18 ch 13 disaster management plan_29.08.10

7/28/2019 18 Ch 13 Disaster Management Plan_29.08.10

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Environmental Management Plant Dam (Barrage) Break Modelling and Disaster Management Plan

Tawang H.E. Project Stage-I 13-2

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projects. The extreme nature of dam (barrage) break floods means that flow conditions will far

exceed the magnitude of most natural flood events. Under these conditions flow will behave

differently to conditions assumed for normal river flow modelling and areas will be inundated

that are not normally considered. This makes dam (barrage) break modelling a separate study for

the risk management and disaster management plan. Therefore, one of the main objectives of

dam (barrage) break modelling or flood routing is to simulate the movement of a dam (barrage)

break flood wave along a valley or indeed any area downstream that would flood as a result of

dam (barrage) failure. The key information required at any point of interest within this flood zone

is generally:

Travel time of flood water

Peak water level extent of inundation

Peak discharge

Duration of flooding

The nature, accuracy and format of information produced from a dam (barrage) break

analysis will be influenced by the end application of the data. The present study for the Tawang

H.E. Project Stage-I comprises of the following hydrodynamic simulations due to occurrence of:

- Design flood with Dam (barrage) break with initial reservoir level at FRL

- Design flood discharge with no dam (barrage) break,

- Design flood without dam (barrage) in place (virgin condition).

13.3 DAM BREAK MODELLING PROCESS

Generally, dam (barrage) break modelling can be carried out by either i) scaled physical

hydraulic models or ii) mathematical simulation using computer. A modern tool to deal with this

problem is the mathematical model, which is most cost effective and approximately solves the

governing flow equations of continuity and momentum by computer simulation. A flow chart for

mathematical modelling is given in Figure 13.1.

Mathematical modelling of dam (barrage) breach floods can be carried out by either one

dimensional analysis or two dimensional analysis. In one dimensional analysis, the information

about the magnitude of flood, i.e., discharge and water levels, variation of these with time and

velocity of flow through breach are assessed in the direction of flow. In the case of two


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dimensional analysis, the additional information about the inundated area, variation of surface

elevation and velocities in two dimension can also be forecast.

One dimensional analysis is generally accepted when valley is long and narrow and the

flood wave characteristics over a large distance from the dam (barrage) are of main interest. On

the other hand, when the valley widens considerably downstream of dam (barrage) and large area

is likely to be flooded, two dimensional analysis is necessary. The basic theory for dynamic

routing in one dimensional analysis consists of two partial differential equations originally

derived by Adhmar Jean Claude Barr de Saint-Venant in 1871. The equations are:

i. Conservation of mass (continuity) equation

(Q/X) + (A + A0) t/t - q = 0

ii. Conservation of momentum equation

(Q/t) + {(Q2/A)/X } + g A {(h/X ) + Sf+ Sc } = 0

where Q = discharge;

A = active flow area;

A0 = inactive storage area;

h = water surface elevation;

q= lateral outflow;

x = distance along waterway;

t = time;

Sf= friction slope;

Sc = expansion contraction slope and

g = gravitational acceleration.

Selection of an appropriate model to undertake dam (barrage) break flood modelling is

essential to ensure in achieving the right balance between modelling accuracy and cost in terms

of time spent developing the model setup. In the instant case HEC-RAS version 4.0 model

released by Hydrologic Engineering Center of U.S. Army Corps of Engineers in March 2008 has

been selected. HEC-RAS is an integrated system of software, designed for interactive use in a

multi-tasking environment. The system is comprised of a graphical user interface, separate

hydraulic analysis components, data storage and management capabilities, graphics and reporting

facilities. The model contains the advanced features for dam break simulation.


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HEC-RAS uses an implicit finite difference scheme. The common problem of instability

in the case of unsteady flow simulation can be overcome by suitable selection of following:

1. Cross section spacing along the river reach

2. Computational time step

3. Theta weighing factor for numerical solution

4. Solution iterations

5. Solution tolerance

6. Weir and spillway stability factors

13.4 DESCRIPTION OF THE PROJECT

Tawang HE Project Stage-I is a run-of-the-river scheme planned across Tawang Chhu

river, near Nuranang Chhu Powerhouse in Tawang district of Arunachal Pradesh. The project

envisages construction of 26 m high barrage (Type - RCC raft with piers) across Tawang Chhu

River. The Full Reservoir Level (FRL) and Minimum Draw Down Level (MDDL) for the project

are at EL 2090 m and 2087 m, respectively. The gross storage of the reservoir at FRL is 1.672

