SATGE I —
PROJECT DESCRIPTION
The
engineering works comprising the Mahaweli Ganga Development, to be constructed
under Stage I, include the Polgolla complex, the Bowatenna complex, the Sudu
Ganga training works and the remodelling of the Elahera weir and the
Elahera-Minneri-Kantalai canal. When these works are completed in 1975-76,
additional water from the Mahaweli Ganga and the Amban Ganga could be made
available to make up the deficiency in the water supply to existing irrigated
lands of 52,400 ac. in areas H and IH including Huruluwewa and 74,185
ac. in areas Dl, D2 and 0. The works to be constructed in Stage I are
designed to meet the ultimate water requirements of 217,585 ac. of existing and
new lands in areas H and IH (123,400 ac.) and Dl and G1(94,185
ac.) which would be developed in Stages II and III respectively.
In
addition to the engineering works, Stage I includes detailed investigations of
existing and new lands in areas H and IH to be developed in Stage II which, inter-alia,
including preparation of the feasibility report, detailed cost estimates,
aerial survey, land classification and development, irrigation and drainage
water cropping patterns and water requirements, settlement of farmers and
agriculture extension services. Drainage and land levelling equipment is
provided for on-farm development of 7,000 ac. of sugarcane in area Dl.
Polgolla Complex
The
Polgolla complex consists of a low diversion dam across the Mahaweli Ganga
about a mile downstream of Kandy city, a 5-mile long diversion tunnel to divert
the Mahaweli flows to the Dhun Oya, thence into the Sudu Ganga, a tributary of
the Amban Ganga, a 40 MW hydro-electric plant at the end of the tunnel with a
by-pass outlet for irrigation releases and a tail-race canal outfalling into
the Dhun Oya, a tributary of the Sudu Oanga.
The diversion dam would be
a concrete ogee structure, 565 feet long at crest and 55 feet high above
foundation, with ten hook type spillway gates, 21 feet high and 40 feet wide,
and a 24 feet wide roadway on the crest. The spillway would be capable of
passing a discharge of 210,000 cusecs, which corresponds to a flood of 100 year
frequency, without causing any significant damage to developments upstream and
without affecting the railway bridge located about 6,100 feet upstream of the
Polgolla site. The dam would be operated to provide a minimum continuous
release of 150 cusecs downstream for sanitation, wild life and domestic use and
to head up water up to the maximum operat ing pool level at elevation 1,446
feet when the maximum head across the dam would be 23 feet. The minimum operating
pool level would be at elevation 1,438 feet which would provide a regulating
storage of 2,000 ac. ft. The geology of the site is favourable for constructing
a low dam. The site had been selected after examining other alternatives both
upstream and downstream, the main consideration being the suitability of the
tunnel intake and not the geology. Rock outcrops are exposed at a few points
both upstream and downstream of the site. The rocks are mostly quartzo- feldspathic-biotitic
gneisses having almost vertical dips. Seven holes with an aggregate length of
312 feet have been drilled along the axis of the dam. The core data revealed
that weathering was deeper, particularly on the right bank, but sound rock was
available at a maximum depth of 20 feet to adequately support the structure.
The foundations would require grouting and an upstream grout curtain would be
constructed to avoid uplift pressures under the downstream floor.
The
diversion tunnel would be 26,450 feet long with a horse-shoe section 19
ft. bin, in diameter, capable of diverting 2,500 cusecs of the Mahaweli flows
into the Amban Ganga through the Dhun Oya and Sudu Ganga. It would be a
pressure tunnel and would be lined about 30% of its length~ The
tunnel alignment has been selected after considering several alternatives.
