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TRANSKEW – Measurements of sand transport induced by skewed waves and currents

Coordinator

Paulo Silva

Programme

In the framework of HYDRALAB-III project, funded under the Sixth Framework Programme (FP6)

Dates

17/09/2017 - 03/12/2017

Participating Institutions

  • Université de Grenoble
  • Utrecht University, Faculty of Geosciences, The Netherlands
  • University of Twente, The Netherlands
  • School of Engineering, King’s College, University of Aberdeen, United Kingdom
  • Department of Civil Engineering, Polytechnic Institute of Viseu, ESTGV, Portugal
  • Deltares

Transnational Access to Hydralab Major Research Infrastructure
Access to the Large Oscillating Water Tunnel (LOWT) of WL|Delft Hydraulics

The knowledge and modelling of sand transport induced by waves and currents still has considerable shortcomings. Two large knowledge gaps in sand transport processes are: i) sand transport induced by skewed waves and ii) sand transport due to combined influence of waves and currents.

As waves travel and shoal towards a beach, apart from becoming peaky with sharp wave crests and long wave troughs, they also attain a sawtooth shape, with a steep front face and more gently sloping rear face. The corresponding orbital velocity near the bottom shows a similar (time) variation. Under the steep front face, between the past wave trough and the following wave crest, the velocity varies rapidly from a maximum negative (offshore) value to a maximum positive (onshore) value, giving rise to rapid fluid accelerations. The accelerations induced during the rear face of the wave are much smaller. This effect is related with the so-called acceleration skewness. According to the experimental findings of King (1990) and Watanabe and Sato (2004), this skewed acceleration oscillatory flow is responsible for an onshore net sediment transport and for onshore sand bar migration (Hoefel and Elgar, 2003).

Many researchers have conducted experiments with regular and irregular symmetric and asymmetric oscillatory flow (e.g. Ribberink and Al Salem, 1994; Wright, 2002; Van der Werf, 2006). However, only a few experiments have been performed with accelerated skewed waves: the King (1991) data set refers to a series of experiments performed in the oscillatory flow tunnel (oft) of Scripps Institute of Oceanography under half-cycle sawtooth waves; Dibajnia and Watanabe (1998) conducted experiments in the oft of the University of Tokyo with irregular skewed waves in sheet-flow (flat-bed) conditions and Watanabe and Sato (2004) considered an oscillatory regular flow with and without a steady flow with the same tunnel. These experimental studies focussed on the net sand transport and did not analyse in detail the effects of the wave nonlinearities, such as the velocity and acceleration skewness, on the sediment transport processes.

Except for storm conditions, a large part of the shoreface is covered with ripples, typically several decimetres long and centimetres high. Sand transport in the rippled bed regime is very complex due to the interaction processes between wave-induced oscillatory flow, net currents, ripples and suspended sand. If the ripple steepness exceeds approximately 0.1, vortex shedding takes place at the ripple’s lee-side. These vortices are highly effective in entraining sand from the bed into suspension and since these are ejected into the flow near flow reversal there exist strong phase lags between the sand concentrations and the oscillatory flow. Due to these phase lag effects, net sand transport can be in the offshore direction, i.e. against the wave direction. The present understanding and modelling of sand transport over rippled beds is mainly limited to wave-alone conditions (e.g. Clubb, 2001; Van der Werf, 2006). In the past, wave plus current ripple regime experiments were carried out in large oft’s by e.g. Ramadan (1994) and Ribberink (1995). However, these were mainly focussed on the ripple dimensions and the time-averaged suspended concentration profile. Very few studies have been done on the net sand transport rates and of the detailed flow and suspended sand dynamics in this specific regime.

The presently proposed experiments are aimed at obtaining a new consistent dataset of ripple dimensions, flow velocities, suspended concentrations and net transport rates in order to fill the existing lack of data. The experiments should provide new insights into the detailed flow and sand behaviour. Different series of regular oscillatory flows will be devised with different acceleration skewness and velocity skewness with and without different superimposed net currents. The tests will be carried out in the sheet flow regime as well as in the rippled bed regime.
The oscillatory flows to be tested will mimic the degree of asymmetry found in real ocean waves before and after wave breaking. The comparison of the experimental results with previous ones obtained with regular 2nd order Stokes waves (Ribberink & Al Salem, 1994; Van der Werf & Ribberink, 2006) will allow studying the importance of i) the wave acceleration skewness (in contrast to wave velocity skewness), and ii) the superimposed net current in terms of the bedform regime and the sediment transport processes.
The data will directly be used for the validation and further development of sand transport models in the framework of (parallel) research programmes in which members of the present researchers group participate (e.g. the STW/EPSRC-project SANTOSS).

further information on HYDRALAB-III – https://hydralab.eu//