MCum (=167.2 ha m). The water spread area of the reservoir at FRL is 12.46 ha. The catchment

area at the project site is 2937 sq km. The Standard Probable Flood (SPF) for the project has been

estimated as 4263 cumec. The upstream elevation of the barrage is given in Fig.13.2 and the

salient features of the project are given in the Table 13.1

Table 13.1 Salient features of the Project

A LOCATION

State Arunachal Pradesh

District Tawang

River Tawang Chhu

Barrage Site Near Nuranang Chu Powerhouse

Nearest BG rail head Guwahati & Nagaon

Nearest airport Guwahati & Tezpur

Latitude 273520Longitude 915903

B HYDROLOGY

Catchment area 2937 sq km

Average annual rainfall (at Murga Bridge) 1710 mm

Maximum temperature 31.1 C

Minimum temperature -2.9 C

Max. 10 daily discharge 299.6 cumec

Min 10 daily discharge 28.2 cumec


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SPF 4263 cumec

C RESERVOIR

Full reservoir level (FRL) EL 2090 m

Min. Draw Down Level EL 2087 m

Gross Storage

at FRL 167.2 Hamat MDDL 131.43 Ham

Area Under submergence at FRL 12.46 Ha

D BARRAGE

Type RCC RAFT WITH PIERS

Top elevation EL 2092 m

Crest elevation EL 2068 m

Downstream Floor Level EL 2061 m

Length at top 130.5 m

Thickness of d/s Raft 5 m

Upstream Floor Level EL 2066 m

Upstream Floor Thickness 2 m

Thickness of Pier 3.5 m

Height 26 m

E SPILLWAY

Discharge capacity 12680 cumec

Type Orifice type

Crest elevation EL 2068 m

Number (including one emergency bay) 9

Size (W x H) 9.5 x 14.75 m

Energy dissipation Stilling Basin with End sill

13.5 METHODOLOGY: DATA INPUT AND MODEL SETUP

Undertaking a dam (barrage) break analysis requires following range of data:

(i) Cross sections of the river from barrage site and up to location downstream of the barrage

to which the study is required

(ii) Stage-volume relationship for the reservoir

(iii) Salient features of the all hydraulic structures at the barrage site and also in the study

reach of the river

(iv) Design flood hydrograph

(v) Stage-discharge relationship at the last river cross section of the study area

(vi) Mannings roughness coefficient for different reaches of the river under study

(vii) Rating curve of all the hydraulic structures in the study reach of the river

(viii) Topographic map of the downstream area for preparation of inundation map after dam

(barrage) break studies


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13.5.1 Dam (barrage) break model set up in generalThe dam (barrage) break model set up consists of a single or several channels, reservoirs,

dam (barrage) break structures, and other auxiliary dam (barrage) structures such as spillways,

sluices etc. The river is represented in the model by cross section at regular intervals. However,

due to the highly unsteady nature of dam (barrage) break flood propagation, it is advisable that

the river course be described as accurately as possible through the use of closely spaced cross

sections, particularly where the cross section is changing rapidly. Further, the cross sections

should extend as far as the highest modeled water level, which normally will be in excess of the

highest recorded flood level.

The reservoir is normally modeled as a storage area to describe the storage characteristics

by the use of storage-volume at different levels. This point will often also be the upstream

boundary of the model, where inflow hydrograph may be specified. The downstream boundary

will be either a stage discharge relation or time series water level as in case of tidal waves etc.

For dam (barrage) break model setup and other hydrodynamic model set up for Tawang H.E.

Project Stage-I, the different components of the project have been represented in the model as

following:

13.5.2 Tawang Chhu River

The Tawang Chhu river for a length of 15.1 km downstream of Tawang HE Project

Stage-I barrage site has been represented in the model by cross sections taken at barrage axis has

been connected to a storage area representing the reservoir. As the dam (barrage) breach flood

levels far exceed the normal flood level marks and the flood spreads beyond the normal river

course, the Mannings roughness coefficient for the dam (barrage) break studies should be

assumed normally more than the other hydro-dynamic studies. The Mannings roughness

coefficient for entire study reach of the river has been taken as 0.040 considering the boulder

river beds with grassy banks of hilly terrain.

13.5.3 Reservoir

The reservoir has been represented in the model by storage area of the graphical editor of

the model and its Elevation-volume relationship has been specified therein. The stage volume

relationship of the reservoir as used in the model set up is given in Table 13.2


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Table13.2 Elevation-Volume relationship of reservoir

ELEVATION

(m)

CUMULATIVE VOLUME

(cubic meter)

2064 0

2065 900

2066 6100

2067 17000

2068 33100

2069 55600

2070 83300

2071 116900

2072 156500

2074 249800

2076 3618002078 493500

2080 641000

2082 807900

2084 997800

2086 1205000

2088 1429100

2090 1672000

2092 1927000

13.5.4 Dam (barrage) and Spillway

The dam (barrage) of the project has been represented in the model by its crest length and

crest level at the cross section just downstream of the reservoir. For the dam (barrage) break

study the breach plan data has been specified at barrage location. The breast wall spillway of the

project have been represented as gated inline structures at the barrage location, with its crest

level, gate size and number of gates specified therein. The HEC-RAS model set up for dam

(barrage) and spillway is given in Fig. 13.2

13.5.5 Upstream Boundary

Normally upstream boundary for any hydrodynamic study is the flood hydrograph. This

flood hydrograph can be corresponding to a flood of specific return period i.e. Standard Probable

Flood (SPF) or Probable Maximum Flood (PMF). For the dam (barrage) break model simulation,

the design flood (GLOF+SPF) given in Table 13.3 has been considered as the upstream

boundary. The same has been impinged in to the reservoir as inflow hydrograph.


