Sediment transport under irregular waves

Helmendach, S.S. (2013) Sediment transport under irregular waves.

Abstract:There is an increasing desire to understand and successfully model nearshore processes, especially in the nearshore zone where many different hydrodynamic and sediment transport processes take place. Different wave conditions and bed shapes for example can cause sediment to move at the bottom, the place where the largest sediment transport often occurs (Malarkey & Davies, 2012). In order to simulate and gain knowledge, about the processes that occur in the nearshore area, mainly regular, sinusoidal, waves have been used for experiments and irregular, realistic, waves have been left aside. This study is focused on the improvement of the knowledge of these irregular waves. Therefore, the main objective of this research is: to increase the understanding of the nearshore sediment transport processes occurring under irregular non-breaking wave conditions, with the use of the boundary layer model. To obtain a better understanding of the irregularity processes and its effects on sediment transport, a regular wave that represents an irregular wave would be easier to implement in existing morphological models that simulate sediment transport. Therefore, the second objective of this research is: to develop, or to approach, a representative regular wave for an irregular wave signal. An analysis on regular wave knowledge of today showed that hydrodynamic processes, such as wave propagation and orbital motions, could cause and have influences on the streaming (progressive- and wave shape streaming) near the seabed, in the boundary layer. The processes also contribute to friction and bed shear stresses at the seabed, causing sediment to move and be brought into suspension. Asymmetry in the wave shape, velocity skewed waves, can contribute to the sediment transport by transporting the remaining sediment that is in suspension, after it was entrained during one part of the flow cycle and did not settle down prior to the following half-cycle, in the opposite direction during the following half-cycle, which is also called the phase lag effect (Grasso et al., 2011; Van der A et al., 2010). In this study, the boundary layer model of Kranenburg (2013) is first validated on net sediment transports of irregular wave flume experiments. Subsequently, research is done to which extent the net sediment transports of irregular and regular waves differ for wave flume- and oscillatory flow tunnel simulations and how these can be explained by hydrodynamic and sediment transport related processes. Finally, research is done on the influence of skewed wave groups on the net sediment transport in oscillatory flow tunnel simulations. Firstly, using fine and medium sediment irregular wave-flume experiments, of Schretlen (2012) and Dohmen-Janssen (2002) respectively, for the boundary layer model wave-flume simulations, the net sediment transport results were considered a good quantitative reproduction for net sediment transports, despite of a slight overestimation in the net sediment transport of a few experiment condition runs. Secondly, for the indication of differences in net sediment transport between irregular and regular waves, two representative regular wave approaching methods for an irregular wave were introduced, the “full signal influence approach” and the “partial signal influence approach”. For both methods, Stokes second order solution for the horizontal velocity is used to create the representative regular wave and the original, irregular wave, wave peak period, wave energy and velocity skewness are retained. However, in the latter principle the two methods differ. Where for the full signal influence approach the entire irregular wave signal is used to define the velocity skewness, for the partial signal influence approach only the highest one-third of the peaks (positive/onshore) is used to define the velocity skewness. Irregular wave simulations (thirteen in total), with fine sediment, of the boundary layer model showed that for flume simulations, both the representative regular waves have equal onshore net sediment transports as their irregular waves, only a slight overestimation occurs. For the oscillatory flow tunnel simulations however, the irregular waves show an offshore net sediment transport, while both the representative regular waves show onshore net sediment transports. Furthermore, between the two representative regular waves there was no significant difference noticeable and therefore a new representative regular wave approaching method is introduced first, before explaining the difference between irregular and regular waves in net sediment transport, according to hydrodynamic and sediment transport related processes. In the new “high wave, signal influence approach” method, only the wave energy of one-third of the highest waves in the irregular wave signal is used to define the velocity amplitudes (u1 and u2) and the velocity skewness. Thirdly, the difference in net sediment transport between the irregular and regular wave, is found in the wave-related component of the intra-wave horizontal sediment flux where phase lag effects after each single irregular wave, and in case the irregular wave contains a sequence of high irregular waves a accumulation of these phase lag effects (pumping effect) occurs. The influence of velocity-skewed waves brings the sediment offshore. For the difference in net sediment transport between oscillatory flow tunnel and flume simulations for irregular waves, the vertical momentum advection is becoming less important with an increasing wave energy (third method) or when the wave signal is irregular. But the vertical sediment advection and the horizontal momentum advection do get more important with more wave energy in a regular wave and with an irregular wave (both including phase lag), and decrease the amount of phase lag effect and also the contribution to the pumping effect occurring for irregular waves (amount of sediment concentration due to offshore flow), resulting in more onshore-directed sediment transport. Finally, net sediment transport simulations, in the oscillatory flow tunnel, showed that three different skewed wave groups, with single irregular waves, have no influence on (the direction of) the net sediment transport.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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