hydro inflicted 1
TRANSCRIPT
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THE HYDRO -
INFLICTED BY SEDIMENT
&
POSSIBLE REMEDIES
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GENERAL
1.0 Water - The resource
The availability of water on earth as a
whole is practically constant at about
1400 M km3, out of which about 95% is
in oceans and and seas and is saline
and 4% is in the form of snow and ice.
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Thus about 1% is fresh and unfrozen
but 99% of this also is in deep aquifers
and only 1% is available in lakes, rivers,
soil and atmosphere.
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In India, annual precipitation including
snowfall is estimated as 4000 km3. The
precipitation which inflows in the rivers
is only about 1800 km3, and most of it
appears as flood flows during monsoon
months.
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2.0 Water use Upto the end of nineteenth century, water needs of the mankind were met by harnessing non-monsoon flows. Urbanization,Industrialization, population increase, changes in life style are the factors responsible for increase both in the uses and the users of water in the twentieth century.
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Resultantly, harnessing of monsoon
flows became a necessity. Storages
through dams were created to store
monsoon flows. Multipurpose projects
were taken up for controlling floods,
generating hydropower, providing water
for irrigation, domestic and industrial
purposes.
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3.0The Sediment Constraint
Therefore, the engineers and planners
of water resources projects, responsible
for harnessing monsoon flows are
confronted with problems in operation
and maintenance of the projects due to
sediment mixed with the flows.
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Depending on catchment
characteristics, the sediment problem is
quite serious for long term sustainability
of the water resources projects
developed on Himalayan rivers, which
contribute about 2/3rd of the country’s
water resources.
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As the experience has been, sediment
problems in water resources do not
have unique solution, every problem
has to be carefully examined and
project specific solution evolved and
tested on a model before adoption.
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4. Specific Problems affecting hydro - power
4.1 Land erosion
About 80% of total annual runoff is concentrated during 3 to 4 months of monsoon season and mostly in a few flood peaks. Therefore, sediment concentration of 30000 to 40000 ppm is not uncommon during such events. Hydro - sector is not equipped to face such events.
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4.2 River morphologyThe river patterns in various reaches of a river are classified as straight, mean-dering, braided and wandering. Yellow river in China and Kosi in India are the examples of wandering rivers. Kosi river has moved through a total distance of 70 miles (115 km) westwards during a period of last 200 years causing damage to life and property.
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4.3 Silting of Reservoirs Some reservoirs silt at a very fast rate
depending on rate of incoming sediment and the storage capacity. Some reservoirs in China have lost 2 to 3% of storage capacity every year. Storage reservoirs in India lose capacity at the rate of 1 to 0.5% every year. In Himalayas, diversion dams such as Ichari, Maneri, Pandoh have been silted upto the spillway crest in 2 to 7 years of operation.
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4.4 Degradation and AggradationA storage reservoir causes aggradation upstream of the dam and degradation on the downstream. The change in upstream course endangers the safety of river training works of the barrage and the change in downstream causes bank erosion and foundation problems due to scour for the structures located in the close proximity of the barrage / dam.
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4.5 Problems due to landslides
The occurrence of landslides in the hills is a common phenomenon.
Generally, cloud bursts activate the potential sites of landslides. Such sites are very common in Himalayas. These landslides sometimes block the river courses.
Some devastating events on this account are -
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Unprecedented Flood of 1.8.2000 in Sutlej
An unprecedented flood occurred in river
Sutlej on 1.8.2000 which left a trail of
destruction in Shimla and Kinnaur districts
of Himachal Pradesh, killing more than 150
people, washing away 14 bridges, houses,
vehicles and machinery at the projects.
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The swollen river damaged the 1500
MW Nathpa Jhakri Power Project
(NJPC) under construction, the 120 MW
Bhaba Hydel Project under operation
and 22.5 MW Ghanvi Hydel Project and
the Chaba Power Project in Shimla
district.
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Flood in Bhagirathi in 1978
An unprecedented flood of about 3700
cumec passed in Bhagirathi at Tehri
discharge site with heavy sediment
concentration in 1978 because of a
landslide in upper reaches of Bhagirathi
causing a blockade of the river.
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The overtopping and breach of the
blockade caused the flood wave. This
resulted in heavy loss and delay of
couple of years in completion of Maneri
Bhali Stage-I project which was under
construction at that time
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Alaknanda Tragedy 1970
The team of Alaknanda Enquiry
Committee inferred in their report that a
severe cloud burst on 20th July, 1970 in
Kunwarikhal hill region was the primary
cause of the Balakuchi landslide
tragedy.
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The Ganga flowed with a high sediment
concentration in suspension to Haridwar
from where all season Upper Ganga
Canal takes off. The canal got silted up
upto Pathri Power House which is
situated at Mile-7 of the Canal. The bed
was raised by 9.8 ft (2.99m) near head
regulator to 6.0 ft (1.83 m) at the Power
House.
