Deltares - R&D Highlights 2009

Contact: aline.telinde@deltares.nl +31 (0)88 335 7761 Future fl ood risk in the Rhine basin Climate change will increase winter precipitation and, in combination with earlier snowmelt, cause a shiſt in peak discharge in the Rhine basin from spring to winter. This will probably lead to an increase in the frequency and magnitude of extreme fl oods. It is expected that the probability of fl ooding in the Rhine basin in 2050 will be two to fi ve times as high as it is today, assuming that no additional fl ood management measures are implemented. Population growth and rising property values in fl ood-prone areas further enhance future fl ood risk. Flood risk projections involve numerous uncertainties. For example, extreme value analyses of fl ood probabilities in 2050 are diffi cult to make and diff erent methods are currently being debated. Furthermore, there are no projections available for the basinwide potential damage. Extreme value plots of yearly maximum discharges of the Rhine at the German-Dutch border. 1000 years of daily meteorological input data obtained by a weather generator served as input for the rainfall-runoff model 68

The central aim of this research is to focus on the probability of extreme fl ood events and to simulate the eff ect of climate change on future fl ood peaks in the Rhine basin. This information can be used to develop and evaluate cross-boundary fl ood management strategies. Recent scientifi c literature has described the need to move away from traditional fl ood frequency analysis, since climate change undermines the basic assumption of stationarity. We therefore adopted a process-based approach including diff erent climate change projections, a rainfall generator, and hydrological (daily) and hydrodynamic (hourly) modelling, the aim being to estimate changes in low-probability fl ood peaks and the impact of measures. The embankments along the Lower Rhine in Germany and in the Netherlands have very high safety norms of 1/500 and 1/1250 years, respectively. Our results showed that, in the most extreme climate change scenario, associated design discharges increase by 17%. Due to the diff erence in safety levels, however, upstream fl ooding in Germany may occur, reducing peak discharges downstream in the Netherlands by up to 15%. The only eff ective measure at the basin-wide scale that can reduce fl ood probability is drastic dike heightening, while measures such as extra retention basins, reforestation and bypasses can be benefi cial at the local scale. However, in a risk-based approach, more could be gained by damage reduction than fl ood defence measures in terms of reducing fl ood risk. Calculations to estimate potential damage in the Rhine basin are in progress. The Rhine at Cologne Further reading A.H. te Linde, J.C.J.H. Aerts et al, Simulating low probability peak discharges for the Rhine basin using resampled climate modeling data. Water Resources Research, 46 (2010) W03512 Finance BSIK KvR Climate changes Spatial Planning. A7 ACER. Adaptive capacity to extreme events in the Rhine basin 69

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