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Presentation on theme: "FLOOD REGIMES OF MID-SIZED AND MIXED LAND-USE CATCHMENTS: CAN WE ASSESS THE URBAN CONTRIBUTION ? B. Radojevic (1), P. Breil (2), B. Chocat (3) (1) UNESCO."— Presentation transcript:

1 FLOOD REGIMES OF MID-SIZED AND MIXED LAND-USE CATCHMENTS: CAN WE ASSESS THE URBAN CONTRIBUTION ? B. Radojevic (1), P. Breil (2), B. Chocat (3) (1) UNESCO (2) CEMAGREF Lyon (3) URGC – INSA Lyon International Symposium on Flood Defense, Toronto, Canada, May 6-8, 2008

2 Urban sprawling: a world wide trend (UNEP, 2003 ) Urban growth of Lyon city (with courtesy from Lyon city council, 2005)

3 Data from French National Institute for Statistics and Economic Studies Urban area Urban unit Periurban area Population growth in Lyon

4 What could be the impact of land-use change on flood discharge? Recurrence interval (years) x 4 x % imperviousness x 4 x 2 The ten years flood is doubled both for : -a change of 70% from forest to vineyard land use - an impervious rate of 20% Peak flood ratio of Post to Pre land use change Adapted from GALEA et al., 1993 Adapted from HOLLIS, Recurrence interval (years) Rural change Urban change 50 %

5 Flood event in Yzeron basin

6 Outline of the presentation  Objective of the study  Study area  Method  Results  Conclusion

7 Objective of the study Vulnerability in terms of flood frequency Hazard In terms of flood frequency > < Flood risk concept The flood risk meets the local objective when the hazard frequency is smaller than the vulnerability frequency and vice versa Each aspect of the flood risk can be expressed as a recurrence interval in year units

8 Study Area

9 Land-use in the l’Yzeron basin

10 Instrumentation within the basin Craponne Taffignon

11 Increase in flood frequency Years ’70’Years ’90’ m3/s

12 Stationary Test – number of floods (according to Lang, 1995)

13 Stationary test on number of floods - rural part (according to Lang, 1995)

14 Stationarity of the number of max. daily rainfall at Bron station

15 Daily max. rainfall regime for the rain gauge Bron Daily intensity:  The most intense in the ’90’  The lowest in the ’70’

16 Method  Built a semi-distributed hydrological model with the land use on the 90’  Use the rainfall and stream-flow data to calibrate the 90’ model  Validate the 90’ model  Built a semi-distributed hydrological model with the land use on the 70’  Use the 90’ fitted parameters and the 90’ rainfall series to simulate the 70’ stream flows  Make projection of the land use evolution and simulate the stream flow evolution- virtual series

17 Method état urba. débits corresp. pluies 70 débits corresp. pluies 90 simu. corresp. pluies 90 années 70 oui (1) (amont) non oui (3) années 90 non oui (2) oui (4) influence urbanisation influence variabilité pluies entre (1) et (2) oui entre (1) et (3) non oui entre (1) et (4) oui entre (2) et (3) oui non entre (2) et (4) non entre (3) et (4) oui non Model quality Influence of rainfall Impact of urbanisation

18 Model development  Dividing the l’Yzeron basin in hydrological units  Calibration of the rainfall – runoff model CANOE  Validation of CANOE

19 Land-use change in the l’Yzeron basin

20 The land use change over 17 years forestFarming-grass landperiurbanurbanmainly forest

21 Land use change in ‘70 and ‘90 (grid based estimation) Upstream (Craponne) Total basin (Taffignon)

22 Drainage network

23 Definition of hydrological units  if the % of urban grid of sub-basin is:  higher 50% option ‘strictely urban’ of CANOE was applied  if the % of périurban grid of sub-basin is:  higher 50% option ‘urbain-rural’ of CANOE was applied  if the % of rurale grid of sub-basin is:  higher 50% option ‘strictely rural’ of CANOE was applied

