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Increasing holding capacities of main reservoirs: Boosting hydropower generation  

23 May 2021

By Maheesha Mudugamuwa     Above-normal rains received during the past few weeks have allowed hydropower stations to step up power generation, somewhat easing concerns raised earlier that were a result of scanty showers from January to April. However, delays in dredging the sludge in the main reservoirs, resulting in a decline in the water quantities held by the respective reservoirs, have affected these hydropower generation levels.   The World Bank’s (WB) report on the “Sri Lanka Integrated Watershed and Water Resources Management Project (P166865)” has highlighted that the degradation of the Upper Mahaweli Watershed has contributed to reduced water flows and storage capacities in several strategic reservoirs serving multiple developments purposes, soil erosion, and reduced soil fertility, and that sedimentation of the country’s main reservoirs trigger multiple downstream impacts, and hence needs urgent addressing.  At the end of last year, the Irrigation Department commenced a project to increase the height of the Kotmale Reservoir Dam by 30 m to contain more water. Under the new project, the water capacity of the Kotmale Reservoir will increase to 391 million cubic metres (MCM) from the present 174 MCM. The Kotmale hydropower turbines generate 210 MW, and it will increase to 240 MW when the height of the dam is elevated by 30 m. The length of the dam is 600 m and the height is 87 m.     Dredging, sediment removal not necessary yet?     When contacted by The Sunday Morning, CEB Chairman Eng. Vijitha Herath said the hydropower reservoirs had not yet reached spill level and therefore, there was no necessity to begin dredging and removing sediments.  “The reservoirs should first reach their spill level for us to begin dredging and to remove sediments. Since none of the hydropower reservoirs have reached that level yet, the necessity has not yet arisen,” he added.   In the meantime, a civil engineer attached to the Water Management Secretariat (WMS) of the Mahaweli Authority of Sri Lanka (MASL) confirmed to The Sunday Morning that none of the hydropower reservoirs were subjected to capacity expansions by way of removing sediments as yet.   “We have not yet commenced any such project of that nature. Earlier, we had conducted sediment removal in Kala Oya. Dredging won't make much of a difference in big reservoirs. However, it's good for small reservoirs, and it actually made a considerable difference in the capacity measurements, and we have updated the latest statistics,” the civil engineer, who wished to remain anonymous, said, adding that the Department had not conducted a survey on reservoir capacities and sediment collection.  The WMS civil engineer noted that the most important thing is to protect the catchment areas. He said that without properly maintaining the catchments, there wouldn't be enough water capacities in the reservoirs.   Meanwhile, when contacted, an official from the Irrigation Department's Digana office confirmed that preliminary discussions were underway to get World Bank funds to commence a sediment removal project in Sri Lanka, but the project, according to the officer, was not yet finalised.     SL’s hydropower generation     In February this year, hydropower generation was recorded at around 19% of the island’s total power generation, as it dropped due to low levels of water in the hydropower reservoirs. As per the latest statistics of the Ceylon Electricity Board (CEB), hydropower generation has now increased to around 32%.  A cheap and renewable source of energy, hydropower plays an important role in Sri Lanka’s power sector. According to the Long-Term Generation Expansion Plan (LTGEP) 2020-2039 of the CEB, approximately 48% of the total existing CEB system’s capacity consists of 17 hydropower stations and 32% of the total energy demand was met by major hydro plants in 2018.  As of now, the country has already harnessed 80% of its total hydro resources; there are limitations to harnessing the rest due to environmental concerns. As such, a major concern expressed by a local energy expert we spoke to, is that since there is no space to expand the existing hydropower reservoirs in the country, it is a challenge to increase the already installed capacities.    Projects pending commissioning    According to the CEB, only a few hydropower projects are yet to be commissioned. These projects include the Broadlands Hydropower Project, which is a run-of-the-river-type project being built in the Kelani River. It is expected to generate 126 GWh of electrical energy annually.  The Broadlands Hydropower Project was the first large-scale hydropower plant project to obtain Clean Development Mechanism (CDM) registration in Sri Lanka in December 2012. The plant is to maintain a firm water release rate in order to safeguard the sport of white water rafting in the Kitulgala area, due to which there will be a reduction in the plant’s estimated annual energy generation. The China National Electric Equipment Corporation (CNEEC) was selected as the main contractor for the project. The total project cost is $ 82 million.  