Kothmale Reservoir/Dam

The Kotmale project is one of five major headworks projects being undertaken under the Accelerated   Mahaweli Ganga scheme. it is the most upstream of these projects and develops the hydro potential of a major right bank tributary of the Mahaweli Ganga, the Kotmale Oya.

The Kotmale Hydro-electric project

The Kotmale project was one of the first projects taken up under the Accelerated programme for development of the Mahaweli ganga. The basic elements of the project are a dam on the Kotmale Oya ( a tributary of the Mahaweli ganga) and a tunnel system leading to a power station with the outfall of the mahaweli ganga. The primary function of the project is the generation of electric power. Additional benefits will arise from an increase in the amount of irrigation water available at Polgolla due to regulation of flows in the Kotmale Oya.

Consultancy services :

Preliminary studies of the Kotmale project were carried out by the Government of Sri Lanka with the assistance of US Agency for International Development (USAID) in 1961 and subsequently by UNDP-FAO from 1964 to 1968. a feasibnility study of the project was carried out by the water and Power Development Consultancy Services (India) Limited (WAPCOS) from 1973 to 1976.

Sir William Halcrow and Partners in association with Messrs Kennedy & Donkin and the Central Engineering Consultancy Bureau (CECB) were appointed in 1979 fto provide consultancy services

Construction work commenced in February 1979. The reservoir was impounded in November 1984 and commercial power generation commenced in June 1985. The project was ceremonially commissioned in August 1985.

Scope

The Kotmale project envisages the construction of a 87 m (285 ft.) high Rockfill dam with a concrete membrane across the Kotmale Oya, an important right bank tributary in the upper reaches of the Mahaweli ganga about 25 miles upstream of the Polgolla barrage constructed under Project I of the Master Plan.

The Kotmale project was mainly for the development of hydro power and the regulated discharge from the reservoir to increase the flow diverted at the Polgolla barrage into the proposed Moragahakanda reservoir for augmenting the irrigation suppled in systems. The reservoir would reduce flood peaks and their frequency, thus alleviated the floods in the Gampola area below it.

The dam would create a reservoir having an effective storage capacity of about 174 m. cm. (141,000 acre feet) enabling regulation of a large proportion of the recorded mean annual flow of the Kotamale oya at the dam site. The water impounded by the reservoir would be conveyed through an underground water conductor system to an underground power station located at about 7.2 km. (4.5 miles) from the dam for generation of electric power. After power generation, this water will be discharged through the outfall into the mahaweli ganga at the Atabaghe Oya confluence.

In addition to the generation of power, the regulated waterwill improve the pattern of inflows of the Mahweli ganga at the existing Polgolla diversion dam. This will firm up the power benefits from Ukuwela power station and serve to increase the irrigation water supplies from the Bowatenne dam.

Location and Access

The dam site is at Kadadora located about 6.6 km (4.1 mls) upstream of the confluence of Kotmale Oya with Mahaweli ganga at a place where the river enters a narrow and deep valley with steep banks. It is close to the Pussellawa-Ulapane highway and is 12 miles from Gampola town and 88 miles from Colombo. The nearest railhead is Ulapone, 7 miles away.

The power house is underground and situated in the belly of the Atabage Mountains, about 6.4 km (4 mls) downstream of the Kotmale Oya – Mahaweli ganga confluence. the project headworks lie on the right bank of the mahaweli ganga and is partly in the Kandy district and partly in the Nuwara Eliya district. Access to the main project headworks can be thourgh two routes. One is from the new access road constructed from the Gampola – Nuwara Eliya road just outside Gampola town, which passes close to the underground power station and joins the Ulapane- Pussellawa road. The other is though the latter road from Ulapane which passes close by the right abutment of the dam.

The Kotmale Oya has its beginning in the South Central massif at an elevation of 2134 m.(7000 ft.) and is about 70 km. (43.3 mls) in lengh draining a toatal area of 58,534 ha.(226 sq.mls) and dropping 1,585 m (5,200 ft) before meeting the Mahaweli ganga. Rainfall data from 1907 onwards are available for this region.

Main features :

- A rockfill dam 87.0 m high and 600 m long
- A chute spillway with a capacity of 5500m3/Sec. consisting of 3 radial gates 14x15m
- A hourse shoe shaped 6.4, diameter low pressure tunnel system nearly 7.0 Km. long with a capacity of 113.3m3/Sec (4000 cusecs)
- A high pressure shaft with diameter varying from 4.8 – 5.5 m
- An underground power station with an installed capacity of 3x67 MW.
- An access tunnel providing access to the power station and a tail-race tunnel to discharge the water after generation of power.

