International Conference on Hydropower for Sustainable Development
Feb 05-07, 2015, Dehradun
FACING THE FLOOD FURY “OVERTOPPING OF DAM OF 900
MW BAGLIHAR HEP IN J&K” - A CASE HISTORY
ZAHOOR AHMAD CHAT
Technical Head – Hydro, Rodic Consultants Pvt. Ltd.
Gurgaon Haryana – India
ABSTRACT
Hydropower is the main source of clean , renewable and sustainable source of energy
available at attractive tariff, in spite of the impediments faced during its construction and the
criticism being generated against it’s development. The estimated Diversion Flood discharge ,
especially for Mega Hydropower Projects with high Dams having Flood Diversion Tunnels ,
is extremely crucial for their Design , Planning , Construction, Completion Time and Cost
Implications. Usually conservative Diversion Flood Discharge is adopted for the Diversion
Works after optimization , particularly in EPC & Turn Key Contracts, being works of
temporary nature . In view of large construction periods required for Mega Hydropower
Projects with Dams , there is every possibility of receiving a flood of magnitude higher than
the adopted diversion discharge , which can cause huge damages to main works of Dam
complex and consequently result in sharp time and cost over run of the Project. A similar
situation was faced during construction of Baglihar Hydro Electric Project in J&K – India ,
when a minimum height of 65m out of 143m high Dam was achieved and a high discharge
varying from 3000 – 6250 cumecs , against designed diversion discharge of 3000 cumecs,
was consistently received at Dam Site from June to August -2005 with repeated rapid
fluctuations in discharge causing several drawdowns , developing huge negative pore
pressures and disturbing the hill slopes near Diversion Tunnels , thus blocking both Diversion
Tunnels, one after the other, in a period of one month. This caused huge impounding and
triggered overtopping of the Dam till a temporary outlet was punctured through the main
Dam for the first time in history & was followed by construction of four gated construction
sluices in the Dam to cater to summer discharges. Due to overtopping consecutively during
two monsoon seasons of 2005 & 2006, followed by operation of 4 high level construction
sluices during next two monsoon seasons of 2007 & 2008 , with huge water column falling
from a minimum height of 65 m, about 20m – 30m scouring of bed rock below the Dam
foundation was caused along with large scale damages to side slopes , developing a deep
crater on the right bank , which finally required about 7 lac cum of concrete & other works
for restoration at an approximate cost of INR 1200 Cr and delaying the commissioning of
project by about 22 months . The paper deals with overcoming of this herculean challenge and
methodology adopted for the diversion of mighty Chenab River by puncturing the main Dam ,
followed with construction of sluices and closure of these temporary outlets for completion
of the project in absence of operational diversion tunnels.
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1.
HYDROPOWER A SUSTAINABLE SOURCE FOR DEVELOPMENT
Electric energy is indispensable for development in any sphere and it’s per capita consumption
is the main indicator for assessing the development of a Country. The average per capita
consumption of electricity in India is 717 kwh as compared to World average of 2800 kwh ,
China 2500 kwh and USA 14000 kwh, which clearly indicates that India is severely lagging
behind, has a long way to tread and needs to construct mega power projects expeditiously on
war footing for its overall development. Electricity produced by various sources have their
own inherent merits & demerits. For sustainable & Inclusive clean development,
Hydropoweris definitely the first choice for a renewable source of energy. Hydropower
generation does not involve consumption of any fossil fuels, which increase the carbon
footprints in our ecology & environment and does not generate any hazardous wastes during
operation , which require safe disposal. However, some criticism is being generated around
the Globe against development of Hydropower as well , when all the developed countries
have almost fully harnessed their own Hydropower Potential. Criticism comes mainly from a
few groups of environmentalists, some fossil fuel producers / business groups , NGO’s etc.
