Flood regimes of mid sized and mixed land use catchments can we assess the urban contribution
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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]

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FLOOD REGIMES OF MID-SIZED AND MIXED LAND-USE CATCHMENTS: CAN WE ASSESS THE URBAN CONTRIBUTION ?

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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) [email protected]

(2) CEMAGREF [email protected]

(3) URGC – INSA [email protected]

International Symposium on Flood Defense, Toronto, Canada, May 6-8, 2008


Urban sprawling a world wide trend unep 2003

Urban sprawling: a world wide trend(UNEP, 2003 )

Urban growth of Lyon city

(with courtesy from Lyon city council, 2005)


Flood regimes of mid sized and mixed land use catchments can we assess the urban contribution

Population growth in Lyon

Periurban area

Urban area

Urban unit

Data from French National Institute

for Statistics and Economic Studies


What could be the impact of land use change on flood discharge

2.5

2

1.5

1

0.5

0

What could be the impact of land-use change on flood discharge?

Adapted from GALEA et al., 1993

Rural change

  • 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

100

0.1

1

10

Recurrence interval (years)

Adapted from HOLLIS, 1975

20

20

20

20 % imperviousness

15

15

15

50 %

10

10

10

Urban change

x 4

x 4

5

5

5

4

4

4

x 2

x 2

2

2

2

1

1

1

0.1

0.1

0.1

1

1

1

10

10

10

100

100

100

200

200

200

2

2

2

25

25

25

50

50

50

Recurrence interval (years)


Flood regimes of mid sized and mixed land use catchments can we assess the urban contribution

Flood event in Yzeron basin


Outline of the presentation

Outline of the presentation

  • Objective of the study

  • Study area

  • Method

  • Results

  • Conclusion


Objective of the study

<

Vulnerability

in terms of flood frequency

Hazard

In terms of flood frequency

>

Objective of the study

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


Flood regimes of mid sized and mixed land use catchments can we assess the urban contribution

Study Area


Land use in the l yzeron basin

Land-use in the l’Yzeron basin


Instrumentation within the basin

Instrumentation within the basin

Taffignon

Craponne


Increase in flood frequency

Increase in flood frequency

m3/s

Years ’70’

Years ’90’


Stationary test number of floods according to lang 1995

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


Stationary test on number of floods rural part according to lang 1995

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


Stationarity of the number of max daily rainfall at bron station

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


Daily max rainfall regime for the rain gauge bron

Daily max. rainfall regime for the rain gauge Bron

  • Daily intensity:

  • The most intense in the ’90’

  • The lowest in the ’70’


Method

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


Method1

débits

débits

simu.

corresp.

corresp.

corresp.

état urba.

pluies 70

pluies 90

pluies 90

années 70

oui (1)

non

oui (3)

(amont)

années 90

non

oui (2)

oui (4)

influence

influence

urbanisation

variabilité

pluies

entre (1) et (2)

oui

oui

ent

re (1) et (3)

non

oui

entre (1) et (4)

oui

oui

entre (2) et (3)

oui

non

entre (2) et (4)

non

non

entre (3) et (4)

oui

non

Method

Influence of rainfall

Model quality

Impact of urbanisation


Model development

Model development

  • Dividing the l’Yzeron basin in hydrological units

  • Calibration of the rainfall – runoff model CANOE

  • Validation of CANOE


Flood regimes of mid sized and mixed land use catchments can we assess the urban contribution

Land-use change in the l’Yzeron basin


Flood regimes of mid sized and mixed land use catchments can we assess the urban contribution

forest

mainly forest

Farming-grass land

periurban

urban

The land use change over 17 years


Land use change in 70 and 90 grid based estimation

Land use change in ‘70 and ‘90 (grid based estimation)

Upstream (Craponne)

Total basin (Taffignon)


Drainage network

Drainage network


Definition of hydrological units

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


Flood regimes of mid sized and mixed land use catchments can we assess the urban contribution

Rural

Peri-urban

Urban

Distribution of hydrological units

70’ land use model

90’ land use model


Semi distributed rainfall runoff model canoe

Impervious areas

Impervious areas

Direct runoff to water courses

Un-Direct runoff to water courses

Semi-distributed Rainfall-Runoff Model CANOE

Permeable areas

( Forest, grassland,..)

Production function

Runoff coef.

Production function

Horton’s infilt. law

Production function

Runoff coef.

Transfer function

Nash cascade

Transfer function

Nash cascade

Transfer function

Linear reservoir

3 hydrographs summation


Results of calibration downstream urban part taffignon

Results of Calibration – downstream urban part - Taffignon

Automne

Winter

Spring

Summer


Description of comparison

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)


Characteristic of selected runoff for description of flood regime qcx d

m3/s

1h

QCX1h

3h

6h

1h

QCX1h

12h

QCX1h

24h

1h

time t

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.

QCX(d)

Threshold level for duration d

Selected durations

1h, 3h, 6h, 12h and 24h


Model validation rural part

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


Model validation urban rural parts

Model validation – urban + rural parts

Null hypothesis H0 tested :

Is the simulated population equivalent to the observed population?

ok

“H0 acceptable”

“H0 accepted”

“H0 rejected”


Flood regimes change between 70 and 90 urban units on 1970 6 and 1990 19

Flood regimes change between ’70’ and ’90’(urban units on 1970: 6% and 1990: 19%)

Only the small floods are affected T=1

Null hypothesis H0 tested :

The 90’population is equivalent with the 70’ population?

“H0 rejected”

“H0 rejected”

“H0 accepted”


Flood regimes change for future development of 24 and 33 urban area

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?

Imperviousness rate of 24 %

Imperviousness rate of 33 %

“H0 accepted”

“H0 rejected”


Flood regimes of mid sized and mixed land use catchments can we assess the urban contribution

Full bank flow

24 % urbanized

(simulated)

+6%

1996 - 19% urbanized

(observed)

+14%

  • - 6% urbanized

  • (simulated)

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


Conclusion on flood hazard evolution

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.


Flood regimes of mid sized and mixed land use catchments can we assess the urban contribution

Flood vulnerability assessment (I)

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 regimes of mid sized and mixed land use catchments can we assess the urban contribution

Flood vulnerability assessment (II)

Flooded areas can be split into vulnerability categories from

forest, grassland and farming , periurban and urban types


Flood regimes of mid sized and mixed land use catchments can we assess the urban contribution

Flood vulnerability Evolution

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.


Conclusion perspectives

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 theincrease 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!


Thank you

Thank you !


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