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Gohna Lake Flood
A heavy landslide in Garhwal in the valley of
Birahi Ganga took place on the 22nd
September 1893. The slip was so huge that
it created a 900 ft (275m) high dam across
the river. Distance from Gohna to Haridwar
along the river valleys is about 240 kms.
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The dam was roughly 900 feet high,
2000 feet at top and 11,000 feet at base
along the valley, and 3,000 feet at top
and 600 feet at bottom across the
valley. Depth of water in the Lake
formed on the 13th - 14th December,
1893, was 450 feet.
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Finally the dam gave way at 11.30 PM
on the night of the 25th August 1894. It
was estimated that about 300 Mcum
(10,000 Mcft) of water passed over the
dam during 4.5 hours which caused
destruction up to Haridwar.
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5. Impact of sediment on Hydro Power Projects
Hydropower projects are generally of two types (I) storage schemes and (ii) run-of-river types. In storage schemes the reservoir acts as a large setting tank and most of the sediment settles in the reservoir and clear water without harmful sediment enters the intake and is carried to the power house.
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Bhakra & Pong Dam are classic examples of
large storage dams on Himalayan rivers. Yet
on some other large storage dams in
Himalayas such as Salal Dam reservoir on
Chenab in Jammu & Kashmir, Tarbela dam in
Pakistan, Indravati reservoir in Orissa,
hydropower plant equipment has suffered
damages.
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In run-of-river schemes, if operated
during monsoon, a lot of sediment with
water enters the power plant, causing
hydro-abrasion damages of varying
degree. The problem is more serious in
case of hydro plants located on rivers
coming from Himalayas.
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Two types of approaches are adopted
to minimize the abrasion damages.
One is to provide a desanding
arrangement and the other is to make
the equipment abrasion resistant. None
has been found fully effective in
preventing abrasion damages.
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6. PERFORMANCE OF DESANDING BASINS
In all run-of-the river hydro power
projects in Himalayas, arrangements
mostly in the form of a desanding basin
have been provided to extract a specific
sediment particle size.
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The performance evaluation of
desanding basins generally designed to
exclude particles of size upto 0.15 to 0.5
has shown that these have performed
the designed function satisfactorily.
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Inspite of the provision of these basins,
in most of the cases, the turbine
runners, the cooling system and other
under water parts of the power plants
have been found severely damaged
mainly due to the abundance of fine
subangular particles of quartz in the
water passing through the turbine.
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The sediment impact on some
significant projects in Himalayas is
reported to be alarming -
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6.1 Yamuna Stage-II (Chibro P.H.)
It is a run-of-river scheme on river Tons,
a tributary of river Yamuna, with
underground power house at Chibro
(240 MW, 4 x 60 MW, Francis vertical
axis turbines)
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The scheme utilizes a head of 123 m
and a discharge of 175 cumec. It is
provided with a hopper type desander
(83m long), located under the river bed.
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It is designed to exclude particles of
size upto 0.3 mm with a flushing
discharge of 75 cumec.
The desander is operating with designed efficiency. The effect of sediment on under water parts is observed to be moderate. The repair cycle of turbines is observed to be 6.0 years
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6.2 Maneri Bhali Stage-I
It is run-of-river scheme on river
Bhagirathi, a tributary of river Ganga. It
is designed for a generation of 90 MW
(3 x 30 MW + 10% over load)
utilising a drop of 180m with 70 cumec
discharge.
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A hopper type desanding basin is designed to exclude particles of size upto 0.3mm with a removal efficiency of 90%.
All the runners were found severely damaged after the first monsoon season after 2600 hours of operation. These were repaired but the runners had to be replaced / repaired after every 3000 to 5000 hours of operation.
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Restrictions were imposed on operation during monsoon when silt content exceeds 1200 ppm. This reduced generation by about 40%. A new profile runner has been developed and its performance has been found much better. Still the problem of severe silt damage to various components persists. The repair cycle of runner is less than one year.
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6.3 Baira-Siul Hydro Electric Project (Himachal Pradesh)
Baira-Siul project is situated in Chamba.
It utilizes a combined flow of three hilly
streams of Baira, Siul and Bhaled; a net
head of 282 m and a design discharge
of 88 cumec to generate 198 MW (3x66
MW), vertical axis Francis turbines.
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Desanding arrangement is provided at
three locations. One in the Bhaled
feeder tunnel. Another desander has
been provided in the power tunnel
taking off from Baira reservoir to
exclude particles of size above 0.2mm.
The 3rd in HRT to exclude 90% of
sediment upto 0.2mm & above.
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The project came in operation in 1980-
81 and heavy damages were observed
in the runners and guide vanes after two
years due to high concentration of fine
sediment (angular quartz content of size
0.008 to 0.25mm is 75 to 98%).
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6.4 Sanjay (Bhaba) Vidyut Pariyojna (Himachal Pradesh)
It is a run-of-river project on river Bhaba, a tributary of river Sutlej. Three, twin jet vertical axis pelton wheel turbines each of 40 MW capacity have been installed. It utilizes a design discharge of 17.5 cumec and a net head of 825m to generate 120 MW.
