Diversion of Diyawini Oya into the surge chamber of Samanalawewa Power Station

Abstract

Investigations of the development of hydro power in the upper Walawe river basin using the head difference of about 350 m between the two pineplaines of the south eastern part of the central high lands of Sri Lanka had commenced in mid 1950's. Several studies have been conducted since then on the possible ways of developing this potential .These studies lead to the finalization of Samanalawewa hydro power project in 1985. The scheme envisaged the development of the potential in two stages .Under the stage I major components such as main dam, low pressure tunnel ,surge chamber ,a single penstock add power plant of 120MW capacity housing two generating units were to be developed. Work under stage I was completed and plant was commissioned in 1992. Stage II of the scheme envisaged the construction of Diyawini Oya reservoir, a second penstock and an extension to the stage I power house to install two additional generating units to bring the total installed capacity of Samanalawewa hydro power project to 240MW. According to the studies the installation of additional 120MW capacity in the second stage of the project was aimed at providing the needs of additional peaking capacity in the CEB generating system at a future date .The studies conducted in 1985 expected such requirement to be in the CEB system in early 21st century. After completion of the wet blanketing of the reservoir to arrest the leakage which developed on the right abutment of the dam Ceylon Electricity Board wished to evaluate the feasibility of the development of stage II of the scheme in the early years of 21" century as envisaged ill 1985. In 1999, CEB requested Central Engineering Consultancy Bureau to review the feasibility of implementing the stage II of Samanalawewa hydro power project. During this review the capacity of the existing system to cater for the conditions after the installation of additional units the feasibility of construction of Diyawini Oya reservoir and the installed capacity to be developed in the near future were evaluated. Most of components of the existing project are found to have sufficient capacity to cater for the increased flows after stage II . Some modifications are required in the surge chamber in order to connect the Intake from the Diyawini oya reservoir. A provision has already been made available for this purpose in the surge chamber. Review of the feasibility of the development of Diyawini reservoir by CECB indicated that the water tightness of the reservoir is uncertain in the light of the latest geological information available .It was also noted that any treatment of reservoir to improve the water tightness will be prohibitively costly. Accordingly CEB apparently had given up the proceeding of stage II studies. However it was interesting to investigate the possibility of using Diyawini Oya waters diverted in the direction of the surge chamber and released in to the surge chamber at a feasible point. This way additional water quantity from the stream can be pumped in to the surge chamber and it is added to generate electricity at the power station. It is very important to check the electrical energy loss when pumping such a quantity of water and then compared with the energy gain at the end. All relevant calculations were done and it indicates that the project is well profitable and impacts on Environmental and social aspects are in favor of the project.