This document summarizes a study that used hydrological modeling and extreme value analysis to simulate flood peak probabilities in the Rhine River basin under current and future climate change scenarios. The study used the GRADE model to generate rainfall-runoff data and simulate the effects of measures like dike heightening and detention areas. The results found that detention areas can significantly reduce flooding depending on the event size, and that climate change is expected to increase the frequency of extreme peak flooding events. The combined effects of climate change and adaptation measures on extreme flood risks in the Rhine basin were analyzed.

1. Simulating flood-peak probability in the Rhine basin and the effect of climate change Institute for Environmental Studies (IVM) Aline te Linde (IVM - VU / Deltares) Jeroen Aerts (IVM - VU) Oxford – October 2, 2008

2. Outline Introduction Method - GRADE Results - Climate change scenarios - Extreme value analysis Conclusions

3. Introduction ACER Recent floods / droughts  major damage Climate change Need for cross-boundary cooperation GOAL: test robustness of new cross-boundary adaptation strategies

4. Rhine basin Length: 1,320 km Area 160,800 km 2 Mean Q 2,206 m 3 /s Maximum observed 12,600 m 3 /s Safety levels vary from 1/200 to 1/1250 58 million inhabitants (10 million flood plain) High economic relevance Flood management strategies since beginning 19 th century

5. Introduction IKSR – Flood Action Plan D – NL Working Group on Floods EU Floods Directive Do not take into account climate change Research available* Assumption – infinite dike height Large uncertainty probability extreme events Do not take into account effect of measures Flood management Climate change * (Kwadijk 1993, 1998; Middelkoop, 2001; Kleinn, 2003, 2005; Te Linde, 2007) Simulate low probability floods, combine impact of climate change impact of dike height

6. Method

7. Method - Hydrological modelling Rainfall - runoff (HBV / VIC) Implementing climate change scenario Landuse change 1D Hydrodynamic model (SOBEK) Measures Dike heightening Dike relocation Landuse change flood plain (friction) Bypass Detention area Flooding (calibrated on 2D model) Field capacity Wilting point

8. GRADE – Generator of rainfall and discharge extremes Developed by Deltares, Waterdienst, KNMI Implement Climate change scenarios Measures X Locations

9. Climate change impact Lobith – mean monthly change Transient run

10. Detention area – flooding

11. Extreme value analysis yearly max. Q – Gumbel fit 100 yrs observed 1000 yrs resampled

12. GEV distribution fit

13. Results

14. Conclusion Method GRADE + extreme value analysis possibility to analyse ensemble of events / bandwidth narrows confidence interval extreme value distribution fit Impact of Detention area  effect strongly depends on event size Climate change  peak events (flooding) expected to occur more frequently Flooding  Q > 12,000 m 3 /s - upstream flooding – lowers max. Q up to 20% (Dike heightening will increase extreme peak discharge downstream) Simulate combined effect

15. This reseach is part of a ‘Climate Changes Spatial Planning’ project Thank you Adaptive Capacity to Extreme events in the Rhine basin (ACER) More information on: www.klimaatvoorruimte.nl (english version) and www.adaptation.nl\acer [email_address]