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7/28/2019 Dam Lecture4
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URBAN FLOOD MODELING
Simulation of flood in a dense
urban area using 2D Shallow
water equations
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Flood Event
City: Southern French city of
Nimes
Event : 03 Oct 1988
Cause: Downpour of 420 mm in
8 hours
Return period: 150-250 years Depths observed: 3 m
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Modeled Zone
Suburban area: Richelieu
Dimensions: 1400 m along N-S, 1050-220 m along E-W
Boundaries: Northern and eastern sideby railway embankment,western by hills
Building type: Military barracks, hospital,regular network of narrow
streets Long. slope: >1%
Flood cause: Storm runoff
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Data for Model Development
X-sections: 200 of 60 streets
Typical profile: 11 points
Flood marks: Map and 99 marls Hydrograph: Rainfall-Runoff
transformation
No. of hydrographs: Two, east and west Sewage network : Decoupled;
interaction capacity
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Mesh Generation
Junction profiles: Generated,
linear interpolation
on altitude
Buildings: Impermeable
DEM: 25000 pts
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Reference Calculation
Dx streets: 25 m Mesh density: 100 cells at
crossroads, 30-60 in
each street Mannings n and 0.025 and 0.1 m2/s
Time step: 0.01 sec
Simulation time: 10 hrs Outflow b.c: Fr=1
Initial condition: dry bed
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Flood Progression
High velocity (3-4 m/s) and supercriticalflow on streets aligned along N-S axis.
Small velocities(0.5-0.7 m/s) and
subcritical flow occurs in streets alignedalong E-W axis.
The time to peak in streets correspondwith time to peak of the eastern
hydrograph. Flow at crossroads is generally complex
with mixed flow regime.
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Comparison Parameter
marksf loodofNon
depthswaterpeakmeasuredand
computedwbdifferenceAveragedh
nHH
n
dhmeasuredcomputed
.
/
/)(dh
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Comparison with Observations
Flooded zone extent correctly simulated. Measurement with peak values of depth.
About 40% overestimated and 60%
underestimated. The max. difference is 1.6 m and the average
difference is 0.41 m
Good agreement in the northern zone
Strong underestimated in the narrow streets (43
cm)
Slight overestimation in the southern part (16 cm)
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Sensitivity Tests 1: Inflow and
Storage Effects
1A: Inflow increased by 20%
To check hydrological uncertainties.
Depths increase by 12.5 cm
Higher increase in the upstream zone thanin the downstream and the southern part.
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Sensitivity Tests 1: Inflow and
Storage Effects
1B: Rainfall vol. taken into account
Rainfall (61 mm/hr) falling over thesimulated zone added to the inflowhydrographs
The rainfall vol. (212,000 cu.m) is verysmall compared to the flood hydrographs
(3600,000 cu.m) Limited effect. Peak water depths
increased by about 4 cm.
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Sensitivity Tests 1: Inflow and
Storage Effects
1C: Creation of a Storage Zone
Reference case computation assumedwatertight buildings with no storage inlawns, parks, basements etc.
The military barracks (lecole dartillerieaerienne) are the largest open space.
Volume stored is about 80,000 cu.m. A very small reduction of peak water
depths at d/s is observed (about 1 cm)
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Sensitivity Tests 2: Roughness and
Kinematic Viscosity Coefficients
2A: Effect of Kin. Viscosity
represents turbulence and the
heterogeneity over the vertical.
=khu*, k=0.01, k=0.1 m2/s.
Small to no effect on computed depths.
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Sensitivity Tests 2: Roughness and
Kinematic Viscosity Coefficients
2B: Effect of Mannings n
n represents the effect of friction on the bottom,
walls, irregularities, obstacles to flow.
n increased to 0.033 from 0.025.
Depths overall increase by 10 cm.
Depth increased at Faita-Sully junction thus
decreasing the discharge in the d/s sections. Flow regime strongly altered at crossroads and
changes from supercritical to subcritical (fig).
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Underestimation decreased in the central
zone, overestimated depths in the
northern zone and in the southern zone. Globally the depths increase and improve
but locally there is worsening.
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Sensitivity Tests 2: Roughness and
Kinematic Viscosity Coefficients
2C: Different Mannings n n=0.05 selected for the central part
comprising of narrow streets meeting atright angle to each other. This accounts forincreased friction due to walls andpresence of parked cars. Elsewhere n is
same as that of reference calculation Results improved significantly in the
central zone where dh reduced to 23 cmfrom 43 cm.
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Sensitivity Tests 3: Downstream
Boundary Conditions
3A: Zero depth gradient boundary
The d/s b.c is changed to a zero depth
gradient condition for subcritical flow andno d/s influence for supercritical flow.
Depth increases in the streets in the
vicinity of the d/s boundary
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Sensitivity Tests 3: Downstream
Boundary Conditions
3B: Representation of backwater effect
To simulate backwater effect of theoutlying areas upon the modeled zone,
flow prevented from leaving through S1,S2 and S10.
Flow strongly affected in the whole
southern zone and flow depths increaseas far up as the southern part of thecentral zone
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Whereas the choice of b,c affects the flow
only in the close vicinity of the exit the
choice of exit has a far greater influence inthe upstream zone extending upto four
streets backwards.
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Sensitivity Tests 4: Simplification of
the Street Profile
3B: 4 point, 5 point and 7 point profiles
To reduce the data requirement and
calculation times
For 11, 7, 5, 4 points cells at the junction
are 64, 16, 4 and 1 respectively.
The general form of the results remains
same except that depths are increased by
about 10 cm.
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The 7 and 11 point calculation results are
very similar
In 5 and 4 point versions of the
calculations flow detail at a junction is
averaged out e.g if a flow at a junction is
mainly subcritical with a small supercriticalarea. The model calculates an average
subcritical flow depth.
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RESULTS SUMMARY
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Conclusions
2D Shallow water equations in completeform were used, without interaction withsewage network to model urban flood.
The results showed a standard deviationof 50 cm which is on the higher side butreflects the uncertainty in flood marks,insufficient topographical data, missinginformation about mobile obstructions, wallirregularities etc.
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Kinematic viscosity seems to exercise
negligible to no influence on he results.
Manning,s n strongly affects the flow but
no single value can be determined to
correctly represent all the zones.
Assigning each zone an n reflecting itsstructural characteristics seems to be the
best strategy.
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Recommendations
The input hydrographs should be preciselycalculated for an accurate peak waterdepth map creation.
If the rainfall volume is small compared tothe input hydrograph volume than thereeffect is going to be negligible and can beneglected.
If the storage volume is small compared tothe input hydrograph volume they can besafely neglected.
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Different friction coefficients should be
applied to various homogeneous urban
zones, depending on the width of thestreets and fixed and mobile obstacles that
may increase the resistance to flow.
Collecting information about the flooded
areas just downstream from the studied
zone is important in establishing anaccurate outflow boundary condition.
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A 5 point representation for a street profilecan represent a fairly good estimate for
the general overview of the flood dynamicsreducing data needed and calculationtimes
However, if information about local depths
is available than a more precisedescription of the streets is required tocalibrate the model.
Use of a 2d code to assess the floodprogression through an urban zone is aconvenient tool for hydraulic engineersand urban planners.