dam lecture4

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

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/

/)(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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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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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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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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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.

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