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Lower Siang Hydroelectric Project Chapter-7: Prediction of Impacts

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CHAPTER-7

PREDICTION OF IMAPCTS

7.1 INTRODUCTION

Prediction is essentially a process to forecast the future environmental

conditions that might be expected to occur in the study area because of

implementation of the project. The present chapter outlines the downstream

impacts on account of commissioning of Lower Siang hydroelectric project.

7.2 IMPACTS ON RIVER LENGTH WITH NORMAL FLOW

The key impact on hydrologic regime due to construction of the proposed

Lower Siang hydroelectric project is on account of change in the free flowing

condition of the river. With the construction of the proposed project, a

reservoir length of 77.5 km along main river Siang and 28.5 km along river

Siyom at FRL, with an area of 51.51 km2 (5151 ha) and gross storage

capacity of 1421 Mm3 (at FRL) will be formed.

The river which in the present stage (pre-project scenario) is flowing freely

over a stretch of 106 km, will get converted into a reservoir. The conversion

of free flowing river into a reservoir will have an adverse impact on riverine

ecology.

Normally, under such circumstances, adverse impacts on water quality are

anticipated on account of increase in residence time in the reservoir.

However, in the catchment area of the proposed Lower Siang hydroelectric

project, pollution loading is quite low, on account of low population density,

low cropping intensity with minimal use of agro-chemicals and absence of

industrialization in the area. Thus, adverse impacts on water quality due to

conversion of river into a reservoir is not anticipated.

7.3 MODIFICATION IN HYDROLOGIC REGIME

The proposed Lower Siang hydro electric project, envisages generation of

(9*300) 2700 MW of hydro power. The average 10 daily flow for the

available data for the period 1978-79 to 1988-89, 1990-91 to 1992-93 and

2001-02 to 2004-05 is given in Table-7.1.


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TABLE-7.1 Average 10 daily flow at the dam site with 18 years data

Month Discharge (cumec) I 4964.7 II 6420.0

June

III 7597.8 I 8916.9 II 8809.4

July

III 8884.1 I 7408.4 II 7109.6

August

III 7901.4 I 7605.6 II 6887.2

September

III 5816.0 I 5025.3 II 4074.8

October

III 3094.1 I 2407.1 II 1848.6

November

III 1553.2 I 1416.0 II 1301.6

December

III 1137.9 I 1087.8 II 1014.1

January

III 960.5 I 985.5 II 965.9

February

III 1061.0 I 1186.6 II 1267.2

March

III 1398.0 I 1767.9 II 2065.7

April

III 2285.5 I 2869.2 II 3355.1

May

III 4000.8

The discharges on account of power generation with average discharges are

given in Table-7.2. In case the inflow is more than 5440 cumec, the water

will flow over the spillways also.


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TABLE-7.2 Discharges on account of maximum power generation with

average inflows Max. Base Load (with Min. 180MW) for 21 hrs

Peaking of 2700MW for 3 hours

Month

D/stream Discharge(cumec) D/stream Discharge (cumec) I 4896.80 5440.00 II 5440.00 5440.00

June

III 5440.00 5440.00 I 5440.00 5440.00 II 5440.00 5440.00

July

III 5440.00 5440.00 I 5440.00 5440.00 II 5440.00 5440.00

August

III 5440.00 5440.00 I 5440.00 5440.00 II 5440.00 5440.00

September

III 5440.00 5440.00 I 4932.00 4932.00 II 3952.34 4932.00

October

III 2831.54 4932.00 I 2046.40 4932.00 II 1408.11 4932.00

November

III 1070.51 4932.00 I 913.71 4932.00 II 782.97 4932.00

December

III 595.89 4932.00 I 538.63 4932.00 II 454.40 4932.00

January

III 393.14 4932.00 I 421.71 4932.00 II 399.31 4932.00

February

III 508.00 4932.00 I 651.54 4932.00 II 743.66 4932.00

March

III 893.14 4932.00 I 1315.89 4932.00 II 1656.23 4932.00

April

III 1907.43 4932.00 I 2574.51 4932.00 II 3129.83 4932.00

May

III 3867.77 4932.00


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In the pre-project scenario, the average ten daily discharges in monsoon

season ranges from 6420 to 8884 cumec. As the reservoir would be

maintained at MDDL of El 225.5 m, there would not be any storage. Thus,

the discharge in downstream of dam in monsoon season will remain the

same.

The average discharge for Lower Siang hydro electric project is higher than

the rated discharge for a period of 130 days from 10th June to 20th October.

Thus, in monsoon months, peaking power can be generated for 24 hours.

In non-monsoon season, the number of hours for which the reservoir can be

operated in peaking condition will be much lower. In lean season, i.e. from

November to March peaking operation shall be 3 to 10 hours only. In order

to ensure peaking of minimum 3 hours, the water availability for rest of the

period will reduce to the extent that remain insufficient for survival of aquatic

life. To avoid such situation, it is proposed to operate the power station with

minimum discharge equal to at atleast 50% of the inflow in the river which

would generate atleast 180 MW of power. This will result in continuous

discharge of 350 cumec. This corresponds to about 50% of flow in the lean

season. Considering the entire period from November to March, the

discharge for base load works out to 25.11% of the total lean season flow.

The details are given in Table-7.3.

TABLE-7.3 Details of lean season discharges

Month Average inflow (cumec)

D/stream Discharge for Maximum Base Load (cumec)

D/stream Discharge for Minimum Base Load (cumec)

I 2407.1 2046.40 1205 II 1848.6 1408.11 950

November

III 1553.2 1070.51 814 I 1416.0 913.71 750 II 1301.6 782.97 660

December

III 1137.9 600 600 I 1087.8 550 550

January II 1014.1 540 540


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Month Average inflow (cumec)

D/stream Discharge for Maximum Base Load (cumec)

D/stream Discharge for Minimum Base Load (cumec)

III 960.5 500 500 I 985.5 500 500 II 965.9 500 500

February

III 1061.0 520 520 I 1186.6 651.54 620 II 1267.2 743.66 650

March

III 1398.0 893.14 750

7.4 MODELING STUDY FOR FLOODS DISCHARGE IN PRE-DAM AND POST DAM SCENARIOS

In this section an assessment of inundation area due to flood caused after

construction of dam over and above the inundation due to the heaviest flood

that may occur without dam being constructed has been made. The probable

maximum flood hydrograph which has been used as the upstream boundary of

the dam model set up and applied at chainage “0” km of the reservoir branch is

given in Table-7.4.

TABLE-7.4 Probable Maximum Flood Hydrograph (PMF)

Time (hour

Discharge cumec)

Time (hour)

Discharge (cumec)

Time (hour)

Discharge (cumec)

Time (hour)

Discharge (cumec)

0 30500 51 52591 102 37348 153 32894 1 31652 52 51088 103 37187 154 32877 2 31665 53 50374 104 36974 155 32860 3 32459 54 49659 105 36760 156 32833 4 32470 55 49472 106 36466 157 32807 5 32486 56 49670 107 36318 158 32780 6 32512 57 50337 108 36171 159 32767 7 32650 58 50116 109 35998 160 32753 8 32788 59 49941 110 35826 161 32727 9 32926 60 49952 111 35611 162 32700 10 33063 61 50022 112 35504 163 32674 11 33288 62 50092 113 35396 164 32661 12 33512 63 50231 114 35282 165 32648 13 33960 64 51188 115 35167 166 32638 14 33909 65 52543 116 34994 167 32627 15 34858 66 54096 117 34908 168 32617 16 35359 67 55879 118 34821 169 32611 17 35860 68 57833 119 34718 170 32606


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Time (hour

Discharge cumec)

Time (hour)

Discharge (cumec)

Time (hour)

Discharge (cumec)

Time (hour)

Discharge (cumec)

