4 th International Symposium on Flood Defence, 6 th – 8 th May 2008, Toronto, Canada Efficiency of distributed flood mitigation measures at watershed scale S.Chennu a, J.-M.Grésillon a, J.-B.Faure a, E.Leblois a, C.Poulard a, D.Dartus b a Cemagref, Lyon, France b IMFT, Toulouse, France

2 4th International symposium on flood defence, May 2008, Toronto Best Practices on Flood Prevention, Protection and Mitigation, 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 Flood management principals Source: Gilard, Cemagref

3 4th International symposium on flood defence, May 2008, Toronto What is the best strategy to protect dispersed zones of interest ? Watershed scale : Complex structure  Drainage network, topography  Dispersed zones of interest Uncoordinated flood defence transfers the problem from one region to another Test and identify efficient flood mitigation strategies at watershed scale via modelling Appropriate strategy : 1.Retaining 2. Storing3. Draining Flood management principals

4 4th International symposium on flood defence, May 2008, Toronto 1.Approach Models Watershed 2.Application Calibration of models Construction of hydrological regime Dispersed mitigation measures 3.Results and discussions 4.Conclusions Outline

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

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

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

8 4th International symposium on flood defence, May 2008, Toronto 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) Lateral surface runoffs along the watershed slope Application : Calibration Hydraulic model : route river flow

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

10 4th International symposium on flood defence, May 2008, Toronto Application : Calibration Time Discharge (m 3 /s) Inflow design flood But at watershed scale : Rainfall heterogeneity, Initial saturation condition, Drainage network Hydrological regime scale Simulated spatial rainfall fall inputs to construct a reference regime (Q ref )

11 4th International symposium on flood defence, May 2008, Toronto Application : Construction of hydrological regime  4 control points representing zones of interest Charbonnières amont Craponne Taffignon Charbonnières aval  Reference regime (Q ref = no dams) from stochastic rainfall scenarios of the rainfall simulator Q ref T (yr) Q (m 3 /s) T (yr) Q (m 3 /s) T (yr) Q (m 3 /s) T (yr) Q (m 3 /s)  Introduction of dry dams for flood peak attenuation(Q dam ) Q dam

12 4th International symposium on flood defence, May 2008, Toronto Application : Dispersed mitigation measures Inflow design hydrograph : 10 yr, 50 yr, 100 yr ……. Outlet Spillway Normal flow Dry dams High flow Q dam Outlet dimension: When the dam is full, outflow discharge(Q dam ) – Q 2 (m 3 /s), Q 10 (m 3 /s)……. Q dam Dry dam

13 4th International symposium on flood defence, May 2008, Toronto Application : Dispersed mitigation measures Constant volume & equal weight for each placement Outlet dimension : Q dam = 2 yrs return period (m 3 /s) = storage volume of individual dams ~ ~ Upstream dams Intermediate dams Downstream dams

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

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

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

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

18 4th International symposium on flood defence, May 2008, Toronto Results and Discussions : Spatial evaluation Mitigation factor : M =  (Q ref - Q dam ) / Q ref * l i / L Dry dams 1 to 6 (Upstream) 7 to 9 (Intermediate) 10 to 11 (Downstream) 1 to 11 (All dams) Mitigation factor 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 For a rare event (100 years return period)

19 4th International symposium on flood defence, May 2008, Toronto 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

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