but more often for their vested interests . If Hydropower Projects are developed strictly
compliant to National / International Standards , Policies & Practices for clean development
with suitable R&R Programs / Packages & Efficient Environment Impact Management
harmonized with its conservation , the demerits of Hydropower Projects such as Issues related
to R&R, Submergence , Forests , Wild Life , Changes in Flow Patterns & Land Use, Aquatic
life , Muck Generation , Capital Intensive, Large Gestation Periods , etc are finally set aside
& rendered insignificant after completion & commissioning of Hydro Projects , on account
of their Extreme Flexibility , Consistency, Reliability , Easy Accessibility , Low O&M Costs ,
Low Attractive Tariffs , Longer Life , Low Life Cycle Cost per Mw , Negligible Greenhouse
Emissions , No Hazardous Waste Generation , CDM Benefits, Non-Consumptive use of Water
, Lesser Risk Factors , etc. Difficulties, Impediments, Geological Surprises, Challenges etc.
are faced mainly during construction periods of Hydropower Projects and thereafter they
mostly run smoothly in rest of their operational life , with a feasible techno-economical
option of life extension , if maintained properly and by conducting renovation, modernization
& uprating works.
Fossil fuels are finite and bound to finish in very near future but water resources are going to
flow continuously. Fossil fuel reserves are under our control & dependent on our usage and if
kept unconsumed, they can be preserved for future use at any time for some finite period but
non utilization of water, without harnessing electricity, is merely continuous waste flow of
currency into Oceans and not the mere flow of water. Our earnest attempt should be to utilize
every drop of water optimally for its power generation through various methods of Run of the
River , Storage , Pumped Storage, Lake bed and reservoir tapping schemes etc. Only about 30
% of the World Hydel Potentialstand harnessed (1311.3 Gw) and the pace of its development
especially in India is reducing with every passing year. India has a total estimated Hydel
Potential of about 1,48,701 Mw , when only 35,745.8 Mw (24.03%) stand harnessed by end
of 2013-14 and rate of its development viz-a-viz Thermal & otherSources is retarding with
every passing year and as on 31st March - 2014, the Hydro mix which normally should have
been 40% has reduced to just 16.68 % as against 50.61 % in 1962-63 .
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2.
HYDROPOWER DEVELOPMENT IN J&K
Jammu & Kashmir is supposed to be rich in Hydropower , with estimated Potential of about
20000 Mw but projects having a total potential of 16475 Mw have so far been identified with
a negligible potential of just 2813.46 mw (14.06 %) harnessed till date , that too mostly
through 7 Central Government Projects (NHPC) having a total capacity of 2009 Mw (71.4%) .
Maximum identified Hydropower Potential is on River Chenab (11283 Mw) , followed by
Jhelum (3084 Mw) , Indus (1608 Mw) and Ravi (500 Mw). About 14 Mega Projects of about
10500 Mw , with high dams have been identified on Chenab itself, out of which three
projects, Salal 690 Mw (NHPC) , Dulhasti 390 Mw (NHPC) , and Baglihar Stage – I , 450
Mw (State owned) totaling 1530 Mw , stand already commissioned.
J&K State is a part of Indus Basin and is governed by the Indus Water Treaty (IWT) of 1960
signed between India & Pakistan , as all the three Western Rivers, as referred in IWT
(Chenab , Jhelum & Indus) , on which India has only the restricted use, flow through J&K ,
while as the other remaining three Rivers of Indus Basin (Satluj, Beas & Ravi) , referred to as
Eastern Rivers in the IWT , on which India has complete unrestricted use , flow through other
Northern States of Himachal , Punjab & Haryana. This has caused huge impediments in the
development of Hydropower in J&K , while as other neighboring Northern States of Indus
Basin have developed their Hydropower Potential to a great extent with large mega projects
built on these rivers.
3.
BAGLIHAR HYDRO ELECTRIC PROJECT (BHEP)
BHEP is the first State owned Mega Hydel Project , located on River Chenab in Ramban area
of Jammu Province. It is a run of the river Project , with a total capacity of 900 Mw , to be
developed in two stages of 450 Mw each , with a common concrete gravity Dam . The work
on Stage-I was started in the year 1999 , with proposed commissioning in December – 2004
which was later on firstly extended to December – 2006 due to various impediments during
execution and then finally the Project was formally commissioned on 10th October , 2008 due
to extensive flood dmages in August -2005, causing blocking of both diversion tunnels
leading to over topping of Dam . The Stage – II of the project is presently under final stages of
completion and is scheduled for commissioning in 2015-16.
