Samanalawewa leak – Engineering blunder or a blessing?

Dr Janaka Ratnasiri
Sunday, August 26, 2012
Samanalawewa Reservoir
  • This article was originally published as a three-part series in The Island (August 26th-28th, 2012). Read: Part 1, Part 2, and Part 3.

In an article published in Daily Mirror of 29.09.2011 on Samanalawewa leak, Minister Patali Champika Ranawaka has described the leak as a serious engineering blunder in Sri Lankan history. However, this leak provides a continuous supply of water to a downstream community dependent on agriculture for centuries and for them, the leak is indeed a blessing.

The reservoir and the power plant

Samanalawewa is a reservoir built during 1986-91 across the Walawe Ganga (River) flowing to the south of the central hills in Sri Lanka. It starts at an elevation of about 1,800 m on the southern wall of the Adam’s Peak range (not from the Peak) to the southern coast traversing a distance of 138 km and discharging an annual average volume of 2,200 Mm3. It is the fourth largest river in the country in terms of volume discharged and has a catchment of 2,456 km2 served mostly by the two inter-monsoonal rains peaking in April and November. The annual average river flow at site is 18 m3/sec and the minimum flow recorded is 3.5 m3/sec. Samanalawewa is the second reservoir built across Walawe, the first being the Uda Walawe Reservoir built in 1965 for the purpose of “developing” the southern plains which was at that time covered with virgin jungle. With the availability of irrigated water, the region is now converted into paddy fields and cash-crop plantations such as sugar cane and banana with corresponding growth of new settlements.

The southern plains of the country rises to an elevation of about 100 m gradually and then within a matter of a kilometre rises to over 400 m elevation forming the southern wall. The upper region terrain is steeply hilly and undulated, and Walawe and its main tributary Belihul Oya flow along deep valleys in it. The two waterways, after their confluence, cascade down to the lower plains within a short distance.  Walawe is damned just below the confluence forming the Samanalawewa. Flowing along the lower elevation is Katupath Oya which joins Walawe after merging with another tributary Diyawini Oya. At one location, Katupath Oya flows only a short distance away from Walawe but more than 300 m below. Water from the reservoir is taken down this height difference into Katupath Oya through a power plant driving a generator with installed capacity of 120 MW.  The power plant was initially estimated to generate 460 GWh of electricity annually, which was about 15% of the total hydro energy output in 1990.

Early studies

The potential of Walawe to generate hydro-electricity had been known to Sri Lankan engineers even in the fifties. According to a news item in The Island of 05.06.2000, the original proposal for this project was made by American consultants in 1966, and later revised by Australians in 1973. Thereafter, British and Swiss consultants were engaged on the project. In late seventies, a feasibility study including detailed design and some preparatory works was carried out by Russian consultants and a local agency. However, the government awarded the contract for implementing the project to Japanese and British companies in mid-eighties. Apparently, changes in government appeared to have resulted in changes in consultants.

In the article by Minister Ranawaka referred to above, he says that “the Russian scientists, who carried out prefeasibility work in the 1970s, concluded that the area was of Karstic cave type geological formation”. Karst is a landform that is formed primarily by dissolution of carbonate rocks such as limestone, dolomite, marble, gypsum and salt. Features of karst landscape include sinkholes, caves, sinking streams and large springs. In simple language, karst land can be described as a honey comb.

Construction without investigations

The Minister further says “to ascertain the existence of such a geological formation, they (Russsians) suggested that a service tunnel be constructed first, through the right bank, if at all we were to go ahead with the construction of the dam. However, when the Japanese firm undertook the construction work, it opted to ignore the conclusions of the Russian scientists. No doubt, it will remain a serious historical engineering lapse, on the part of the then policy makers associated with the project”.

This is not the only occasion when the Japanese took over projects on which initial studies had been done by others. Also in the eighties, the German Agency GTZ had developed a master plan for the electricity supply in Sri Lanka, but before the government could implement it, Japanese came and offered funds to develop a project to generate electricity after tapping water from five waterways in the hill country drying up several waterfalls. A modified version was implemented after much controversy. The GTZ plan is not spoken about now. In the meantime, another master plan for the power sector was developed by the Japanese Agency JICA together with CEB in 2006.

