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Wave dissipation over vegetation fields
Suzuki, T. (2011). Wave dissipation over vegetation fields. PhD Thesis. Delft University of Technology: Delft. ISBN 978-94-91211-44-7. xii, 176 pp.

Thesis info:
    Delft University of Technology (TUDelft), more

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Document type: Dissertation

Keywords
    Drag coefficient; Vegetation; Wave dissipation

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Abstract
    It has been widely recognized that ongoing climate change, most likely due to human interference with nature, may accelerate sea level rise and increase storm intensity. It is therefore urgent to design countermeasures to alleviate the impact of climate change on coastal regions. Apart from the view point of coastal protection, it is also very important for coastal engineers to keep an eye on environmental issues in the coastal region. In this context, vegetation fields such as salt marshes, sea grasses and mangrove forests in coastal regions have started to attract the attention of coastal engineers due to their function as wave attenuator. However, the wave attenuation function of a vegetated field is not well understood yet. To utilize coastal vegetation fields as a part of coastal management in practice, it is crucial to accumulate more knowledge about the physical processes, especially the hydraulic processes, and these need to be modeled in a practical sense. Hence, this thesis is intended as an investigation of the process of wave dissipation over vegetation fields through various approaches, specifically theoretical, physical and numerical studies.

    Prior to practical modeling of wave attenuation, theoretical background and literature reviews are conducted at the beginning of this research. Basic theories and major research efforts on the drag force are introduced indicating the importance of obtaining the bulk drag coefficient in different conditions. In the end formulations for wave dissipation over a multiple-cylinder field are developed, based on the literature. In the physical experiments, hydrodynamics around a salt marsh are investigated. Five different configurations of a salt marsh including two different density conditions for rigid cylinders and a real vegetation case are tested in a wave flume in terms of wave dissipation, wave reflection, velocity fields and pressures acting on the cliff. After the physical experiments, a 2DV spatial averaged model is developed. This model is validated by the physical model results. It can simulate wave dissipation appropriately by incorporating the defined porosity and surface permeability for artificial vegetation and natural vegetation, even in a strongly varying topography. Also velocities and pressures acting on a cliff are reproduced well. In the case studies, impact on a dyke in different wave conditions, impact on seedlings from a view point of vortices and impact on cliff erosion are discussed.

    Even when the model is appropriate, it is often difficult to choose an appropriate bulk drag coefficient to estimate vegetation induced wave attenuation. This is partly due to the complexity of the turbulence structure over the vegetated field. To understand the bulk drag coefficients, a three dimensional numerical simulation model CADMAS-SURF/3D is further developed incorporating an Immersed Boundary Method and a Large Eddy Simulation technique. From the validation, all the results obtained show good agreement with the reference data in literature. After the validation, case studies were conducted with regard to the bulk drag coefficient. From the study, it was concluded that the drag reduction is not just a function of density but also a function of the parameter 2a/S, where 2a is the stroke of the motion and S is the cylinder spacing. When 2a is less than S, the effect of the density is neglected because the wake does not reach the other cylinders even when the density is high. Additionally it was found that the drag reduction is not only related to 2a/S and but also to b, which is a frequency parameter described as Re/KC or D2/nT. In this study it was not possible to define a clear relationship quantitatively between wave conditions and cylinder array condition because of data limitations. However, a certain tendency was obtained.

    For the engineering practice of wave dissipation simulation, it is important to develop a computationally less expensive model. A 2DH model based on a third generation action balance wave model (SWAN) with vertical schematization of the vegetation is developed in this thesis. This is the most detailed implementation of wave dissipation of vegetation in a 2D numerical model to date. After the implementation and the validation, sensitivity analyses are conducted in terms of frequency spectrum shape, directional spreading (directional spectrum shape) and layer schematization. This model is capable of calculating not only the effect of horizontal variation of vegetation but also of wave diffraction around vegetation patches. This is quite important when the wave height behind the vegetation patch is of importance. Subsequently two practical engineering applications based on the developed SWAN model developed are presented. One is about the effectiveness of mangroves in attenuating cyclone-induced waves including a quantitative case study for a mangrove island near the upcoming Dhamra Port in Orissa, India. The other is about wave attenuation by a field of willows in front of a river dike in the Dutch Noordwaard polder. In both cases, wave attenuations in extreme conditions are calculated and some conclusions are drawn. The first focuses on wave propagation with different vegetation factors and hydraulic conditions in a practical topography while the latter focuses on the capability of a willow field to attenuate waves in a specific condition and discusses uncertainties and model limitations which can be also very important to a practical design.

    Finally, each model developed in this thesis is evaluated in terms of its applicability. In practice, many effects related to wave dissipation over vegetation fields have to be considered. In general, the 2DH model in this study (SWAN) is most suitable to a practical application compared to the other models developed in this thesis (2DV and 3D model) when applied to fields due to computational resources needed and applicability to wave characteristics. However, the 2DH model has more limitations than the other models. For instance, the model has difficulties in modeling in a situation of strongly varying topography. Often cliffs, which are common and important features of most salt marshes, can be seen at the edge of the salt marsh and experiencing erosion. Thus it is important to know the hydrodynamics around the cliff, not only in terms of wave dissipation but also velocity fields, pressures to the cliff and forces to the cylinders, in view of eco-engineering and sustainable coastal management applications. The 2DV model is capable of producing these results. In the 3D model, wake interference which is important to obtain the bulk drag coefficient in a multiple cylinder field, is calculated. Therefore, it is important to use different models depending on the purpose of the calculations.

    Practical models and a relationship between the bulk drag coefficient and wave parameters are developed in this thesis, but further investigation is still necessary on wave dissipation over vegetation fields. Specifically for a better understanding of the effective bulk drag coefficient, experimental flume studies with a wide range of parameters are highly recommended since there are not enough data for such an analysis.


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