The Court of Miracles of Hydrology, 18-20 June, Engref Paris 2008 1 Do monsters resist to rating curves scrutiny ? U.R Hydrologie-Hydraulique, Lyon M. Lang Example of flowing conditions during floods (Ardèche & Volane, 20 Oct. 2001) (picutres by M. Lang, Cemagref Lyon) The Court of Miracles of Hydrology, 18-20 June, Engref Paris 2008 2 Degree of rating curve extrapolation in France Class 0 1 2 3 4 1 8 31 7 53 Size (%) T > 100 yrs 10 < T < 100 yrs 2< T < 10 yrs 1 < T < 2 years T < 1 yr Criteria . . . .. . Class • 60% of the stations have not been gauged beyond the 2 year flood peak • Less than 10% of the stations have not been gauged beyond the 10 year flood • Large basins with smooth floods have been better gauged (measurements of discharge are easier) Return period of the maximum gauged flow few rating curves on extreme discharges
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The Court of Miracles of Hydrology, 18-20 June, Engref Paris 20081
Do monsters resist to rating curves scrutiny ?
U.R Hydrologie-Hydraulique, Lyon M. Lang
Example of flowing conditions during floods (Ardèche & Volane, 20 Oct. 2001)
(pic
utr
es b
y
M.
Lan
g, C
ema
gre
f L
yon
)
The Court of Miracles of Hydrology, 18-20 June, Engref Paris 20082
Degree of rating curve extrapolation in France
Class
0
1
2
3
4
1831753Size (%)
T > 100
yrs
10 < T <
100 yrs
2< T < 10
yrs
1 < T < 2
years
T < 1 yrCriteria
......Class
• 60% of the stations have not been gauged beyond the 2 year flood peak
• Less than 10% of the stations have not been gauged beyond the 10 year flood
• Large basins with smooth floods have been better gauged
(measurements of discharge are easier)
Return period of the maximum gauged flow
�few rating curves on extreme discharges
The Court of Miracles of Hydrology, 18-20 June, Engref Paris 20083
• Active policy of gauging measurement
main source of information
• Data collection on flood marks
several marks to assess the water surface profile, for model calibration
• Hydraulic extrapolation of the rating curves
no empirical calculation, specific behaviour for overflowing discharge
• Feedback on gauging equipment and gauging methodologies
knowledge of the various limitations
• Exchange groups on new technologies
ADCP, video information
How to improve the discharge assessment on large floods ?
The Court of Miracles of Hydrology, 18-20 June, Engref Paris 20084
Review of rating curves by hydraulic modelling
• Morphological data (main channel, flood plain, bridges, thresholds)
• Hydrometrical data (discharge data series, rating curves, gauging values, flood marks)
Data collection
• Sensitive analysis on the downstream condition
• Determination of the roughness coefficients
� main channel : calibration with gauged values
� flood plain : a priori values from field visit & Ven Te Chow tables
Calibration of the hydraulic model
0
5
10
15
20
25
30
35
0 10 20 30 40 50 60 70
Discharge(m3/s)
K=
1/n q
q-10%
q+10%
Overflow of the main channel
Domains : 1 2 3 4
�Representative Manning
coefficient from domain 2
The Court of Miracles of Hydrology, 18-20 June, Engref Paris 20085
Review of rating curves by hydraulic modelling
• Sensitive analysis on the gauging values (H, Q)
(H-5 cm, Q+10%) and (H+5 cm, Q-10%)
Extrapolation of the hydraulic model
0
20
40
60
80
0 0.5 1 1.5 2 2.5
Water depth (m)
Dis
cha
rge
(m3/s
)
Maximal curveMedian curveMinimal curve
Q+10%
QQ-10%
H
H-5 cm H+5 cm
135
140
145
150
155
160
67000 67500 68000 68500 69000 69500 70000 70500
Pk (m)
Lev
el (
m N
GF
)
Q=3420 m3/s (Kmin)
Q=2470 m3/s (Kmed)Q=1730m3/s (Kmax)marks of the 1992 flood
Bed level (m NGF)
New
bridge
Railroad
bridge
Old
bridge
• Validation with past flood levels
The Court of Miracles of Hydrology, 18-20 June, Engref Paris 20086
Review of rating curves by hydraulic modelling
• Small to medium error on gauge discharge : 3-5% to 10%
• Large error on the extrapolated domain of the rating curve
� with good hydraulic conditions : about 30% for the 10 year flood
� without gauged values or with complex hydraulic conditions : 60-100 % !