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4.
MAIN FEATURES OF BHEP
The project proposed a 144.5 m highconcrete gravity dam (later on lowered to 143m) above
main foundation level (El.700m) to be constructed in 21 blocks (9 overflow blocks) involving
a concrete quantity of 21.45 lac cum with u/s & d/s coffer dams of 28m & 10 m height
respectively.
The Dam was designed for a flood discharge of 16500 cumecs (1 in 1000 year return period) ,
for which 5 bays of Main Spillway (crest El. 808m) in blocks 10 to 14 , 3 bays of Chute
Spillway (crest El. 821m) in blocks 4 to 6 and 1 bay of Auxiliary Spillway abutting block 14
(crest El. 837m) located within right training wall, were provided to cater to the discharges of
10500 cumecs, 5947 cumecs& 53 cumecs (total 16500 cumecs) respectively. The MDDL ,
DSL , FRL and Dam Top levels were fixed at 835m (later on raised to 836m) , 835m (later on
raised to 836m) , 840m & 844.5m (later on lowered to 843m) respectively.
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The intakes for both the stages with sill level of 818m (later on raised to 821m) and each
designed for a discharge of 430 cumecs , were to be constructed under stage-I. The various
levels for the Dam Complex were changed (as given above) on the determination of Neutral
Expert Prof Raymond Lafitte appointed by the World Bank in terms of IWT , to resolve the
issues raised by Pakistan. Two circular HRT’s , one for each stage , about 2 km long of
10.15mØ , were to run parallel and about 50m of HRT-II from intake - II was to be
constructed under stage-I and kept concrete plugged till completion of HRT-II. 3 Steel lined
Penstocks of 5.5m Ø about 200m long taking off from an u/s Surge Shaft of 27.5 m Ø , 77 m
high (restricted orifice type) provided with a separate d/s Gate Chamber , were to feed 3
Francis Turbines of 150 Mw each , with average net Hydraulic Head of about 125 m installed
in an underground Power House with Transformer Hall of size 121mx24mx50m &
122mx15mx25m respectively. The Power house for Stage-II , earlier proposed as a mirror
image of Stage – I as it’s extension , was later on shifted towards upstream side by about
200m on geological considerations. An underground d/s collection chamberwith circular Tail
Race Tunnel 10.15m Ø , 197 m long was to deliver water back to River Chenab.
5.
DESIGN FLOOD DISCHARGES
BHEP was designed for a diversion flood of 3000 cumecs based on a very low return period
of 25 years. Two Diversion Tunnels (DT-I & DT-II) of size 10.5m(W) x 10.7m(H) of
modified horse shoe shape having length of 385m & 530m , each designed for 1500 cumecs ,
were to be constructed at inlet sill level of 710m & 710.5m respectively having outlet sill at a
common level of 709m.
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The Diversion Flood and Design Flood Discharges are very critical design parameters for all
the Hydropower and other River Valley Projects, having huge impact on their Design,
Planning, Construction Methodology and Cost. Since Diversion Works are temporary in
nature , required during construction period only, attempts are commonly made to optimize on
their Planning & Design , especially in EPC / Turn Key Projects. Gestation Period of
Hydropower Projects is usually not less than 3 to 4 years with Diversion Floods generally
worked out for a small return period, which is usually less than the average flood discharge
worked out for that particular stream and a flood of higher return period can also hit the
project during its construction phase , thereby making it vulnerable to huge damages. Detailed
risk assessment and cost benefit analysis need to be thoroughly undertaken, more importantly
for Mega Hydropower / River Valley Projects with high Dams, especially on Rivers proposing
Temporary River Diversion through Tunnels and without any Construction Sluices.