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Desanding arrangement in the form of
two basins each 50m long has been
provided to exclude particle size larger
than 0.2mm. The desander is followed
by a balancing reservoir / settling tank
which helps to exclude finer particles.
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The project was commissioned in 1989
and it worked satisfactorily for six years
but trouble started in Nov. 1995 when
the bucket of a runner was found
broken. Inspection revealed damages
in most of the buckets of all the three
units.
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The runners of all the units were
changed in the period from 1996 to
1999 and repaired runners are kept as
spares. Now, the buckets and nozzles
are being regularly repaired.
Petrographic analysis of sediment has
indicated that it has 76% quartz and
finer quartz particles are responsible for
severe damage.
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Bogged down by frequent repairs, two
spare runners are under consideration
for purchase
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6.5 Tanakpur Hydro Electric Project (Uttaranchal)
It is a run-of-river project on river Sarda.
It utilizes 22m head to produce 120 MW
(3 x 40 MW, Kaplan turbines). The desanding arrangement comprises
a sediment excluder in front of the head regulator and a hopper type desanding basin in the power channel.
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The desanding basin has been designed to exclude particles of size 0.5mm and above with 95% removal efficiency.
The impact of sediment on the turbines has not been severe because of low head.
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However, the cooling water system, which originally tapped water from penstock of each unit, was frequently damaged because of high sediment concentration (upto 6000 ppm) comprising 60 to 70% of quartz particles. Now, the cooling system has been changed into closed circuit cooling system and making up the loss by pumping from a tube well.
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6.6 Masyangdi Hydro Power Station (Nepal)
It is a run-of-river plant with installed
capacity of 69 MW (3 x 23 MW) utilizing
a head of 95m. The turbines are
vertical axis Francis type.
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The desander to exclude suspended
load has been provided in the form of a
settling basin of size 400 x 75 x 12m. It
is designed to exclude particles of size
upto 50 micron (0.05mm).
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Still fine sediment remains in the turbine
discharge whose concentration varies
from 4000 to 8000ppm. The sediment
size distribution from turbine has shown
that 80% of particles are finer than 50
micron, which damages the turbine
runner blades and other internal parts
(guide vanes, cooling system etc.).
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6.7 Thimruk Power Plant (Nepal)
It is a 12 MW (3x4 MW) capacity run-of-river plant. The design discharge is 7 cumec and net head 205m. The turbines are horizontal axis Francis type.
The desanding arrangement has been provided in the form of two parallel basins. The basin was designed for excluding 90% particles of size 0.2mm or more.
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The project was commissioned in 1994.
The operation experience has indicated
that turbine overhaul frequency of once
a year was not sufficient to avoid
irreparable damage. Regular repair and
maintenance of turbine runner, upper
and lower covers, labyrinth seal and
guide vanes has been carried out
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It can, therefore, be inferred that
exclusion of a particular size of
sediment through the desanding
basin is not adequate to avoid
sediment damage.
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7. POSSIBLE REMEDIES Based on the extensive damages
inflicted by harmful sediment to various hydro-power projects, both storage type & run-of-the river type, located on Himalayan rivers, following suggestive remedies need consideration of hydro-power engineers & planners, particularly for run-of-the river schemes -
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Location of Intake structure should be
based on extensive hydraulic model
studies wherein the u/s river approach
must invariably be given equal
weightage, if not more, as that for a
Diversion Dam / Barrage
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Operation of the power-plant with
head-pond reservoir maintained
close to MDDL condition. It helps in -
– saving the live storage
– Flushing of sediment deposited in reservoir
&– improves efficiency of the desander due to
lower flow - velocities on account of lower pressure / gravitational -head.
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The design of sediment exclusion
device should account for modified
sediment content as per Nozaki’s
criterion which interalia accounts for
abrasion potential of the sediment.
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Pre-flushing of diverted water u/s of the
approach zone of the main sediment
exclusion device. This would help in -
– Reducing concentration of inflow sediment in
the desander
– Reducing the probability of choking in
hopper type desanders
– improve efficiency of the main sediment
exclusion device.
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Well designed vortex-tube systems can
be very effective for pre-flushing
provided topography permits. For small
hydro - these tend to provide a rather
easier, efficient & economical
alternative for a conventional desander.
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The selected turbine specific speed
should be one or two steps lower.
Although it would increase the machine
cost and also affect the efficiency yet it
would reduce the hydro-abrasion
significantly.
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Abrasion resistant coatings may help
though such coatings tend to involve
substantial cost. Non-metallic coatings,
such as Dura tough, ceramic, epoxy
and polyuthene based plastics are
being adopted selectively with limited
success stories.
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Creation of a large storage - capacity
u/s of the cascading development along
any river. However, such a creation
must be based, if techno-economically
feasible, on the principle of sustainable -
environment.
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CONCLUSION
Therefore, an integrated approach of
dealing the sediment from the
catchment to the water use location of a
hydro power plant will in most cases be
more effective and economical.