4 th International Symposium on Flood Defence, 6 th – 8 th May 2008, Toronto, Canada Efficiency of distributed flood mitigation measures at watershed scale

4th International Symposium on Flood Defence,6th – 8th May 2008, Toronto, Canada

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

[email protected]

24th International symposium on flood defence, 6 - 8 May 2008, Toronto

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

Flood management principals

Source: Gilard, Cemagref


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. Storing 3. Draining

Flood management principals


1. Approach

• Models

• Watershed

2. Application

• Calibration of models

• Construction of hydrological regime

• Dispersed mitigation measures

3. Results and discussions

4. Conclusions

Outline


Approach : Models

To simulate physical processesRainfall generatorSpatially distributed rainfall

Spatially distributed hydrological model (MARINE)Lateral surface runoff

θ= θs

θ= θi

1 D hydraulic model (MAGE)River flow

+

Hydraulic structures


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


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

In mm0

1 – 30

31 – 60

61 – 90

91 – 120

> 120

Example of a cumulated 24 hours rainfall event

Simulated events : • 3 hour time step• 72 hours• 500 m X 500 m grid


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


Hydraulic model : route river flow


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


Shallow water Saint Venant (1870) and Manning – Strickler (1923) head loss equations

Calibrated November 1990 event



TimeDis

char

ge

(m3 /

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 (Qref)


Application : Construction of hydrological regime

4 control points representing zones of interest

Charbonnières amont

Craponne

Taffignon

Charbonnières aval

Reference regime (Qref = no dams) from stochastic rainfall scenarios of the rainfall simulator

Qref

T (yr)

Q (

m3 /

s)

T (yr)

Q (

m3 /

s)

T (yr)Q

(m

3 /s)

T (yr)

Q (

m3 /

s)

Introduction of dry dams for flood peak attenuation(Qdam)

Qdam


Application : Dispersed mitigation measures

• Inflow design hydrograph : 10 yr, 50 yr, 100 yr …….

Outlet

Spillway

Normal flow

Dry dams

High flow

Qdam

• Outlet dimension: When the dam is full, outflow discharge(Qdam)

– Q2 (m3/s), Q10 (m3/s)…….

Qdam

Dry dam


Application : Dispersed mitigation measures

Constant volume & equal weight for each placement

Outlet dimension : Qdam = 2 yrs return period (m3/s)

= storage volume of individual damsΣ 16 v i= Σ 1

3v i= Σ 12v i

~ ~

Upstream dams

Intermediate dams

Downstream dams


Results and Discussions : Local evaluationCharbonnières amont


Results and Discussions : Local evaluation

Charbonnières aval



Craponne



Taffignon


Results and Discussions : Spatial evaluation

Mitigation factor :M = (Qref - Qdam) / Qref * li / L

Dry dams

1 to 6(Upstream)

7 to 9 (Intermediate

)

10 to 11 (Downstream)

1 to 11 (All dams)

Mitigation factor

0.0225 0.0088 0.0039 0.0348

• 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)


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


Thank you for your attention

Documents

4 th International Symposium on Flood Defence, 6 th – 8 th May 2008, Toronto, Canada Efficiency of distributed flood mitigation measures at watershed scale