18 36863 69 60115 120 34615 171 32592 19 37814 70 59641 121 34478 172 32577 20 38765 71 58890 122 34409 173 32562 21 40180 72 58047 123 34340 174 32555 22 40441 73 56861 124 34265 175 32547 23 40702 74 55119 125 34190 176 32540 24 41137 75 53238 126 34095 177 32533 25 41572 76 52038 127 34048 178 32526 26 42164 77 51600 128 34001 179 32522 27 42755 78 51162 129 33933 180 32518 28 43938 79 50724 130 33865 181 32521 29 45399 80 50287 131 33806 182 32524 30 46860 81 49898 132 33751 183 32530 31 48670 82 49509 133 33696 184 32536 32 50690 83 48731 134 33641 185 32542 33 53276 84 47797 135 33586 186 32541 34 53085 85 46843 136 33530 187 32539 35 52855 86 45140 137 33473 188 32538 36 52730 87 44289 138 33417 189 32537 37 52445 88 43437 139 33388 190 32536 38 52240 89 42668 140 33360 191 32027 39 52034 90 41899 141 33310 192 31518 40 51622 91 41075 142 33262 193 31009 41 52424 92 40663 143 33213 194 30500 42 53504 93 40251 144 33188 43 55051 94 39819 145 33163 44 56597 95 39386 146 33121 45 58673 96 39025 147 33078 46 58340 97 38694 148 33036 47 57694 98 38362 149 33015 48 56794 99 38096 150 32994 49 55894 100 37829 151 32961 50 54094 101 37508 152 32927

For the present case the study reach of the river is about 51.13 km

downstream of the Siang Lower dam axis. In order to have no influence of the

downstream boundary in the study reach of the river the same has been

applied at a location 60 km downstream of the dam site. The downstream

boundary (stage-discharge relationship) has been worked out using Manning’s

equation.


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Comparison of Maximum Discharge and Water Level

For the different hydrodynamic scenario simulated so far, the maximum

discharge and water level occurring at different locations of Siang river

downstream of Lower Siang dam have been compared in Tables-7.5 and 7.6

respectively.

TABLE-7.5

Comparison of maximum discharge obtained in different cases Maximum discharge (cumec) Chainage (m)

d/s of Lower Siang dam

PMF with dam

PMF in pre-project scenario

Percentage variation w.r.t pre-project scenario

SIANG 494.00 58172 60088 3.19 SIANG 1482.00 58171 60035 3.10 SIANG 2470.00 58169 59985 3.03 SIANG 3458.00 58168 59936 2.95 SIANG 4446.00 58167 59891 2.88 SIANG 5381.67 58166 59852 2.82 SIANG 6265.00 58165 59815 2.76 SIANG 7148.33 58163 59779 2.70 SIANG 8031.67 58162 59743 2.65 SIANG 8915.00 58161 59723 2.62 SIANG 9798.33 58159 59729 2.63 SIANG 10700.83 58158 59735 2.64 SIANG 11622.50 58156 59740 2.65 SIANG 12544.17 58154 59748 2.67 SIANG 13465.83 58152 59756 2.68 SIANG 14387.50 58150 59763 2.70 SIANG 15309.17 58148 59771 2.72 SIANG 16207.00 58146 59777 2.73 SIANG 17081.00 58143 59785 2.75 SIANG 17955.00 58139 59794 2.77 SIANG 18829.00 58134 59804 2.79 SIANG 19703.00 58128 59815 2.82 SIANG 20635.00 58119 59827 2.85 SIANG 21625.00 58110 59838 2.89 SIANG 22615.00 58112 59848 2.90 SIANG 23605.00 58119 59855 2.90


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Maximum discharge (cumec) Chainage (m) d/s of Lower Siang dam

PMF with dam


Percentage variation w.r.t pre-project scenario

SIANG 24595.00 58124 59858 2.90 SIANG 25563.89 58130 59856 2.88 SIANG 26511.67 58134 59849 2.87 SIANG 27459.45 58138 59834 2.83 SIANG 28407.22 58140 59811 2.79 SIANG 29355.00 58140 59778 2.74 SIANG 30302.78 58139 59733 2.67 SIANG 31250.55 58136 59674 2.58 SIANG 32198.33 58131 59623 2.50 SIANG 33146.11 58123 59655 2.57 SIANG 34100.50 58111 59684 2.64 SIANG 35061.50 58096 59706 2.70 SIANG 36022.50 58077 59719 2.75 SIANG 36983.50 58077 59723 2.76 SIANG 37944.50 58089 59716 2.72 SIANG 38905.50 58097 59699 2.68 SIANG 39866.50 58103 59670 2.63 SIANG 40827.50 58105 59628 2.55 SIANG 41788.50 58102 59565 2.46 SIANG 42749.50 58093 59505 2.37 SIANG 43678.89 58076 59522 2.43 SIANG 44576.67 58053 59528 2.48 SIANG 45474.45 58024 59521 2.52 SIANG 46372.22 57990 59501 2.54 SIANG 47270.00 58000 59467 2.47 SIANG 48167.78 58004 59418 2.38 SIANG 49065.55 58003 59353 2.27 SIANG 49963.33 57997 59273 2.15 SIANG 50861.11 57984 59291 2.20

It can be seen from Table-7.5, that there is attenuation of flood peak in the

study river reach to some extent, i.e., about 2.2 to 3.2%. Further due to

large capacity of the reservoir the PMF peak gets mitigated by about 2000

cumec in comparison to virgin condition of river, by storage available

between FRL and MWL of Lower Siang reservoir.


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TABLE-7.6 Comparison of maximum water level obtained in different cases

Maximum water level (m) Chainage (m) d/s of Siang Lower dam

Bed Level (m)

PMF with dam


Change in water level w.r.t. pre-project scenario

SIANG 0.00 149.40 194.90 195.37 0.47 SIANG 988.00 * 194.23 194.68 0.45 SIANG 1976.00 193.41 193.85 0.44 SIANG 2964.00 192.37 192.80 0.43 SIANG 3952.00 190.87 191.28 0.41 SIANG 4940.00 158.83 188.33 188.72 0.39 SIANG 5823.33 186.36 186.74 0.38 SIANG 6706.67 184.49 184.86 0.37 SIANG 7590.00 182.74 183.10 0.36 SIANG 8473.33 181.10 181.45 0.35 SIANG 9356.67 179.58 179.90 0.32 SIANG 10240.00 140.76 178.16 178.45 0.29 SIANG 11161.67 177.92 178.21 0.29 SIANG 12083.33 177.37 177.65 0.28 SIANG 13005.00 176.61 176.89 0.28 SIANG 13926.67 175.64 175.90 0.26 SIANG 14848.33 174.33 174.57 0.24 SIANG 15770.00 149.96 172.40 172.61 0.21 SIANG 16644.00 170.85 171.05 0.20 SIANG 17518.00 169.38 169.57 0.19 SIANG 18392.00 167.98 168.17 0.19 SIANG 19266.00 166.65 166.84 0.19 SIANG 20140.00 145.67 165.40 165.59 0.19 SIANG 21130.00 163.86 164.04 0.18 SIANG 22120.00 162.34 162.51 0.17 SIANG 23110.00 160.84 160.99 0.15 SIANG 24100.00 159.43 159.57 0.14 SIANG 25090.00 138.19 158.36 158.49 0.13 SIANG 26037.78 157.28 157.40 0.12 SIANG 26985.55 156.17 156.29 0.12 SIANG 27933.33 155.03 155.14 0.11 SIANG 28881.11 153.85 153.95 0.10 SIANG 29828.89 152.64 152.73 0.09 SIANG 30776.67 151.39 151.47 0.08 SIANG 31724.45 150.08 150.16 0.08 SIANG 32672.22 148.73 148.80 0.07 SIANG 33620.00 137.66 147.32 147.38 0.06


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Maximum water level (m) Chainage (m) d/s of Siang Lower dam

Bed Level (m)

PMF with dam


Change in water level w.r.t. pre-project scenario

SIANG 34581.00 145.78 145.85 0.07 SIANG 35542.00 144.24 144.31 0.07 SIANG 36503.00 142.70 142.77 0.07 SIANG 37464.00 141.15 141.23 0.08 SIANG 38425.00 139.61 139.68 0.07 SIANG 39386.00 138.32 138.39 0.07 SIANG 40347.00 137.04 137.12 0.08 SIANG 41308.00 136.09 136.16 0.07 SIANG 42269.00 135.86 135.93 0.07 SIANG 43230.00 121.97 134.41 134.49 0.08 SIANG 44127.78 133.34 133.42 0.08 SIANG 45025.55 132.68 132.76 0.08 SIANG 45923.33 131.53 131.60 0.07 SIANG 46821.11 130.63 130.70 0.07 SIANG 47718.89 129.73 129.79 0.06 SIANG 48616.67 129.08 129.14 0.06 SIANG 49514.45 128.42 128.48 0.06 SIANG 50412.22 127.74 127.80 0.06 SIANG 51310.00 114.71 127.03 127.09 0.06

From the Table-7.6, it can be concluded that :

i) The water level variation in the river reach with dam is from EL

194.90 to 127.03 m.

ii) The reservoir dampens the PMF by about 2000 cumec and water

level along the river reach with dam is about 6 cm to 40 cm less in

comparison to virgin river condition.

