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DSD-INT 2019 Modelling Human Interventions and Salt Marsh Dynamics in the Wadden Sea - Siemes

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Presentation by Rutger Siemes, HKV Lijn in Water BV, The Netherlands, at the Delft3D and XBeach User Day: Coastal morphodynamics, during Delft Software Days - Edition 2019. Wednesday, 13 November 2019, Delft.

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DSD-INT 2019 Modelling Human Interventions and Salt Marsh Dynamics in the Wadden Sea - Siemes

  1. 1. MSc thesis: Modelling Human Interventions and Salt Marsh Dynamics in the Wadden Sea 13 November Rutger Siemes
  2. 2. Content • Introduction • Model description • Methods & Results of: – Hydrodynamics – Morphodyanmics – Structures • Discussion
  3. 3. Introduction • Salt marsh development • Why stimulate salt marsh growth? • How do we stimulate salt marsh growth • Why we used Delft3D FM Schematization of a growing marsh (A), dynamic marsh (B) and eroding marsh (C) (Reed, van Wesenbeeck et al. 2018).
  4. 4. Introduction: Study area Salt marsh which started eroding after maintenance of nearby structures was stopped Top view of study area; Remains of sedimentation fields are still visible on the tidal flat
  5. 5. Introduction: Research aim Study how engineering solutions can be used to steer the morphological development of salt marshes. • Analysing the dynamic processes motivating morphological development on and around an eroding salt marsh and; • Studying the impact of human interventions on these processes.
  6. 6. Model domain The area of the model domains and their main characteristics. Coupled flow-wave module
  7. 7. Simulated period Simulating 1 month: Oktober 2017 – Contains both storm conditions and daily conditions Observed water level at measurement station ‘Lauwersoog’ during the simulated period.
  8. 8. Hydrodynamics: Hydrodynamic forcing • Flow module – Boundary forcing: Dutch Continentinental shelft model (D3D-FM). – Surface forcing: ‘High Resolution Limited-Area Model’ (HiRLAM). Hourly spatially varying wind vectors and air pressure. • Wave module – Boundary forcing: Estimated based on wind vectors and wave heights at measurement station. – Surface forcing: From flow module, thus HiRLAM
  9. 9. Hydrodynamics: Validation Comparison between observed and modelled water levels at measurement station ‘Lauwersoog’ over time (left) and in a scatter plot (right) Comparison between observed and modelled wave heights at measurement station ‘Wierummer Wad’ over time (left) and in a scatter plot (right)
  10. 10. Morphodyanmics: set up • Sediment transport formulation: Van Rijn (1993) – Both bed load and suspended load transport – Cohesive sediment: mud fraction (𝑃 𝑀)
  11. 11. Morphodyanmics: sensitivity analysis A sensitivity analysis with the length-averaged volumetric change of the salt marsh as indicator. This is performed for the significant wave height along the open boundary (𝐻𝑠,𝐵𝐶), the Manning coefficient (M), mud fraction (𝑝 𝑚) and median grain size (𝐷50).
  12. 12. Morphodyanmics: Reference model Modelled bed-level change (m) at the study area (1 month).
  13. 13. Structures: implementation Name Structure form as referred to in Figure 3 Description Base 1 1 A groyne of 2.5m high. Wave attenuation by transmission coefficient. Base 2 1 A groyne of 2.5m high. Wave attenuation represented as over a dam. Long 1 2 A groyne of 2.5m high. Wave attenuation by transmission coefficient. Long 2 2 Wave attenuation by transmission coefficient. Flow module not affected. Combi 1 1+3 Combination of Base 1 and a traditional sedimentation field. Wave attenuation represented as over a sheet Combi 2 2+4 Combination of Long 1 and a proposed sedimentation field. Wave attenuation by transmission coefficient Locations of the structures implemented
  14. 14. Structure: results Change in bed-level within the study area for the various structures.
  15. 15. Structure: results Daily and length averaged volumetric change (a) on the salt marsh, (b) on the salt marsh front and (c) on the tidal flat.
  16. 16. Discussion • Hydrodynamics – Flow module performs good – Wave module performs okay • Morphodynamics – Processes robust and credible – Magnitude of erosion and accretion uncertain, calibration required – No accurate representation cliff erosion • Structures – To stimulate salt marsh growth, sedimentation fields are recommended
  17. 17. Thank you for your attention

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