Solution to water deficit in NWP and NCP-by BANDULA KENDARAGAMA

Source:Island

Former Section Engineer (Dam), Kothmale Hydro-Power Project and International Dams Consultant, Melbourne, Australia

(Based on an interview with Srilal D. Perera (Construction Project Manager), Melbourne, Australia)

Sri Lanka is now a middle-income country with consistently high economic growth over the recent years. The government’s public investment plans include several development plans and projects for the coming years, to further accelerate economic growth and promote social and human development.

Being a tropical island located close to the equator, Sri Lanka is highly susceptible to adverse effects of climate change. The Global Climate Risk Index (CRI) 2018, which indicates how countries have been affected by the impacts of weather-related loss events (storms, floods, earthquakes, heat waves etc.,) ranks Sri Lanka as the fourth among the most affected countries of the world. This is an alarming situation. Adverse effects of climate change are evident from the severe and long duration droughts as well as severe flooding occurring almost every year in several parts of the country. Water availability is becoming more variable and uncertain, even as demand for bulk water supplies for agriculture, new industries and tourism and clean water for drinking increases. Studies confirm that these impacts are likely to exacerbate, and the wetter areas of the country would eventually become wetter, and the drier areas drier and drier.

The government of Sri Lanka has taken steps to address these challenges with the successful completion and commissioning of Moragahakanda and Kalu Ganga reservoirs, and the ongoing and planned construction of dams and associated feeder canals, hydraulic facilities, Hydro Power enhancements to comply with Paris Convention that His Excellency the President of Sri Lanka has consented under renewable energy promotion in Sri Lanka; as well as, other water resources development investments envisaged under the Public Investment Plan (PIP). The Government is also implementing several other programs and investments to mitigate flooding and flood damages in several river basins which are vulnerable to flooding.

Planning and investments in additional water storages, and transfer infrastructure to transfer water from water surplus river basins of the wet zone to water deficit river basins, will be a major priority to meet the growing demands and challenges of climate change impacts. Additional investments will be required on a priority basis for developing water resources to provide potable water supplies to people and areas currently affected by chronic kidney disease.

These new investments on water resources will demand diligent planning, protection, management and allocation of water for multiple uses and users within river basin context. As the water infrastructure expands through additional infrastructure bringing water to water deficit geographic areas, to new and current groups of farmers, large and small industries, urban and rural drinking water consumers, mechanisms will be required for diligent planning, allocation, and monitoring of bulk water supplies from the major water conveyance systems beyond the mandate and the role played by the existing Water Management Secretariat (WMS) of the Mahaweli Authority of Sri Lanka (MASL).

The Project

With the present climatic uncertainty, North Western Province (NWP) and the North Central Province (NCP) are among the most affected areas without reliable supply of water for irrigation, about 40,000 ha, and ensuring domestic water supply to families in such provinces. Major diversions of this water are supplied through the Polgolla tunnel (875 Million Cubic Meters (MCM) / Year) for both the seasons of Yala and Maha. This volume of diversion is entirely depending on the storage above the Polgolla barrage. At present only the Kotmale reservoir with 174 MCM supports to regulate this water. However, the Polgolla tunnel is functioned only with 60% of its capacity, and the tunnel has additional capacity to divert around 400 MCM / Year, if the upstream storage is available.

Therefore, a proposal for increasing the height of the Kotmale dam to increase the retention capacity by additional 250 MCM would be one of the feasible proposals to solve the water deficits in NWP and NCP. Also, the increased head and volume would boost the hydro-power generation by about 20%, once the supplementary water starts to pass through Kotmale and Ukuwela power stations.

The Kotmale Hydropower Project was one of the first projects taken up under the Accelerated Mahaweli Development Program (AMDP). It was the most upstream among those headworks in the Mahaweli river basin, and exploited the hydropower potential of Kotmale Oya, a major right bank tributary of the Mahaweli Ganga. The Kotmale dam site is at Kadadora, located about 6.6km upstream of the confluence of Kotmale Oya with Mahaweli Ganga, where the river forms a narrow and deep valley with steep banks. The powerhouse is underground and situated in the belly of the Atabage mountains, about 6.4km downstream of the Kotmale Oya – Mahaweli Ganga confluence.