The water levels, given in Table-7.6, has been used for the preparation of

inundation map. The inundation map is enclosed as Figure-7.1, and this map

can be used for estimating the period of inundation corresponding to a

particular elevation during the preparation of disaster management plan. The

maximum water level at these cross sections due to dam break flood has also

been superimposed over them.


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7.5 IMPACTS ON FISHERIES

7.5.1 Impacts due to modification of flow regime

As mentioned earlier in section 7.3, commissioning of a hydroelectric project,

significantly affects the hydrologic regime. The proposed project too will have

similar impact on hydrologic regime, with a corresponding impact on riverine

ecology including fisheries. The free flowing water regime will be affected

over a stretch of about 106 km, upstream of the dam site. The dam will store

water to enable peaking power generation. As a result, barring for a period

from June to October, the river Siang will have alternate periods of flows i.e.

flow for base load equal to around 50% of river inflow and flow

corresponding to peaking flow equal to 4932 cumec.

During project operation phase, the hydrologic regime in river Siang would

be highly modified, on account of:

• Hydrograph getting completely modified

• Modification of floods including suppression and alteration of flood

peaks.

• Conversion of free flowing stretch of river into a reservoir.

The modification of downstream river flow characteristics (regime) by an

impoundment can have a variety of negative effects upon fish species. These

include:

• loss of stimuli for migration • loss of migration routes and spawning grounds • decreased survival of eggs and juveniles • diminished food production.

Regulation of stream flow during the migratory period can alter the seasonal

and daily dynamics of migration. Regulation of a river can lead to a sharp

decrease in a migratory population, or even to its complete elimination.

7.5.2 Impacts due to regulation of flow for hydropower generation

As outlined in Table-7.2, a minimum flow of 500 to 2046 cumec (Refer Table-

7.3), with as average of 50% lean season flow. Thus, minimum flow will

always remain in the river Siang throughout the year. This will facilitate the


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endemic fishery downstream as water availability is ensured throughout the

year. The lotic habitat will change in to the lacustrine habitat upstream dam

site.

7.5.3 Impact of dam on the fish communities

As a part of the study, discussions were held with the local fishermen and

market summary, it was evident that the occurrences of fishes downstream

site at Ranaghat where daily catch of about 1000 kg is received from river

Siang. However, ecological conditions of the catchment area provide scope

for possibility of upward migration of fishes from Siang during monsoons,

when there is high discharge in river Siang. Various streams like Yamne,

Siyom and Bolenge upstream and the Sipro, Sibot, Sille, Siku and Sibo

Korang which confluence with river Siang, downstream of dam site before the

river emerges into plains. These rivers shall provide migratory routes for

spawning and breeding of fishes.

The building of a storage dam will have a major impact on fish population,

migrations and other fish movements can be stopped or delayed, affect,

accessibility to their habitat, which plays an important role in population

sustainability. The modification in discharge regime or water quality can also

have indirect impact upon fish species.

In the project operation phase, migration of Tor sp. will be affected or

curtailed. All the endemic hill stream fishes will be affected due to habitat

destruction such as loss of breeding grounds, spawning grounds, substratum,

food and shelter by submergence of river bed and change in water chemistry

from flowing water to impounded water. One of the major impact of the

construction of a high dam on fish populations is the decline of migratory

species.

Impoundments can have an effect on the timing of fish downstream

migration. Such delays can have rather drastic effect by exposing fish to

intensive predation, to nitrogen super saturation and several other hazards

such as exposure to disease organisms and parasites. The delay can also


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result in a significant portion of the juvenile population residualizing and

spending several months in fresh water (Ebel, 1977).

Dam construction will lead to impoundment resulting in transformation of

lotic to lentic habitats. The proposed dam will hold back a huge volume of

water converting into lacustrine/semi lacustrine water body. There is

likelihood of many species being wiped out and invasion of many fish, algal

and fungal species. A large reservoir is not conducive for sustenance of

streamlined species, particularly loaches and cat fish (Glyptothorax spp.,

Botia spp., Nemacheilus spp, Pseudecheneis sp. Sisor sp.).

7.5.4 Habitat fragmentation

The proposed dam of 86 m height (above river bed) would lead to the habitat

fragmentation, which directly affects fish migration of Mahaseer (Tor putitora

and T. tor), Labeo gonius and Acrossocheilus hexagonolepis, main migratory

fish in river Siang as these fish undertake upstream migration for the

breeding purpose in monsoon season.

7.5.5 Impacts on migratory activity due to Modification of discharge

The modification of downstream river flow characteristics (regime) by an

impoundment can have a variety of negative effects such as: loss of stimuli

for migration, loss of migration routes and spawning grounds, decreased

survival of eggs and juveniles, diminished food production. Regulation of

stream flow during the migratory period can alter the seasonal and daily

dynamics of migration. Regulation of a river can lead to a sharp decrease in a

migratory population, or even to its complete elimination.

Any reduction in river discharge during the period of migratory activity can

diminish the attractive potential of the river, hence the numbers of brooder/

spawner entering the river is reduced. Because of this, regulation of a river

can greatly influence the degree of migration to the non-regulated part of the

river below the dam site. The fluctuations of water-level and velocities due to

dam could have adverse impact on fish: spawning behaviour could be


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inhibited, juveniles could be swept downstream by high flows, sudden

reductions in flow could leave eggs or juveniles stranded.

7.5.6 Migration Pattern

The endemic Mahseer is a fresh water stream fish, having their territorial

regime confined up to foothills migrate up and down for spawning. The

Mahseer is the most important game and food fish in the Himalayan rivers.

The snow melt water from the springs induces spawning. The snow melts in

the months of April to May. During this period these fishes migrates

upstream for spawning. After spawning, fish migrates downstream during

September. The fish migrates for considerable distances upstream in search

of suitable spawning grounds (Badola and Singh, 1984; Nautiyal and Lal,

1984; Singh, 1988).

Adults and juveniles of species such as Schizothorax spp and Tor spp move

upstream and downstream respectively in river Siang including its streams

Yamne and Siyom. Majority of the tributaries serve as the routes through

which the fish can have easy access to the spring-fed placid streams that

provide congenial environment for the fish to breed. The presence of gravel,

pebbles, sand and bankside vegetation is prerequisite for Mahseer to build

their spawning nests. Mahseer requires stable, well oxygenated, gravel

habitats to spawn. The eggs laid in the gravels require well-oxygenated

water (Sharma, 1984).

The proposed Lower Siang hydroelectric project on river Siang at Bodak will

have adverse impact on the migration of Mahseer fishes (Tor tor Hamilton

and Tor putitora Hamilton). Various tributaries, e.g., Yamne, Siyom and

Boleng join river Siang upstream dam site. The Sipro, Sibot, Sille, Siku and

Sibo korang confluence with river Siang downstream of the dam site and

before the river emerges into plains.

It is proposed to operate the Lower Siang hydroelectric project with a base

load of 180 MW with a corresponding discharge of 350 cumec and base load

would be increased suitably as river inflow increases. The depth of flow due


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to minimum discharge corresponding to base load at various sections

downstream of dam site is given in Table-7.7. The depth of flows at

discharge corresponding to base load at various sections as per ground

truthing by installation of G&D site is given in Table-7.8.