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Table 13.3 : Design Flood (GLOF+SPF)

Time

(hour)

Discharge

(cumec)

Time

(hour)

Discharge

(cumec)

Time

(hour)

Discharge

(cumec)

0 185 33 2377 66 664

1 190 34 2718 67 615

2 208 35 3112 68 574

3 244 36 3519 69 536

4 301 37 3887 70 502

5 382 38 4160 71 470

6 487 39 12680 72 441

7 637 40 10584 73 415

8 859 41 8694 74 390

9 1168 42 7310 75 367

10 1548 43 6194 76 341

11 1967 44 5323 77 313

12 2390 45 4632 78 285

13 2771 46 4095 79 264

14 3064 47 3671 80 248

15 3238 48 3340 81 239

16 3272 49 3064 82 232

17 3178 50 2825 83 226

18 2996 51 2603 84 22419 2782 52 2383 85 221

20 2578 53 2163 86 218

21 2409 54 1951 87 215

22 2281 55 1759 88 212

23 2191 56 1592 89 207

24 2125 57 1441 90 203

25 2074 58 1309 91 199

26 2033 59 1193 92 198

27 1996 60 1091 93 197

28 1962 61 1000 94 197

29 1936 62 918

30 1933 63 846

31 1984 64 780

32 2127 65 718


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13.5.6 Downstream Boundary

The normal depth has been used as the downstream boundary for the dam (barrage) break

model set up. The downstream boundary has been applied at the cross section of Tawang Chhu

river 15.1 km d/s of Tawang HE Project Stage-I barrage site.

13.5.7 Selection of breach parameters for Dam (barrage) Break study

For any dam (barrage) break study it is extremely difficult to predict the chances of

failure of a dam (barrage), as prediction of the dam (barrage) breach parameters and timing of the

breach are not within the capability of any of the commercially available mathematical models.

However, assuming the dam (barrage) fails, the important aspects to deal with are, time of

failure, extent of overtopping before failure, size, shape and time of the breach formation.

Estimation of the dam (barrage) break flood will depend on these parameters.

The breach characteristics that are used as input to the existing dam (barrage) break

models are i) Final bottom width of the breach, ii) Final bottom elevation of the breach, iii) Left

and right side slope of the breaching section, iv) Full formation time of breach, and v) Reservoir

level at time of start of breach. The breach formation mechanism is, to a large extent, dependent

on the type of dam (barrage) and the cause due to which the dam (barrage) failed.

A study of the different dam (barrage) failures indicate that concrete arch and gravity

dams breach by sudden collapse, overturning or sliding away of the structure due to inadequate

design or excessive forces that may result from overtopping, earthquakes and deterioration of the

abutment or foundation material. The manner in which the failure is to commence can be

specified as one of the following:

At a specified stage (water surface elevation) of the reservoir and duration

At a specified time

13.5.7.1 Critical condition for dam (barrage) break study

The critical condition for a dam (barrage) break study is when the reservoir is at Full

Reservoir Level (FRL) and design flood hydrograph (GLOF+SPF) in the present case is

impinged. Accordingly, in the present study keeping the reservoir at FRL of 2090 m, the

reservoir routing has been carried out by impinging the design flood hydrograph. The flood

discharge capacity of spillway is 12680 cumec while the peak ordinate of (GLOF+SPF) is also

12680 cumec. As the capacity of spillway is sufficient for the design flood it is possible to


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synchronize the outflow with all segments or the ordinates of the design flood. Accordingly the

release through spillway has been synchronized with the ordinates of design flood keeping the

reservoir at FRL throughout the simulation period of 94 hours. The discharge through spillway

gates and the reservoir level as obtained during reservoir routing of SPF is shown in Fig. 13.3.

The dates given on the horizontal axis of the plot are the relative dates only, as used in HEC-RAS

model set up. From the Fig. 13.3 it can be seen that the design flood can be safely passed by

suitable operation of spillway gates. It is important to mention here that the reservoir storage at

FRL is only 1.672 Mcum.

13.5.7.2 Breach parameters selected for sensitivity analysis of dam (barrage) break simulation

Considering the criteria for selection of breach parameters and critical condition for the

dam (barrage) break study as discussed earlier, two different cases of breach parameters as given

in (Table 13.4) have been identified for sensitivity analysis of dam (barrage) break simulations.

