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.

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.

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.

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.

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 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.

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 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.

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,

vehicles and machinery at the projects.

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 of the Balakuchi landslide

tragedy.

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.

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.

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. 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.

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.

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.

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.

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)

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.

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

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.

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.

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.

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.

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%).

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.

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.

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.

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.

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). The desanding arrangement comprises

a sediment excluder in front of the head regulator and a hopper type desanding basin in the power channel.

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.

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.

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 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.

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

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 hydro-power engineers & planners, particularly for run-of-the river schemes -

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

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.

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.

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 pre-flushing

provided topography permits. For small

hydro - these tend to provide a rather

easier, efficient & economical

alternative for a conventional desander.

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.

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.

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.