TABLE-7.7

Depth of flows at Deepest Bed Level corresponding to minimum discharge of 350 cumec for minimum base load at various sections

Distance downstream of dam site (km) Depth of flow (m) 4.94 2.88 10.24 13.13

(River bed is too deep) 15.77 3.37 20.14 2.41 25.09 2.69 33.62 1.21 43.23 6.08 51.31 4.19

TABLE-7.8

Depth of flows at Deepest Bed Level corresponding to minimum discharge of 350 cumec for minimum base load at various sections

as per ground truthing by installation of G&D site Distance downstream of dam site (km)

Depth of flow (m)

33.62 2.81 43.23 7.77 51.31 5.88

Water depth at a section just downstream of Dam axis

The water depth with the discharge of 350 cumec at a section downstream of dam axis has been assessed as 2.88 m as per analysis carried out by MIKE 11. Alternatively, the water depth has been measured by taking soundings at a section near the dam axis which shows that for a discharge of 1608 cumec the water depth varies from 5.46m at the banks to 15.36 m in middle of the river with the water way of 210 m. From these data it has been assessed that the water depth corresponding to the discharge of 350 cumec would be 3.2 m at the middle of the river with the water way of 98 m.


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The downstream flow of 350 cumec will be for very less period (i.e a few

days). The minimum depth of flow for the sections downstream of dam site is

almost 2.81 m which is sufficient depth of flow required to maintain riverine

fisheries as specified in CIFRI report.

The proposed dam height is about 86 m from the river bed which would have

major impact on fish migration. As the water availability is ensured

throughout the year, this might not affect the downward movement of fishes.

The upward migration will not be possible in river Siang due to the proposed

storage dam, which will not only act as barrier for fish migration but shall

also contribute to change in the fish habitat due to formation of reservoir.

The lotic habitat will change in to the lacustrine habitat from the crest of dam

up to 78 km distance upstream. The migration of fish from the river or

stream downstream to upstream of dam site will be curtailed due to blockage

of migratory route. Therefore, proposed dam can lead to adverse impacts on

the endemic fishes due to fragmentation of habitat, which play a major role

in maintaining the aquatic ecosystem and contribute to the livelihood

opportunities for the local inhabitants.

To protect the endemic fishes, the remaining part of small channels/

tributaries downward from dam site up to 5km downstream Bodak and side

streams should be declared as protected areas. The spawning grounds shall

be protected for the required habitats for their existence by imposing ban on

fishing. The option left behind for propagation of endangered Mahseer is

either in/ex situ/ex conservation. The fish hatchery should be developed,

yearlings and fish seed should be placed in the submergence zone for further

improvement for the population.

7.5.7 Identification of Spawning Grounds

The large number of young ones and fingerlings were visually observed and

counted on the spot that varied from 150 to 300 per square meter in the

undisturbed, virgin places rich in nutrients. Small sized fishes such as Barilius


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sp, Puntius sp, and Nemacheilus sp, strictly breed only in the small hill

streams where as Himalayan trout (Schizothorax sp) and Mahseer (Tor sp)

spawn in the shallow-water pockets on the river bank. Labeo sp prefer the

stagnant water while Schizothorax sp lay eggs in the slow flowing water and

adhered to the stones. There are many spawning sites that occur for the

endemic Mahseer and other hill stream fishes.

Some of the breeding, spawning and nursery grounds in the study area are

located within 5-6 km stretch of River Siang near village Bodak. At base load

of 180 MW, more than 1.0 m water depth will be available throughout the

year at this site.

River Siang consists of gravelled substratum with rocky banks and deposition

of sand at meander curve. Water current (0.6-1.2m/s), including water

temperature (16-190C), DO (>8 ppm) and water depth all are found

favourable with riffle, pools and run habitat and heterogenous sub-stratum.

Two small streams of Sibot and Sirpo Korang (11-15 m wide, 2-4% gradient)

confluence with river Siang at a distance of about 1 km downstream of the

dam site. Thus, the area from Bodak to downstream meander curve where

river is opening into much wider valley, need to be protected for restoration

of fishery and to sustain downstream fish production at Ranaghat area. The

flow regime from Ranaghat upstream also observed suitable for breeding,

spawning and feeding of cat fishes and other carps due to buffer zone and

mix habitat of hillstream and floodplains.

7.6 IMPACTS ON RIVERINE ECOLOGY DURING CONSTRUCTION PHASE The following impacts are envisaged on riverine ecology during construction

phase:

• Silt load will increase turbidity of flowing water downstream and

deteriorate the physical and chemical quality of water. The settling of

sediments on the substratum and flow of turbid water will cause loss

of biodiversity.


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• The immediate impact would be occurred on benthos invertebrates

lives beneath the pebbles, gravel and stones and other surface

attached algae. These benthos and algal matter constitute fish food

both for bottom dweller Schizothorax sp., Glyptothorax sp., and

column dweller Mahseer in lower reaches and rainbow /brown trout

introduced in the upper region.

• The deposition of sand and silt on the river banks will also damage and

stress the growth of riparian flora which provides shelter and food for

numerous insects, dragon fly, damsel fly, butterfly and other species

completing their life cycle in the water and their larvae constitutes

important part of micro and macro fauna of aquatic environment.

7.7 IMPACTS ON WATER LEVEL DUE TO PROJECT OPERATION

In monsoon months, peaking power can be generated for 24 hours. As the

scheme is run of the river, during monsoon season, there will not be any

significant impact on downstream flow volume. In non-monsoon seasons, the

number of hours for which the reservoir can be operated in peaking condition

will be much lower, i.e. for 3 to 10 hours. In the remaining times, reservoir

shall be filled upto FRL. During lean season, to avoid drying of river stretch, it

is proposed to operate the hydro electric project at minimum base load of

180 MW when the flow in the river are minimum. This will result in

continuous minimum discharge of 350 cumec. The channel routing for

discharge for peaking and base load operations was conducted using Mike-II

software. The water levels w.r.t. discharge variation and ground levels is

given in Table-7.9. The water level with respect to discharge variation and

ground level as per ground truthing by installation of G&D site is given in

Table-7.10.


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TABLE-7.9 Water levels w.r.t. discharge variation and ground levels

Water Level (El. m)

Remarks

Section No.

Distance from

Lower Siang

Dam Axis (km)

For 5063 cumec for

3 hrs

For 328 cumec for 21

hrs

General Bed

Level (El. m) River

Bank

S-4 25.09 147.96 140.88 138.19 152.50 End of Hilly terrain

S-3 33.62 142.61 138.87 137.66 150M D. Ering Sanctuary at El 150m

S-2 43.23 133.33 128.05 121.97 137.59 D. Ering Sanctuary at El 137m


TABLE-7.10 Water levels w.r.t. discharge variation and ground levels as per

ground truthing by installation of G&D site Water Level

(El. m) Remarks Section

No.

Distance from

Lower Siang Dam Axis (km)

For 4932 cumec

for 3 hrs


hrs

General Bed

Level (El. m) River

Bank

S-3 33.62 142.25 140.31 137.50 150m D. Ering Sanctuary at El 150m



7.8 IMPACTS ON SOCIO-ECONOMIC ENVIORNMENT

As mentioned earlier in Chapter-6, there are 23 villages in the study area

villages, including 10 villages on the right bank and 13 villages on the left

bank. The impacts on various aspects of socio-economic environment are

presented in the following paragraphs.


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Impacts on Agriculture

During the field studies and public consultations, it was learnt that in the

down stream study area villages, the locals had historically cut the forest

areas along the river bank and had begun sedentary cultivation. The locals

cultivate mainly paddy. In addition vegetables are also grown. It was also

observed that wherever irrigation was possible, the fields were irrigated by

lifting water from river Siang. In irrigated lands too, mainly paddy was

grown. In lands at slightly higher elevation, where water could not be lifted

for irrigation, farmers practiced jhum cultivation within their designated plots

of lands.

At/near Partung and Borguli village, which is roughly about 33.62 km

downstream of the dam, there are two islands in the flood plain of river

Siang. Towards left bank, main island is generally known as Pekar chapori,

the general ground level of this island is in the range of about ±149 m, which

is sufficiently above the normal flood levels. Similarly towards right bank, the

main island is named as Pasighat Reserved Forest. The general ground level

of this island is in the range of about ±150 m. This island also does not get

affected during normal monsoon season. The likely water levels at this

location due to peak load discharge after the implementation of the project

would therefore be sufficiently below the general ground levels of these two

main islands.