The basic elements of the Kotmale Hydro-Power Project are a concrete face rockfill dam (87m high and 600m long) having the active storage capacity of 174 MCM and a tunnel system leading to 201MW power plants with outfall to the Mahaweli Ganga. The primary function is the generation of electric power. Additional benefits arise from an increase in the amount of irrigation water available at the Polgolla diversion, as well as mitigation of floods in the Gampola area as a result of the regulation of flows in the Kotmale Oya. The Project construction work commenced in 1979 and was completed in 1985, with financial assistance provided by the Government of Sweden.

The dam has been provided with basic facilities for future rising, to three alternative levels 715m, 725m and to 735m above the sea level (capacity of 370 MCM at 728m FSL). The provision for the extension of foundation from the downstream face, and the land around the reservoir rim up to 735 m elevation, have already been acquired since the beginning to enable the future upgrading.

However, the raising of Kotmale dam would be an engineering challenge and need in depth study to check the stability of the Concrete Face Rockfill Dam (CFRD), during and after construction, instrumentation, reliability of foundation and geology, supplementary grout curtain, spill gates and chute with elevated ogee (Flip bucket), added pressure head to tunnels, height of the surge shaft and power station equipment etc., Also, the heightening of the dam shall not affect the ongoing annual supply of water to NCP, as it would generate critical social problems and, therefore, the live construction method statement with the active storage of water available for downstream use would be the most practical approach.

Therefore, a comprehensive feasibility study shall be originated to find a solution for the critical water deficit in NWP and NCP, and for harnessing addition of renewable energy, considering above referred to facts at earliest possible.

The original consultant of this project was Sir William Halcrow & Partners (Halcrow Water) of the United Kingdom (UK); and Skanska (Sweden) with joint collaboration of Central Engineering Consultancy Bureau (CECB) , then local Consultant to the Mahaweli Ministry, had been involved in a preliminary feasibility study, the project design and supervisions during the first construction phase of this development in the year 1979 to 1984 period. Hence, the re-engagement of previously involved qualified consultants would be highly recommended for future studies and development, as well as the previous data, knowhow and experience that retained with them would be essential for accomplishing this challenging task.

In addition, consideration should be given to appointing a Panel of Experts from local & overseas professional bodies, who have extensive knowledge and experience in CFRDs, gates and valves, geology, geotechnical engineering, instrumentation and dam safety.

Consideration should be given to employing a minimum number of expatriate engineers and a maximum number of local engineers.

Recommendations

Following studies are needed to execute to enhance the project more feasible to meet the challenges in climate change, and the supply of more renewable hydro-energy, especially from the power plants newly built in Mahaweli Basin from the Kelani water.

1. Raising Kotmale Dam (15, 25, 30m). Note all lands are already acquired and there is no social impact;

2. Check the possibility of building dams at locations 11, 12, 14 etc., in Master Plan above Polgolla near Ginigathhena (Koladeniya, Carolina, Trapalga, Rosalla, to increase the retention above Polgolla, as the water about 1,000 MCM / Year is spilling down Polgolla during flood;

3. Enhance the capacity of Kandalama-Huruluwewa Canal to carry about 200 MCM / Year, at a rate of 10m3/second;

4. Built a new tunnel from Bowatenna Reservoir to Dambulu Oya where the capacity is 30m3/second. This would enhance the deficit of water in NWP (150 MCM / Year) and the water need in Anuradhapura;

5. Expand the capacity of power generation in Ukuwela Powerhouse adding one unit; and

6. Revisit the canal availability from Kalawewa to Nachhaduwa, Tissawewa and Basawakulana etc. via Yoda Ela.

7. A supplementary study to check the availability of water from Kelani Basin to Mahaweli Basin would further enhance this proposal.

The writer has B.Sc. Eng. (Hons), M.Eng. (Structural Engineering & Construction), MASCE, MIE Aust, CPEng NER, APEC Engineer, IntPE(Aus).