It is not clear why the contract with the Russians was terminated prematurely and given to consultants from Japan and UK. It could be that Japan had agreed to provide funding which was not forthcoming from the Russians. Still, that is no reason why the findings of the Russians put aside. It is also not clear why the local authorities remained silent when Japanese had their way. After all, the final decision should have been made by the local authority, the owner of the project. Had the government took the correct decision at this stage to undertake further studies on the sub-surface structure before commencing work on the project, the disaster to follow could have been averted.   

The dam and the leak

According to various reports on the project available in the internet, the development of the project could be traced and information about it sourced. The construction work of the dam was started in year 1986 and the main work was completed in 1991. The dam was rock filled 100 m high and 530 m long. The total storage capacity is 218 Mm3, with 60 Mm3 of dead storage. The design maximum water level is 460 m amsl. The water is taken along a 5.1 km long low-pressure tunnel and 840 m high-pressure penstocks feeding the turbines. 

The initial impounding of the reservoir was commenced in June 1991. However, when the water level was only 16 m deep, a leakage appeared as a small spring in the right bank about 300 m downstream of the dam.  The impounding was stopped at that time and all the experts would have put their heads together to find out what had gone wrong. After carrying out many investigations, the potential leakage paths were considered to be along the karstic rock units and through major faults existing in the right bank. Thus a 1315 m long and 100 m deep grout curtain cutting across the fault zones and running in to the right bank was constructed. The grout curtain consumed 13,450 tons of cement and involved 53,600 m of drilling for grouting.

On completion of the right bank grout curtain construction, the official impounding of the reservoir was commenced in March 1992.  However, impounding could not be continued because of a sudden burst in the right bank in October 1992, washing away of large portion of the right bank downstream face. The impounding was suspended and the reservoir water level was maintained 30 m below the maximum reservoir water level until the remedial measures were decided.

Laying clay blanket – Example of Tarbela Dam

With the failure of the grout curtain, authorities decided to consider laying a clay blanket along the bed of the reservoir. The authorities at this stage decided to obtain the necessary expertise from Pakistan who had used this technology to arrest a seepage problem in their Tarbela Dam constructed in early seventies across the mighty Indus River.

Tarbela Dam is world’s largest earth and rock filled dam which is 2,743 m long and 143 m high. The reservoir created is 80 km long and holds 14.3 Gm3 of water. It drives a hydro power plant comprising 14 generators with a total installed capacity of 3,478 MW. The engineering consultant TAMS of USA was responsible for the design, supervision and commissioning of the project. The dam was built on alluvial soil and therefore some seepage under the dam was anticipated. As a remedial measure, an impervious clay blanket was laid on the reservoir bed stretching to 1.74 km upstream. The clay blanket was able to reduce the seepage which was about 5.5 m3 initially to below 0.15 m3 after several years.

At Samanalawewa, work started on laying a wet blanket in 1998 under the supervision of Pakistani Consultants. It was planned to dump graded earth material under water along the 700 m of suspected ingress zone, where the three major faults are crossing the riverbed. However, even after dumping about 500,000 m3 of clay on the suspected ingress areas no response in either the leakage flow rate or the ground water level was observed. The leakage continued at a rate of about 1.8 m3/sec, which is approximately 1/10 the annual average flow rate of the river at the site. Unlike in the Tarbela Reservoir, there was no decline of the leakage with time. Hence, further clay dumping was terminated in 1999.

Investigations by local engineer

In the meantime, a civil engineer, Dr Kamal Laksiri, attached to CEB and who was involved with the project had gone beyond his normal call of duty and had studied in detail the geological aspects of the problem and had developed a mathematical model forecasting the possible leakage path, earning a doctoral degree in the process. He has been able to narrow down the possible ingress region which is about 100 km upstream from the dam on the right bank. Subsequently, with the intervention of the Minister, he had been able to show evidence that the region he had forecasted is indeed the point of ingress of the leakage path, by using a radio-isotope technique with the assistance of the Atomic Energy Authority. This experiment is described in detail in the Minister’s article referred to earlier. Apparently, no further work has been carried out in this respect. The uncertainty is there however as to whether water enters the right bank only at one point or more.

Dr Laksiri concludes his doctoral dissertation ( by saying “in general, this study shows that very careful attention is required in selecting a site for water retaining structures, especially in the case of dams and reservoirs. The evidence of karstification needed to be thoroughly investigated as it may not be directly visible. Long term ground monitoring records need to be evaluated in understanding the hydrogeological influence. This is of importance since the karstification always associated with peculiar hydrogeological situations.”