At / near village Pilumukh and Namsing around 43.23 km downstream of the

proposed dam site, general ground level of main island ie D’Ering Sanctuary

is ±137 m. This island also does not get affected during normal monsoon

floods. The likely water levels corresponding to peak load discharge will

therefore be much lower at this location.

At / near village Oyan and Gadum, which is about 51.31 km downstream of

proposed dam site, there is a big island in the flood plain of river Siang

named as Kobo Chapori. The general ground level of this island is in the

range of ±133 m, which is sufficiently above the normal monsoon flood


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levels. The discharges in the river due to peak load after implementation of

this project will therefore have no impact on this island as the likely water

levels will be much lower.

In addition to the main islands as described above, there are number of small

shoals which get exposed during the winter season when the water levels

recedes to lower levels. Some of these shoals will get submerged due to peak

load discharge after implementation of the project. The location and size of

these shoals keep on changing every year depending upon the river

morphology during the monsoon season. As gathered during field surveys

these shoals do not contribute significantly in sustaining the wild life as

compared to the large size shoals and islands in the flood plains of Siang

river in the downstream reaches.

From the above, it may be seen that as general ground levels of the main

islands in the flood plain of Siang river in the downstream reaches, are at

higher levels, these are not likely to get submerged due to the peak load

discharges after the implementation of the project and therefore there is no

adverse environmental impact on the wildlife in existence in the sanctuaries

and grazing of animals in various Chaporis.

Impacts on Jhum Cultivation

Jhum cultivation, which is done at higher elevations at a distance from the

banks, where, water cannot be lifted for irrigation will also not be affected as

water will remain below the river bank, even for peaking discharge.

Livestock

It was learnt during public consultations and discussions with the Animal

Husbandry and Livestock Department that a large number of farmers cultivate

and rear cattle. Locals engage shepherds, who are mostly from Bihar and

Assam, to tend to their cattle. It was learnt that many of the farmers during

lean season make their cattle cross the river to the many islands within the

river. The cattle are left to graze on the pasture and grass lands, which grow


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naturally in these islands/ river banks which are at a higher levels than the

estimates levels upto which water would rise during peaking operation with

4932 cumec of discharge for 3 hours.

7.9 IMPACTS ON D’ ERING WILDLIFE SANCTUARY

The D’ Ering Wildlife Sanctuary is supported with rich wetland biodiversity.

Despite many favourable conditions, this wetland is not free from threats.

The eastern and western sides have extensive growth of macrophytes weeds

Typha, Carex, Hydrilla etc. and elephant grasses varies from 2 m to 4 m

height. The major threat to wetlands is siltation due to deposition of

sediment/silt during rains coming in river Siang in all around the wildlife

sanctuary area.

Other threats include construction and encroachments at Pasighat. Sediment

load from surrounding mountains of the Wildlife area increases load due to

runoff during rains, which require careful consideration to check to the

siltation in wetland water depth and quality. The present survey reveals that

threats to D’ Ering Wetland lies within and not due to the proposed dam.

Proposed dam will help to manage the accidental flood which carries huge

sediments and silt load.

The present investigations revealed that the river Siang harbours 36 fish

species and all are present in the wetland area. Common species viz., Tor

spp., Channa spp., Puntius spp and cat fishes like seenghala, magur,

seenghi, and other carp species. The minnows and other ornamental fishes

shows common occurrence in the swamps, marsh, beels and pools of wetland

than the river. The area falls in the tropical rain forest zone and receives

heavy rainfall. Thus, construction of dam will reduce the impacts due to soil

erosion and siltation caused by regular flood in the area.

Impacts on D’ Ering Wildlife Sanctuary due to hydropower generation

It can be seen from Table-7.9, that at about 33.62 km downstream of dam

site, low level shoals near Chapori will be inundated at the time of peaking

discharge of 3 hours. For next 21 hours, water level will recede. The depth of


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water with respect to islands for peaking discharge will reduce with distance

from the dam site. These areas can be protected by suitably raising the

ground levels along with stone pitching, if required to save the habitats.

As the river may get silted up and change the sectional area or the flows

may get diverted to different channels, the flow pattern in the river will be

monitored every year after the monsoon and corrective steps will be taken to

safe guard the islands/chapori etc.

At 43 km and 51 km downstream of dam site, water level at peaking

discharge will be lower than the level of Chaporis/islands. Similarly, the

water level during the peaking operation will remain below the level of D’

Ering Wildlife Sanctuary, thus no adverse impact on the sanctuary is

anticipated.

The depth of flow at base load will be 1.21m, 6.08m and 4.19m at 33.62 km,

43.23km and 51.31 km respectively downstream of the dam site which is

sufficient to sustain the riverine fisheries. The same has been verified by

installation of G&D site at three sections, the water depth thus seen is more

than that calculated by analytically. The water depth is sufficient as per CIFRI

report.

7.10 IMPACTS ON DIBRU-SAIKHOWA NATIONAL PARK

The Dibru-Saikhowa National Park is in the stretch from 53.556 km to 90 km

downstream of the dam site of Lower Siang hydroelectric project. The

national park is located downstream of the confluence of rivers Siang, Dibang

and Lohit. Thereafter it is situated on the left bank of Brahmputra. A series

of hydropower projects have been envisaged on three tributaries upstream of

confluence point. The discharge in Brahmputra along Dibru-Saikhowa

National Park has been assessed considering the operation pattern of the

lower most hydroelectric projects on the above referred three rivers.

The projects considered are listed as below:

• River Siang : Lower Siang Hydroelectric Project • River Dibang : Dibang Multi-purpose Hydroelectric Project • River Lohit : Demwe Lower Hydroelectric Project


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In order to harness the hydro power potential in upper reaches of

Brahmputra, numbers of Hydroelectric Power Project have been planned on

the three major tributaries of Brahmputra i.e. Siang, Dibang and Lohit.

As the hydro power projects are designed for peaking power for which river

flows are required to be regulated causing variation in river flow during the

day in lean season due to peaking operation of the project. In a particular

river basin, the project located in the most downstream reach of the river

has a major contribution for such variation. The regulated flows of upstream

projects get absorbed in downstream project. As such the river flows

variation due to operational pattern of terminal dams on river Siang, Dibang

and Lohit which join with Brahmputra been considered to study the variation

in flow pattern in Brahmputra. The main features of the three dams

considered in this study are given in Table-7.11.

TABLE-7.11 Main features of the three dams considered in this study

S. No.

Name of the Project Name of the river distance from confluence

Installed capacity (MW)

Height of the Dam (m)

Type of the scheme

1 Lower Siang HE Project Siang(57km) 2700 86 ROR 2 Dibang HE Project Dibang(50km) 3000 288 Storage 3 Demwe Lower HE

Project Lohit (75km) 1750 163.12 ROR

The river discharge pattern for the lean season in downstream reach is

mainly varied due to peaking operation of the project. The original 10 daily

inflow discharge series i.e. pre-dam flows for the average year (50%

dependable year) upto respective Dam site as per the approved DPR for the

three projects is given in Table-7.12.


