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Targeted Retention of Contaminated Sediment in a Green Flood Retention Reservoir

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

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Targeted Retention of Contaminated Sediment in a Green Flood Retention Reservoir

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  1. 4th international Symposium on Flood Defence Managing Flood Risk, Reliability and Vulnerability Toronto, Ontario, Canada, May 6–8, 2008 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

  2. Motivation • 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 Immission Emission Erosion mobilisation of pollutants Deposition floodplains & retention reservoirs conflict situation! Land use(environmental damage/ loss of values…)

  3. Overview • Aim • Procedure • Numerical model “Flood retention reservoir Horchheim” • Boundary conditions and scenarios • Reservoir sedimentation • Conclusions

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

  5. Procedure 1. Flow 2D-numerical modelling (TELEMAC-2D) of filling and emptying phase • present conditions • modified operation • rules • modified reservoir • design Green flood retention reservoir Horchheim 20-, 50- and 100- years flood events 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 3. Mass balance/ deposition patterns Long term accumulation of reservoir sedimentation (100 years)

  6. Numerical model “Flood retention reservoir Horchheim” 21849 elements lmin = 1.25m/ lmax = 23 m • kstriver bed 20 m1/3/s • kstagricultural land 25 m1/3/s • kstgrassland 20 m1/3/s • kstdeveloped area 10 m1/3/s • ordinary flood • retention volume 1.16*106 m³ • area 590000 m2 • HQ100 (inflow gauge) 43.7 m³/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. Reservoir sedimentation – present conditions Deposition accumulated over 100 years 35 h rise 5 h rise

  9. Reservoir sedimentation – modified operation rules Deposition accumulated over 100 years 35 h rise 5 h rise

  10. present conditions modified operation rules 5 h rise

  11. Reservoir sedimentation – modified reservoir design Deposition accumulated over 100 years Vmod = 12500 m³ present conditions 5 h rise modified design 5 h rise

  12. Reservoir sedimentation – varying grain size 20 µm 40 µm HQ 50 35 h time of rise 80 µm 150 µm Deposition height [m]

  13. 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

  14. 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)

  15. Thank you very much for your attention!

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