is especially important as it simultaneously measures sand concentration and 2 velocity components over a 10-cm vertical profile, with 1mm vertical resolution, thereby revealing the detailed intra-wave sediment fluxes in the near-bed region. Early results show that the bar develops due to the combined effect of onshore-directed sand transport from seaward of the break-point and offshore-directed sand transport from shoreward; the former is mainly driven by wave asymmetry with sheet-flow conditions, while the latter is dominated by the offshore-directed suspended sand transport driven by undertow. The Hydralab IV SandT-Pro experiments are integrated within a larger research project on sand transport processes under breaking and irregular waves. Named SINBAD, the project is led by the Universities of Twente and Aberdeen and funded by the Dutch STW and the UK’s EPSRC. SINBAD involves two additional experimental campaigns in the CIEM, one in which similar detailed process measurements to SandT-Pro are made, but focused on a particular stage of the bar development, and a second in which hydrodynamics only are measured over the barred profile made rigid by laying concrete on the sand bed. The results from the combined set of experiments will be important for the future development of well-founded sand transport models, such as the SANTOSS model.

Highlights of important research results

While all instruments are important to the experiment, it is the instruments measuring close to the bed, within the wave bottom boundary layer, which are of primary interest here. Of these, the HR-ACVP is especially important as it simultaneously measures sand concentration and 2 velocity components over a 10-cm vertical profile, with 1mm vertical resolution, thereby revealing the detailed intra-wave sediment fluxes in the near-bed region. The experiments have generated a large dataset, the analysis of which is ongoing. Early results show that the bar develops due to the combined effect of onshore-directed sand transport from seaward of the break-point and offshore-directed sand transport from shoreward; the former is mainly driven by wave asymmetry with sheet-flow conditions, while the latter is dominated by the offshore-directed suspended sand transport driven by undertow. The Hydralab IV SandT-Pro experiments are integrated within a larger research project on sand transport processes under breaking and irregular waves. Named SINBAD, the project is led by the Universities of Twente and Aberdeen and funded by the Dutch STW and the UK’s EPSRC. SINBAD involves two additional experimental campaigns in the CIEM, one in which similar detailed process measurements to SandT-Pro are made, but focused on a particular stage of the bar development, and a second in which hydrodynamics only are measured over the barred profile made rigid by laying concrete on the sand bed. The results from the combined set of experiments will be important for the future development of well-founded sand transport models, such as the SANTOSS model.

Publications
jan s. ribberink, Dominic A. van der A, Joep van der Zanden, Tom O'Donoghue, David Hurther, Ivan Cacares, Peter D. Thorne SandT-Pro: Sediment transport measurements under irregular and breaking waves , 2014 Book of abstracts, 34t Int. Conf. on Coast Eng., Seoul, June 2014

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