The Samanala Dam ( Sinhala: සමනලවැව වේල්ල) is a dam primarily used for

hydroelectric power generation Hydroelectricity, or hydroelectric power, is electricity generated from hydropower (water power). Hydropower supplies 15% of the world's electricity, almost 4,210 TWh in 2023, which is more than all other renewable sources combined and also ...

Sri Lanka Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka, also known historically as Ceylon, is an island country in South Asia. It lies in the Indian Ocean, southwest of the Bay of Bengal, separated from the Indian subcontinent, ...

. Commissioned in 1992, the Samanalawewa Project (''Samanala Reservoir Project'') is the third-largest hydroelectric scheme in the country, producing 405 GWh of energy annually. It was built with financial support from

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

and the

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. It is notable for a large leak on its right bank. Power production continues as planned despite the leakage, and the water from the leak now provides two thirds of the water issued by the reservoir for agriculture in downstream areas.

Location

The Samanala Dam is located in the Uda Walawe basin. It was built at the confluence of the Walawe river and the Belihul Oya, a location above mean sea level. It is near the town of

Balangoda Balangoda is a town in Ratnapura District, Sabaragamuwa Province, Sri Lanka, governed by an urban council located away from Colombo and from Ratnapura on Colombo - Batticaloa Highway(A4). It is one of the largest towns of the Sabaragamuwa Pro ...

, southeast of the capital

Colombo Colombo, ( ; , ; , ), is the executive and judicial capital and largest city of Sri Lanka by population. The Colombo metropolitan area is estimated to have a population of 5.6 million, and 752,993 within the municipal limits. It is the ...

. The ground of the project area is

karstic Karst () is a topography formed from the dissolution of soluble carbonate rocks such as limestone and dolomite. It is characterized by features like poljes above and drainage systems with sinkholes and caves underground. There is some eviden ...

Background and development

With the Mahaweli and Laxapana

hydroelectric Hydroelectricity, or hydroelectric power, is Electricity generation, electricity generated from hydropower (water power). Hydropower supplies 15% of the world's electricity, almost 4,210 TWh in 2023, which is more than all other Renewable energ ...

power projects implemented, the demand for electricity in

rapidly increased. When it became clear that it would not be possible to meet the demand with

coal-fired power plants Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is a type of ...

, the government of Sri Lanka decided to initiate another hydroelectric power project. A reservoir type hydroelectric power plant was planned to be constructed across the

Walawe river The Walawe (, ) is a long river in Sri Lanka which originates on Adam's Peak. It discharges into the Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or approximately 20% of the water are ...

, which would address the shortage of electricity in the country. Detailed investigations for a hydroelectric power plant in this area have been carried out since 1958, but the scheme was initiated only in 1986. The financing for the project was given by the governments of

and the

. Sir Alexander Gibb & Partners did the design work and

Balfour Beatty Balfour Beatty plc () is an international infrastructure group based in the United Kingdom with capabilities in construction services, support services and infrastructure investments. A constituent of the FTSE 250 Index, the company is active ac ...

were the contracts to lay roads, drive a tunnel and build the power station. The cost of the project was estimated to be 60,176 million

yen The is the official currency of Japan. It is the third-most traded currency in the foreign exchange market, after the United States dollar and the euro. It is also widely used as a third reserve currency after the US dollar and the euro. T ...

, but with the remedial measures taken to control the leak, the cost increased to 74,313 million yen. The Samanalawewa power plant was commissioned in 1992. The Samanalawewa project is the second largest hydroelectric scheme in Sri Lanka after the Mahaweli project.

Dam, reservoir and the power station

The Samanala Dam is in height and has a length of at crest level. The volume of the dam is about . Rainfall in the area has an annual average of , and the catchment area is . The dam is of rock fill, central earth core type. The

spillway A spillway is a structure used to provide the controlled release of water downstream from a dam or levee, typically into the riverbed of the dammed river itself. In the United Kingdom, they may be known as overflow channels. Spillways ensure tha ...

of the dam has three gates, each high and wide. It can discharge water at a rate of per second. Its tunnel is in diameter and in length. The reservoir created by the dam has a total live storage capacity of . Its gross storage capacity is , of which is dead storage. The reservoir's full supply level is above main sea level, and the reservoir spreads upstream at this level. The reservoir is u-shaped. It covers an area of 897 ha. However, with the water leakage the water level was reduced to . It is one of the largest reservoirs in Sri Lanka. The power house contains two

Francis turbine The Francis turbine is a type of water turbine. It is an inward-flow reaction turbine that combines radial and axial flow concepts. Francis turbines are the most common water turbine in use today, and can achieve over 95% efficiency. The pro ...

s, each with a capacity of 62 MW, and generates 405 GWh of energy annually. The powerplant is managed by the

Ceylon Electricity Board The Ceylon Electricity Board - CEB (; ), was the largest electricity company in Sri Lanka. With a market share of nearly 100%, it controlled all major functions of electricity generation, transmission, distribution and retailing in Sri Lanka. I ...

Leak

A permeable area of ground was found during the construction of the dam in 1988. Curtain grouting was used in an effort to remedy this. However, as the reservoir was being filled, a large leak occurred on the side of the right bank, about downstream from the dam, causing a

landslip Landslides, also known as landslips, rockslips or rockslides, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, mudflows, shallow or deep-seated slope failures and debris flows. Landslide ...

. Subsequent measures taken to control the leakage were largely ineffective. The leakage of approximately per second continues, but has not affected power production at the plant, which has been in full operation since its commissioning in 1992. The leak is constantly monitored. If the leak remains stable and does not increase further, it does not pose a threat to the sustainability of the project. The karstic ground has created complex geological conditions, and as a result the exact mechanism of the leak cannot be established, so remedial measures have been unsuccessful.

Impact

The project was planned as a single purpose hydropower project. Therefore, the effects on the agriculture and the environment of the area were taken into consideration very little during the development. Although an irrigational release valve (IRV) is there in the dam to supply water to the farmlands in the downstream areas, the yield and cultivable acreage has declined since the Samanalawewa project was commissioned. However, the necessity to constantly release water for agriculture in the downstream areas was reduced due to the leak. Of the water released from the Samanala Dam for agriculture in the downstream areas, two-thirds is from the leak and only one-third has to be supplied via the IRV.

References