24 70’ land use model90’ land use model Distribution of hydrological units

25 Semi-distributed Rainfall-Runoff Model CANOE Production function Runoff coef. Impervious areas Direct runoff to water courses Permeable areas ( Forest, grassland,..) Transfer function Nash cascade Impervious areas Un-Direct runoff to water courses Production function Runoff coef. Production function Horton’s infilt. law Transfer function Nash cascade Transfer function Linear reservoir 3 hydrographs summation

26 Results of Calibration – downstream urban part - Taffignon Automne Winter Spring Summer

27 Description of comparison Influence of urbanisation Comparison between simulated runoff (land use 1970) with observed rainfall series of 1990 (Taffignon) and simulated runoff (land use 1990) with observed rainfall series of 1990 (Taffignon) Model Quality Comparison between simulated and observed runoff series (Taffignon, Craponne)

28 Characteristic of selected runoff for description of flood regime: QCX(d) QCX (d) are discharge values continuously overpassed for selected durations. Shorter is the duration, higher is the discharge and vice versa. QCX(d) allow to describe the pattern of floods. Selected durations 1h, 3h, 6h, 12h and 24h time t 1h 6h 1h 3h 12h 24h QCX1h m3/s Threshold level for duration d QCX(d)

29 Model validation – rural part Null hypothesis H0 tested : The simulated population is equivalent to the observed population? “H0 accepted” “H0 rejected” “H0 accepted” Only large durations (24hours) are rejected from the statistical test. Model is validated for the flood regimes simulation

30 Model validation – urban + rural parts Null hypothesis H0 tested : Is the simulated population equivalent to the observed population? “H0 acceptable” “H0 accepted”“H0 rejected” ok

31 Flood regimes change between ’70’ and ’90’ (urban units on 1970: 6% and 1990: 19%) “H0 accepted” “H0 rejected” Null hypothesis H0 tested : The 90’population is equivalent with the 70’ population? Only the small floods are affected T=1

32 Flood regimes change for future development of 24% and 33% urban area Null hypothesis H0 tested : Is the future population equivalent with the present population? “H0 accepted”“H0 rejected” Imperviousness rate of 24 %Imperviousness rate of 33 %

33 Flood hazard evolution From 6 to 19% of urbanization only small floods are affected, only T=1year. Over 20% of urbanization, also large floods are affected. It means that both transfer and production were affected % urbanized (simulated) 24 % urbanized (simulated) % urbanized (observed) Full bank flow +14% +6%

34 Conclusion on flood hazard evolution  Model results are sensitive to an increase of urbanisation by 13% only (Taffignon station).It is detected over 6%  For rural part of the basin (2/3 of the total basin): No urban influence (even small floods are not effected). For urban part mainly floods with a small return period are affected.  Simulation results indicate the increase in flood frequency does not result only from the land use change. It means the rainfall regime is a major factor but …  Expected urban development on 2025 should have a very sensitive effect on flood peak increase. The effect on large floods would be very sensitive for 33% urbanisation.  Unexpected compensation effects of the periurban growth exists and should be considered as a mitigating potential if managed.

35 Flooded area boundaries are determined from a DEM analysis considering at least all grid cells connected to a water course with no more than a given height (e.g.1 meter) above the full bank altitude DEM- Digital Elevation Model Flood vulnerability assessment (I)

36 Flooded areas can be split into vulnerability categories from forest, grassland and farming, periurban and urban types Flood vulnerability assessment (II)

37 As a consequence of the land use change in the vicinity of the stream corridor the average acceptable flooding return period has doubled from years 79 to 96; meaning the need for protection. Flood vulnerability Evolution

38 Conclusion & Perspectives  The urban development increases upstream flood frequencies.  The periurban development has sensitive effect on large flood frequencies since a 33% urbanized area.  The flood risk is not proportional to imperviousness rate but rather to spatial distribution in mixed land use catchments  Mainly the vulnerability of flooded areas can explain the increase in flood risk. Vulnerability is however manageable under 20 % and should allow to reduce flood risk.  Over 20% urban it seems necessary to have a better characterization of the hydrological functioning of periurban areas, which is not trivial!

39 Thank you !


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