Hydro Inflicted 1

THE HYDRO INFLICTED BY SEDIMENT & POSSIBLE REMEDIES

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.

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.

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.

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.

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.

4.2 River morphology The river patterns in various reaches of a river are classified as straight, meandering, 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.

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.

4.4 Degradation and Aggradation A 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.

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 -

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,

machinery at the projects.

vehicles

and

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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.

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.

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

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 tragedy.

of

the

Balakuchi

landslide

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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.

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.

5.

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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.

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 hydroabrasion 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

preventing abrasion damages.

in

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.

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.

The

sediment

impact

on

some

significant projects in Himalayas is reported to be alarming -

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.

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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

6.2

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It

Maneri Bhali Stage-I

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.

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 198081 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%).

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

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).

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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.

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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.

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).

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.).

6.7 ■

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Thimruk Power Plant (Nepal)

It is a 12 MW (3x4 MW) capacity run-ofriver 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

indicated

that

experience turbine

has 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.

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 hydropower 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.

Well designed vortex-tube systems can be very effective for preflushing permits.

provided

topography

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

significantly.

the

hydro-abrasion

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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.

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.