Presentation by Willem Bodde (Witteveen & Bos) at the XBeach X (10th Year Anniversary) Conference, during Delft Software Days - Edition 2017. Friday, 3 November 2017, Delft.

1. XBeachX Conference – 3 November 2017
Willem Bodde – Witteveen+Bos (NL)
Aline Kaji – Witteveen+Bos (NL)
Leonie Straatsma – Witteveen+Bos (NL)
Tijs Cornu – Witteveen+Bos (BE)
Robert McCall, Deltares (NL)
Formal safety assessment of the
Belgian coast using XBeach and
SWASH

2. 2
Laag duin De Panne
1
255

3. Project details
˗ Client: Coastal Division (MDK)
˗ Belgian + Dutch cooperation
˗ 14 months (4 phases)
· January 2016 – March 2017
˗ Extensive field visit
˗ Online GIS system
3

4. Project background
˗ Safety assessment of the Belgian coast
· 1000 year storm
· Dune erosion < predefined safety line
· Overtopping < 1 l/s/m
˗ For the first time: state-of-the-art process-based models:
· XBeach (dune/beach erosion)
· SWASH (overtopping + empirical formulas)
4

5. Content of this presentation
˗ Results and lessons learned from a process-based approach for a complete
formal safety assessment
˗ Recommendations towards a formal safety assessment with XBeach and SWASH
5

6. ˗ 2D/1D SWASH
· Roughly 4000 models
· Weakest cross-sections per
coastal section
Methodology
˗ Methodology set up by Flanders Hydraulics (Suzuki et al, 2016)
˗ Optimisations during the project by Witteveen+Bos and team of experts
˗ 2D XBeach
· 24 model domains covering entire coastline
˗ 1-10 km alongshore (dx=1-30m)
˗ 1-2 km cross-shore (dx=2-20m)
˗ Including structures
6
XBeach 2D
Dune/beach
erosion
SWASH 2D/1D
Wave
overtopping
Dune safety
assessment
Dike/low dune
safety assessment
Low dune

7. Methodology: XBeach
˗ Domain and bathymetry: lateral and seaward extensions
˗ Boundary conditions – SWAN - Hs, Tp, water level, duration 45 hours
˗ Model settings: WTI (Deltares, 2015)
˗ Groundhog Day Release
˗ No parallel computing
7

8. Safety assessment with XBeach 2D
8

9. 2D effects: net sand loss in cross-shore profiles
˗ No cross-shore sediment balance at curved sections
˗ Example: Het Zwin
· Stronger erosion towards the east, deposition towards the west
9

10. Methodology: SWASH
10
Model setup
• Profiles XBeach
• Boundary
conditions
2D calibration
• Conditions at
-5 m TAW
1D calibration
• Conditions at
toe
Overtopping
computations
• Average wave
overtopping
“toe”
Safetyline
Q
Model bathymetry:
- Post-storm profile (XBeach)
- Dike profile (if applicable)
Simplified to 1D, taking into account
directional spreading

11. Results – example dike section
11
Overtopping is highly dependent on the wave train
SWASH results are stochastic

12. Results – Overtopping computations
12
Calculations where formula
is not applicable (dunes, etc)
In general: Good agreement
between SWASH and empirical
formulas
Calculations where formula
is not fully applicable (wide
beach area)

13. Lessons learned
- Identify weak spots better
- Computational effort
- XBeach: 1-6 weeks/model
- SWASH: >4000 models for only 160 sections
- Largely automated, but QA of each model/section still required
- Stochastic versus deterministic approach
- Very good local knowledge is still required (field visit or local experts)
- Cooperation with client/coastal managers is very important for success
13

14. Recommendations towards future formal safety assessments
- Apply at pilot locations (e.g. Scheveningen/Katwijk/Slufter), compare with current empirical
methods
- Increase reliability of XBeach, e.g. parallel computing, validation of settings (jetfac, swrunup)
- Further validate with erosion measurements during storms (e.g. Quick Response Teams)
- Generate standardised models for the North Sea coast
- Provide unambiguous sets of guidelines
- Modelling
- Safety assessment
- For specific (complex) locations, e.g. 2D flooding/storage analysis or 2D wave overtopping
computations are required
14
Wenduine

15. www.witteveenbos.com
Thank you!
Do ‘fancy’ models lead to a better safety
assessment?