This critical aspect attains huge significance after witnessing present climatological changes
triggering unprecedented Cloud Bursts , High Intensity Precipitation in extremely short
periods & deluges with recent cases of high unprecedented cloudbursts /precipitation received
in Leh (J&K) - 2010 , Uttaranchal - 2013 and now in Kashmir Sept-2014 , resulting in
enormous loss to life , land , property and agriculture . The average rainfall for Kashmir in
September is 26.6 mmwith average annual rainfall of about700 mmbut rainfall observed only
in first 7 days of September - 2014 was173 mmin Srinagar , while as in South Kashmir , it
was617mmin Qazigund , surpassing all the previous records.
Major changes in precipitation patterns being observed presently should act as an eye opener
and a wake up warning for all our Hydrologists , Meteorologists , Planners and Designers ,
who need to study such patterns and accordingly update / revise their norms , analysis &
strategy for Estimation of Flood discharges (Diversion & Design Floods) , Planning, Design &
Construction Methodology for Flood related works / structures , even though it may require
adoption of higher flood discharges and increasing the flood frequency periods for their
estimation / design. Seeing the damage potential of floods during construction of High Dams,
as was witnessed during construction of BHEP-I in 2005, the cost impact by increasing the
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Design Diversion Flood will be very nominal and will work out techno-economically more
feasible in the long run.
6.
THE FLOOD FURY OF BHEP-I
A situation of floods higher than the Design Diversion Flood was faced during construction
phase of BHEP-I , causing colossal damages. During winter -2005, heavy snowfall occurred
in the catchment of River Chenab when an unprecedented snow depth of more than 1 m was
observed at Dam Site of BHEP (El. 710 masl). This was followed by persistent heavy rains in
the monsoon months of June , July & August. Heavy precipitation and huge snow melt
resulted in large sustained discharges varying from 3000 to 4000 cumecs consistently in June
till early July with repeated fluctuations causing several rapid drawdowns , thus disturbing the
hill slopes , which were already geologically weak . This discharge was, however,
successfully catered by DT’s with some additional heading , as DT’s were designed to cater to
a discharge of 3000 cumecs . After severe precipitation in first week of July heavy flood
discharge of about 6250 cumecs was observed at site on 7th July, causing huge impounding
behind the Dam after damaging & overtopping the u/s cofferdam when Dam was already
completed to a height of about 65m above the main foundation level (El. 700m). The flood
receded rapidly within a few days developing huge negative pore pressures due to rapid draw
down, in the already weak & disturbed right bank slopes above DT’s & Intakes due to
repeated drawdowns in June, thereby triggering massive slides near these areas and
completely blocking DT-II .
The situation immediately got stabilized to some extent due to reduction in flood discharge
and the single operational Diversion Tunnel (DT-I) continued to function with additional
impounding behind the Dam . Intense precipitation again occured in the following month of
August and a discharge of about 4000 cumecs again hit the site , when only one Diversion
Tunnel (DT-I) with a discharge carrying capacity of 1500 Cumecs was functional. This
created immediate impounding behind the Dam at alarming levels and the already weakened
slopes above Diversion Tunnel & Intakes due to slides in floods of July , again gave way
blocking this only operational DT-I and Intake – I as well , thereby leading to overtopping of
the Dam. At this time the Dam was completed in different blocks with minimum levels
ranging above 760m (block-14) and about 11 laccum of concrete, out of total estimated
quantity of 21.45 lac cum required for the Dam, was already executed.
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SLIDES ABOVE INLETS OF DIVERSION TUNNELS
The road connectivity to the Dam site on either sides got snapped due to massive slides
including washing away of all the bridges and infrastructure around the Dam Site. The
important Bye-pass road for critical Nasri Slide Zone of main National Highway (NH-1A)
located in between Towns of Batote & Ramban , passing along the left bank at Dam Site got
slided off.
