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Experiments by Invited Researchers

 

Measurement of wave-induced sand transport over rippled beds

Project acronym: HyIII-FZK-08
Name of Group Leader: Jebbe van der Werf, University of Twente
User-Project Title: Measurement of wave-induced sand transport over rippled beds
Facility: Large Wave Flume, GWK
Proceedings TA Project: Boundary layer flow and sand transport under full scale surface waves
Data Management Report: There is no Data Management Report available for this project

User-Project Objectives

Summary:

The main objective of the experiments was to increase the insight into and to obtain reliable data on the sand transport processes under full-scale regular asymmetric surface wave conditions in both the low velocity ripple regime and the high velocity flat-bed/sheet-flow regime. More specifically, the focus was on the following processes:

  • bedform types and dimensions,
  • vertical structure (including the wave boundary layer) of the time-dependent flow velocities, suspended sand concentrations and suspended sand fluxes, and
  • total net transport rates.

A secondary objective was test and compare new instruments and measurement set-ups for these type of experiments.

For the detailed near-bed measurements, a special subframe was designed to achieve the required high spatial (0.001 – 0.01 m) and temporal (0.001 - 0.1 s) measurement resolutions, while at the same time avoiding flow disturbances and being rigid enough to withstand the wave forces.

During the experiments, measurements were done of the flow velocity, sediment concentration and sediment transport rates. The flow velocities were measured in the free-stream (with 4 Electro Magnetic Flowmeters (EMF), 4 Ultrasonic Velocity Profilers (UVP) and Vectrino) and inside the wave boundary layer (with 4 UVPs and Vectrino). The results of the sediment concentration measurements are of suspended sediment (measured with Acoustic Backscatter Sensors (ABS), Transvers Suction System (TSS) and an Ultra High Concentration Meter (UHCM)), but also of sand concentrations inside the sheet-flow layer (measured with 3 Conductivity Concentration Meters (CCM)). For the detailed flow velocity and sand concentration measurements near the bed the instruments UVP, Vectrino, and CCM were used, to be able to measure with the required high spatial (10-4 – 10-3 m) and high temporal (10-3 – 10-2 s) resolutions. Net transport rates were determined by applying the mass conservation principle to measured profiles of the bed over the full length of the flume before and after a test. During the test, a 1-2 m long section of the bed below the measuring frame was monitored with a ripple profiler (sonar).

All measurements are done for a range of wave conditions, with variations in wave height, wave period and wave shape and various sand transport regimes from ripples to sheet-flow.

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