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
Urban sprawling: a world wide trend (UNEP, 2003 )
Urban growth of Lyon city
(with courtesy from Lyon city council, 2005)
Lyon on 1975 Lyon on 2000
20 km
Lyon on 1975 Lyon on 2000Lyon on 1975 Lyon on 2000
20 km20 km
Data from French National Institutefor Statistics and Economic Studies
Urban area
Urban unit
Periurban area
Population growth in Lyon
What could be the impact of land-use change on flood discharge?
1
5
10
15
20
0.1 101 100 200
Recurrence interval (years)
24
2 25 50
x 4x 2
1
5
10
15
20
0.1 101 100 200
24
2 25 501
5
10
15
20
0.1 101 100 200
20 % imperviousness
24
2 25 50
x 4x 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, 1975
0.1 1 10
Recurrence interval (years)
0
0.5
1
1.5
2
2.5
100
Rural change
Urban change
50 %
Flood event in Yzeron basin
Outline of the presentation
Objective of the study Study area Method Results Conclusion
Objective of the study
Vulnerability in terms of flood
frequency
HazardIn 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 riskcan be expressed as a recurrence interval in year units
Study Area
Land-use in the l’Yzeron basin
Mon
ts du
lyon
nais
Riv. Charbonnières
Riv.Yzeron
Lyon
RhôneSaône
Espaces en-forêt-rural-périurbain-urbain
1 km2 NM
onts
du ly
onna
is
Riv. Charbonnières
Riv.Yzeron
Lyon
RhôneSaône
Espaces en-forêt-rural-périurbain-urbain
Espaces en-forêt-rural-périurbain-urbain
1 km21 km2 NN
Instrumentation within the basin
Craponne
Taffignon
Increase in flood frequency
Years ’70’ Years ’90’
m3/s
Stationary Test – number of floods (according to Lang, 1995)
Taffignon, 130 km2S= 4.87m3/s; QCXd (d= .04jours); Coef1= .10 , Coef2= 5.00
0
5
10
15
20
25
30
35
1988 1989 1990 1991 1992 1993 1994 1995 1996 1997
Année
No
mb
re d
e cr
ues
M95%
Mexp
M5%
Taffignon, 130 km2S= 4.87m3/s; QCXd (d= .04jours); Coef1= .10 , Coef2= 5.00
0
5
10
15
20
25
30
35
1988 1989 1990 1991 1992 1993 1994 1995 1996 1997
Année
No
mb
re d
e cr
ues
M95%
Mexp
M5%
Craponne, 49 km2S= .75m3/s; QCXd (d= .04jours); Coef1= .10 , Coef2= 5.00
0
20
40
60
80
100
120
140
1965 1970 1975 1980 1985 1990 1995 2000 2005
Année
Nom
bre
de c
rues
M95%
Mexp
M5%
Stationary test on number of floods - rural part (according to Lang, 1995)
Stationarity of the number of max. daily rainfall at Bron station
Test de stationnaritéréalisé sur les pluies journalières du poste de Bron
0
20
40
60
80
100
120
1965 1970 1975 1980 1985 1990 1995 2000
Année
No
mb
re d
e p
luie
s
M95%
Mexp
M5%
Daily max. rainfall regime for the rain gauge Bron
Comparaison IdF de durée 1 jour, station Bron
0
10
20
30
40
50
60
70
80
90
100
0,1 1 10 100
T (ans)
I (m
m/jo
ur)
bron 20-30 1 jourbron 40-50 1 jourbron 69-78 1 jourbron 78-88 1 jourbron 88-97 1 jour
Daily intensity:
The most intense in the ’90’
The lowest in the ’70’
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
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 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
Model quality
Influence of rainfall
Impact of urbanisation
Model development
Dividing the l’Yzeron basin in hydrological units
Calibration of the rainfall – runoff model CANOE
Validation of CANOE
Land-use change in the l’Yzeron basin
forest Farming-grass land periurban urbanmainly forestforest Farming-grass land periurban urbanmainly forest
The land use change over 17 years
forest Farming-grass land periurban urbanmainly forest
Fo
res
t
gra
ss
lan
d &
cro
ps
su
bu
rba
n
urb
an
seventies
nineties0.00
0.10
0.20
0.30
0.40
0.50
% a
rea
ForêtPrairie
périurbainurbain
Etat sol 70
Etat sol 90U700.00
0.10
0.20
0.30
0.40
0.50
0.60
% s
urf
ace
bv
Land use change in ‘70 and ‘90 (grid based estimation)
Upstream (Craponne)
Total basin (Taffignon)
ForêtPrairie
périurbainurbain
Etat sol 70
Etat sol 90U700.00
0.10
0.20
0.30
0.40
0.50
% s
urf
ac
e b
v
1979 19966% 19%
decadeimperviousness %
Drainage network
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
70’ land use model 90’ land use model
RuralPeri-urbanUrban
Distribution of hydrological units
Semi-distributed Rainfall-Runoff Model CANOE
Production function Runoff coef.
Impervious areasDirect runoff to water courses
Permeable areas( Forest, grassland,..)