The situation grew serious with every passing day due to huge column of water of about 4000
Cumecs falling from about 65m high Dam with huge velocity . The whole area was severely
vibrating by the impact of such an enormous water fall with maximum damage caused on
right side, as the concrete level for main Dam was lowest on this side at El. 760m , El. 761m
& El.762m in block Nos: 14, 16 & 13 respectively and the central / left side blocks were
comparatively at higher levels above El.770m, thereby concentrating maximum discharge on
this lower right side. The discharge mainly concentrated through the right training wall cum
Auxiliary Spillway Channel completed up to El. 760 m and this wall cum channel protruding
largely away from Dam body, resulted in huge scour of 20 m to 30 m in the bed rock below
the foundation level up to El.670m and on the hill slopes with the development of a deep
scour hole , named later on as “Black Hole”. The plunge pool concrete works which were in
progress before floods got completely washed away.
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7.
NEW RIVER DIVERSION PROPOSALS
Services of experienced divers were utilized to work out the strategies for removing the
underwater blockades at the mouths of DT’s but all attempts to make the DT’s functional at
such large water depths proved to be a failure as the depth of debris exceeded 30m.
Geotechnical experts also unanimously opined that removal of muck near the DT’s would
trigger further slides , thereby will worsen the situation endangering the Dam safety. At the
first instance , it was decided to make a new Diversion Tunnel (DT-III) on left bank of 7m
size , about 528m long and even though having full knowledge of the poor rock mass quality
on this left bank (as the main Diversion Tunnels DT-I & DT-II were originally proposed on
this left bank but were later on shifted to right bank during execution on account of poor
geology) , still an attempt was made to construct new DT-III on left bank , but the work had
to be suspended only within few weeks after executing about 50m of this tunnel, owing to
huge slope debris and large scale rock mass failures due to poor geology , making it difficult
to proceed ahead . The water continued to overflow the Dam accelerating damages in D/S
plunge pool and right side slopes. All hopes to revive the project were dashed to ground and
some desperate opinions were given even to abandon the project and blast the dam works
already executed.
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After the failure of constructing new DT-III, it was finally decided to puncture the main dam
itself and make a temporary out let through it for diversion of water catering to immediate lean
flows , with some additional construction sluices for monsoon flows , to allow completion of
Dam above main spillway level. This novel decision was unprecedented & the first of its kind
in history and was apt to have serious safety implications for the main dam and the workers,
due to blasting and plugging later on , under huge hydraulic pressure. After conducting
detailed consultations with National & International level experts and weighing all the merits
and demerits of such a bold & a dare devil proposition , the Project Authorities finally
decided to take a chance & go ahead with a temporary circular bottom outlet through the dam
body in block 11 of 6.8m Ø, at inlet & outlet sill elevations of 733m & 732m respectively for
a lean discharge of 580 cumecs, when a level of 764m was already achieved in this block. It
was also decided that for catering to additional normal monsoon discharge of 2300 cumecs ,
go for four high level gated construction sluices, at two levels, having size of 3.5m (W) x
8.2m (H), two each in dam blocks 12 & 13 at sill levels of 766m & 763m respectively , when
the concrete level already achieved in these two blocks was 765m & 762m , i.e. 1 m below
the proposed sill levels of sluices , just to provide cushion for fresh concrete to be monolithic
with Sluice structure .
8.
TEMPORARY BOTTOM OUTLET AND CONSTRUCTION SLUICES
After deciding to go for a Temporary Bottom Outlet and four Construction Sluices , the work
had to be resumed on war footing . The whole infrastructure that was washed away due to
floods & overtopping of dam had to be immediately rebuilt. A new tail track and rotec
concrete conveyance system with other machinery & equipment , infrastructure etc. were put
at dam site along with construction of new bridges and approaches . The existing Tunnel of
Nashri Bye Pass Road on left bank later on required to be extended by about 700m to make
this Bye Pass road functional near the damaged portion. After finalizing all the designs and
making backup infrastructure ready, the work on this temporary bottom outlet was finally
taken up in September - 2005under extremely controlled conditions and regularly digitally
monitoring the dam behavior during blasting & construction , maintaining vibrations and
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crack development completely well below the permissible limits. This work was taken up
under the strict supervision of experts from Central Mining Research Institute (CMRI) –
Dhanbad , Bihar. Last main blast was taken with remote control simultaneously from d/s face
as well as from u/s reservoir face (under water by using plastic mufflers), to ensure maximum
impact and finally the puncture was completed on 14th January -2006, where after water
level started receding behind the Dam and the water which continued to overflow the Dam
continuously for more than five months , completely stopped overflowing within three days,
being a lean period . During the period since starting of work on Bottom Outlet in September
– 2005 till making it functional on 14th January – 2015 , all the designs & requisite backup
arrangements for execution of Gated Construction Sluices and Spillways on war footing ,
were kept in place to pave way for immediate resumption of the work on the main Dam and
Construction Sluices . The construction sluices were made fully operational well before
monsoon of 2007 , but overflow of Dam continued during monsoon period of 2006 as well.