In both the two cases, the initial breach elevation has been taken corresponding to the top of

barrage (EL 2092 m). The final bottom elevation of the breach has been taken corresponding to

average ground level. The breach side slope has been taken as zero for concrete gravity dam

breach and 0.50 for compacted rockfill breach. The breach development time has been taken as

10 minutes for the instantaneous breaching of concrete blocks. The breach development time has

been taken as 30 minutes for breaching of compacted rockfill portion.

Table 13.4: Breach parameters considered for sensitivity analysis

Breach Elevation

(m)

Breach

Width (m)

Breach

Development

Time

(Minutes)

Max. Discharge

just d/s of

barrage

(cumec)

RemarksCase

No.

Initial Final

1. 2092 2068 26 10 14910 2 concrete blocks with pier to

pier central line width of 26 m

considered to break up to the

foundation level at EL of

2068 m.

2. 2092 2080 20 30 12654 The compacted rockfill with

bottom breach width of 20 mand side slope of 0.5

considered to breach up to

average ground level at EL

2080 m.

As case-1 results the maximum discharge just downstream of barrage (Table 13.4), the

same has been finalized for detailed outputs of dam (barrage) break simulation.


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13.5.7.3 Dam (barrage) break simulation: Breach width 26 m, breach depth 24 m

For the present case, two concrete blocks with total width of 26 m considered to break till

the foundation at EL 2068 m, 39 hours after the occurrence of design flood. As the reservoir is

very small with storage of 1.672 Mcum only at FRL the assumption of only two concrete blocks

getting breached is completely justified. In the model set up the first ordinate of design flood

hydrograph has been assumed to occur on 02 July 2010 at 0000 hour (which is a relative time

used for mathematical modelling only). Accordingly, the dam (barrage) has been assumed to

breach on 03 July 2010 at 1500 hours, when the peak of design flood also occurs. The maximum

discharge through breach has been found as 2631 cumec occurring on 03 July 2010 at 1500 hours

i.e. (39 hours after the impingement of design flood hydrograph). The time series plot of

discharge through dam (barrage) breach is given in Figure 13.4. From Figure 13.4 it can be seen

that due to very small capacity of the reservoir gets depleted instantly resulting the maximum

discharge through breach of the order of 2631 cumec with the time base of the breach outflow

hydrograph of only 30 minutes.

The dam (barrage) break flood hydrograph just downstream of barrage (comprising of

total discharge through spillway and dam (barrage) breach) with peak 14910 cumec is given in

Fig. 13.5. The maximum discharge, water level and flood travel time at different locations of the

Tawang Chhu river downstream of the dam are given in Table 13.5. From the Table 13.5., it can

be seen that the dam (barrage) breach flood peak just downstream of the barrage is 14910 cumec

which gets reduced to 14740 cumec at a distance 15100 m downstream of barrage. This very

little attenuation of flood peak is primarily due to very steep slope of the river (of the order of

1:100) in the study reach. The flood wave propagation for such steep slope is expected to be like

kinematic wave with very little or nil attenuation of flood peak. The velocity of the flood wave in

the study reach of the river is about 90 km/hour due to very steep slope of the river course.

Table 13.5 Maximum discharge, water level and flood wave travel time at different

locations of Tawang Chhu river for dam (barrage) break of Tawang HE

Project Stage-I (breach width 26m, breach depth 24 m)

(Note: Tawang Chhu 400 denotes the location of river cross sections 400 m d/s of barrage axis)

River

Chainage (m)

d/s of Tawang

HEP Stage-I

barrage axis

Profile

Max

Disch-

arge

Bed

level

Max.

Water

level

Travel

Time

(m) (m3/s) (m) (m) (Date: hr:min:sec)

Tawang Chhu 0 Max WS 14910 2066.29 2078.28 3-7-2010 15:00:00


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Tawang Chhu -400 Max WS 14885 2057.08 2067.73 3-7-2010 15:01:00






























13.5.7.3.1 Dam (barrage) Breach flood hydrographThe dam (barrage) break flood hydrograph just downstream of the Tawang HE Project

Stage-I barrage for breach parameters corresponding to case-1 (Table 13.4) given in Figure 13.5

is also tabulated in Table13.6.


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Table 13.6: Dam (barrage) break flood hydrograph

Date (hr:min:sec) Discharge (cumec) Date (hr:min:sec) Discharge (cumec)

2-7-2010 00:00:00 10 3-7-2010 18:00:00 73562-7-2010 01:00:00 190 3-7-2010 19:00:00 62322-7-2010 02:00:00 207 3-7-2010 20:00:00 5355