Transfer function Nash cascade
Impervious areasUn-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
Results of Calibration – downstream urban part - Taffignon
44 .32
35 .456
26 .592
17 .728
8 .864
0
44 .32
35 .456
26 .592
17 .728
8 .864
0
d éb it t2 -1 ta f f9 0 n o v
0 600 1200 1800 2400 3000 3600 4200 4800 5400 6000
0 600 1200 1800 2400 30 00 3600 4 200 480 0 5400 6000
Ta 2 1 .3 .9 1 -9 0
34 .986 8
27 .989 4
20 .992 1
13 .994 7
6 .9973 6
0
d éb it t2 -1
34 .986 8
27 .989 4
20 .992 1
13 .994 7
6 .9973 6
0
D ébit t2-1
27 .0574
21 .6459
16 .2344
10 .823
5 .411 48
0
27 .0574
21 .6459
16 .2344
10 .823
5 .411 48
0
0 600 1200 1800 2400 30 00 3600 4200 4800 5400 6000 6600 7200
D ébit t2-1 16 .11 .92
0 600 1200 1800 2400 3000 3600 4200 4800 5400 6000
20 .8589
16 .6871
12 .5153
8 .34356
4 .17178
0
20 .8589
16 .6871
12 .5153
8 .34356
4 .17178
0
Automne
Winter
Spring
Summer
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)
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
1h6h
1h
3h
12h
24h
QCX1h
QCX1h
QCX1h
m3/s Threshold level for duration d
QCX(d)
Model validation – rural part
01
234
567
89
0.1 1 10 100
T (years)
QC
X (
m3/
s)
24 hours QCX-flow frequency distribution
observations
model
0
5
10
15
20
25
0.1 1 10 100
T (years)
QC
X (
m3/
s)
1hour QCX-flow frequency distribution
observations
model
0
2
4
6
8
10
12
14
16
0.1 1 10 100
T (years)
QC
X (
m3/
s)
3 hours QCX-flow frequency distribution
observations
model
Null hypothesis H0 tested : The simulated population is equivalent to the observed population?
“H0 accepted”
“H0 rejected”
“H0 accepted”
flood duration 10% 5% 1%1h yes yes yes3h yes yes yes6h yes yes yes
12h yes yes yes24h no no no
Wilcoxon-Mann-Whitney / unilateral testsignificance level
Only large durations (24hours) are rejected from the statistical test. Model is validated for the flood regimes simulation
Model validation – urban + rural parts
0
10
20
30
40
50
60
70
80
0.1 1 10 100
T (years)
QC
X (
m3/
s)
1 hour QCX-flow frequency distribution
observations
model
0
10
20
30
40
50
60
0.1 1 10 100
T (years)
QC
X (
m3/
s)
3 hours QCX-flow frequency distribution
observations
model
0
5
10
15
20
25
30
35
0.1 1 10 100
T (years)
QC
X (
m3/
s)
24 hours QCX-flow frequency distribution
observations
model
QCX durations 10% 5% 1%1h no no yes3h yes yes yes6h yes yes yes
12h yes yes yes24h no no no
Wilcoxon-Mann-Whitney / bilateral testsignificance level
Null hypothesis H0 tested : Is the simulated population equivalent to the observed population?
“H0 acceptable”
“H0 accepted” “H0 rejected”
ok
0
10
20
30
40
50
60
70
0.1 1 10 100
T (years)
QC
X (
m3
/s)
90' model
70' model
3 hours QCX-flow frequency distribution
0
10
20
30
40
50
60
70
0.1 1 10 100
T (years)
QC
X (
m3
/s)
90' model
70' model
6 hours QCX-flow frequency distribution
Flood regimes change between ’70’ and ’90’(urban units on 1970: 6% and 1990: 19%)
0
10
20
30
40
50
60
70
80
0.1 1 10 100
T (years)
QC
X (
m3
/s)
90' model
70' model
1 hour QCX-flow frequency distribution
QCX durations 10% 5% 1%1h no no yes3h no yes yes6h yes yes yes
12h yes yes yes24h yes yes yes
Wilcoxon-Mann-Whitney / unilateral testsignificance level
“H0 accepted”
“H0 rejected”
“H0 rejected”
Null hypothesis H0 tested : The 90’population is equivalent with the 70’ population?
Only the small floods are affected T=1
Flood regimes change for future development of 24% and 33% urban area
050
100150200250300350400
0.1 1 10 100
T (years)
QC
X (
m3/
s)90' model 19% urban
model 33 % urban
1 hour QCX-flow frequency distribution1 hour QCX-flow frequency distribution
0
20
40
60
80
100
0.1 1 10 100
T (years)
QC
X e
n m
3/s
90' model 19% urban
model 24 % urban
Null hypothesis H0 tested : Is the future population equivalent with the present population?
“H0 accepted” “H0 rejected”
flood duration 10% 5% 1%1h yes yes yes3h yes yes yes6h yes yes yes12h yes yes yes24h no no yes
Wilcoxon-Mann-Whitney / unilateral testsignificance level
flood duration 10% 5% 1%1h no no no3h no no no6h no no no12h yes yes yes24h yes yes yes
Wilcoxon-Mann-Whitney / unilateral testsignificance level
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
grassland 2.7 0.5 0.5 1.4 0.2periurban 0.3 1.3 5.0 1.7 6.3
urban 0.8 1.4 10.0 7.7 14.3total 4.5 4.5 11.1 21.6
area in km2Vulnerability amount in
years (area * VC)
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!
Thank you !