Ignoring scientific advice

The question that comes to the mind of any layman is when the Russians warned about weak right bank and even suggested digging a service tunnel, why was that information ignored and the authorities went ahead with the construction work as planned by the Japanese, which cost the country many billions of rupees. The Japanese Consultants have a moral obligation to meet the extra expenditure incurred on remedial measures which were necessitated by their ignoring the warning given by the Russians, as mentioned in the minister’s article. It seems that the authorities concerned instead of seeking the advice of our own geologists in this regard, have ignored the advice given to them.

The late Professor Vithanage, well known Sri Lankan geologist, apparently had warned the authorities on the unsuitability of the proposed location for a dam, but to be ignored by them. This seems to be a clear case of professional negligence, both on the part of local authorities and foreign consultants. The Minister himself says in his article that “it is unfortunate that no action has been taken against the foreign consultants or contractors for the misleading advice and poor work they were responsible for”. I believe the Minister has the powers to hold back the repayments on the loan even now if the investigations show that the Japanese Consultants were indeed at fault.

The 2011 September issue of Sri Lanka Engineering News says referring to the Samanalawewa leak and the findings of Dr Laksiri and AEA, “the achievement was even more valuable considering that it was done with the help, knowledge and the skills of the Sri Lankan Engineers and scientists”. Had the collaboration of engineers and scientists was sought from the inception, without discarding scientists’ advice, the country could have saved much money and damage to the environment and livelihoods of thousands of people who were displaced by the project.

Alternative methods for ground investigations

Actually, if anyone wanted to investigate the nature of the sub-surface structure in the concerned area, there is no need to incur heavy expenditure doing operations like tunneling or even drilling, because less expensive techniques such as multi-channel high resolution seismic soundings are available for such applications. Any stratification of the ground underneath or the presence of cavities or rocks could be easily detected using this technique. The apparatus for these studies are commercially available for such applications as petroleum prospecting and highway construction.

In fact, Sri Lanka’s highway research laboratory had such equipment even in the nineteen sixties or seventies. It was a pity that no one had thought about it at that time. Generally, our policy makers are happy to turn to foreigners to get advice at enormous cost when such advice could be obtained locally at a nominal cost if not free of charge. Apparently, more than 15 experts from around the world had spent many years trying to solve this problem, but with little success.

Had the detailed investigations as suggested by Dr Laksiri were carried out and if the site was proven to be unsuitable to build a large reservoir, the logical step the authorities could have taken was to build a small weir and a pond to feed a run-of-the-river type power plant using the 300 m head available at the site. The two recently built hydro power plants Kukule (70 MW) and Upper Kotmale (150 MW) both have low weirs, 20 m and 35 m high, respectively. But, they produce more electricity than what Samanalawewa is generating at present. On an average, Kukule generates 306 GWh and Upper Kotmale generates 409 GWh, whereas Samanalawewa (120 MW) has been generating an average of 270 GWh only, fluctuating between 150 and 320 GWh (see figure).

Lack of environmental studies

Samanalawewa is a clear case where inadequate or no environmental impact assessment (EIA) studies had been carried out prior to commencing the project. The EIA process became mandatory in Sri Lanka only in 1993 under the National Environment Act of 1980. Sine Samanalawewa project was executed much earlier, no one had thought it important to study the impacts on the environment from the project as well as impact of the environment on the project. If someone took the initiative to do so, most of the problems that cropped up during its construction as well as during its operation phases would not have arisen.

Samanalawewa is not the only case of a project that had failed due to poor understanding of the environment of the project area. The (Lower) Kotmale power plant too developed a leak in the under-ground pressure shaft soon after the project was commissioned and the remedial measures taken which comprised lining the shaft with steel would have cost an enormous amount of money. The Nilwala Flood Protection Scheme had resulted in the abandoning of thousands of hectares of paddy land in Matara District due to the fact that those who implemented the project had no idea of the chemistry of the underlying soil, and did not know what would happen to the soil when the water was drained out. The more recent case is the submerged rock in the mouth of the Hambantota Port which was even highlighted in the editorial of Sunday Island of 29.07.2012. Its presence was apparently not known initially to those who planned the project and its removal without causing damage to the structures already built has resulted in the government incurring a colossal amount of money unnecessarily. All these heavy losses had been incurred due to project designers not carrying out proper environmental studies prior to finalizing the project designs.

