chapter-7 prediction of impacts - arunachal pradesh...

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 the 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, on 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 for the available 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 during 90% dependable year

are given in Table-7.2.


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TABLE-7.2 Discharges on account of power generation in 90% dependable year

Base Load of 180MW Peaking of 2700MW Month Hours Discharge

(cumec) Hours Discharge

(cumec) I 5.33 328.0 18.67 5462.00 II 24.00 5462.00

June

III 24.00 5462.00 I 24.00 5462.00 II 24.00 5462.00

July

III 24.00 5462.00 I 24.00 5462.00 II 24.00 5462.00

August

III 24.00 5462.00 I 24.00 5462.00 II 24.00 5462.00

September

III 24.00 5462.00 I 1.58 328.0 22.42 4994.16 II 24.00 4813.38

October

III 6.50 328.0 17.50 4813.38 I 10.71 328.0 13.29 4813.38 II 14.08 328.0 9.92 4813.38

November

III 14.23 328.0 9.77 4851.46 I 17.28 328.0 6.72 4921.15 II 17.37 328.0 6.63 4966.13

December

III 18.66 328.0 5.34 5053.90 I 20.93 328.0 3.07 5063.32 II 21.00 328.0 3.00 4957.07

January

III 20.86 328.0 3.14 4890.15 I 20.31 328.0 3.69 4859.53 II 21.00 328.0 3.00 4920.26

February

III 20.57 328.0 3.43 4911.37 I 19.98 328.0 4.02 4833.60 II 18.83 328.0 5.17 4821.33

March

III 18.12 328.0 5.88 4813.38 I 16.08 328.0 7.92 4813.38 II 18.48 328.0 5.52 4813.38

April

III 13.48 328.0 10.52 4813.38 I 10.32 328.0 13.68 4813.38 II 9.56 328.0 14.44 4813.38

May

III 11.73 328.0 12.27 4994.16


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In the pre-project scenario, the discharge in monsoon season ranges fro

6420 to 8884 cumec. This will reduce to 5462 cumec in the post-project

scenario. Thus, even at present, the discharge in monsoon season is quite

high.

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 seasons, 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 the

remaining times, reservoir shall be filled upto FRL. This can result in drying of

river stretch downstream of dam site of the Lower Siang hydro electric

project. To avoid drying of river stretch, it is proposed to operate the hydro

electric project at a base load of 180 MW. This will result in continuous

discharge of 328 cumec. This corresponds to about 13.63% to 33.96% 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 flow Discharge for Base Load (cumec)

Percentage of flow

I 2407.1 328 13.63 II 1848.6 328 17.74

November

III 1553.2 328 21.12 I 1416.0 328 23.16 II 1301.6 328 25.20

December

III 1137.9 328 28.83 I 1087.8 328 30.15 II 1014.1 328 32.34

January

III 960.5 328 34.15 I 985.5 328 33.28 II 965.9 328 33.96

February

III 1061.0 328 30.90 I 1186.6 328 21.64


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Month Average flow Discharge for Base Load (cumec)

Percentage of flow

II 1267.2 328 25.88 March III 1398.0 328 23.46

Average 1306.25 328 25.11

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 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


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Time (hour)

Discharge cumec)

Time (hour)

Discharge (cumec)

Time (hour)

Discharge (cumec)

Time (hour)

Discharge (cumec)

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.

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 Table-7.5 and 7.6

respectively.


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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 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


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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 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 2.20 to3.19%. 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 PMF in pre-project scenario

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 PMF in pre-project scenario

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 39386.00 138.07 138.14 0.07 SIANG 40347.00 136.54 136.62 0.08 SIANG 41308.00 135.09 135.16 0.07 SIANG 42269.00 133.86 133.93 0.07 SIANG 43230.00 121.97 133.01 133.09 0.08 SIANG 44127.78 132.34 132.42 0.08 SIANG 45025.55 131.68 131.76 0.08 SIANG 45923.33 131.03 131.10 0.07 SIANG 46821.11 130.38 130.45 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, is used for the preparation of inundation

map. The same 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 as outlined in Table-7.2.

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 flow 328 cumec and flow

corresponding to peaking flow.