Eighty-five percent of the funding for this project was taken in the form of a loan from the Industrial and Commercial Bank of China (ICBC) while the balance 15% was financed by Hatton National Bank (HNB). The construction work and electro-mechanical work are progressing in parallel at the main dam site, main tunnel, diversion tunnel, and powerhouse site. The project was scheduled to be completed in 2019 and was to be operational last year, as per the CEB’s LTGEP 2020-2039.  The 30 MW Moragolla Hydropower Project is another such project yet to be commissioned. Located downstream of the Kotmale Power Station and approximately 3.5 km downstream from the confluence of the Mahaweli Ganga with the Kotmale Oya, the reservoir’s total storage capacity is 4.66 MCM and the annual mean energy expected to be generated is 97.6 GWh.  The Moragolla Hydropower Project was first identified in the “Report on a Survey of Resources of the Mahaweli Ganga Basin, Ceylon, Hunting Survey Corporation, 1962”, prepared in co-operation with the Survey General of Ceylon. The location was highlighted as one of the many potential hydropower sites in the “Master Plan for the Electricity Supply of Sri Lanka, German Agency of Technical Co-operation, 1988”. The Central Engineering Consultancy Bureau of Sri Lanka (CECB), in association with Al-Habshi Consultants, was appointed by the CEB to conduct a feasibility study on the project, utilising financing obtained from the Kuwait Fund for Arab Economic Development in 2009. Nippon Koei Co., Ltd., in a joint venture with Nippon Koei India (Pvt.) Ltd., was appointed by the CEB in 2012 to conduct a review of the feasibility study and prepare detailed design and bidding documents for the construction process. At present, the project is in the pre-construction phase and is expected to be operational by December 2022.  The Uma Oya Hydro Power project is one of the largest remaining sites of hydro potential. The project is a multipurpose development project and it will transfer water from the Uma Oya to Kirindi Oya in order to develop hydropower and to irrigate the dry and less-developed south-eastern region of the central highlands.  The project was implemented by the Ministry of Mahaweli Development and Environment in co-ordination with the Ministry of Power and Energy and the CEB. Its total capacity is 122 MW and its expected annual energy generation is 290 GWh. Financial assistance for the project was provided by the Government of Iran and currently, the project is under construction. It was expected to be completed by December 2020.     Catchment erosion a concern     As highlighted in the Mahaweli, Agriculture, Irrigation, and Rural Development Ministry’s report on the “Integrated Watershed and Water Resources Management Project”, which is funded by the International Bank for Reconstruction and Development (IBRD) and thereby has to comply with World Bank Operational Policies, the Global Climate Risk Index ranks Sri Lanka as the second-most climate change-affected country in the world.  Additionally, as per the University of Moratuwa’s study titled “Climate Change Impacts and Adaptation Measures in Giritale Reservoir in Polonnaruwa Sri Lanka”, authored by M. Kamran and N.T.S. Wijesekera, rainfall is projected to increase by 48% during the Southwest Monsoon by 2050.  Meanwhile, the WB report on the “Sri Lanka Integrated Watershed and Water Resources Management Project (P166865)” highlighted that “increasingly, variable rainfall with more intense events will tend to be more erosive, contributing to reduction of soil carbon in the catchment while reducing reservoir capacity through sedimentation”. It went on to state that catchment erosion has severely affected the capacity of reservoirs.    As per the CEB’s LTGEP 2020-2039, the major hydropower schemes already developed are associated with the Kelani and Mahaweli River Basins, and five hydropower stations with a total installed capacity of 369.8 MW have been built in the Laxapana Hydropower Complex where two cascaded systems are associated with the two main tributaries of the Kelani River, Kehelgamu Oya, and Maskeliya Oya.    Furthermore, Castlereigh and Moussakelle are the major storage reservoirs in the Laxapana Hydropower Complex located at the Kehelgamu Oya and Maskeliya Oya, respectively, which are the main tributaries in the area. The Castlereigh Reservoir, with an active storage capacity of 52 MCM, feeds the Wimalasurendra Power Station of capacity 2x25 MW at Nortonbridge, while the Canyon Power Station of capacity 2x30 MW is fed from the Moussakelle Reservoir that has a storage capacity of 108 MCM, according to the LTGEP.   


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