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 [email protected](2) CEMAGREF Lyon [email protected](3) URGC – INSA Lyon [email protected]International Symposium on Flood Defense, Toronto, Canada, May 6-8, 2008
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FLOOD REGIMES OF MID-SIZED AND MIXED LAND-USE CATCHMENTS: CAN WE ASSESS THE URBAN CONTRIBUTION ?
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 [email protected] (2) CEMAGREF Lyon [email protected] (3) URGC – INSA Lyon [email protected]. - PowerPoint PPT Presentation
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FLOOD REGIMES OF MID-SIZED AND MIXED LAND-USE CATCHMENTS: CAN WE
Imperviousness rate of 24 % Imperviousness rate of 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
1979 - 6% urbanized(simulated)
24 % urbanized(simulated)
1996 - 19% urbanized
(observed)
0
10
20
30
40
50
60
70
80
90
0.1 1 10 100
Flood recurrence interval (in years)
dis
char
ge
(in
m3/
s)
Full bank flow
+14%
+6%
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.
Flooded area boundaries are determined from a DEM analysis considering at least all grid cellsconnected 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)
Flooded areas can be split into vulnerability categories from
forest, grassland and farming , periurban and urban types
Flood vulnerability assessment (II)
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
Land use type
Negociated acceptable flooding return period
Year 79 Year 96 (VC) Vulnerability coeff in years Year 79 Year 96Forest 0.7 1.3 0.5 0.4 0.7
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!