Completion of the proposed project in the basin, would render river Siang as

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 328 cumec, which on an average

is 25.11% of the average lean season flow. Thus, minimum flow will always

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


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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 are 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, quality,

quantity and accessibility of their habitat, which plays an important role in

population sustainability. Changes 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 put to threat 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

effects 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, the main

migratory fishes 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 mgratory 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. d 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 Bodeke 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, 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 328 cumec. The depth of


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flow due to discharge corresponding to base load at various sections

downstream of dam site is given in Table-7.7.

TABLE-7.7 Depth of flows at discharge corresponding to 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 0.92 51.31 1.43

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

almost 1m, i.e., 0.92 m which is sufficient depth of flow required to maintain

riverine fisheries.

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


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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

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 1.0m 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 substratum.

Two small streams of Sibot and Sirpo korang (11-15m 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,


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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.

• The immediate impact would be occurred on benthos invertebrates

lives beneath the cobbles, graves and stones and other surface attach

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. 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. This can result in

drying of river stretch downstream of dam site of the Lower Siang hydro

electric project. To avoid drying of river stretch, it is proposed to operate the

hydro electric project at a base load of 180 MW. This will result in continuous


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discharge of 328 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-7.8. A typical

section is enclosed as Drawing-No.-7.1.

TABLE-7.8 Water levels w.r.t. discharge variation and ground levels

Water Level (El. m)

Minimum Ground

Level (El. M)

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 Chapori

or Island, if any

Remarks

S-4 25.09 147.96 140.88 138.19 152.50 End of Hilly

terrain

S-3 33.62 142.61 138.87 137.66 144.00

Chapori at El 141.50 to El.143.0

D. Ering Sanctuary at El 143 to El 152m

S-2 43.23 126.86 122.89 121.97 130.60

Island at El. 127.20

D. Ering Sanctuary at El 130 to El 133.7m

S-1 51.31 120.70 1116.14 114.71 123.50

Island at El. 121.70

D. Ering Sanctuary at El 125.3 to El 128.9m

7.8 IMPACTS ON SOCIO-ECONOMIC ENVIORNMENT

As mentioned earlier in Chapter-6, there are 25 villages in the study area

villages, including 16 villages on the right bank and 9 villages on the left

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

presented in the following paragraphs.

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


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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, 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.

As per Table-7.8, the minimum ground level of Paker Chapori (33.62 km

downstream of dam site) is at elevation of 141.50 m and the river bed level

at this site is elevation of 137.66m. The increase in water level for base load

and peaking discharges at Paker Chapori shall be 1.21m and 4.95m

respectively. Thus, water level for base load and peaking discharges at Paker

Chapori shall be at elevation of 138.87m and 142.61m respectively. Thus, at

peaking discharge the Paker Chapori will be under a water depth of 1.11 m.

However, river bank is at an elevation of 144.00 m. Thus, no adverse impact

on agriculture land is anticipated.

At 43.23 km downstream of dam site ground elevation of Chapori is at

elevation of 127.20 m and the river bed level at this site is at elevation of

121.97m. The increase in water level for base load and peaking discharges at

this site shall be 0.92 m and 4.89 m respectively. Thus, water level for base

load and peaking discharges at this site shall be at elevation of 122.89 m

and 126.86m respectively. At peaking discharge water level will be

marginally below the minimum level of island, which is at elevation of 127.20

m. The river bank is at an elevation of 130.60 m. Thus, no adverse impact

on agriculture land is anticipated.

At 51.31 km downstream of dam site, which is the end of Daying Ering

sanctuary, island is at elevation of 121.70 m and the river bed level at this

site is at elevation of 114.71m. The increase in water level for base load and

peaking discharges at this site shall be 1.43m and 5.98m respectively. Thus,

water level at for base load and peaking discharges at this site shall be at

elevation of 116.14m and 120.69m respectively. At peaking discharge water


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level will be below the minimum level of island, which is at elevation of

121.70 m. The river bank is at an elevation of 123.50 m. Thus, no adverse

impact on agriculture land is anticipated.

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. In these islands, the cattle are left to graze on the

pasture and grass lands, which grow naturally.

