Sediment transport due to irregular waves
Trouw, K. (2013). Sediment transport due to irregular waves. PhD Thesis. KU Leuven, Faculteit Ingenieurswetenschappen: Leuven. ISBN 978-94-6018-717-9. xii,186 pp.
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Document type: Dissertation
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| Keywords |
Transport > Sediment transport
Water waves > Irregular waves
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| Abstract |
Sediment transport due to irregular waves is an illustration of the problems that occur when small differences of two big numbers have to be determined. It is even difficult to estimate the direction of the net sediment transport. Both for ripples and sheet flow conditions, the transport can be opposite to a small mean current or, for asymmetrical waves, opposite to the mean wave direction.This thesis aimed to give a contribution for the knowledge of sediment transport due to waves, using both physical and numerical models.For physical modelling, it is important to know the scaling effects. It is also important that the waves in the wave flume are correctly generated. Errors may lead to wrong conclusions, even about the direction of the net sediment transport. This was illustrated by the constructed numerical model, that was validated by the physical models described below. The best scaling is obtained using the so called Sand Model, in which the settling velocity of the sand is scaled with the scale of the orbital velocity. However, for fine sand, this leads to grain sizes which are too small (resulting in cohesive material). If the Sand Model is used, the vertical axis of the concentration profile has to be scaled with the ripple height. To do this, the ripple height at prototype scale should be derived from empirical formulations to estimate the ripple height.In the Delta flume of Deltares, the measuring frame of Proudman Oceanographic Laboratory (UK), that was normally used at the sea bottom, was tested under controlled circumstances. The frame contained sensors to measure at different heights above the bottom the velocity and the sediment concentration and also to record the evolution of the ripples beneath the frame. Also at the walls of the flume, sensors were installed for comparison. The comparison indicated that the measurements of the frame compared well with the undisturbed wall measurements, except for the turbulence characteristics relative far from the bed. The experiments also allowed to validate the existing formulations to calculate ripple dimensions. For regular waves, the formulae of Nielsen (1992) gave good results. For irregular waves, the correspondence was worse, better results were obtained with Van Rijn (1993). This might be explained by the use of comparable experiments by Van Rijn to derive his formulations. The vertical sediment concentration profiles were compared to learn about the influence of the grain size, the wave height and the wave period. Also the relative horizontal distance between sensor and ripple crest was analysed. Also it was examined which turbulence characteristics could be derived from the available instrumentation.In the wave tunnel of Deltares, measurements were carried out with instruments that allowed to measure velocities and concentrations in the sheet flow layer. Also PIV measurements (measurement of the velocity of a cluster/pattern of sediment particles) were organized by the University of Edinburgh. The measurements indicated that the effect of increasing the orbital velocity was to greatly enhance the magnitude and sharpness of the concentration peaks generated at backward flow reversal and to increase the magnitude of the peaks which appeared to be associated with flow maxima. The suspension events near backward flow reversals exhibited a lag with increasing elevation above the bed. This lag decreased with increasing peak orbital velocities. A pair of concentration sensors made it possible to obtain velocities and concentrations inside the sheet flow layer. The lifting up of the bed is clearly visible near flow reversals, with an important increase of the concentrations above the original bed level, and a decrease of concentrations below this level. The combination of velocities and concentrations makes it possible to predict sediment transport in the sheet flow layer. The PIV measurements were able to derive a velocity field on a reliable way, when comparing with traditional point measurements. However, the vertical velocities seem contain the settling velocity of the sand particles in the water. It was also proven that the ADV could be used to derive turbulence characteristics.Finally, in the CFD (Computation Flow Dynamics) software a script was written to calculate the time varying sediment concentration above ripples. The model was validated with the results of the physical experiments. Applying the model, learned that the number of waves in a wave group mainly influences the averaged concentration at relative high levels above the bed and the instantaneous concentration profile. The modeling also confirmed that one must be careful with the interpretation of measurements in one vertical, since they are not representative for the whole ripple length. Averaging the near bed concentration at the time interval of the highest waves gives an important difference with averaging them over the time interval of the lowest waves. This is important since wave groups induce bound long waves, with on offshore flux for the highest waves and an onshore flux for the lowest waves. The net transport can become onshore directed. |
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