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TABLE-7.12 Discharge pattern for Brahmputra river (50% dependable year)

Month 10-

daily No. of days

Lower Siang

(cumec)

Dibang project

(cumec)

Demwe Lower

(cumec)

D/s area discharge (cumec)

Total (cumec)

June 01--10 10

4323.32 1110.40 2276.85 819.48 8530.05

11--20 10

6359.21 1110.40 2207.42 1161.58 10838.61

21--30 10

8906.69 1110.40 2297.57 1620.43 13935.09

July 01--10 10

7175.42 2947.10 3214.95 1798.37 15135.84

11--20 10

7014.25 2947.10 2392.96 1627.33 13981.64

21--31 11

7243.23 2947.10 2324.52 1655.26 14170.10

August 01--10 10

9085.08 1303.20 2074.42 1646.19 14108.89

11--20 10

7802.25 1303.20 1929.44 1397.80 12432.69

21--31 11

6617.27 1303.20 1893.41 1185.38 10999.27

September 01--10 10

7045.47 875.80 1437.51 1106.21 10464.99

11--20 10

7273.70 875.80 1280.91 1118.67 10549.09

21--30 10

7010.78 875.80 1301.17 1076.46 10264.21

October 01--10 10

4924.28 727.80 1321.37 1213.14 8186.58

11--20 10

4947.96 727.80 1003.60 1161.98 7841.34

21--31 11

3601.31 727.80 919.03 913.00 6161.14

November 01--10 10

2813.13 344.40 842.73 695.91 4696.17

11--20 10

2172.66 344.40 798.07 576.72 3891.85

21--30 10

2183.15 344.40 764.32 572.67 3864.54

December 01--10 10

1626.12 315.10 711.68 461.51 3114.41

11--20 10

1645.19 315.10 677.33 458.86 3096.47

21--31 11

1348.16 315.10 642.80 401.18 2707.23

January 01--10 10

1068.13 330.80 297.62 295.14 1991.69

11--20 10

1011.47 330.80 293.69 284.60 1920.56

21--31 11

1010.92 330.80 317.29 288.61 1947.61


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Month 10-

daily No. of days

Lower Siang

(cumec)

Dibang project

(cumec)

Demwe Lower

(cumec)

D/s area discharge (cumec)

Total (cumec)

February 01--10 10

1083.41 652.40 314.97 356.77 2407.54

11--20 10

870.23 652.40 300.00 317.07 2139.70

21--28 8

1158.77 652.40 359.33 377.59 2548.09

March 01--10 10

1142.56 395.30 330.60 325.05 2193.51

11--20 10

1361.10 395.30 545.84 400.51 2702.75

21--31 11

1429.06 395.30 1339.02 550.32 3713.69

April 01--10 10

1810.54 385.20 724.19 507.97 3427.90

11--20 10

1363.39 385.20 717.38 429.00 2894.97

21--30 10

2296.14 385.20 806.98 606.85 4095.17

May 01--10 10

2887.31 1629.40 1306.30 491.11 6314.11

11--20 10

3028.64 1629.40 2284.55 685.88 7628.46

21--31 11

2500.36 1629.40 1632.61 480.56 6242.92

The details of catchment area for dams of down Siang hydroelectric project

Dibang Multi-purpose project and Demwe Lower hydroelectric project are

given in Table-7.13.

TABLE-7.13 Details of catchment area

Lower Siang HE Project

Dibang HE Project Damwe Lower HE Project

S. No.

Rainfed Snowfed Rainfed Snowfed Rainfed Snowfed

A Upstream of the Dam 151084 99510 10636 640 15296 4878 B Downstream of the Dam and upto confluence with Brahmputra 1125 - 2075 - 7920 -

The discharge for the catchment downstream of the three dams and upto the

Brahmputra confluence is given in Table-7.14. These inflows upto the dam

site has been added with the runoff discharge in the downstream reach of the

respective basin upto Brahmputra confluence (intermediate catchment)


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which has been calculated in proportion to the upstream and downstream

portions of catchment area.

TABLE-7.14 Discharge for intermediate catchment downstream of the three dams

and upto Brahmputra confluence Month Discharge (cumec)

I 819.48 II 1161.58

June

III 1620.43 I 1798.37 II 1627.33

July

III 1655.26 I 1646.19 II 1397.80

August

III 1185.38 I 1106.21 II 1118.67

September

III 1076.46 I 1213.14 II 1161.98

October

III 913.00 I 695.91 II 576.72

November

III 572.67 I 461.51 II 458.86

December

III 401.18 I 295.14 II 284.60

January

III 288.61 I 356.77 II 317.07

February

III 377.59 I 325.05 II 400.51

March

III 550.32 I 507.97 II 429.00

April

III 606.85 I 491.11 II 685.88

May

III 480.56


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The power potential studies as per approved DPR of Lower Siang HEP and

Dibang multi-purpose project and Demwe Lower HEP are given in Tables-

7.15 to 7.17 respectively.

TABLE-7.15

Power potential studies as per approved DPR of Lower Siang HEP Base Load of

180MW Peaking of 2700MW Spillage Inflow Month

10-daily

Nos of

days HRs Discharge HRs Discharge Jun 01--10 10 5.33 328.0 18.67 5462.00 0.00 4323.32 11--20 10 24.00 5462.00 894.87 6359.21 21--30 10 24.00 5462.00 3442.35 8906.69

Jul 01--10 10 24.00 5462.00 1711.63 7175.42 11--20 10 24.00 5462.00 1550.46 7014.25 21--31 11 24.00 5462.00 1779.44 7243.23

Aug 01--10 10 24.00 5462.00 3621.05 9085.08 11--20 10 24.00 5462.00 2338.22 7802.25 21--31 11 24.00 5462.00 1153.25 6617.27

Sep 01--10 10 24.00 5462.00 1581.73 7045.47 11--20 10 24.00 5462.00 1809.96 7273.70 21--30 10 24.00 5462.00 1547.04 7010.78

Oct 01--10 10 1.58 328.0 22.42 4994.16 0.00 4924.28 11--20 10 24.00 4813.38 132.69 4947.96 21--31 11 6.50 328.0 17.50 4813.38 0.00 3601.31

Nov 01--10 10 10.71 328.0 13.29 4813.38 0.00 2813.13 11--20 10 14.08 328.0 9.92 4813.38 0.00 2172.66 21--30 10 14.23 328.0 9.77 4851.46 2183.15

Dec 01--10 10 17.28 328.0 6.72 4921.15 1626.12 11--20 10 17.37 328.0 6.63 4966.13 1645.19 21--31 11 18.66 328.0 5.34 5053.90 1348.16

Jan 01--10 10 20.93 328.0 3.07 5063.32 1068.13 11--20 10 21.00 328.0 3.00 4957.07 1011.47 21--31 11 20.86 328.0 3.14 4890.15 1010.92

Feb 01--10 10 20.31 328.0 3.69 4859.53 1083.41 11--20 10 21.00 328.0 3.00 4920.26 870.23 21--28 8 20.57 328.0 3.43 4911.37 1158.77

Mar 01--10 10 19.98 328.0 4.02 4833.60 1142.56 11--20 10 18.83 328.0 5.17 4821.33 1361.10


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Base Load of 180MW

Peaking of 2700MW Spillage Inflow Month

10-daily

Nos of

days HRs Discharge HRs Discharge 21--31 11 18.12 328.0 5.88 4813.38 1429.06

Apr 01--10 10 16.08 328.0 7.92 4813.38 1810.54 11--20 10 18.48 328.0 5.52 4813.38 1363.39 21--30 10 13.48 328.0 10.52 4813.38 2296.14

May 01--10 10 10.32 328.0 13.68 4813.38 2887.31 11--20 10 9.56 328.0 14.44 4813.38 3028.64 21--31 11 11.73 328.0 12.27 4994.16 2500.36

TABLE-7.16 Power potential studies as per approved DPR of Dibang Multipurpose

Project

Environ Flow Peaking of 3000MW Spillage Inflow Month

10-daily

Nos of

days HRs Discharge HRs Discharge

Jun 01--10 10 24.00 15.00 24.00 1308.30 1110.4

11--20 10 24.00 15.00 24.00 1335.83 1110.4

21--30 10 24.00 15.00 24.00 1353.66 1110.4

Jul 01--10 10 24.00 15.00 10.30 1370.90 2947.1

11--20 10 24.00 15.00 10.30 1370.90 2947.1

21--31 11 24.00 15.00 10.30 1370.90 2947.1

Aug 01--10 10 24.00 15.00 19.24 1368.06 1303.2

11--20 10 24.00 15.00 19.24 1368.06 1303.2

21--31 11 24.00 15.00 9.55 1371.18 1303.2

Sep 01--10 10 24.00 15.00 8.61 1419.58 875.8

11--20 10 24.00 15.00 8.66 1416.47 875.8

21--30 10 24.00 15.00 8.72 1413.17 875.8

Oct 01--10 10 24.00 15.00 8.79 1409.68 727.8

11--20 10 24.00 15.00 8.48 1418.36 727.8

21--31 11 24.00 15.00 8.48 1375.84 0.00 727.8

Nov 01--10 10 24.00 15.00 8.48 1329.69 0.00 344.4

11--20 10 24.00 15.00 8.48 1332.59 0.00 344.4

21--30 10 24.00 15.00 8.48 1335.64 344.4

Dec 01--10 10 24.00 15.00 8.48 1338.85 315.1


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Environ Flow Peaking of 3000MW Spillage Inflow Month