2-7-2010 03:00:00 243 3-7-2010 21:00:00 46582-7-2010 04:00:00 299 3-7-2010 22:00:00 41162-7-2010 05:00:00 379 3-7-2010 23:00:00 36882-7-2010 06:00:00 483 4-7-2010 00:00:00 33542-7-2010 07:00:00 632 4-7-2010 01:00:00 30762-7-2010 08:00:00 851 4-7-2010 02:00:00 28352-7-2010 09:00:00 1156 4-7-2010 03:00:00 26132-7-2010 10:00:00 1532 4-7-2010 04:00:00 23932-7-2010 11:00:00 1951 4-7-2010 05:00:00 21732-7-2010 12:00:00 2375 4-7-2010 06:00:00 19612-7-2010 13:00:00 2758 4-7-2010 07:00:00 17682-7-2010 14:00:00 3054 4-7-2010 08:00:00 16012-7-2010 15:00:00 3232 4-7-2010 09:00:00 14492-7-2010 16:00:00 3271 4-7-2010 10:00:00 13162-7-2010 17:00:00 3181 4-7-2010 11:00:00 11992-7-2010 18:00:00 3002 4-7-2010 12:00:00 10962-7-2010 19:00:00 2789 4-7-2010 13:00:00 10042-7-2010 20:00:00 2585 4-7-2010 14:00:00 9222-7-2010 21:00:00 2415 4-7-2010 15:00:00 8492-7-2010 22:00:00 2286 4-7-2010 16:00:00 7832-7-2010 23:00:00 2194 4-7-2010 17:00:00 7213-7-2010 00:00:00 2127 4-7-2010 18:00:00 6663-7-2010 01:00:00 2076 4-7-2010 19:00:00 6173-7-2010 02:00:00 2035 4-7-2010 20:00:00 5753-7-2010 03:00:00 1997 4-7-2010 21:00:00 5373-7-2010 04:00:00 1963 4-7-2010 22:00:00 5033-7-2010 05:00:00 1937 4-7-2010 23:00:00 471

3-7-2010 06:00:00 1933 5-7-2010 00:00:00 4423-7-2010 07:00:00 1982 5-7-2010 01:00:00 4163-7-2010 08:00:00 2122 5-7-2010 02:00:00 3913-7-2010 09:00:00 2368 5-7-2010 03:00:00 3683-7-2010 10:00:00 2706 5-7-2010 04:00:00 3423-7-2010 11:00:00 3099 5-7-2010 05:00:00 3143-7-2010 12:00:00 3506 5-7-2010 06:00:00 2863-7-2010 13:00:00 3876 5-7-2010 07:00:00 2653-7-2010 14:00:00 4152 5-7-2010 08:00:00 2493-7-2010 14:15:00 6062 5-7-2010 09:00:00 2393-7-2010 14:30:00 8219 5-7-2010 10:00:00 2323-7-2010 14:45:00 10366 5-7-2010 11:00:00 2263-7-2010 15:00:00 14910 5-7-2010 12:00:00 2243-7-2010 15:05:00 14111 5-7-2010 13:00:00 221

3-7-2010 15:10:00 12633 5-7-2010 14:00:00 2183-7-2010 15:15:00 12233 5-7-2010 15:00:00 2153-7-2010 15:30:00 11694 5-7-2010 16:00:00 2123-7-2010 15:45:00 11171 5-7-2010 17:00:00 2073-7-2010 16:00:00 10648 5-7-2010 18:00:00 2033-7-2010 16:15:00 10169 5-7-2010 19:00:00 1993-7-2010 16:30:00 9697 5-7-2010 20:00:00 1983-7-2010 17:00:00 8754


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13.5.8 Maximum water level in Tawang Chhu due to occurrence of design flood without

dam (barrage) break

In order to assess the maximum water level at different locations of Tawang Chhu river

downstream of barrage due to occurrence of design flood, without any dam (barrage) break, the

design has been routed through the reservoir assuming initial water level at FRL. As the capacity

of reservoir at FRL is only 1.672 Mcum no mitigation in design flood peak can be expected.

Further, the capacity of spillway is sufficient for the design flood, hence, it is possible to

synchronize the spillway outflow with all segments or the ordinates of the design flood.

Accordingly the release through spillway has been synchronized with the ordinates of design

flood keeping the reservoir at FRL throughout the simulation period.

The maximum discharge, water level and flood travel time at different locations of the

Tawang Chhu river downstream of the barrage are given in Table 13.7. From the Table 13.7 it

can be seen that there is no attenuation in the flood peak. This is due to very large time base of

the peak segment of the design flood hydrograph. Due to very steep slope the flood translation

characteristic is just like kinematic wave without any attenuation of peak. The velocity of the

flood wave propagation in the study reach of the river is about 70 km/hour.

Table 13.7 Maximum discharge, water level and flood wave travel time in Tawang Chhu

due to occurrence of design flood without dam (barrage) breach

River

Chainage (m)

d/s of Tawang

HEP Stage-I

barrage axis

Profile

Max

Disch-

arge

Bed

level

Max.