As outlined in Table-7.8, the increase in water level during peaking discharge

at Paker Chapori will be 1.11 m. The water level in peaking discharge shall be

at elevation of 142.61m against the minimum elevation of 141.50 m. Thus,

Paker Chapori will be under submergence for a period of 3 hours of peaking

operation in lean season. These areas can be protected by suitably raising

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

The Paker Chapori is under submergence at present in rainy season form

June to October in 90% dependable year, will continue to do so, even under

project operation phase.

On the downstream side, the depth of flow in peaking discharge will reduce.

At 43.23 km downstream of dam site ground elevation of Chapori is 127.20

m and the river bed level at this site is 121.97m. Water elevation for peaking

discharge at this site shall be at elevation of 126.86m, which is below the

minimum level of island (127.20m).


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At 51.31 km downstream of dam site ground level of island is at elevation of

121.70m and the river bed level at this site is at elevation of 114.71m. Water

level for base peaking discharge at this site shall be at elevation of

120.69m, which is below the minimum level of island (121.70m).

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 of the Lake 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

were 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-7.8, that at about 33.62 km downstream of dam

site, Chapori will be under a water depth of 1.11 m at the time of peaking


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discharge of 3 hours. For next 21 hours, water level will recede below the

minimum level of Chapori. The depth of 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 marginally 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, 0.92m and 1.43m at 33.62

km, 43.23km and 51.31 km downstream of the dam site, which is sufficient

to sustain the riverine fisheries.

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 Project 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 gets 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-7.9.

TABLE-7.9 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 ion Table-7.10.


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TABLE-7.10 Discharge pattern for Brahmputra river

Month 10-

daily No. of days

Lower Siang

(cumec)

Dibang project

(cumec)

Demwe Lower

(cumec)

D/s area discharge (cumec)

Total (cumec)

Jun 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

Jul 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

Aug 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

Sep 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

Oct 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

Nov 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

Dec 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

Jan 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


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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)

21--31 11

1010.92 330.80 317.29 288.61 1947.61

Feb 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

Mar 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

Apr 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 these dams are given in Table-7.11.

TABLE-7.11 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-7.12. These inflows upto the dam

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


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respective basin upto Brahmputra confluence which has been calculated in

proportion to the upstream and downstream portions of catchment areas.

TABLE-7.12 Discharge for the 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-

7.13 to 7.15 respectively.

TABLE-7.13 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


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-7.14 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-7.15 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 11--20 10 24.00 35.70 16.80 1779.13 0.00 1280.91


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Base Load of 40MW


10-daily

Nos of days

HRs Discharge HRs Discharge cumec cumec 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

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


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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 cumecs and 21hrs-328 cumecs release from dam

Figure-7.1 shows time series of released water from dam. 328cumecs water

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

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

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

within half an hour.

Figure-7.1 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 cumecs is shown in Figure-7.2.The

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

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

Figure-7.3. The longitudinal profile for maximum discharge of 3500 cumecs


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

7.4.

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

Figure7.2 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


Figure7.3 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 7.4 : 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-7.16.

TABLE-7.16 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 ( kms) Pre-dams

Max Min 1 61.76 1920 5510 663

The variation in water levels due to regulated discharge of three

distributaries is given in Table-7.17.

TABLE-7.17


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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

For 663 cumec for 21

hrs

General Bed

Level (El. m) River

Bank Chapori

or Island, if any

Remarks

S -65 61.76 116.05 112.9 112.09 125.70

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-7.17, 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 112.09 m. The increase in

water level for base load and peaking discharges at this site shall be 0.81m

and 3.96m respectively. 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 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 increase in water level for base load and peaking

discharges at this site shall be 4.9m and 9.21m respectively. Thus, at

peaking discharge the water level at this site shall be lower as compared to


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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.

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 107.87m. The increase in water level for base load and peaking discharges

at this site shall be 1.82m and 4.92m 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-7.18. The concentration of

sediment due to flushing of sediments is given in Table-7.19.


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TABLE-7.18 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-7.19 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 as per the G&D site data in the months of June

to August are given as below:

• August 2008 : 1087 mg/l • September 2008 : 433 mg/l • June 2009 : 414 mg/l • August 2009 : 490 mg/l

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-7.20.


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TABLE-7.20 Increased sediment level due to flushing

Years 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

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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chapter-7 prediction of impacts - arunachal pradesh...

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