Although 28 bore~ holes with an aggregate length of 3,709 feet have been
drilled in the region for examining alternative alignments, the geological data
along the final alignment is based on 5 drilled holes only with a total
length of about 1,000 feet. The rocks along the tunnel trace consist mainly of
gneisses with inter-beds of quartzite’s and granulites except in the tail-end
where hard crystalline limestones and calc gneisses were found. Several closed
faults and shear zones traverse the tunnel area and are represented by long and
narrow drainage valleys at the ground surface between mountain ridges. At these
sections, problems of fractured rocks aiid seepage water are likely to be
encountered during construction. Several geologists visited the site from time
to time during 1959-68 and expressed favourable views on the feasibility of the
tunnel. Experience on the construction of the Maskeliya
tunnel, which is also 5 miles long and passes through similar geology,
supports the conclusion that no unsurmountable problems would be expected
during the construction of the Polgolla tunnel. Nevertheless the data of
only 5 bore holes along a 5-mile long tunnel does not provide sufficient
information to international contractors to prepare accurate and competitive
bids and it would be prudent to carry qut additional drilling before the tender
documents are issued. The Irrigation Department is carrying out the additional
drilling suggested at their cost.
The
hydro-electric5 plant is to contain two vertical shaft turbo generator sets
with a rated capacity of 20 MW each and designed for a nominal discharge of
1,000 cusecs and an operating head of 268 feet. The maximum head would be 300
feet and the minimum 230 feet. At the downstream tunnel portal there would be a
steel penstock, 13 feet in diameter at the beginning and bifurcates into 219’
diameter pipes and about 1,300 feet in length, which would enter a manifold
from which two outlets would convey the water to the turbine units with a
third outlet to serve as a by-pass discharging directly into the tailrace which
joins the Dhun Oya.
The
Polgolla power plant would increase the firm power capacity of the system by
about 20 MW and would contribute 180 million kWh of energy annually. In order
to connect the power station to the existing 132 KV transmission network, the
Ceylon Electricity Board would construct a short double-circuit 132 KY
transmission line from the Switching Station located near
Bowatenna Complex
The
Bowatenna complex consists of a high diversion dam on the Amban Ganga at
Bowatenna,. a 4-mile-long diversion tunnel on the left
bank followed by about a mile long lined canal up to Dambulla Oya, a
bifurcation structure at Dambulla Oya to release part of the flows to the
Dambulla Oya which discharges into the Kalàwewa tank and the remaining part
into a short earthen canal which will feed the Kandalama tank. (A reservoir
with a net capacity of 7,500 ac. ft. will be constructed across Dambulla Oya
under Stage 11).
The
diversion dam would be a concrete structure. 741 feet long at the crest and 100
feet high with six radial gates and an earth embankment on the left bank having
a spillway of length 222’ and a 16 feet wide roadway on the crest. The spillway
would have a capacity of 125,000 cusecs which corresponds to a flood of 100
year frequency. The maximum operating pool level of the dam would be at
elevation 820 feet and the minimum at 800 feet which would provide an active
storage of 21,000 ac. ft. The geology of the site is favourable. Rock outcrops
are exposed which are fractured and joined, consisting of gneisses and
quartzite’s. Boreholes data along the dam axis shows some fracturing at depths,
indicating the need for grouting.
The
diversion tunnel was 22,468 feet long with an unlined horse-shoe section 13
feet in diameter capable of discharging 1,500 cusecs; about 30% of this tunnel
will be protected by concrete lining and shotcreting. Six boreholes have been
drilled along the 5 mile length of the tunnel. In the upper part of the
tunnel, the rock formations consist of gneisses and quartzite which are
fractured and joited. In the lower half of the tunnel the rock consists of calc
gneisses and crystalline limestone. The groundwater table lies closer to the
ground surface in the lower half of the tunnel alignment and may give rise to
difficult problems of care of water during construction. Geologists from USOM,
FAO and UNDP mentioned in
The
tunnel would lead to an open-cut with depth of cut varying from 65 to 15 feet
in a length of 4,160 feet to Dambulu Oya. The canal would have a
capacity of 1,500 cusecs and would be lined with concrete. At Dambulu Oya there
would be a bifurcation structure for diverting 750 cusecs into the
Dambulu7 Oya which feeds the Kalawewa tank while the remaining discharge of 250
cusecs would be conveyed through a short unlined canal, the Kandalama tank.
