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Efficiency of Distributed Flood Mitigation Measures at Watershed Scale

This study explores the effectiveness of distributed flood mitigation strategies at the watershed level. It focuses on testing and identifying efficient measures to protect dispersed areas prone to flooding. The approach involves modeling various mitigation strategies on a complex watershed structure to understand the impact of different measures. By calibrating models and simulating physical processes, the research aims to provide insights into the best strategies for flood management at a larger scale. The study emphasizes the need for coordinated development and management actions to tackle flooding challenges effectively.

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Efficiency of Distributed Flood Mitigation Measures at Watershed Scale

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  1. Efficiency of distributed flood mitigation measures at watershed scale S.Chennua, J.-M.Grésillona, J.-B.Faurea,E.Lebloisa, C.Poularda, D.Dartusb a Cemagref, Lyon, France b IMFT, Toulouse, France sandhya.chennu@cemagref.fr 4th International Symposium on Flood Defence, 6th – 8th May 2008, Toronto, Canada

  2. Flood management principals Source: Gilard, Cemagref Best Practices on Flood Prevention, Protection and Mitigation, 2003. Water directors of European Union • From defensive action to risk management • Natural phenomena, human interference should be avoided • Flood strategy should cover the entire river basin • Promote coordinated development and management actions 4th International symposium on flood defence, 6 - 8 May 2008, Toronto

  3. Flood management principals Appropriate strategy : Retaining 2. Storing 3. Draining Watershed scale : Complex structure • Drainage network, topography • Dispersed zones of interest Uncoordinated flood defence transfers the problem from one region to another What is the best strategy to protect dispersed zones of interest ? Test and identify efficient flood mitigation strategies at watershed scale via modelling 4th International symposium on flood defence, 6 - 8 May 2008, Toronto

  4. Outline • Approach • Models • Watershed • Application • Calibration of models • Construction of hydrological regime • Dispersed mitigation measures • Results and discussions • Conclusions 4th International symposium on flood defence, 6 - 8 May 2008, Toronto

  5. Approach : Models Rainfall generator Spatially distributed rainfall Spatially distributed hydrological model (MARINE) Lateral surface runoff θ= θs 1 D hydraulic model (MAGE) River flow θ= θi + Hydraulic structures To simulate physical processes 4th International symposium on flood defence, 6 - 8 May 2008, Toronto

  6. Approach : Yzeron watershed • 150 km², Altitude: 82 m – 915 m • Tributaries: Yzeron and Charbonnières • Densely populated at the downstream • 5 discharge stations and 8 rain gauges 4th International symposium on flood defence, 6 - 8 May 2008, Toronto

  7. Application : Calibration Approximately 9000 rainfall events ( = 1000 years data) ~ Example of a cumulated 24 hours rainfall event Stochastic Rainfall generator : Cemagref, Lyon (Leblois, 2004) Input : 12 years rainfall records from 5 rain gauges Turning Bands Method (Mantoglou and Wilson, 1982) Output : Spatially distributed rainfall fields respecting the statistical characteristics of local rainfall Simulated events : • 3 hour time step • 72 hours • 500 m X 500 m grid 4th International symposium on flood defence, 6 - 8 May 2008, Toronto

  8. Application : Calibration Lateral surface runoffs along the watershed slope Hydraulic model : route river flow MARINE : Event based spatially distributed model IMFT, Toulouse (Estupina-Borrell, V., 2004) Input : Topography, Drainage network, Land use, Soil texture and Soil depth, Soil humidity Observed rainfall : uniformly distributed during calibration Green-Ampt infiltration model (1911) and kinematic wave theory for overland flow (Henderson and Wooding, 1964) 4th International symposium on flood defence, 6 - 8 May 2008, Toronto

  9. Application : Calibration Calibrated November 1990 event Routes the simulated lateral surface runoffs along the drainage network to the outlet of the watershed MAGE : 1 D hydraulic model ,Cemagref, Lyon (Giraud, F.M. et al. 1997) Input : Topography of drainage network, Roughness coefficient + Simulated lateral surface runoffs Shallow water Saint Venant (1870) and Manning – Strickler (1923) head loss equations 4th International symposium on flood defence, 6 - 8 May 2008, Toronto

  10. Application : Calibration Inflow design flood Discharge (m3/s) Time Hydrological regime scale But at watershed scale : Rainfall heterogeneity, Initial saturation condition, Drainage network Simulated spatial rainfall fall inputs to construct a reference regime (Qref) 4th International symposium on flood defence, 6 - 8 May 2008, Toronto

  11. Application : Construction of hydrological regime Qref Charbonnières amont Q (m3/s) T (yr) Charbonnières aval Q (m3/s) Q (m3/s) Q (m3/s) Taffignon T (yr) T (yr) T (yr) Qdam Craponne • 4 control points representing zones of interest • Reference regime (Qref= no dams) from stochastic rainfall scenarios of the rainfall simulator • Introduction of dry dams for flood peak attenuation(Qdam) 4th International symposium on flood defence, 6 - 8 May 2008, Toronto

  12. Application : Dispersed mitigation measures Dry dams High flow Qdam Normal flow Spillway Dry dam Qdam Outlet • Inflow design hydrograph: 10 yr, 50 yr, 100 yr ……. • Outlet dimension: When the dam is full, outflow discharge(Qdam) • Q2 (m3/s), Q10 (m3/s)……. 4th International symposium on flood defence, 6 - 8 May 2008, Toronto

  13. Application : Dispersed mitigation measures Upstream dams Intermediate dams Downstream dams ~ ~ = storage volume of individual dams Constant volume & equal weight for each placement Outlet dimension : Qdam = 2 yrs return period (m3/s) 4th International symposium on flood defence, 6 - 8 May 2008, Toronto

  14. Results and Discussions : Local evaluation Charbonnières amont 4th International symposium on flood defence, 6 - 8 May 2008, Toronto

  15. Results and Discussions : Local evaluation Charbonnières aval 4th International symposium on flood defence, 6 - 8 May 2008, Toronto

  16. Results and Discussions : Local evaluation Craponne 4th International symposium on flood defence, 6 - 8 May 2008, Toronto

  17. Results and Discussions : Local evaluation Taffignon 4th International symposium on flood defence, 6 - 8 May 2008, Toronto

  18. Results and Discussions : Spatial evaluation Mitigation factor : M =  (Qref - Qdam) / Qref * li / L For a rare event (100 years return period) • Maximum mitigation assured in the presence of all the dams (high storage volume) • Upstream dams protect a large region compared the intermediate and downstream dams • Downstream dams protect only a limited reach length 4th International symposium on flood defence, 6 - 8 May 2008, Toronto

  19. Conclusions • Dispersed flood management strategy to protect dispersed zones of interest • Importance of working at regime scale and not just test one design flood • Well defined efficiency range of mitigation strategy beyond which no attenuation is possible • Upstream mitigation measures protect a large region compared to downstream measures • For the given event the upstream dry dams assured a major contribution to flood mitigation 4th International symposium on flood defence, 6 - 8 May 2008, Toronto

  20. Thank you for your attention 4th International symposium on flood defence, 6 - 8 May 2008, Toronto

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