URBAN FLOOD MODELING Simulation of flood in a dense urban area using 2D Shallow water equations.

URBAN FLOOD MODELING

Simulation of flood in a dense urban area using 2D Shallow

water equations

2

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

3

Modeled Zone

• Suburban area: ‘Richelieu’• Dimensions: 1400 m along N-S, 1050-

220 m along E-W• Boundaries: Northern and eastern

side by railway embankment, western by hills

• Building type: Military barracks, hospital, regular network of narrow streets

• Long. slope: >1%• Flood cause: Storm runoff

5

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<10% Qmax

8

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

• Manning’s ‘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 supercritical flow on streets aligned along N-S axis.

• Small velocities(0.5-0.7 m/s) and subcritical flow occurs in streets aligned along E-W axis.

• The time to peak in streets correspond with time to peak of the eastern hydrograph.

• Flow at crossroads is generally complex with mixed flow regime.

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Comparison Parameter

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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 than in the downstream and the southern part.

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1B: Rainfall vol. taken into account• Rainfall (61 mm/hr) falling over the

simulated zone added to the inflow hydrographs

• The rainfall vol. (212,000 cu.m) is very small compared to the flood hydrographs (3600,000 cu.m)

• Limited effect. Peak water depths increased by about 4 cm.

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1C: Creation of a Storage Zone• Reference case computation assumed

watertight buildings with no storage in lawns, parks, basements etc.

• The military barracks (l’ecole d’artillerie aerienne) 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 Manning’s ‘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 Manning’s ‘n’• n=0.05 selected for the central part

comprising of narrow streets meeting at right angle to each other. This accounts for increased friction due to walls and presence of parked cars. Elsewhere n is same as that of reference calculation

• Results improved significantly in the central zone where dh reduced to 23 cm from 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 and no 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 the

outlying 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 increase as far up as the southern part of the central 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 in the 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 supercritical area. The model calculates an average subcritical flow depth.

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RESULTS SUMMARY

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Conclusions

• 2D Shallow water equations in complete form were used, without interaction with sewage network to model urban flood.

• The results showed a standard deviation of 50 cm which is on the higher side but reflects the uncertainty in flood marks, insufficient topographical data, missing information about mobile obstructions, wall irregularities 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 its structural characteristics seems to be the best strategy.

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Recommendations• The input hydrographs should be precisely

calculated for an accurate peak water depth map creation.

• If the rainfall volume is small compared to the input hydrograph volume than there effect is going to be negligible and can be neglected.

• If the storage volume is small compared to the input hydrograph volume they can be safely neglected.

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• Different friction coefficients should be applied to various homogeneous urban zones, depending on the width of the streets 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 an accurate outflow boundary condition.

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• A 5 point representation for a street profile can represent a fairly good estimate for the general overview of the flood dynamics reducing data needed and calculation times

• However, if information about local depths is available than a more precise description of the streets is required to calibrate the model.

• Use of a 2d code to assess the flood progression through an urban zone is a convenient tool for hydraulic engineers and urban planners.

URBAN FLOOD MODELING Simulation of flood in a dense urban area using 2D Shallow water equations.

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