1. Targeted Retention of Contaminated Sediment in a Green Flood Retention Reservoir Development of an Integrated Management Strategy for Green Flood Retention Reservoirs and Polders Sven Wurms 4 th international Symposium on Flood Defence Managing Flood Risk, Reliability and Vulnerability Toronto, Ontario, Canada, May 6–8, 2008

2. Motivation Emission conflict situation! Land use (environmental damage/ loss of values…) Immission Integrated management strategy for green flood retention reservoirs, polders and floodplains Quantity/ quality of deposited sediments? Factors influencing deposition in retention reservoirs as well as on floodplains? Flood event Erosion mobilisation of pollutants Deposition floodplains & retention reservoirs

3. Overview 1.Aim 2.Procedure 3.Numerical model “Flood retention reservoir Horchheim” 4.Boundary conditions and scenarios 5.Reservoir sedimentation 6.Conclusions

4. Aim Integrated management strategy 1.Quantity and spatial distribution of reservoir sedimentation 2.Can this be influenced by modified operation rules (outflow discharge) modified reservoir design? maximum retention (enhance situation downstream) minimum retention (enhance situation within reservoir) Options concerning deposition of potentially contaminated sediments in green flood retention reservoirs spatially targeted retention (enhance situation within reservoir) retain present situation and adapt land use

5. 1. Flow 2D-numerical modelling (TELEMAC-2D) of filling and emptying phase present conditions 2. Transport 2D-numerical modelling (SUBIEF-2D) of suspended sediment transport sediment is considered to be medium of conservative contaminant transport (grain diameter: 20 µm) only sedimentation, no erosion Green flood retention reservoir Horchheim 20-, 50- and 100- years flood events 3. Mass balance/ deposition patterns Long term accumulation of reservoir sedimentation (100 years) Procedure modified operation rules modified reservoir design

6. Numerical model “Flood retention reservoir Horchheim” ordinary flood retention volume1.16*10 6 m³ area590000 m 2 HQ 100 (inflow gauge)43.7 m³/s 21849 elements l min = 1.25m/ l max = 23 m k st river bed20 m 1/3 /s k st agricultural land25 m 1/3 /s k st grassland20 m 1/3 /s k st developed area10 m 1/3 /s

7. Boundary conditions and scenarios Flow BC: Two sets of hydrographs (5 h and 35 h time of rise) Transport BC: Assumption: Linear relation between inflow discharge and suspended sediment concentration with a maximum of 1 g/l.

8. scenariooperation rulereservoir design present conditionsconstant outflow (30 m³/s)present condition modified operation rules reduced outflow to use entire ordinary retention volume and maximise deposition present condition modified reservoir design constant outflow (30 m³/s) modified design for spatially targeted deposition HQ time of rise present conditions/ modified designmodified operation rules outflow [m³/s] volume [m³] duration [h] outflow [m³/s] volume [m³] duration [h] 205 h 30 798048.3 1.16 * 10 6 98 505 h110940141276 1005 h2605802115.465

9. Reservoir sedimentation – present conditions 5 h rise 35 h rise Deposition accumulated over 100 years 5107sediment inflow [t]13629 145deposition [t]535 2.8deposition [%]3.9

10. Reservoir sedimentation – modified operation rules 5 h rise 35 h rise 10489sediment inflow [t]21218 5107deposition [t]5737 48.7deposition [%]27.0 Deposition accumulated over 100 years

11. present conditionsmodified operation rules 5 h rise

12. Reservoir sedimentation – modified reservoir design present conditions 5 h rise modified design 5 h rise 5107sediment inflow [t]5107 145deposition [t]163 2.8deposition [%]3.2 Deposition accumulated over 100 years V mod = 12500 m³

13. 20 µm 40 µm 80 µm 150 µm Sediment deposition Grain size [µm] inflow [t] deposition [t] 202560 115 (4,5%) 402560 233 (9,1%) 802560 535 (20,9%) 1502560 761 (29,7%) Deposition height [m] HQ 50 35 h time of rise Reservoir sedimentation – varying grain size

14. Conclusions Knowledge of deposited masses of potentially contaminated sediments provides a basis for an integrated management strategy for green flood retention reservoirs Possibilities of taking influence on sedimentation in green flood retention reservoirs could be shown Maximizing/ minimizing of reservoir sedimentation can be done efficiently by modifying operation rules Deposition pattern can be influenced up to a certain extent by modifying reservoir design Numerical transport simulations should be performed for every grain size of interest due to varying sedimentation pattern

15. Related works presented on ISFD 2008 Christoffels, E. et al. An Integrated Management Strategy for Green Flood Retention Reservoirs, Polders and Floodplains - Taking account of contaminants (Poster) Schönau, S. et al. Erosion and Sediment Yield Estimation for Flood Protection (Presentation)

16. Thank you very much for your attention!