Puncturing of Main Dam body (Block-11) VIEW OF BOTTOM OUTLET FROM
INSIDE THE DAM for Temporary Bottom Outlet (Sept.- 05)
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9.
PLUGGING OF BOTTOM OUTLET AND CONSTRUCTION SLUICES
After completing the major works of main Dam & spillways by January-2008, the new
challenge was now to divert the water out of the plunge pool to resume work in this area by
plugging the Temporary Bottom Outlet and Construction Sluices, with water proposed to be
diverted through extreme left bay of Chute Spillway for which a Temporary Escape Channel
of about 50m was constructed by extending the Chute Spill Channel beyond the d/s coffer
dam area. There was no direct access to control the flow in Bottom Outlet for its plugging and
the plugging had to be done under flowing water conditions. There was no alternative but to
close this out let from its inlet on Reservoir side, where during closing of this Outlet &
Construction Sluices , the water level would increase above sill level of Chute Spillways (El.
821m) . The plugging was started on 23rdMarch , 2008 . A heavy steel Bulkhead of size
larger than the size of this Outlet , was lowered from the Reservoir side , in order to utilize
the hydraulic head available behind the dam to partially close down the Outlet. Additional
lateral support to this Bulkhead was induced due to earth pressure developed by dumping
debris behind this bulkhead from the Reservoir end. This procedure worked out successfully
and when the Bulkhead was lowered to cover the opening , it effectively reduced discharge
through the outlet , thereby enabling access into this outlet from d/s face and adjoining
Inspection Gallery.
Huge leakages were taking place around the edges of this Bulkhead , causing severe
turbulence and water flashes inside this bore , which were temporarily plugged by using
packing materials and constructing a 2.5m thick temporary bulkhead of bags filled with dry
concrete , inside this outlet to act as a solid shield after setting of dry concrete under moist
conditions, in order to reduce turbulence of water during plugging. Additional concrete was
laid after a few days behind this bulkhead and piped openings were provided on the side of
this bulkhead for release of leakage water. The existing concrete surface inside the outlet was
scrapped off loose materials, cleaned and chemically treated before carrying out concrete
plugging. Concreting was done firstly behind this bulkhead by accessing through the
Inspection Gallery and thereafter concreting was done rapidly from d/s face of outlet towards
u/s direction and from Inspection Gallery simultaneously both in u/s as well as in d/s direction
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, followed with pressurized chemically treated concrete grouting to plug any micro openings
and hair pin cracks , for its water tightness and ensuring bond with the cold concrete for its
structural strength & durability. Later on pipe openings except one were closed by
pressurized concreting and the leftover pipe was kept unplugged to safely drain the leakages ,
if any. The plugging was successfully completed on 18th July , 2008.
LOWERING OF BULKHEAD FROM RESERVOIR SIDE
LEAKAGES
FROM BULKHEAD INSIDE THE OUTLET PLUGGING OF TEMPORARY
BOTTOM OUTLET
Temporary Bulkhead For Plugging
Drainage Pipes For Plugging
The plugging of four Construction Sluices was taken up on 15th September , 2008
immediately after plugging of Temporary Bottom Outlet and it was made rather easier due to
lowering down of gates through the gate grooves already provided in them. The leakages
through the gates were plugged in a similar way as that adopted for plugging leakages in
temporary bottom outlet. These sluice ducts were concrete plugged and chemically grouted
under controlled conditions to ensure water tightness and bond with the cold concrete joints
and the plugging was completed on 21stFebruary , 2009 where after water got completely
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safely diverted through temporary extended escape channel of Chute Spillway which had
already become operational in May -2008 , immediately after initial plugging of temporary
bottom outlet.
