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.

1. MSc thesis:
Modelling Human Interventions and Salt
Marsh Dynamics in the Wadden Sea
13 November
Rutger Siemes

2. Content
• Introduction
• Model description
• Methods & Results of:
– Hydrodynamics
– Morphodyanmics
– Structures
• Discussion

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. 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. 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. Model domain
The area of the model domains and their main characteristics.
Coupled flow-wave module

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. 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. 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. Morphodyanmics: set up
• Sediment transport formulation: Van Rijn (1993)
– Both bed load and suspended load transport
– Cohesive sediment: mud fraction (𝑃 𝑀)

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. Morphodyanmics: Reference model
Modelled bed-level change (m) at the study
area (1 month).

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. Structure: results
Change in bed-level within the study area for the various structures.

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. 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. Thank you for your attention