1d and 2d flood routing and risk...

1D AND 2D FLOOD ROUTING AND RISK ASSESSMENT

ByPankaj Mani

NATIONAL INSTITUTE OF HYDROLOGYROORKEE – 247 667

INDIA

Flood Routing

• Flood routing is the process of modeling a flood wave tounderstand what it will do at various points along a waterway(numerical evaluation of flood: what will happen in future?).

• Concern: What will be the peak of flood? When will the flood peak reach us? How much area will be inundated?

• This is important in an emergency when accurate floodingforecasts are necessary to determine whether a flood posesa risk to health and safety.

• Mathematical model permits analysis of flood in its completescale, in space and time including consideration of futurechange in the river and floodplain regime. 2

Governing Equations

• Hydraulic routing is based on the solution of partialdifferential equations of unsteady open-channel flow. Theequations used are the St. Venant equations or the dynamicwave equations.• Continuity equation (1D)

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Governing Equations

• Hydraulic routing is based on the solution of partialdifferential equations of unsteady open-channel flow. Theequations used are the St. Venant equations or the dynamicwave equations.• Momentum equations (1D)

• Similar equations are for 2D flow

(1) friction slope (2) bed slope (3) pressure gradient (4) velocity head gradi(5) local acceleration

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Real world

(+) Big areas, High system complexity (branches, unions,structures), longer time scale, computationally efficient

(-) No interaction of flow across river and floodplain, unable tocompute inundation extent.

1D Modeling (macro scale)

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Real world

(+) small areas, detailed physical description, integrated treatment of river and flood plain, flood extent

(-) low system complexity (unable to include structures), smaller time scale, reduced computational efficiency

2D Modeling (micro scale)

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Real world

Couple Modeling

1D(+) Big areas, High system complexity (branches, unions, structures), 2D(+) small areas, detailed physical description, integrated treatment of river and flood plain, flood extent

Stream & Floodplain description in MIKE FLOOD

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Case Studies(Dam break analysis using NWS DAMBRK)

Flood inundation map when both the dams fail under PMF - Case A3

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Case Studies(Dam break analysis NWS DAMBRK)

Flood inundation map when only Panchet dam fail under PMF

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Case Studies(Flood plain zoning HEC-RAS)

Gha

gra

R

Ganga R

Ganga R

Ganga R

Gan

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R

Son

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Pun

pun

R

Raj

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idge

Bux

ar

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Gha

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Ganga R

Ganga R

Ganga R

Gan

dak

R

Son

e R

Pun

pun

R

Raj

endr

a B

idge

Bux

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0 10 20 30 40 50 6085

90

95

100

105

110

115

with modified river bank Plan: Plan 04

Main Channel Distance (km)

Ele

vatio

n (m

)

Legend

WS 1983

WS 100 yr

WS 50 yr

WS 25 yr

WS 2000

Ground

Kh.

..

Mog

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Bas

antp

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Gau

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Itahu

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Son

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babu

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Alin

agar

Elg

in b

ridge

Ghaghara 1

The flow parameters are used for evaluating the performance of anti erosion works. 12


The flow parameters are used for evaluating the performance of anti erosion works. 13

Case Studies(Narmada river flow modeling MIKE 11)

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Case Studies(Narmada river flow modeling MIKE FLOOD)

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Combinations of Events

SNCase No.

Local catchment Rainfall

Bargi dam conditions Failure of U/S dams

Remarks

BargiDambreak

Inflow flood Outflow fromgates

Inflow floodU/S dams

Design Basis Events1 100 yr2 100 yr 100 yr Full open3 100 yr 100 yr 10% closed4 100 yr 100 yr Full closed

5A 100 yr 100 yr Full open5B 100 yr +1SD 100+SD Full open6A 1000 yr 1000 yr 10% closed6B 1000 yr +1SD 1000 yr+1SD 10% closed7A 1000 yr 1000 yr Full closed7B 1000 yr +1SD 1000 yr+1SD Full closed8A 1000 yr Yes 1000 yr Full open Failure of Bargi

dam if over tops.(hydrological

8B 1000 yr +1SD Yes 1000 yr+1SD Full open1000 yr +1SD Yes 1000+SD Full closed


Combinations of Events

SNCase No.

Local catchment Rainfall

Bargi dam conditions Failure of U/S dams

Remarks

BargiDambreak

Inflow flood Outflow fromgates

Inflow floodU/S dams

Design Basis Events

9A

1000 yr Yes 1000 Full closed Failure of Bargidam if over tops.(hydrologicalfailure)

9B 1000 yr +1SD Yes 1000 yr+1SD Full closed10A 1000 yr +1SD Yes 25 yr Full open Seismic failure of

Bargi dam10B 1000 yr +1SD Yes 25 yr Full open

11A 1000 yr +1SD Yes 25 yr Full closed

11B 1000 yr +1SD Yes 25 yr Full closed12A 1000 yr +1SD PMF Full open PMF of U/S

damsFailure if U/S damsover tops but Bargidam does not fail12B 1000 yr +1SD PMF Full open PMF of U/S



The flow parameters are used for computation of extent of inundation, flood level and water depth near plant site a


Calibrated Manning’s n for the flood plain

Thus the sensitivity analysis of ‘n’ in two severe most flooding scenario may be used to quantify the general effect of uncertainty in estimate of ‘n’ on maximum water level at plant site in the range of ±0.1 m.

