University of Twente Student Theses


Simulate the erosion process of a dike made of cohesive material in the GWK with XBeach

Luijkx, G.W. (2017) Simulate the erosion process of a dike made of cohesive material in the GWK with XBeach.

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Abstract:At the Forschungszentrum Küste six experiments are conducted on dike erosion with six different cohesive materials. A new dike design should be evaluated with the experiments done in the flume. The goal for this thesis is to get to know if it would be possible to model erosion in a numerical model XBeach. First there has been looked at the erosion process of the dikes built in the flume. Only three of the tested materials did erode. The erosion process of a dike could simply be divided in two stages. In the first stage the toe of the dike is eroding due to the breaking of the wave steep slope. This slope will be hit directly by the wave, which will dig a hole underneath the material. When the hole has become big enough the material of the dike will collapse. By the force of the waves the material will dissolve in the water. The way of eroding is for all the materials similar, but there is one big difference, which is the eroding rate of the different materials. A very important part of the erosion process is the run-up and run-down of waves. In order to see whether XBeach is a good simulation for this process the model will be validated with measurements done in many flumes around Europe. The outcome of this validation is that XBeach isn’t very good in modelling the run-up on a slope. Furthermore there has been looked into the erosion module of XBeach and done a small sensitivity analysis. The erosion module of XBeach consists of two main branches: the sediment transport and the bottom updating/avalanching. For the sediment transport there has been looked in to the two main parameters, the maximum allowed sediment concentration and the time material needs to be absorbed by the water. For both values the change in the erosion isn’t very big, therefore it would be recommended to use the default value in the final simulation of the dike in XBeach. The second part of the erosion module of XBeach is the avalanching. Avalanching is the module of XBeach which accounts for the suddenly collapse of material. There are three important parameters: the maximum slope for both dry and wet material and the maximum amount of falling material per second (Dmax). Especially the value which is chosen for both the maximum dry and wet slope has a large effect on the erosion process in XBeach. Compared to the experiments in the flume the maximum slope for dry material is not steep enough to simulate the process. It seems that the default value for Dmax is nearly large enough to let erode all the material. Therefore it would be recommended to use the default setting for the Dmax. In the present version of XBeach it isn’t possible to simulate cohesive material. There are many differences between the way of eroding in the flume and the way XBeach is simulating the erosion process. XBeach is only capable of roughly giving an indication of the first 1 to 1.5 hour of the experiments. After this the slope is too steep to work with for XBeach. At first sight the physical process in XBeach looks completely different from the process in the flume. But in the numerical process of XBeach there are parts which have the opportunity to be used for different processes. The greatest challenge will be to make the model work with materials which are cohesive. This of course will affect all the other processes which are happing in the model.
Item Type:Essay (Bachelor)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering BSc (56952)
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