Water

level

Travel

Time













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Tawang Chhu -11000 Max WS 12596 1696.94 1704.82 3-7-2010 15:09:00Tawang Chhu -11500 Max WS 12596 1675.19 1686.96 3-7-2010 15:10:00








13.5.9 Maximum water level in virgin condition of Tawang Chhu due to occurrence of

design flood

It is important to know the water level for occurrence of design flood in the virgin condition

of the Tawang Chhu river i.e., without barrage. This will indicate inundation levels under virgin

conditions. In this condition the design has been impinged at chainage 0 of the Tawang Chhu

river (location just downstream of barrage) without considering the Tawang HE Project Stage-I.

The maximum discharge, water level and flood travel time at different locations of the

Tawang Chhu river are given in Table 13.8. From the Table 13.8 it can be seen that the

maximum discharge and water level in this case is exactly same as that of routing of design flood

without dam (barrage) break. The velocity of the flood wave propagation in this case is also

about 70 km/hour.


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Table 13.8 : Maximum discharge, water level and average travel time in virgin condition of

Tawang Chhu river due to occurrence of design flood

River

Chainage (m)

d/s of Tawang

HEP Stage-I

barrage axis

Profile

Max

Disch-

arge

Bed

level

Max.

Water

level

Travel

Time








Tawang Chhu -3000 Max WS 12597 1926.11 1938.65 3-7-2010 15:04:00Tawang Chhu -3500 Max WS 12597 1914.97 1925.74 3-7-2010 15:04:00























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13.5.10 Inundation map

An inundation map depicting the downstream areas likely to be inundated by the dam

(barrage) break was prepared. The map was prepared with the help of maximum water level

given in Tables 13.5, 13.7 and 13.8 for different simulated conditions. The inundation map has

been prepared using 1:50000 SOI toposheets. It is clear from this study that in the event of the

barrage break, none of the villages / settlements will be affected because they fall out of the

inundation zone. However, infrastructure assets like short length of road and existing bridge

located on the margins of the likely flooded area may be affected. due to dam (barrage) break

flood (Fig. 13.6). In such a scenario, loss of property could be anticipated in the downstream. The

uncertainties associated with the inundation map, specially breach width, breach depth and

breach development time may cause uncertainty in flood peak estimation and arrival times.

13.6 DISASTER MANAGEMENT PLAN

Dam (barrage) failure though unlikely to happen, poses serious threat to human lives,

property and infrastructures located downstream from the dam (barrage). In order to save a large

numbers of injuries, huge damage to property an integrated disaster management approach is

essential. This approach includes disaster prevention, mitigation, and preparedness. However,

failure of dam (barrage) is a low risk but high impact hazard as they do not occur often but can be

extremely catastrophic. However, over the recent years failure rate has fallen below 0.5%, in

which most of the failures involve small dams. The failures of dam (barrage) are directly related

to the type of dams (barrages).

From the result it is evident that up to about 15.1 km D/S of the Tawang H.E Stage I

barrage, time required in reaching the flood wave elevation to the maximum is of the order of

few minutes. It hardly leaves any possibility of any rescue or evacuation. Since the time available

is very short, the Disaster Management Plan should concentrate on preventive actions.

Surveillance and monitoring programmes are required to be implemented during design

and investigation, construction, first reservoir filling, early operation period and operation &

maintenance phases of the life cycle of barrage. It is desirable that all gates, electricity, public


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announcement system, power generator backups etc. are thoroughly checked before arrival of the

monsoon. As it is clear from the results that upstream water level has significant effect on the

dam (barrage) break flood, the following flood conditions may be considered for different level

of alertness:

i) If u/s water level is at or below FRL (El. 2090 m) and flood is of the order of 20% to 30%

PMF, it may be considered as normal flood condition and normal routine may be

maintained.

ii) If water level is rising above FRL, it may be considered as Level-1 emergency. In this

condition all concerned officials should be alerted so that they may reach at the barrage site to

take suitable actions. Preventive actions may be carried out simultaneously. A suitable

warning and notification procedure may be laid. The local officials should be informed about

the situation.

iii) If u/s water level rises above the barrage top (El. 2092 m), it may be considered as Level-2

emergency. At this point only a few minutes are available for taking any action. All the staff

from the barrage site should be alerted to move to a safe place. The district administration

and the Corporations head office should be informed about the possibility of barrage failure.

The following measures can be taken to avoid the loss of lives and property:

13.6.1 Preventive Measures

Once the likelihood of an emergency situation is suspected, action has to be initiated to

prevent a failure. The point at which each situation reaches an emergency status shall be

specified and at that stage the vigilance and surveillance shall be upgraded. At this stage a

thorough inspection of the barrage shall be carried out to locate any visible signs of distress. The

anticipated need of equipment shall be evaluated and if these are not available at the barrage site,

the exact locations and availability of these equipments shall be identified. A plan shall be drawn

on priority for inspection of the barrage. The barrage, its sluices and non-overflow sections will

be properly illuminated.