Another bifurcation structure constructed at the end of this canal will divert
100 cusecs of the flow to 1-Iuruluwewa along a 16 mile canal which leads to
Habaraua Oya for feeding Huruluwewa.
Sudu Ganga Training Works
Water
released from the Polgolla tunnel and hydro-electric plant would be conveyed to
Bowatenna through the natural channels of Dhun Oya, Sudu Ganga and Amban Ganga
rivers. The additional flow of up to 2,500 cusecs in these channels would
affect their natural regime, particularly in the Dhun Oya and the upper reaches
of the Sudu Ganga, where the existing capacity of the channels is less than
2,500 cusecs. Channel improvements and training works would be required to
prevent erosion and damage to existing improvements. Inspection of the channels
has also indicated that parts of the cultivated land along the banks of the
Sudu Ganga, which would be subjected to flooding, may have to be acquired and 3
road bridges, on the Sudu Ganga would have to be remodelled to improve their
Waterways. Some drop structures may have to be constructed to stabilize the
channel.
The
Elahera-Minneri-Kantalai-Yoda Ela canal (referred to as the Elahera canal) is
an old canal which has been restored and remodelled several times in the past
two decades. It takes -off from the Elahera weir on the Amban Ganga and runs
along a contour to the Minneriya tank with a by-pass feeder to the Giritale
tank. It takes off again from the Minneriya tank, passes above the Kaudulla
tank, augmenting the supplies to the latter through a special feeder and then
spills into the Yoda Ela canal which discharges into the existing Kantalai
tank. The total length of the canal from the Elahera weir to Kantalai tank is
about 38 miles consisting of 22 miles from Elahera to Minneriya and 16 miles
from Minneriya to Kantalai. Its present capacity is 1,000 cusecs in the’ first
4.9 miles, 1,250 cusecs from 4.9 to 6.3 miles and 1,500 cusecs ifl the remaining
15.7 miles to the Minneriya tank. From the Minneriya tank, the canal has a
single bank on the right side and intercepts all the drainage from the left
bank. Here it is in effect, a catch-water drain with large pools at places
where the natural streams enter the canal. In view of its high right bank, it
can carry over 2,500 cusecs of peak floods.
The entire canal from
Elahera weir to Kantalai is poorly maintained. Below Minneriya tank, there is a
heavy growth of weeds and bushes which restrict the waterway and cause heavy
water losses.
In order to meet the full
irrigation requirements of areas Dl and G, and also to fill the tanks in time
for regulation and storage, the capacity of the Elahera canal would be
increased to 1,600 cusecs up to the Minneriya tank. Beyond this point, no
increase in the theoretical capacity is required, but considerable work has to
be done to restore the waterway by clearing the weeds and bushes and widening
the restricted sections. After the canal is fully remodelled, it would require
heavy annual maintenance to maintain its capacity as the numerous natural
streams entering the canal throughout its length would cause silting, encourage
weed growth and create large standing pools. The alternative is to construct a
regular canal with two banks, crossing all the natural drainage lines by
siphons underneath the canal. This would require 146 drainage siphons to cross
the 146 natural streams draining a catchment’s of 40 square miles on’ the left
bank from Elahera to Minneriya and another 65 drainage siphons to cross the 65
natural streams draining a catchment’s area of 17 square miles on the left bank
from Minneriya to Kantalai. The main advantages of this alternative would be
the reduction of water losses and annual maintenance costs. However, the main
disadvantages are the heavy capital cost of construction, the hazards of damage
to the numerous cross drainage works during heavy rains, which would increase
the annual maintenance cost and might also make the canal inoperative at times
and, above all, the waste of an important source of water from the left bank
catchments of the canal varying from 60,000 to 20,000 ac. ft. annually, which
can be regulated in the tanks and utilized for irrigation.
The
Elahera weir would be remodelled to raise its pond level by an additional 2
feet and a new head regulator would be constructed on the left bank, adjacent
to the existing regulator, with a capacity of 700 cusecs to provide additional
supplies to the Elahera canal. The first 1,000 feet of
Elahera canal runs through a deep cut in rock and is lined with concrete.
