Bowatenna reservoir /dam
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 Para 5 above, inspected the site and endorsed the feasibility of the tunnel. For supplying adequate information to contractors on the geology of the tunnel, additional drill holes were made along the axis.
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.
Elahera-Minneri-Kantalai-Yoda Ela Canal
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.