10-daily

Nos of

days HRs Discharge HRs Discharge

11--20 10 24.00 15.00 8.48 1363.58 315.1

21--31 11 24.00 15.00 8.48 1390.54 315.1

Jan 01--10 10 24.00 15.00 8.48 1423.08 330.8

11--20 10 24.00 15.00 8.68 1415.49 330.8

21--31 11 24.00 15.00 8.94 1401.73 330.8

Feb 01--10 10 24.00 15.00 9.26 1385.55 652.4

11--20 10 24.00 15.00 9.53 1372.29 652.4

21--28 8 24.00 15.00 9.83 1358.05 652.4

Mar 01--10 10 24.00 15.00 10.10 1345.79 395.3

11--20 10 24.00 15.00 10.31 1336.46 395.3

21--31 11 24.00 15.00 10.56 1325.88 395.3

Apr 01--10 10 24.00 15.00 14.77 1311.54 385.2

11--20 10 24.00 15.00 14.69 1310.95 385.2

21--30 10 24.00 15.00 14.69 1310.95 385.2

May 01--10 10 24.00 15.00 15.08 1310.82 1629.4

11--20 10 24.00 15.00 15.09 1310.82 1629.4

21--31 11 24.00 15.00 15.08 1310.82 1629.4

TABLE-7.17 Power potential studies as per approved DPR of Demwe Lower HEP

Base Load of 40MW


10-daily

Nos of days

HRs Discharge HRs Discharge Cumec cumec Jun 01--10 10 24.00 35.70 24.00 1779.13 462.01 2276.85 11--20 10 24.00 35.70 24.00 1779.13 392.59 2207.42 21--30 10 24.00 35.70 24.00 1779.13 482.89 2297.57

Jul 01--10 10 24.00 35.70 24.00 1779.13 1394.95 3214.95 11--20 10 24.00 35.70 24.00 1779.13 578.14 2392.96 21--31 11 24.00 35.70 24.00 1779.12 509.69 2324.52

Aug 01--10 10 24.00 35.70 24.00 1779.13 259.59 2074.42 11--20 10 24.00 35.70 24.00 1779.13 114.62 1929.44 21--31 11 24.00 35.70 24.00 1779.13 78.58 1893.41

Sep 01--10 10 24.00 35.70 18.91 1779.13 0.00 1437.51


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Base Load of 40MW


10-daily

Nos of days

HRs Discharge HRs Discharge Cumec cumec 11--20 10 24.00 35.70 16.80 1779.13 0.00 1280.91 21--30 10 24.00 35.70 15.52 1650.73 0.00 1301.17

Oct 01--10 10 24.00 35.70 20.05 1539.64 0.00 1321.37 11--20 10 24.00 35.70 15.09 1539.61 0.00 1003.60 21--31 11 24.00 35.70 13.77 1539.62 0.00 919.03

Nov 01--10 10 24.00 35.70 12.58 1539.61 0.00 842.73 11--20 10 24.00 35.70 11.89 1602.73 0.00 798.07 21--30 10 24.00 35.70 11.36 1539.59 0.00 764.32

Dec 01--10 10 24.00 35.70 10.54 1539.66 0.00 711.68 11--20 10 24.00 35.70 10.01 1539.60 0.00 677.33 21--31 11 24.00 35.70 9.47 1539.64 0.00 642.80

Jan 01--10 10 24.00 35.70 4.47 1552.90 0.00 297.62 11--20 10 24.00 35.70 4.39 1582.07 0.00 293.69 21--31 11 24.00 35.70 4.40 1606.09 0.00 317.29

Feb 01--10 10 24.00 35.70 4.44 1626.71 0.00 314.97 11--20 10 24.00 35.70 4.42 1665.45 0.00 300.00 21--28 8 24.00 35.70 4.92 1708.28 0.00 359.33

Mar 01--10 10 24.00 35.70 4.46 1745.41 0.00 330.60 11--20 10 24.00 35.70 4.88 1654.79 0.00 545.84 21--31 11 24.00 35.70 19.97 1547.90 0.00 1339.02

Apr 01--10 10 24.00 35.70 10.74 1539.63 0.00 724.19 11--20 10 24.00 35.70 9.32 1539.64 0.00 717.38 21--30 10 24.00 35.70 14.09 1650.31 0.00 806.98

May 01--10 10 24.00 35.70 15.59 1650.72 0.00 1306.30 11--20 10 24.00 35.70 24.00 1650.74 598.1019 2284.55 21--31 11 24.00 35.70 21.54 1779.14 0.00 1632.61

From these studies, the quantum and duration of minimum and maximum

out flow from the power station of the respective project has been adopted.

Since the power stations are located at considerable distance of more than

50 kms from the Brahmputra confluence point beyond which, the impact due

to flow variation is being studied, the change in flow due to channel routing

has also been taken into account. As a sample case, channel routing of river


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Siang for the minimum and maximum flows of Lower Siang HE Project has

been carried out using MIKE-II software. The results of the same are given in

the following paragraphs.

Study for Water Level at different locations of Siang River (downstream of Siang HE Project) for 3hrs 5063 cumec and 21hrs-328 cumec release from dam

Figure-7.3 shows time series of released water from dam. 328cumecs water

released for 21 hrs then it increases up to 5063 cumec to generate peaking

of 2700 MW at a head higher than rated head of 55m within half an hour.

5063 cumec water is released for 3 hrs and again reduces to 328 cumec

within half an hour.

Figure-7.3 Time series of released water from dam

The longitudinal profile of water level and discharge at 9 o clock when

released from dam reached 5063 cumec is shown in Figure-7.4.The

longitudinal profile of water level and discharge at 12 o clock when release

from dam starts to reduce from 5063 cumec to 600 cumec is shown in


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Figure-7.5. The longitudinal profile for maximum discharge of 3500 cumec at

88790 m from dam axis throughout dam release period is shown in Figure-

7.6.

0.0 10000.0 20000.0 30000.0 40000.0 50000.0 60000.0 70000.0 80000.0 90000.0[m]

100.0

110.0

120.0

130.0

140.0

150.0

160.0

170.0

180.0

190.0

200.0

210.0

220.0

230.0

240.0

Wat

er L

evel

(m)

0.0

500.0

1000.0

1500.0

2000.0

2500.0

3000.0

3500.0

4000.0

4500.0

5000.0

5500.0

Dis

char

ge (C

umec

s)

SIANG 0 - 93790

0

4940

1024

0

1577

0

2014

0

2509

0

3362

0

4323

0

5131

0

6176

0

6737

0

7553

0

8879

0

9379

0

Water LevelLeft BankRight BankBed LevelDischarge

Figure7.4 Longitudinal profile of water level and discharge at 9 o clock when release from Siang HEP reached 5063 cumecs

0.0 10000.0 20000.0 30000.0 40000.0 50000.0 60000.0 70000.0 80000.0 90000.0[m]

100.0

110.0

120.0

130.0

140.0

150.0

160.0

170.0

180.0

190.0

200.0

210.0

220.0

230.0

240.0

Wat

er L

evel

(m)

0.0

500.0

1000.0

1500.0

2000.0

2500.0

3000.0

3500.0

4000.0

4500.0

5000.0

5500.0

Dis

char

ge (C

umec

s)

0 0 0 00 00

SIANG 0 - 93790

0

4940

1024

0

1577

0

2014

0

2509

0

3362

0

4323

0

5131

0

6176

0

6737

0

7553

0

8879

0

9379

0


Figure7.5 Longitudinal profile of water level and discharge at 12 o clock when release from dam starts to reduce from 5063 cumecs to 328 cumecs


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0.0 10000.0 20000.0 30000.0 40000.0 50000.0 60000.0 70000.0 80000.0 90000.0[m]

100.0

110.0

120.0

130.0

140.0

150.0

160.0

170.0

180.0

190.0

200.0

210.0

220.0

230.0

240.0W

ater

Lev

el (m

)

0.0

500.0

1000.0

1500.0

2000.0

2500.0

3000.0

3500.0

4000.0

4500.0

5000.0

5500.0

Dis

char

ge (C

umec

s)

21 7 2010 02:57:00

SIANG 0 - 93790

0

4940

1024

0

1577

0

2014

0

2509

0

3362

0

4323

0

5131

0

6176

0

6737

0

7553

0

8879

0

9379

0


Figure 7.6 : Shows maximum discharge of 3200 cumecs at 88790m from dam axis throughout dam release period For the combined flows downstream of Brahmputra confluence, the

maximum discharge at the outlet of all the three projects have been

considered and after channel routing, the reduced discharge in Brahmputra

are given in Table-7.18.