LAYOUT OF TEMPORARY ESCAPE CHANNELTEMPORARY
CHANNEL OF CHUTE OF CHUTE SPILLWAY IN OPERATION
ESCAPE
After great impediments, the plugging of blocked Diversion Tunnels was also taken up on 1st
December , 2007 and completed on 6th May 2008 , without lowering of DT Gates , as these
gates were already blocked under debris andimpounding of Reservoir up to MDDL (EL.
836m) was completed in August / September , 2008. The main Project works except Plunge
Pool were completed by September-2008 and to avail the Power Generation during ensuing
winter period, the Project was successfully commissioned formally and dedicated to Nation by
Sh. Manmohan Singh Ji, Hon’ble Prime Minister of India on 10th October, 2008 , as the lean
discharge till March – 2009 could easily be catered through extended Chute Spill Channel
without the requirement of Plunge Pool.
IMPOUNDING THE RESERVOIR UPTO MDDL (EL. 836 M) & SLOPE
TREATMENT OF SLIDED AREA (AUG-SEP’08)
10.
PLUNGE POOL AND RESTORATION WORKS
Immediately after commissioning of Project on 10th October-2008, works on the Plunge Pool
were resumed .The d/s coffer dam was reconstructed and plunge pool was dewatered in lean
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season from November-2008. After complete dewatering of plunge pool , the real extent of
flood fury was exposed by witnessing the actual horrible status with Dam bed rock under
scoured to the extent of 20m to 30 m (upto El. 670m) below the foundation level with Dam
almost hanging for a major portion of it’s length and development of huge crater towards the
right bank , nicknamed as “Black Hole” .
Plunge Pool before Overtopping–(july-05)
The damages in the plunge pool had got aggravated due to overtopping of Dam consecutively
for two monsoon seasons of 2005 & 2006 and then due to diversion of monsoon flows
continuously for two years during 2007 & 2008 through high level Construction Sluices ,
with a fall from about 65m above main foundation level , in absence of any other alternative .
This scoured and damaged Plunge pool & Black Hole with extremely damaged right bank
slopes were rebuilt with additional concreteof about 7 lac Cum (final quantity of concrete in
Dam complex as 28.5 lac cum against earlier estimated 21.45 lac cum) and works were
completed by April – 2009 and the main Spillways were actually made ready to operate in
May -2009with the increase in discharge.
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REHABILITATION OF ERODED FOUNDATION & BLACK HOLE CONCRETE
The additional direct financial impact on the project, due to these flood damages , as approved
by CEA was INR 1200 Cr. after completion of restoration works. The commissioning of the
Project also got delayed from December - 2006 (earlier revised commissioning date prior to
floods) to October – 2008, thereby causing a direct generation loss for about 22 months as
well.
11.
CONCLUSION
The strategy for River Diversion Works for Hydro Projects especially with high Dams
provided with Diversion Tunnels and without any construction sluices, need to be changed /
reassessed in view of present climatological changes triggering intense unprecedented
precipitations & snow fall causing instantaneous discharges much higher than designed
diversion discharges , providing very limited reaction time during execution. These Mega
Projects, which have longer gestation periods are prone to potential risks due to floods during
construction period , requiring detailed risk management, planning and analysis with the
option of increasing their diversion discharges and provision of some additional low level
gated construction sluices to reduce redundancy factors. The slopes near intakes of Diversion
Tunnels, where maximum suction and disturbances are caused, need to be specially protected
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up to elevations much above the top levels of u/s cofferdam, to ward off any failures against
subsidence , slides & development of negative pore pressures due to any draw down
conditions and / or overtopping of cofferdam during floods. A little extra expenditure on these
safety measures / precautionary works will be worth to save the Project as a whole from the
colossal damages with huge cost & time over run , as were witnessed in BHEP.
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