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Impact of climate change

The impact of climate change has been considered by increasingthe rainfall by 15% and thus the corresponding river flow iscomputed and its effect on flooding has been estimated as 2.02 m. 22


Computation of safe grade elevation

Maximum flood level (Case-31XA) RL 437.18 mIncrease in level due to future climate change 2.02 mAdd due to sensitivity analysis wrt n values for floodplain 0.10 mSafety margin 2.00 mSafe grade elevation RL 441.30 m

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• Breach in FBC

• Breach in BML

• Catchment flooding

• Local site rainfall

Case Studies(Flooding at a plant site MIKE FLOOD)

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Case-22Catchment flooding – 1000+σ+15%increase, local site rainfall – 1000+ σ +15%increase,

Full flow divert from FBC & BML 26


Maximum flood level (Case-21) = RL 218.15 mIncrease in level for climate change= 0.10 mSafety margin = 1.00 mSafe grade elevation = RL 219.25 m, say 219.3 m

Identify Flood Protection Measures

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Maximum flood level (Case-21) = RL 218.15 mIncrease in level for climate change= 0.10 mSafety margin = 1.00 mSafe grade elevation = RL 219.25 m, say 219.3 m

Identify flood protection alternatives

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Study the effects of floodplain dynamics on flow characteristics (if the existing village roads are raised for approach)

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Major concern of 2D flow(Computation efficiency)

Case No. Grid size (m)

Study area represented byRow x column

Area(km2)

CPU time(sec)

Results file size

1 10 1024 x 864 88.47 50167 3.17 GB2 30 341 x 288 88.39 2354 361 MB3 60 171 x 144 88.65 624 90.6 MB4 90 114 x 96 88.65 284 40.2 MB5 120 85 x 72 88.13 215 22.5 MB6 150 68 x 58 88.74 173 14.5 MB7 180 57 x 48 88.65 155 10.0 MB8 180 57 x 48 88.65 156 10.0 MB

System configuration.: Intel(R) Core(TM), i7-2600 [email protected], 3.39 GHz, 4 GB of RAMSource: Mani et al., 2015 (communicated)

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Case Description of DEM Maximum value (at mid of plant area)

grid size Source MSL Flood Elevation (m)

FloodDepth

(m)

Inundation extent

(km2)Case-1 10 m Detailed topographical

survey at 10 m grid size and re-sampling of 10 m DEM for generation of

courser DEM

215.39 2.28 13.3576Case-2 30 m 215.37 2.27 15.5862Case-3 60 m 215.36 2.26 17.1324Case-4 90 m 215.36 2.26 20.1447Case-5 120 m 215.34 2.24 21.5712Case-6 150 m 215.35 2.22 20.25Case-7 180m 215.43 2.14 22.4532Case-8 180m Spot level and Contours

extracted from 1:50,000 scale SOI toposheets.

218.25 1.17 48.924

Measure concern of 2D flow(Computation efficiency)

Source: Mani et al., 2015 (communicated) 31

• Hazard is defined as the probability of occurrence of, generally, anundesirable event while the risk is the exposure to this hazard.

• Thus, for any damage, hazard is the causing factor while the risk islinked with the exposure of man and material towards the causingfactor.

• The limited available natural resources like land and water arecontinuously over-exploited to meet the demand of ever-increasingpopulation.

• The encroachment of the basin area affects the flood magnitude andfrequency in spatial and time domain and hence intensify thehazard. On the other hand, encroachment in the floodplainsincreases the exposure and vulnerability.

Hazard and Risk

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• Through the hydrodynamic modeling, the point-specific hazardestimates, i.e., floods of various exceedance probabilities, aretransformed into spatial domain in terms of depth, flow velocity andduration of flooding.

• The other parameters influencing the flood hazard include floodwave height, debris flow and sediment load, etc.

• The flow parameter that are generally used to classify the hazard indexare flow depth (d), velocity (v) and duration of flooding (T). The crossproduct of depth and flow velocity (dv), signifying the momentum hasbeen used in many studies.

Hazard and Risk Assessment

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Hazard classification

Hazard category for different flooding parametersHazard category

Depth of flooding

(m)

Depth* flow velocity (m2/sec)

Flood duration(hour)

Hazard index

Very low 0-0.2 0 -0.3 0-25 0Low 0.2-0.6 0.3-0.7 25-50 1

Medium 0.6-1.5 0.7-1.2 50-100 2High 1.5-3.5 1.2-1.6 100-175 3

Very high >3.5 >1.6 >175 4Source: Mani et al., Nat Hazards (2014) 70:1553–1574

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Source: Mani et al., Nat Hazards (2014) 70:1553–1574 35


Hazard classification scheme as function of depth, depth-velocity and durationSN Function of depth, depth x velocity and

durationHazard index

Hazard category

1 0<d< 0.2 and 0<dv < 0.3 and 0<T<50 0 Very low2 0<d< 0.2 and 0<dv < 0.3 and T>50 1 Low3 0.2<d< 0.6 or 0.3<dv < 0.7 and 0<T<25 1 Low4 0.2<d< 0.6 or 0.3<dv < 0.7 and T>25 2 Medium5 0.6<d< 1.5 or 0.7<dv < 1.2 and 0<T<25 2 Medium6 0.6<d< 1.5 or 0.7<dv < 1.2 and >25 3 High7 1.5<d< 3.5 or 1.2<dv < 1.6 and 0<T<25 3 High8 1.5<d< 3.5 or 1.2<dv < 1.6 and T>25 4 Very high9 d>3.5 or dv >1.2 and T>0 4 Very high


Hazard classificationHazard map when only catchment flooding is considered.

Hazard map when local site flooding source in addition to catchment flooding is considered.


Thanks

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1d and 2d flood routing and risk...

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