Samanalawewa Reservoir leakage: Need for urgent decisions

The year was 1991. All the Mahaweli major reservoirs had been completed. Kotmale, Victoria, Randenigala and Rantembe power plants were in full operation. Their electricity production was instantly absorbed into the industrial boom at the time. Yet, 70% more of the country’s households were without an electricity connection, awaiting expansion of the power distribution network.  

Without giving ear to professional advice (just as it is today), the Government at the time, had cancelled the Trincomalee power plant in 1987 and the Mawella power plant in 1991. Without calling for binding competitive bids (just as it is today), the Government was “negotiating” with a one-man Canadian (fly-by-night) company to build a $ 500 million private power plant in Trincomalee, and the man ran away three years later, filing an arbitration claim against the Government. History repeats.

So, after the Rantembe hydropower plant in 1990, there were no new power plants to serve the growing demand. Power cuts were staring at the economy. In this backdrop, the prospect of getting electricity from the Samanalawewa hydropower generators from 1992 onward, was a god send. But that was not to be, as the story unfolded.

After five years of construction, in June 1991, with all the pomp, pageantry and with the Minister cutting the ribbon, “impounding” of the reservoir began. Water was being filled-up into the brand-new reservoir. Instead of sending water down through Ambalanthota to the sea, Walawe Ganga and Belihul oya were now feeding the Samanalawewa reservoir. Water levels in a reservoir are always stated with respect to the mean sea level. Once filled-up to 430 meters above sea level, electricity production was to begin.

 The first leak and the second

Alas! Even before the water level reached the “electricity production” level of 430 meters, a water leak appeared to the right-hand bank of the dam. Left and right of a river are defined as a person standing facing the direction of river flow. For those familiar with Kelani Ganga, Wellampitiya is on its left bank, Peliyagoda is on the right bank. The leak in Samanalawewa was from its right bank. The newly built dam itself was not leaking, but the natural embankment that was expected to be water-tight, was leaking. Water-filling was immediately stopped, and the leak was investigated. Then it was decided to start filling again. 

The date was 22 October 1992. Water level reached 439 meters, and there was a sudden burst. A large leakage of water amounting to about 7,500 litres per second gushed down the river. Gradually the leak reduced and stabilised at 2,500 litres per second and has been leaking ever since. Over the years, the leakage further reduced to about 2,200 litres per second. Yet, it is a large volume of water, escaping under the mountainous terrain, carrying with it debris, in turn indicating that further cavities are forming underneath the visibly harmless lush green surface.  

The leak amounts to about one week’s water used in an average household, gushing down the holes every second. 

After expert investigations, it was decided at that time, to allow the water level to rise, but was not allowed to rise up to the maximum of 460 meters. The power plant commenced producing electricity in 1993. The safety of the embankment, the dam and the life and property living downstream, was the main concern. Simply put, Sri Lanka operated the Samanalawewa reservoir for 30 years, without fully understanding what exactly was going in there, under the surface. Are the cavities enlarging? Will the embankment get washed down some day? Will it cause a catastrophic failure? What if it fails and a massive wave of water sweeps down the Walawe Ganga causing untold devastation. Who will take the blame?

To the credit of CEB’s right-thinking management at that time, and various Governments that came and went, numerous studies were conducted, opinions sought, and remedies applied. Even the Chief Engineer of Tarbela Dam in Pakistan, producing a massive 4,800 megawatt of power (40 times the size of Samanalwewa), who had a similar problem and implemented solutions, was consulted, in addition to studies by Sri Lanka’s top academics, geologists and engineers. 

 Recent studies 

For the first time after 30 years, the drought in 2023 caused the reservoir to be fully drawn down, back to its minimum level of 430 meters. As many may remember, weeks after hesitation, electricity was produced and water was drawn down, to supply irrigation requirements further downstream of the Samanalawewa dam, along the Walawe River. At or around 430 meters, electricity production stops, although there is more water in the reservoir. This situation provided for the first time in 30 years, an opportunity to conduct further investigations on how these leaks occur. This is because some leaks were now visible, showing where the water enters the cavities. 

Investigations conducted jointly by CEB and University of Peradeniya, reconfirmed that leaks are from two major sources; (1) leaks along a construction access tunnel that has not been fully sealed after construction, (2) “sink holes”, which are cavities at reservoir bottom, opening pathways for water to leak downstream.

At a recent public seminar held at the Institution of Engineers, experts expressed their observations, investigations, calculations, and results. The call for immediate action was emphatic and unanimous, taking advantage of the water levels in Samanalawewa that are still very low, despite recent rains. Experts required water levels to be reduced further, to make direct investigations on some leakage paths that are still not visible, and for immediate rectification work to be done. Rectification will require soil, mud and concrete, depending on the final decision of geologists and engineers.

 The mini hydro saga

There are two mini hydro power plants on Walawe River, that were built to use water downstream of the Samanalawewa dam; water that is not used to produce electricity from CEB’s Samanalawewa power plant. Sri Lanka’s mini hydropower plants (all privately-owned) had an average capacity factor of 43% in 2021. Apologies for the jargon — the capacity factor indicates the equivalent time a power plant produces electricity over a year, given the variation of river flow. Prices paid to private mini hydros that signed up in the 2013-2016 era, for electricity they produce, was based on a capacity factor of 42%. However, the two minihydros downstream of Samanalawewa have reported a capacity factor of 55%, ably supported by the godsend water “leaking” from the reservoir.

Therefore, the two minihydros, both threatened of a complete washout in case of a disaster, who may well have recovered their investment at least twice over, should be the most interested in making sure, the reservoir is safe as it could be. Although some newspaper reports wrongly indicated that the minihydro owners are “against” the leak being plugged, they should be the most interested in pushing the Government to plug the leak and make their investments secure.

 Needed: Urgent action

Meanwhile, as the Institution of Engineers was raising its voice, the Government in its customary style, called for a report on the safety of all dams in the country, without specifically naming Samanalawewa that is presumed to be in grave danger. Simultaneously, the Ministry of Power and Energy announced that rectification work will not be done until 2025, without giving any reasons for delaying the crucial actions to make the reservoir safe, strongly recommended by geologists and engineers. 

Action can be postponed, excuses can be given, but disaster cannot be postponed.

 Libya orders arrest of eight dam officials 

Meanwhile on 10 September 2023, while Sri Lanka’s politicians and officials were busy, brushing aside the latest findings on Samanalawewa leakage, two dams overburdened with heavy rainfall burst in Libya, and an estimated 4,000 people died. Already eight officials, present and former administrators of dams, have been arrested, facing charges of criminal negligence. 

In recent history, Sri Lanka too had one major burst. The Kantale dam burst at 3:30 a.m. on 20 April 1986, killing at least 126 people and destroying more than 1,600 houses and property. 

The Institution of Engineers have done their job to draw the attention of CEB, Ministry of Power and Energy, and the entire Government bureaucracy, about the imminent danger and the need for urgent action. If they do not listen or listen and postpone, what else can a professional body do?