TABLE-7.18

Discharge at D/s reaches in leanest period of the year at Brahmaputra Confluence

Discharge at D/s reaches in the leanest period of the year (cumec)

Post Dams

S. No.

Distance from Dam site of Lower Siang Project ( km) Pre-dams

Maximum Minimum 1 61.76 1920 5510 663

The variation in water levels due to regulated discharge of three

distributaries is given in Table-7.19.


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TABLE-7.19 Water levels w.r.t. discharge variation and ground levels

Downstream of Brahmaputra Confluence Water Level

(El. m)

Minimum Ground Level (El. M)

Section No.

Distance from

Lower Siang

Dam Axis (km)

For 7610 cumec for

3 hrs


hrs

General Bed

Level (El. m)

River Bank

Chapori or Island,

if any

Remarks

S -65 61.76 117.05 113.70 111.89 125.70

Kobo Chapori at El 123.20

Confluence with Brahmputra River

S-64 67.37 114.66 110.35 105.45 117.30

Kobo Chapori at El 117.20

Dibru-Saikhowa N. Park

S-63 75.53 112.79 109.69 107.87 115.50

Island at El 114.40m

Dibru-Saikhowa N. Park

S-62 88.79 INSIGNIFICANT VARIATION

As per Table-7.19, the ground elevation of 61.76 km downstream of dam

site, which is confluence point of Lower Siang with river Brahmaputra is

123.20 m and the river bed level at this site is 111.89 m. The depth of water

for base load and peaking discharges at this site shall be 1.81 m and 5.16 m

respectively. At peaking discharge, water level at this site shall be lower as

compared to the minimum elevation of the Kobo Chapori. River bank is at a

higher elevation. Thus, no adverse impact on agriculture land on river banks

or on Kobo Chapori is anticipated.

At 67.37 km downstream of dam site ground elevation of Kobo Chapori

(Dibrusaikhowa National Park) is 105.45 m and the river bed level at this site

is 112.40 m. The depth of water for base load and peaking discharges at this

site shall be 4.9 m and 9.21 m respectively. Thus, at peaking discharge the

water level at this site shall be lower as compared to the minimum elevation

of the Kobo Chapori. River bank is at a higher elevation. Thus, no adverse


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impact on agriculture land on river banks or on Kobo Chapori/Dibrusaikhowa

National Park is anticipated.

At 75.33 km downstream of dam site ground elevation of Kobo Chapori

(Dibrusaikhowa National Park) is 114.40 m and the river bed level at this site

is 107.87 m. The depth of water for base load and peaking discharges at this

site shall be 1.82 m and 4.92 m respectively. Thus, at peaking discharge the

water level at this site shall be lower as compared to the minimum elevation

of the Kobo Chapori. River bank is at a higher elevation. Thus, no adverse

impact on agriculture land on river banks or on Kobo Chapori/Dibrusaikhowa

National Park is anticipated.

It is seen that after the confluence, the Brahmputra flows as a normal river

and some other tributaries join it. Thus, no adverse impact on Dibrusaikhowa

National Park is anticipated due to discharge at peaking power.

7.11 IMPACTS DUE TO SEDIMENT FLUSHING

A reservoir requires flushing of sediments to be done every year during

monsoon months. This is expected to increase the sediment levels for a short

duration of say 3 days. The quantum of sediment to be flushed during

reservoir life at various years is given in Table-7.20. The concentration of

sediment due to flushing of sediments is given in Table-7.21.


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TABLE-7.20 Quantum of reservoir to be flushed during project operation phase

Years Volume of water used in flushing

Storage Available

Storage Available

Storage Available

Storage Available

at FRL at at FRL at

EL 225 EL 225

Accumulated Volume of sediment

New Zero

Levels attained

Amount of sediment flushed

through low level sluices

MCM M3 m3 MCM MCM MCM EL (m) MCM At 0 1421000000 1194251064 1421 1194.251064 0 149.4 0

After 5 131089.724 1280103276 1053354340 1280.103276 1053.35434 140.8967244 155.1982 51.48947706 After 10 131089.724 1151399132 924650196.5 1151.399132 924.6501965 269.600868 162.225 51.62060092 After 15 131089.724 1034164256 807415320.7 1034.164256 807.4153207 386.8357438 171.65 49.59750847 After 20 131089.724 930478817.7 703729882.2 930.4788177 703.7298822 490.5211823 175.649 48.38243677 After 25 131089.724 838764529.4 612015593.9 838.7645294 612.0155939 582.2354706 181.055 45.84237668 After 30 131089.724 758015114.1 531266178.6 758.0151141 531.2661786 662.9848859 183.94 41.43498052 After 35 131089.724 686830897.2 460081961.7 686.8308972 460.0819617 734.1691028 187.7 35.17495907 After 40 131089.724 624241343.5 397492408 624.2413435 397.492408 796.7586565 28.04759879 After 45 131089.724 569609493.3 342860557.8 569.6094933 342.8605578 851.3905067 193.415 20.68950907 After 50 131089.724 521798619 295049683.5 521.798619 295.0496835 899.201381 14.34649075 After 55 131089.724 479404136.8 252655201.3 479.4041368 252.6552013 941.5958632 196.39 10.38375811 After 60 131089.724 441398261.4 214649325.9 441.3982614 214.6493259 979.6017386 7.283410488 After 65 131089.724 407054882.8 180305947.3 407.0548828 180.3059473 1013.945117 5.484461783 After 70 131089.724 375934728.5 149185793 375.9347285 149.185793 1045.065271 201.65 4.334779743


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TABLE-7.21 Sediment level in water released during flushing

Years Volume of water used in flushing (MCM)

Amount of sediment flushed through low level sluices(MCM)

Sediment level (mg/l)

At 0 0 After 5 131089.724 51.48947706 982.0 After 10 131089.724 51.62060092 984.5 After 15 131089.724 49.59750847 945.9 After 20 131089.724 48.38243677 922.7 After 25 131089.724 45.84237668 874.3 After 30 131089.724 41.43498052 790.2 After 35 131089.724 35.17495907 670.8 After 40 131089.724 28.04759879 534.9 After 45 131089.724 20.68950907 394.6 After 50 131089.724 14.34649075 273.6 After 55 131089.724 10.38375811 198.0 After 60 131089.724 7.283410488 138.9 After 65 131089.724 5.484461783 104.6 After 70 131089.724 4.334779743 82.7

The average sediment level is taken as 606 mg/l. The average discharge is

taken as 5671.5 cumec. Considering the flushing is to be done in three days,

the composite value of sediment level in various years is given in Table-7.22.

TABLE-7.22

Increased sediment level due to flushing

Year Increased sediment level due

to flushing (mg/l) At 0

After 5 988 After 10 990 After 15 952 After 20 928 After 25 880 After 30 796 After 35 677 After 40 541 After 45 401


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Year Increased sediment level due

to flushing (mg/l) After 50 280 After 55 205 After 60 146 After 65 111 After 70 89

Thus, it can be concluded that increase in sediment level will be higher in the

initial years, which will reduce in subsequent years. The average increase in

sediment level will be from pre-project level of 606 mg/l to 987 mg/l during

flushing of sediments. This implies an increase in sediment level of 381 mg/l.

To minimize the impact of increase in sediment level, it is proposed to

increase the water used for sedimentation by 50%, which will bring down the

sediment level to almost pre-project level during the flushing period.

Initially, sediment trap will be more and relatively clearer water will flow

downstream. Sediment rich stream will have a tendency for siltation

downstream and also it will affect the fishery but it will be ensured that

downstream water is released with almost same percentage of concentration

as before. This will be ensured by increasing the discharge for sediment

flushing.

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