13.6.2 Surveillance

Surveillance and monitoring programs are required to be implemented during design and

investigation, construction, early operation period and operation and maintenance phases of the

life cycle of the barrage. It is desirable that all gates, public announcement system, power

generator backups etc. are thoroughly checked before arrival of the monsoon. An effective flood


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forecasting system is required by establishing hourly gauge reading at suitable u/s locations with

real time communicator at the top. An effective dam (barrage) safety surveillance and monitoring

programme also includes rapid analysis and interpretation of instrumentation and observation data

along with periodic inspection, safety reviews and evaluation.

13.6.3 Infrastructural Development

It is essential to improve, modernise and expand the existing network of rainfall and

stream gauging stations in the region. Total financial allocation for the surveillance, monitoring

and infrastructure development would be Rs. 60 lakhs.

13.6.4 Emergency Action & Preparedness Plan

An emergency is defined as a condition of serious nature which develops unexpectedly

and endangers downstream property and human life and requires immediate attention.

Community preparedness is key mitigation factor in the flash flood condition. It involves not only

the emergency action plan and well developed communication but needs awareness programme for

the people residing in downstream areas. Preparedness also involves the development of

infrastructures like escape routes and refuge for people and livestock of flood prone areas.

Following emergency action and preparedness measures are suggested for disaster management

of Tawang HE Project Stage I:

13.6.4.1 Administrative and Procedural Aspects

The Administrative and Procedural aspects of emergency action plan consist of a

flowchart depicting the names, addresses and telephone numbers of the responsible and

coordinating officials. In order of hierarchy, the following system will usually be appropriate. In

the event of potential emergency, the observer at the site is required to report it to the Engineer-

in-charge through a wireless system, if available, or by the fastest communication system

available. The Engineer-in-charge shall be responsible for contacting the Civil Administration,

viz. Deputy Commissioner. In order to oversee all the operations required to tackle the

emergency situations, a centralized command and control room would be set up by the project

authorities at Jang village near the barrage site. The office would also remain in contact with

offices of downstream projects. .

Each person involved with the emergency plan would be made aware of his/her

responsibilities / duties and the importance of work assigned under the Emergency Action Plan.


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All the villages falling under the flood prone zone or on the margins would be connected through

wireless communication system with backup of standby telephone lines. A centralized siren alert

system would be installed at all the Village Panchayats so that in the event of a warning all

villagers can be alerted through sirens rather than informing everybody through messengers

which is not feasible in such emergency situations. A financial allocation of Rs.70 lakhs has

been made in the project cost for setting up of emergency control room and installation of

siren/hooter alert systems at various locations.

13.6.4.2 Communication System

An efficient communication system and a downstream warning system are absolutely

essential for the success of an emergency plan especially when time is of great essence. The

difference between a high flood and a dam (barrage) break situation shall be made clear to the

downstream people in advance through awareness programmes. All the villages falling under the

flood-prone zone or on the margins are required to be connected through wireless system backed

by stand-by telephone lines. A centralized siren system is to be installed at Panchayats so that

villagers can be alerted in the event of any disaster.

Keeping the disaster scenario in mind, any terrestrial system such as land lines or even

cellular towers, etc. are likely to be the first casualty in earthquakes or floods. The system,

therefore, cannot be made operational soon enough. The fault repairs and restoration of

communication services are usually not possible for a considerable period of time after the calamity

has struck. Moreover, it is critical that the communication systems are restored at the earliest so that

relief/medical teams and other personnel can be arranged at the earliest possible time. All the

subsidiary help depends solely on the communication system. As this criterion is paramount,

existing systems such as telephones and telex, etc. are practically of little use in case of such events

and situations. Similarly, microwave links are expected to be down due to collapse of towers, etc.

Restoration of towers and alignment of equipment is again a time consuming activity.

Keeping in view the urgency of services and their dependability during emergency

relevant to the disaster conditions, satellite based systems present an ideal solution. The satellite

based system usually comprises following components:

i) A small dish of approximately one meter diameter

ii) Associated radio equipment

iii) A power source


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The deployment of the system is not dependent on the restoration of land routes. The

existing satellite based communication systems are designed in such a manner that they are able

to withstand fairly high degree of demanding environmental conditions. Secondly, the restoration

of the satellite based system can be undertaken by carrying maintenance personnel and

equipment by helicopters at a very short notice. Even the fresh systems could be inducted in a

matter of an hour or so because most of these are designed for transportability by air. The

deployment takes usually less than an hour. The power requirements are not large and can be met

by sources such as UPS/batteries/generators. Satellite phones are the other option that could

prove very useful for such situations and must be considered by the project authorities.

The cost of deployment and maintenance of a telecommunication system in disaster prone

areas is not as important as the availability, reliability and quick restoration of the system. The cost

of both satellite bandwidth and the ground components of the satellite communication system has

been decreasing rapidly like that of V-SAT (Very Small Aperture Terminal) based systems

supporting a couple of voice and data channels. Some highly superior communication systems in

VSAT without time delay are marketed by National agencies like HECL, HFCL and HCL Comet.

