Nature-based flood defenses: panacea or illusion

1. Nature-based flood defenses:
panacea or illusion?
Dr. Bregje K. van Wesenbeeck
Bregje.vanWesenbeeck@deltares.nl

2. Asian Tsunami (2004)

3. Hurricane Katrina (2005)

4. Japanese tsunami (2011)

5. Hurricane Sandy (2012)

6. Typhoon Haiyan (2013)

7. Disaster aftermath
Role of ecosystems in reduction of impact
and damage

8. Asian Tsunami (2004)

9. Hurricane Katrina (2005)

10. Japanese tsunami (2011)

11. Hurricane Sandy (2012)

12. Typhoon Haiyan (2013)

13. Transition in flood risk management
Use of natural processes and ecosystem services for
flood defence and flood risk mitigation

14. Type of ecosystems

15. Why?
• Call for sustainability and quality of life
• Hard infrastructure is expensive
• And does not apply everywhere

16. Risk reduction cascade
Spalding et al. 2014
Cumulative interventions
Initial
risk
Residual
risk
Wetlands
Levees/
Flood walls
Building codes/
zoning
Early warning/
Evacuation plans
Structural and non-structural combinations

17. Combinations of structural
More space needed

18. What about risk?

19. Space and risk
Van Wesenbeeck et al. In prep

20. Ready for implementation?
Design
Construction
Maintenance
&
management
State of quantitative engineering knowledge:
• Design (effectiveness)
• Hydraulic
• Geotechnic
• Construct (uncertainty)
• Manage (adaptive)

21. State of knowledge: salt marshes
Model data
Flume data
Field data
Functioning under extreme conditions
Vegetation versus elevation

22. Elevation

23. Elevation clay platform with vegetation

24. Models without vegetation
Smale et al. 2013

25. Testing on realistic scales

26. With extreme conditions

27. With and without vegetation
Moller et al. 2014

28. Modelling extreme conditions woody vegetation
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0.90
1.00
1.10
-20 -10 0 10 20 30 40 50 60 70 80 90 100
x (m)
WaveHeight(m)
0.0
1.0
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Groundlevel(m)NAP
VF000
HS1
HS2
HS3
Van Wesenbeeck et
al. submitted

29. Modelling extreme conditions grassy vegetation
Dijkstra et al. in prep

30. Modelling extreme conditions
• Veg factor: density * diameter * height * Cd
(Mendez and Losada 2004)
• Cd as calibration factor
• However, Cd changes with water levels and wave
characteristics

31. Models: bulk drag
Remmers et al. In prep

32. Future research and developments
• Improve predictability (validated models and generic rules)
• Sensitivity analyses of model parameters
• More controlled tests on realistic scales
• Full parameterization versus quick assessment models

33. Take home
• Ecosystems contribute to reduction of flood risk
• Ecosystems are likely to be most effective in combination with
other measures
• Vegetation matters
• Design rules can be derived but need to be improved
• Long term trends, such as sea level rise, will ask for constant
management of soft protection strategies