10th Asian Regional Conference of IAEG (2015)

Natural damming in the India Himalaya and its Impact on Hydropower Development(Extended Abstract)
Yogendra Deva
Indo Canadian Consultancy Services Limited
Email: yogendradeva@gmail.com

adverse foundation conditions including liquefaction
potential, and extremely difficult or eveninsurmountable problems in the construction andmaintenance of subsurface structures like tunnels. It
is, therefore, advisable to stay away fromexceptionally thick alluvial deposits in wide valleys
and, if that is unavoidable, to study and explore sucha site thoroughly. This is not always possible and, if
unavoidable, adversely influences the technoeconomic parameters of the project.

Occurring as a prominent convex arc of mountainchain in north India, the 2400 km long Cenozoic
mobile belt of the Himalaya is the product ofcollision between the Indian and Asian plates. The
Himalaya play host to three major river basins, viz.
Indus, Ganga and Brahmaputra that give rise to a
sprawling alluvial plain all along its base. Divided
into three major tectonic zones, viz, Indus-Shyok Belt,
Main Himalayan Belt and Frontal Fold Belt, a host oftectonic planes like Karakoram Fault, Indus-Tsangpo
Suture, Main Central Thrust and Main Boundary
Thrust and Frontal Thrust, along with numerous
transverse faults, dissect the Himalaya into discrete
blocks and segments.

In particular, some of the major hydroelectric
projects in Indian Himalaya that faced thegeotechnical investigation and construction challenge
include the 390 MW Dul Hasti Project across theChenab River in Jammu & Kashmir, 780 MW
Nyamjang Chhu Project across the Nyamjang Chhu
River in Arunachal Pradesh and 300 MW Baspa-II
Project across the Baspa River in Himachal Pradesh.

The Main Himalayan Belt is host to majorhydropower development in India that, at present, isfocused in the Lesser Himalaya bound between theMain Central Thrust and Main Boundary Thrust, andis stepping out in to the Higher Himalaya north of theMain Central Thrust. The investigation andimplementation of the hydropower projects in thisbelt are governed by complex geotechnicalimplications arising out of unique site specificlithological and tectonic models. Not very
infrequently, many of the project sites have comeacross natural dams with silted up lakes posing
serious foundation and tunnelling problems. Some of
the well known cases involving Quaternary natural
damming, characterised by thick lacustrine deposits,
include an over 180m thick lacustrine deposit below
a 400m thick fluvioglacial deposit at Kishtwar in
Chenab Basin, a 100m thick lacustrine deposit at
Zimithang in Tawang Basin, a 150m thick lacustrine
deposit at Sangla in Satluj Basin, etc. On the other
hand, formation of several small and large natural
dams with serious construction and maintenanceimplications is seen as a fairly common ongoingprocess.
Generally, the low bearing capacity and saturated
nature of the lacustrine deposits lead to seriously

The alignment of the 9.78 km long Head Race
Tunnel for 390 MW Dul Hasti Hydroelectric Project
ran across the 2 km x 8 km plateau at Kishtwar thatwas suspected to be overlying a buried course of themighty Chenab River. Problems in drilling beyond
260m through the bouldery fluvio-glacial overfill
using
conventional
machines
constrained
confirmation of bedrock at tunnel grade. The project
construction as EPC contract commenced with buried
valley crossing as grey area. Deep drilling of about
600m using imported wireline drilling machines
proved absence of bedrock even down to the drilled
depth of 100m below the tunnel grade. The modified“Loop” tunnel alignment bypassing the buried valley
is an “S”-shape and is about 10% longer than theoriginal “Straight” alignment (Fig.1). The buried
valley turned out to be over 500m deep and the resultof natural damming close to powerhouse area at Hasti
village. It is believed to have led to the re-routing of
the Chenab and capture of about 11 km length of theMarusudar River.

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10th Asian Regional Conference of IAEG (2015)Fig.1: Straight and Loop alignments of HRT, Dul Hasti HE Project, Jammu & Kashmir, India.
the liquefaction potential of the foundation material.
The ground improvement proposals againstliquefaction, besides adding to the project cost
significantly, turned out to be a major challenge forcivil design of the barrage, desilting tanks, andapproach channel to the Intake structure of the 24 kmlong Head Race Tunnel. The overall site specificrequirements and terrain condition were major
impediments in relocating the proposed barrage atalternative locations.

The thick lacustrine deposit behind a bouldery
natural dam at Zimithang in Tawang District ofArunachal Pradesh, constituted the site for the 11m
high barrage and appurtenant headworks structures
for the Nyamjang Chhu Project (Fig.2). Deep drillingat the barrage axis proved the bedrock depth to beabout 100m. The subsurface geotechnicalinvestigations comprising drilling and geophysics,
and field and laboratory tests involving SPT,
permeability, mechanical properties of sandy andsilty overburden, grain size analysis, etc, confirmed

Fig.2: Natural dam at barrage site, Nyamjang Chhu HE Project, Arunachal Pradesh, India.

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10th Asian Regional Conference of IAEG (2015)& 25 and the average bearing capacity was worked
out to be 0.5 MPa for the design of the barrage.
Among other measures, the main design features
included floor length of the barrage at 120m, aconcrete cut off with a sheet pile at its downstream
end as a safeguard against piping, sand drains forearly release of excess pore water pressure, etc.

The site for the 60m long diversion barrage for the300 MW Baspa Stage-II Hydroelectric Project acrossthe Baspa River near Sangla in Himachal Pradesh lies
over an enormously thick lacustrine deposit believed
to have been formed due to a natural dam. Studies
and subsurface exploration through drilling revealthat the 200m wide and more than a km longlacustrine deposit could be as deep as 150m. The
general ground level of the lacustrine deposit is El
2519m and the river gradient about 1 in 82.
Immediately downstream, the river has a very steep
slope of about 1 in 4 before assuming the general
gradient of 1 in 40 after dropping about 150m. The
SPT ‘N’ values at the barrage site ranged between 18

The presentation takes a critical look at thephenomena of natural damming in the Himalaya andits engineering geological and geotechnicalimplications with special reference to Dul Hasti,
Nyamjang Chhu and Baspa-II Hydroelectric Projects.

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