There are two different types of systems with the above mentioned capabilities available in the

market viz. SCPCDAMA and TDMA. However, the first one named SCPCDAMA has been

recommended for Tawang H.E. Project Stage I. Such systems would be installed at different sites

in the area. The estimated cost of installation of such a communication system has been given in

Table 13.9.

Table 13.9 The estimated cost of setting up of a satellite communication system

Sl.No. Product Amount (Rs. in lakhs)

A. Setting up of V-SAT communication system

1. Product Name : SCPCDAMA (6 sites) 150.00

@ Rs.25.00 lakhs per site

2. Generators 10 Nos. (2 KVA) 15.00

3. UPS 4 Nos. (2 KVA) 5.00

4. Installation and maintenance of system, maintenance 60.00

and running cost of UPS, generators, etc for 7 years.

Total 230.00


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13.6.5 Awareness

A few guidelines to be generally followed by the inhabitants of flood prone areas, which

form part of public awareness for disaster mitigation include:

(i). Listen to the radio for advance information and advice.

(ii) Disconnect all electrical appliances.

(iii) Move household goods and all clothing out of reach of flood water.

(iv) Move vehicles, farm animals and movable goods to the highest ground nearby.

(v) Move all dangerous pollutants and insecticides out of reach of water.

(vi) Do not enter flood waters on foot, if it can be avoided.

13.6.6 Response and Recovery

The entire rescue operation depends on the responses from the administration and project

developers. All technical support and medical support must be supplied to the victims in first

phase of operation. The response and Recovery plan include evacuation plan.

13.6.7 Evacuation Plan

Emergency Action Plan includes evacuation plans and procedures for implementation

based on local needs. These are:

(i). Demarcation and prioritization of areas to be evacuated

(ii) Notification procedures and evacuation instructions

(iii) Safe routes, transport and traffic control

(iv) Shelter areas

(v) Functions and responsibilities of members of evacuation team

(vi) The flood prone zone in the event of barrage break of Tawang H.E. Project shall be marked

properly at the village locations with adequate factor of safety. As the flood wave takes

sufficient time in reaching these villages, its populace shall be informed well in time

through wireless and sirens etc. so that people may climb on hills or to some elevated place

beyond the flood zone which has been marked.

The Evacuation Team would comprise:

i) D.M. / Nominated Officer (To peacefully relocate the people to places at higher

elevation with State Administration)

ii) Engineer-in-Charge of the Project (Team Leader)

iii) S.P./ Nominated Police Officer (To maintain law and order)


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iv) C.M.O. of the area (To tackle morbidity of affected people)

v) Sarpanch / Gram Budha of Affected Villages to execute the resettlement operation

with the aid of state machinery and project proponents

vi) Sub-committees at village level

The entire evacuation team will be well equipped with rescue team, medical team,

medicines, emergency vans, boats, helicopter, and other means of transport. The Engineer-in-

Charge will be responsible for the entire operation including prompt determination of the flood

situation from time to time. Once the red alert is declared the whole state machinery will come

into swing and will start evacuating people in the inundation areas delineated in the inundation

map. For successful execution, annually Demo exercise will be done. DM is to monitor the entire

operation. Total financial outlay for the Recovery, Evacuation and rescue operation would be Rs.

70.00 lakhs.

13.6.8 Medical team

After declaration of red alert, District Administration would arrange a team of doctors within a

few hours. The strength of the medical team depends on the magnitude of disaster. The team will

be lash with all possible medical facilities to cure the emergency cases, injuries and water borne

diseases like diarrhoea etc. Total financial budget for the medical team would be Rs. 50.00

lakhs.

13.6.9 Mitigation and Rehabilitation

In event of the barrage break, project authorities would provide adequate Relief Fund. The

package includes the cost of property lost, sustenance grant, livelihood grant, medical grant and

rights and privilege grant on forest resources. A road infrastructure is affected in case of barrage

failure. Rs.160.00 lakhs has been made in the project cost for accidental and emergency causes.

Besides, for the notification and public awareness Rs 50 lakhs has been allocated. In addition, for

miscellaneous expenses another Rs. 50 lakh has been allocated.

13.7 COST ESTIMATE

The estimated total cost of execution of disaster management plan including the

equipment would be Rs. 740.00 lakhs and the break-up is given in Table 13.10.


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Table 13.10 Cost estimate for the disaster management plan of Tawang H.E Project Stage I

Particulars Total cost (Rupees in lakhs)

Surveillance, monitoring and infrastructural development 60.00

Administrative and Procedural Aspects 70.00

Communication System 230.00Recovery, evacuation and rescue operation 70.00

Medical expenditure 50.00

Mitigation & rehabilitation 160.00

Notification and Public awareness 50.00

Miscellaneous 50.00

Total 740.00

18 ch 13 disaster management plan_29.08.10

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