Samanalawewa Dam


The Samanalawewa Dam, known as the "Samanalawewa Reservoir Project," is a significant hydroelectric dam in Sri Lanka. Commissioned in 1992, it is crucial in generating hydroelectric power and supporting the country's energy needs. This article delves into the construction, specifications, and operational aspects of the Samanala Dam, highlighting the challenges posed by a significant leakage and its implications for power production and downstream agriculture.

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Situated in the Uda Walawe basin, the Samanala Dam is strategically located at the confluence of the Walawe River and the Belihul Oya. Positioned approximately 400 meters above mean sea level, the dam resides near Balangoda, about 160 kilometres southeast of Colombo, Sri Lanka's capital. The project area's ground is characterized as karstic, contributing to unique geological conditions.

Background and Development

The demand for electricity in Sri Lanka experienced rapid growth following the implementation of major hydroelectric power projects such as Mahaweli and Laxapana. Recognizing the limitations of coal-fired power plants, the Sri Lankan government embarked on the Samanalawewa Project to address the escalating electricity shortage. Extensive investigations for the hydroelectric power plant began in 1958, with the project officially initiated in 1986. Financial assistance from Japan and the United Kingdom facilitated the project's development.

Dam, Reservoir, and Power Station

The Samanala Dam stands tall at 110 meters and spans 530 meters at crest level. Its volume measures approximately 4,500,000 cubic meters, and the catchment area receives an average annual rainfall of 2,867 millimetres. The dam, predominantly of rock filled with a central earth core, features a spillway equipped with three gates, each 14 meters high and 11 meters wide. The dam's tunnel has a diameter of 4.5 meters and stretches 5,159 meters in length.

The reservoir formed by the dam boasts a total live storage capacity of 218,000,000 cubic meters, with a gross storage capacity of 278,000,000 cubic meters. Notably, 60,000,000 cubic meters constitute the reservoir's dead storage. The reservoir covers an area of 897 hectares and extends 8 kilometres upstream at its whole supply level, which is 460 meters above mean sea level. Power generation at the Samanala Dam relies on two Francis turbines, each with a capacity of 62 MW, contributing to an annual energy output of 405 GWh.

The Leak Issue

During the construction phase in 1988, a porous area was discovered, prompting the application of curtain grouting as an attempted remedy. However, as the reservoir began filling, a significant leak manifested on the right bank, around 300 meters downstream from the dam, leading to a landslip. Despite subsequent efforts to control the leakage, the measures proved largely ineffective. As a result, the Samanala Dam continues to experience a constant leak of approximately 2,100 litres per second. However, it is essential to note that the leakage has helped the plant's power production, which has operated at total capacity since its commissioning in 1992.

Environmental and Agricultural Impact

The Samanalawewa Project was primarily designed as a single-purpose hydropower project, with little consideration given to its effects on agriculture and the environment in the surrounding area. Consequently, the project's development should have prioritized addressing the potential impacts on agriculture and the local ecosystem. While an irrigational release valve (IRV) was incorporated into the dam's design to supply water for farmlands downstream, the yield and cultivable acreage have declined since the project's commissioning.

However, the continuous leak from the dam has unintentionally reduced the need to release water for agriculture in the downstream areas. Approximately two-thirds of the water released from the Samanala Dam for downstream agriculture comes from the leak, while only one-third is supplied through the IRV. This unintended consequence has altered the water distribution dynamics in the area, affecting the availability and management of water resources for agricultural purposes.

FAQs (Frequently Asked Questions)

  1. Is the Samanala Dam the largest hydroelectric project in Sri Lanka? No, the Samanala Dam is the third-largest hydroelectric scheme in Sri Lanka, following the Mahaweli and Laxapana projects.
  2. Does the leakage from the dam affect power production? Despite the ongoing leakage, power production at the Samanala Dam remains unaffected and operates at total capacity.
  3. How is the leak from the dam monitored? The leak from the Samanala Dam is constantly monitored to ensure its stability and assess any potential threats.
  4. What measures have been taken to control the leakage? Various remedial measures have been attempted to prevent the leakage, but the complex geological conditions have hindered their effectiveness.
  5. What is the impact of the leak on downstream agriculture? The leak has unintentionally reduced the necessity to release water for agriculture downstream, altering the water distribution dynamics in the area.




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