It would be widened and re-modelled to
increase its capacity to 1,500 cusecs.
investigations
— Areas H and IH
Area
H is estimated to contain 42,000 ac. of already irrigated land including
Huruluwewa irrigation area and 71,000 ac. of new land, mainly under forest
which is suitable for and capable of being• irrigated. Area in is estimated to
contain 10,000 ac. of irrigated land; no new land is available. Additional
irrigation, drainage and agricultural investigations are required in these
areas before they can be appraised for further development. These
investigations and studies would involve, but not be limited to, the following
(a) Review
of existing data and reports;
(b) Aerial photography at. a scale of 1 :10,000 to enable production of rectified
prints and mosaics;
(c) Review the existing contour mapping;
(d) Land classification of areas at a scale
of about 1: 10,000;
(e) Water supply, water
management and use studies, including seepage and evaporation losses and use of
measuring devices;
(f) Land development and
on-farm development costs for existing and new land based on typical farm
layouts from sample plots on representative areas covering 10% of the total
irrigable area;
(g) Agronomic
and farm management studies to establish optimum cropping patterns with and
without livestock;
(h) Socio-agro-economic studies of
settlement patterns and suitable farm sizes;
(i) Establish an overall irrigation and drainage
plan including flood control;
(j) Establish costs of recommended layouts;
(k) Study farm-to-market road requirements;
(I) Review and advice on expanded agricultural extension service
requirements and sitting and layout of demonstration farms;
(m) Determine
project agricultural benefits using internal rate of return;
(n) Prepare interim and final feasibility
reports; and .
(o) Recommend type or organization suitable
for execution and operation of Stage II.
On-Farm Development
If
7,000 ac. of sugarcane in area Dl are to benefit fully from the increased water
supply that should be available by end of 1975, the irrigation and drainage
systems will require levelling and sub-soiling. Equipment has been provided in
the project to enable this work to be completed by the end of 1975.
It
would be necessary for on-farm development work to be carried out immediately
on 220 ac. prior to the arrival of equipment being provided under the project.
This work would be done by equipment already in the country, to enable information
to be obtained on the yield of sugarcane per acre, when water is no longer the
constraint. This, in turn, wilt require the sugar cane estate to have a first
call on the available water in the Kantalai tank for irrigating adequately 200
ac. of sugarcane throughout the year. This involves making available sufficient
water to permit a minimum application of 2 in. of water to the root zone every
10 - 14 days.
Improvements to existing
Irrigation Schemes
Nearly
126,585 acres of land in the existing irrigation schemes in systems Dl, 02, G,
1-I, ti-I and under Huruluwewa will receive sufficient irrigation water for two
crops. These areas are presently being cultivated satisfactorily only during
the Maha season. Special Projects are proposed to be set up in these areas in
order to co-ordinate and improve the services available for farmers. Supply of
inputs, extension services, marketing and credit facilities, and education for
farmers on better water management, irrigation and farming practices will be done
using these special projects in order to improve the efficiency of farming and
the productivity in the area. Irrigation and drainage systems in these schemes
will need improvements in order to facilitate the discharge of increased
irrigation water supply. Necessary improvements mainly to the canals,
structures and other waterways will be carried out to improve the efficiency of
the irrigation and drainage network.
Kalawewa Head Works —
Improvements
The
active storage capacity of the Kalawewa Tank will be increased by 20,000 ac.
ft. This will improve the regulation of irrigation supply to areas H and IH
(under Kala Oya and
Bowatenna Power House
It
has been proposed to harness the potential hydropower of the water flowing down
the Bowatenna Dam. A hydro-electric power plant with an installed capacity of
40MW and an average annual power generation of 160 million kWh of energy will
be constructed on the downstream of the dam. A short tunnel will be excavated
through the left abutment of the dam which will be connected to a steel
penstock for conveying water to the Power House. The water,
after generating. hydropower will flow to Amban
Ganga for irrigation systems D and G.