University of Twente Student Theses
Modelling wave attenuation by salt marsh vegetation : a porous layer approach with CoastalFOAM
Heijer, W.J. den (2024) Modelling wave attenuation by salt marsh vegetation : a porous layer approach with CoastalFOAM.
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| Abstract: | Salt marshes known for their wave attenuation capabilities have garnered attention because they can protect low-lying areas sustainably from larger and more powerful waves resulting from extreme weather events. CoastalFOAM is a computational fluid dynamics model (CFD), which is able to simulate sea waves in a numerical wave flume. The model’s capability to accurately compute wave overtopping and wave loads enables the assessment of salt marsh vegetation in front of dikes as a strategy to reduce these failure mechanisms, potentially leading to lower design requirements for the dikes. However, CoastalFOAM is a 2D vertical model, posing challenges when modelling wave attenuation using the conventional 3D cylinder method. Nevertheless, the model can simulate flow velocity reduction in a stone layer by representing it as a porous layer. This leads to the research question: ’How can a porous layer within CoastalFOAM be effectively used to model wave attenuation during storm conditions by salt marsh vegetation at the foreshore of a dike?’. The porous layer, representing the damping capacity of salt marsh vegetation, can decrease the orbital velocity of sea waves. The Darcy-Forchheimer equation is used to quantify this damping effect. For stone layers, physical characteristics are defined by median grain size (D50) and porosity (np). Assuming a comparable scale for D50 and pore size, D50 can be equated to the distance between grass stems. Porosity is considered the ratio of total volume from the ground to stem height to the same volume excluding stems. Although the physical parameters can now be determined, the Darcy-Forchheimer equation includes two calibration parameters: α, β. The values of α and β determine the relative importance of laminar and turbulent flow within the equation. This importance is assessed using the porous Reynolds number (Rep). A flume study on wave attenuation by a salt marsh is used to calibrate the model and run sensitivity tests. Three tests are selected: the base case, a tests with a higher wave and a test with lower vegetation height. The value of (Rep) is higher than 300 for the tests, which indicates that α is negligible, leaving only β to be calibrated. With all parameters of the Darcy-Forchheimer equation known, the calibration of β remains. The optimal value for β equals 4.84 for the base case. The model accurately replicates significant wave height reduction, peak wave period changes, and wave spectra for the base case, demonstrating agreement with the physical flume experiment. However, when simulating higher waves in the second test, the model overestimated wave height reduction, showing a reduction of 23.6% instead of the observed 13.9% reduction. This difference could be attributed to the model’s assumption of rigid grass stems, as real-world grass demonstrates bending. To address this, deriving a relationship between the calibration parameter β and stem bending could improve model applicability. The mowed vegetation test, with a height less than 10% of the original vegetation height, underestimated wave reduction, showing a reduction of 2.5% instead of the observed 7.5% reduction. This difference might come from either the incorporation of bottom friction within the porous layer or the model’s inability to accurately capture plant bending. Decoupling bottom friction and the porous layer, potentially using a gradient boundary condition or two separate porous layers, could resolve the bottom friction issue. Although the model accurately represents wave attenuation for the base case, it currently faces limitations in accounting for variations in hydrodynamic conditions or vegetation characteristics without recalibration. This method can be extended to mangroves or willow forests, which align better with the rigid stem assumption. However, factors such as vertical layers, uniform stem distribution, and emergent vegetation instead of submerged can introduce additional complexities. |
| Item Type: | Essay (Master) |
| Clients: | Royal HaskoningDHV, Rotterdam, Netherlands |
| Faculty: | ET: Engineering Technology |
| Subject: | 56 civil engineering |
| Programme: | Civil Engineering and Management MSc (60026) |
| Link to this item: | https://purl.utwente.nl/essays/103512 |
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