Historical Background

In the history of country, Kotmale enjoys a very special place. This is due for no other reason than the fact that King Dutugemunu, who is acknowledged as our greatest King, spent his youth in the mountains and valleys of Kotmale. This history, the story of Dutugamunu is familiar to us all. The stream he bathed in is here; the village he lived in is here; the field he tilled is here. The story of his life at Kotmale is part of the folklore, idiom and life of this valley and its people.

From that historic epoch in which Dutugemunu lived, his name and message beckons to all generations that follow. Many are the lessons that we can learn and should learn from his life, his work, his vision and his leadership. In my view, the most important of these lessons is the need for thought and action in keeping with the demands of our time.

Inherited Lands :

More than 3,000 families who lived in the Kotmale valley had to lose their homes and lands, some of them inherited through a continuity of nearly twenty centuries. They were given alternate land on higher elevations of Kotmale itself or in Kalawewa, Maduru-oya, and Ulhitiya, which are being developed under the Accelerated Mahaweli programme.

Geology:

The Kotmale area has had a history of earthslips, landslips, and other geological disturbances. Owing to this great care had to be taken in the design of the various features of the project. Soon after construction work was awarded and during the early stages of the dam construction, when excavations of the riverbed were in progress, the presence of a limestone layer underneath the bed rock below the dam and extending up to the reservoir, was discovered. These and other adverse features led to a through investigation of all the geological features in order to see whether there are practical problems liked landslides above the reservoir leakage of water through the dam foundation into adjoining valleys, earth tremors, etc. which could seriously affect the stability of the project.

Mahaweli Maha Seya :

During times of prosperity in ancient times, great cultural monuments and outstanding structures were built. Similarly in keeping with ancient tradition, a large stupa to be named as Mahaweli maha seya is proposed at Kotmale, to depict the progress and development of the country under the Accelerated Mahaweli Programme. This stupa is to also compensate for the eight Buddhist temples which were inundated and to also stand as a tribute to those who sacrificed their ancestral land, which had been cultivated for generations in the Kotmale Valley.

The Seya is built on the right bank hill above the dam site at an elevation of 950 m (4150 ft.) above MSL ie. It overlooks the dam site from a place 257m (845 ft.) above. It consists of a 200 ft. (61,0 m) diameter and 11” thick reinforced concrete shell, which supports a asquare chamber (31 ft.) a Dewatha Kotuwa (12 ft), a pinnacle 44(ft) and a Kothderella (52 ft) respectively. The overall height of the structure is 274 ft. (83.)

The Main Physical Components of the Project

Basic Features

The catchment area of the river at the dam site is 544 sq.kms mostly developed under tea plantations. There were submerged by the reservoir was mostly developed land. The dam is a thick earth core rock fill structure.

The long term mean annual runoff at the dam site is 2,430 thousand acre feet of which 1.100 thousand ac.ft is presently being diverted at Polgolla.

Salient Features :

Catchment area - 544 Km2
Mean river flow - 105 m3/s
Highest recorded river flow 761.5 m3/sec
Gross storage capacity (at 703m) - 174 million m3
Useful storage capacity - 154 million m3
Surface water area - 1374 Ha
Main Dam

Dam Crest Length - 600 m
Dam Height (above foundation) - 87 m (max.)
Crest Elevation - 706.5 metres MSL
Total Volume of rockfill - 4.3 x 10^6 m3
 

Tunnel

Length - 6954 metres
Diameter of tunnel
(inside concrete lining - 6.2 metres
Height of surge chamber - 142 meters

Penstock

Steel circular 5.55-4.80m diameter, 480m length, with surge tank 
Tailrace
Length: 635 m; Maximum Level: 479 m above MSL : 476 m above MSL

Turbines (KAMEWA)

Type - vertical Francis
Design (net head) - 201.5 meters
Speed of rotation - 375 revs/mm

Output: 67MW @ 201.5 m effective head (rated) , 68 MW @ 231 m effective head (Maximum)
Rated discharge : 35 m3/sec
 

Generators (ASEA)

Conventional vertical

Type - Semi umbrella 9000 kvA, 0.85 pf, 3765 A
Generating voltage - 13.8 kv

50 Hz, 16 poles, 375 rev/min

 
Energy production capability - 310 GWH/ year (firm)
Energy production capability - 445 GWH/ year (initial)