University of Twente Student Theses

Improving a non-linear model for simulating sand waves An idealized modelling study on the effect of including suspended sediment and a critical condition for bed load transport on sand wave predictions in the North Sea

Ensing, Erik (2011) Improving a non-linear model for simulating sand waves An idealized modelling study on the effect of including suspended sediment and a critical condition for bed load transport on sand wave predictions in the North Sea.

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Abstract:	Sand waves are bed forms that are observed in shallow seas of about 10 to 55 m deep with strong tidal currents. They only appear on a sandy bottom and they do not develop when coarse sediment covers the sea bed. They have a typical wavelength ranging from 100 to 800 m and their maximum amplitude is in the order of 5 m. Sand waves often pose a problem in shallow seas, because those seas are used for various functions. Sand waves can migrate and thereby expose pipelines, obstruct navigational routes and even expose undiscovered mines and chemical waste that may have been deposited in the past. Therefore, more insight into the behaviour of such sand waves can be put to good use. The model that is used in this research is called the Sand Wave Code (SWC). It is a non linear model, which allows the prediction of sand waves from their initial state up to their final shape. The model can predict the sand wave shape, growth rate and migration speed and usually works with a domain of one wavelength. The domain length is determined in a linear perturbation analysis, and it is equal to the wavelength of the fastest growing mode. The main goal of this research was to improve this model, in order to be able to make better predictions of the behaviour of the sand waves. In this research two improvements of the SWC are carried out. The first is an improvement of the suspended load transport calculation. This was necessary, because it had some shortcomings, for instance it lacked a proper scaling and the no flux boundary condition was not actually implemented at the surface. After improving all of these aspects of the suspended sediment calculation, a critical condition for the initiation of bed load transport was added to the SWC. This critical condition also works with the improved suspended sediment module and the surface gravity waves module of the model. Hereafter, we carried out simulations to test the effect of these changes. The results of these simulations are compared with measurements from three locations in the North Sea. The results showed that the sand wave length was predicted best when suspended sediment and the critical condition were both not taken into account. However, in this case the predicted final sand wave amplitude is way too large. Including suspended sediment did not help to reduce these large amplitudes, they were even increased slightly. The results of the suspended sediment runs did show however, that the simulated concentration profiles are more physically justified. The results of the critical condition for the initiation of bed load transport were as expected. It causes shorter sand waves with a lower crest. The growth rate of the sand waves is also reduced significantly, except in the case where suspended sediment transport was also included. The resulting behavior of the bed load transport over a tidal cycle was as expected, with the bed load transport declining to zero for low flow velocities. The critical condition thus succeeded in reducing the predicted final amplitude of the sand waves, but it gave an underestimation of the sand wave length. To check whether the reduction in amplitude was not only associated with the reduction in sand wave length, simulations have been performed with the same lengths as the ones predicted in the basic simulations. The results of this showed that the critical condition was still able to predict lower values of the final sand wave amplitudes. These lower amplitudes were still too large however, so more processes are needed to help reduce this. The Sand Wave Code has been improved significantly in this research. Especially the suspended sediment calculation has been improved. Many minor shortcomings have been improved and a new scaling has been successfully applied to the calculation. The critical condition has also been implemented successfully, even for the case with surface waves. When comparing the results to measurements at three locations in the North Sea, it can be seen that a better prediction of the final shape is obtained with the critical condition, but not with suspended sediment.
Item Type:	Essay (Master)
Faculty:	ET: Engineering Technology
Subject:	56 civil engineering
Programme:	Civil Engineering and Management MSc (60026)
Link to this item:	https://purl.utwente.nl/essays/61101
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