Road impact on erosion development during overtopping flood events

Bomers, A. (2015) Road impact on erosion development during overtopping flood events.

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Abstract:In the Netherlands, a large number of riverine flood defences are combined with roads located at the crest. However, the influence of this road on erosion development during wave overtopping is not yet known. The objective of the research is therefore to quantify the influence of a road located on top of a flood defence on the erosion development during wave overtopping. Experiments were performed with the wave overtopping simulator, which is a device to perform destructive tests on inner slopes of dikes in order to establish the erosion resistance against overtopping waves. In Millingen aan de Rijn an experiment was performed on a dike with a road on top. Measurements of this experiment were used as boundary conditions and validation data. A coupled hydrodynamic-bed model was developed. With the use of COMSOL Multiphysics a CFD simulation was performed, which models the water released by the wave overtopping simulator and the hydrodynamics along the dike profile. Tests with a discharge of 50 and 100 l/s per meter were simulated. With the computed bed shear stresses, the amount of scour was determined. The simulated eroded dike profiles show some discrepancies compared to the measured data. These discrepancies can be explained by the heterogeneities in soil properties. Local variations of the grass strength were neglected during the simulation, while the berms were damaged due to traffic. In addition, the soil underneath the grass sod also includes debris, roots with different strengths and sand. The model assumes a homogenous clay layer, which is a cohesive sediment and has a higher erosion resistance. Erosion at the crest of the dike increases significant due to a road located on top of the dike. This increase is caused by a combination of the smoother asphalt section and the damaged berms at the transitions. The model is highly sensitive to irregularities in the dike profile. An upward slope with an angle larger than 25˚ leads to a substantial increase of the maximum bed shear stresses. In addition, the overtopping tongue loses less energy due to the lower friction of the road section compared to a grass cover. Moreover, the turbulence of the flow increases due to a sudden increase of the bed roughness at the transition of the road section with the grass sod. The erosion along the slope and toe of the dike decreases slightly. However, this decrease is small compared to the increase of the amount of scour at the crest. It can therefore be concluded that a road located at the crest has a negative influence on the erosion development during wave overtopping and must be included during the safety assessment. It is recommended to use the hydrodynamics at the outlet of the simulator as boundary conditions of the model and to exclude the wave overtopping simulator from the geometry. The hydrodynamics inside this simulator are very complex and dependent on several factors, leading to uncertainty in the model. This can be captured by excluding the simulator. Furthermore, it is recommended to investigate if the maximum bed shear stress can be expressed as a function of bed roughness, slope and wave volume. With this expression it is possible to compute the erosion without doing the CFD simulation, which has a relatively large computation time. This research only investigates the influence of a road on erosion development. To get a complete picture of the influences of a road on the safety level of a dike also failure mechanism such as macro stability must be considered.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
Link to this item:http://purl.utwente.nl/essays/67530
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