I trust our project planners will at least in the future give environmental considerations priority and internalize them from the inception. The current practice appears to be that environmental impacts are looked only at the end of the project design exercise merely to satisfy the statutory requirement of submitting an EIA report to the project approving authority rather than with a genuine interest in minimizing the damage to the environment. Since the consultants undertaking the EIA study are hired by the proponent, they have an obligation to ensure that the project will receive the green light which means that they would recommend only some cosmetic changes and nothing drastic. Also, some proponents may not want to go back on their designs after everything is finalized as any changes at that stage could incur high cost. Therefore, it is important that every project planning team has several senior environmental scientists from the inception of the project who will work closely with the project designers and keep them on the correct track. This would serve a better purpose than getting a team of outside consultants to look at environmental impacts only after completing the final design of the project.

Impact on Kaltota Irrigation Scheme

One of the adverse impacts of the reservoir was the deprivation of water to an ancient irrigation scheme located at Kaltota, which is about 10 km downstream of the dam. Just before Walawe reaches the plains, an ancient dual anicut diverts water to two canals running along each of the two banks providing water to paddy fields stretching many kilometers on either side of the river. The Diyawini Oya carrying the outflow of the power plant joins Walawe only at the end of these fields. Kaltota is described as a highly developed area centuries ago with the people skilled in metallurgy, irrigation and agriculture.  Before the reservoir was built, farmers had free access to water from the river through the canals, which they lost with the impounding of the reservoir. If the entire flow of Walawe was sent through the power plant to maximize power generation, the stretch of the river between the dam and the Diyawini Oya would go dry depriving the farmers any water for their cultivation, except for some little water coming from a few small tributaries in between, but this was not to be.  

There has been a study carried out on “Balancing Irrigation and Hydropower” referring to the Kaltota Irrigation Scheme (KIS) by the International Water Management Institute (IWMI), published as Research Report 94 in 2005 (  According to this report, apparently, prior to building the dam, the CEB had agreed to release water to farmers, recognizing their right for water which they had enjoyed for centuries. However, water was not given freely, but CEB had laid down conditions for the release of water. Apparently, CEB had the say in deciding how much and when. 

The Kaltota community comprising today of about 2,500 persons were persuaded by CEB to change their cropping pattern, grow short duration crops, reduce land preparation periods and adopt water saving techniques. But naturally the farmers were not too happy with such arrangements. However, when the leak developed providing a continuous supply of water down the river, farmers would certainly have considered it as a blessing. The flow from the leak is about 55 Mm3 annually, while the annual water demand of KIS has been estimated to be about 90 Mm3 (IWMI). The CEB has been supplying the balance by opening the bottom sluice gates as and when necessary, though unwillingly it appears, as it reduces the annual energy generation by about 8 per cent. The farmers could at least get more than half their water requirement now without any restriction from the CEB.

Pursuing the blessing

The question the policy makers will have to pose now under such a situation is, should action be taken to close the leak permanently.  The Minister in his article says towards the end “Now it is left to other experts to put their heads together and propose any other means of further investigation to ascertain the nature of caving or cavities hidden in the right bank. Only then will we be able to close down the legendary leakage, permanently”. If the leak is plugged permanently as proposed by the Minister which will cost many more billions of rupees, CEB will have to provide the full water requirement of KIS by opening the bottom sluice gates of the dam.

Does this make sense? Or, isn’t it wise to let the water from the leak flow at the present rate enabling farmers to do their cultivation without any interference from the CEB. If the CEB wishes the farmers to manage their cultivation with this water only without depending on water released through the sluice gates, the farmers need to be given adequate training in efficient water management systems and provided with better farming facilities. Since there will be more energy generation with added revenue to CEB because of the water saved, CEB may even consider granting the farmers some incentive allowances.

Concurrently, it is essential to ensure that the leak is stable and will not grow so as to risk the safety of the dam. In the meantime, geological scientists should be given modern facilities and funding to investigate the nature of the sub-surface in the region, and also elsewhere before any important construction work begins. Let the present leak be there as long as it is stable. The country need not waste any more money on foreign consultants in trying to plug the leak.  




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