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SESSION 2017
FLOODPLAIN REPRESENTATION
AND ACCURACY OF PROCESSES
1D/2D
SUPERVISORS
James CELRAY
Leslie SALVAN
TEAM MEMBERS
Tessie BARJAT
Anaïs MAYAU
Tamsin JONES
Aymeric FRANCOIS
Laura TALBA
Konlavach MENGSUWAN
Andres GONZALEZ INIGUEZ
Miquel SARRIAS MONTON
Oussama MOKHTARI
Chenyu LIU
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INTRODUCTION
Floodplain:
• Area of low-lying ground adjacent to a river subject to flooding
• Need to understand how water flows in these areas
Accuracy of processes:
• Depends upon the model used
• Ability to model ‘real processes’ occurring in the floodplain
• Need to produce a 1D and 2D model – compare
• 3 November 2011 event chosen
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INTRODUCTION
Aim:
Compare 1D and 2D model
processes for the floodplain – find
the limitations of processes
Objectives:
represented.
Vs.
1. Discuss processes of 1D and 2D models
2. Produce an output for HEC-RAS (1D).
3. Produce an output for Telemac (2D).
The var river
4. Compare the representation of the
floodplain for each model
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1D VS 2D MODELS
2D MODELS
1D MODELS
Cross-sections taken
perpendicular to river flow (x)
Uniform water level
across the cross-section
Uniform velocity across
the cross-section
Better for low
gradients
Velocity is not
considered to be uniform
Mathematical
governing
equations
Terrain represented as a
continuous surface (x,y)
Better for steep gradients
Water depth
is not
considered to
be uniform
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DOMAIN DEFINITION - HECRAS
● Lower Reach of the Var River Basin
→ Covering the Floodplain
● Upstream Boundary → La Manda
Bridge (observed discharge data)
● Downstream boundary
→ Mediterranean Sea
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TOPOGRAPHY REPRESENTATION
Original DEM
DEM after bridges extraction
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TOPOGRAPHY REPRESENTATION
WEIRS
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CHOICE OF MODEL - HECRAS
Way to
Mike
11solve
case problem of uniform water level by the implementation of
levees
Meters
Meters
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HEC-RAS (MODEL SET-UP)
Topography
5m resolution
DEM
ArcGIS
+HEC-GeoRAS
Cross
sections
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HEC-RAS (MODEL SET-UP)
Roughness coefficient
RIVER BED
n (s/m1/3)
Dense vegetation
0.080
Some vegetation
0.050
No vegetation
0.033
FLOOD PLAIN
n (s/m1/3)
Agricultural areas
0.050
Industrial areas
0.100
Urban areas
0.150
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HEC-RAS (MODEL SET-UP)
Hydrograph
The hydrograph used as upstream boundary condition was observed on La
Manda bridge on November 2011.
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HEC-RAS (MODEL SET-UP)
Boundary Conditions
● Upstream: Flow Hydrograph (La Manda bridge, 2011)
● Downstream: Rating curve obtained from Manning equation, assuming a
rectangular cross-section.
Initial Conditions
● Initial distributed flow
(20m3/s)
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HEC-RAS (MODEL SET-UP)
Computation aspects
1D Courant–Friedrichs–Lewy condition:
● 17 days computed
● 30s computational time step
● 30min output interval
∆x ≈ 100m
u ≈ 2m/s
Cmax = 1
∆t < 50s
about 5 min to
compute
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HEC-RAS (FLOODPLAIN REPRESENTATION)
10 meters between cross sections
100 meters between cross sections
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HEC-RAS (RESULT)
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TELEMAC (MODEL SET-UP)
MANNING
● Mesh characteristics:
DISTRIBUTION
○ 25 m size
○ part of the sea
0.05
0.08
○ buildings
○ try to remove bridges elevation
● Simulation parameters & equations:
○ 2 boundary conditions
○ Numerical scheme SaintVenant FE
○ Manning’s friction law
0.1
0.15
0.033
BUILDINGS
DISTRIBUTION
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TELEMAC (RESULT)
• Many observed problems for:
false flux observed
bridges impact the flood map
mesh size too big
time step too high
faced turbulence
TURBULENCE
WATER DEPTH &
VELOCITY AT THE
BRIDGE
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TELEMAC (RESULT)
IMAGE
WATER DEPTHS
CALCULATED
AIRPORT
ZOOM
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TELEMAC (RESULT)
IMAGE
VELOCITY
DISTRIBUTION
AIRPORT ZOOM
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CONCLUSION
1D:
● Quicker process time
● Limitations - uniform velocity
● Accuracy - increase number of cross-sections (slower). Depends on modellers
experience
2D:
● Accounts for variability in the river
● Limitations - computation time
● Accuracy - finer mesh
Depends on the purpose - flood forecasting, scheme design
QUESTIONS?
Thank you to all lecturers especially our supervisors James and Leslie for their advice and experience
We are grateful for the opportunity to take part in
HydroEurope
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