Opinion (Spotlight Weekly)

West Seti Dam And Disaster

By AB Thapa

Now it is becoming increasingly clear that the root of the Western Terai submersion problem across a vast stretch extending from Babai to the Banganga could be the the Saryu canal which would be dependant on water drawn from the West Seti storage reservoir. It hardly needs to be explained that the resolution of this problem should be accorded the topmost priority. Very recently a great shift in Indiaís water policy has been observed. The Government of India has sometime ago announced in public its willingness to buy regulated water supplied by Nepal (obviously it will need some time and very hard bargaining to settle the price). Unfortunately at such a critical time, when the full attention of the country should have been drawn to various issues related to the West Seti, everybody is shocked to learn that the government has all of a sudden extended the validity of the agreement to allow the private hydropower developer to build the West Seti project exclusively for power generation. The agreement with the private hydropower developer had expired a long time ago at the end of the last year.

The West Seti dam project appears to be ridden with very serious technical problems that concern the safety of its most important structure, the high dam itself. The WECS had few years back sent its opinion to the Water Resources Ministry stating that the type of the high dam proposed for the West Seti by the private developer could be risky. Some of the evidences put forward by the developers to justify their selection do not appear to tally with the information contained in the recently published scientific documents. So it was suggested to constitute a panel of few renowned international experts recognized to be the authority on their respective discipline to seek their opinion. Any further action in the direction of taking the decision to award the project to the private developer should be deferred until the panel gives its absolutely positive opinion. It is not known who had approved the technical design of the West Seti project on behalf of the government to vouch for the correctness, unfortunately the developers were allowed to go ahead to find buyers for the electricity. Every sensible man will agree that the West Seti dam should be completely safe. The incidence of the Kulekhani high dam should not be allowed to be once more repeated. At that time the Kulekhani dam could be saved which was on the verge of collapse resulting into one of the biggest man made disasters in the world In case of the West Seti reservoir the magnitude of such disaster would be far greater because the volume of the West Seti reservoir might be about 15 times greater than the volume of the Kulekhani reservoir.

Dam Design Problems

It has been proposed to build a 195 m high embankment type dam across the West Seti river. It need not be explained that the embankment type of dam with impervious earthen core is the best suited for the proposed dam site. However, good deposits of suitable materials for impervious core could not be found nearby. It led to find an alternative to the impervious earthen core embankment dam. So it was decided to build a concrete face rockfill dam ( CFRD ). Needless to say that such dam even according to the study report of the private developer is the least expensive. Now the question arises whether it is safe to build this type of dam in Nepal. Almost the same type of problem was encountered about 10 years earlier in 1970s while planning the Kankai High Dam by the German consulting firm SALZGITTER. However, the scale of the problem at that time was far smaller. By comparison with the West Seti dam 195 meters in height, the Kankai high dam is just 85 meters in height. Nevertheless the German Government took this matter very seriously. Despite the fact that there are countless good geotechnical experts in the Germany itself, German Government constituted a small panel under a renowned French professor to review the study report and to make appropriate recommendation. Only after such through recheck it was decided to adopt the embankment type dam with an asphaltic concrete facing. There were already several dams of this type close to the proposed Kankai dam in height in good operation all across the world and it was the strongest justification in support of the decision to select this type of dam with an asphaltic concrete facing. Unfortunately it might not be possible to justify in a similar way the proposed dam type in respect of the West Seti dam.

Why West Seti Dam Could Be Unsafe?

The West Seti Project will have a 195-m high concrete face rockfill dam ( CFRD ). This type of dams are found to be very competitive in cost but until recent time they were considered risky for heights above 150 meters. This type of dam could have also been considered as one of the alternatives in selection of the high dam for the Kankai Project. The French expert called in to advise on Kankai dam appears to have dissuaded the German study team from considering the CFRD as a viable option. It was considered that it could be unsafe to build the CFRDs. So finally at the end an embankment type dam with an asphaltic concrete face was selected.

The CFRD is very sensitive to settlement and deformation of the rock-fill supporting the upstream face. These deformations produce movements of the concrete slab joints by opening them and if the movements exceed certain limits then the resulting leakage is difficult to control. The derfomations of CFRD for dynamic loading are even more difficult to evaluate. In the Michigan Convention of ASCE in 1985 on CFRDs papers were presented to establish Earthquake Severity Index (ESI) with relative vertical settlement. In the ICOLD meeting of 1988 it was suggested that future research should be focused to correlate ESI and the relative settlement as function of yield acceleration.

There is no Precedent

Needless to say that there is a need for great caution in adopting very high CFRD. According to J. Barry Cooks, Consultant USA (Development in High CFRDs, Hydropower & Dams, Issue Four 1997), this type of dams are of empirical design and based on precedent design and experience. Unfortunately only very recently relatively high CFRDs have been introduced.

The proposed developer of the West Seti Project has cited as precedent in their report two CFRDs close to the West Seti dam in height. It is claimed that they are already in good service. They are the Tiensingquiao of China with a height of 180 meters and Aguamilpa of Mexico with a height of 185.5 meters. Unfortunately both these CFRDs can not be satisfactory example to justify the selection of CFRD for the West Seti Project. The Tiensigquiao dam of China might be still under construction. According to the original schedule the dam was expected to be completed towards the May of 1999. So nobody might be yet knowing how that dam would be going to perform. The Aguamilpa reservoir was first filled in August 1993. Since then two abnormal peak seepage values have been observed. These events were of great concern, and several hypothesis were made to explain this behaviour although none has been satisfactory. Now underwater inspections are in progress to try to identify any factor, which could explain the observed behaviour.

There are not any other examples in the world of CFRDs in service with heights close to 195 meters. It is a fact that very high CFRDs are yet to be tested in practice. So a great deal of caution is required to adopt this type of dam. Let us not take the risk of unnecessarily endangering the life and property of innumerable people by recklessly deciding to implement the West Seti Project. There are several such instances when a country had to pay dearly for the unpardonable mistakes committed by the decision makers.

Dam Failures

Insensitivity to advise from outsiders could have terrible consequences. A case in point could be the Malpasset dam near Frejus in Southern France. It is reported that the decision makers were cautioned in time. They were told that the dam site was not suitable so it was recommended that the dam be built elsewhere. However, for reasons of engineering convenience, the advice was disregarded. The dam failed on 2nd December, 1959, causing death of a large number of people.

Construction of the high dam without full study of the environs led to the catastrophe of the Vaiont Dam in Italy. The Vaiont Dam is 261 meters high and the volume of the water contained in its reservoir is 150 million cubic meters ( the volume of the West Seti reservoir would be about 1500 million cubic meters). In September, 1963 the reservoir level reached a height of 180 meters, and an earth movement started along the slope of Mont Toc. That movement accelerated in October and caused a landslide which gave rise to a giant wave that flooded the valley beneath, wiping out several villages and killing more than 2,000 people.

Shoddy workmanship and poor quality of materials are often the prime cause of dam failures. The failure of St. Francis dam in California has been attributed to faulty foundations. Design errors were apparently largely responsible for the collapse of the Teton dam. In our own region ëover-toppingí occurred with the Machau II dam in India in 1979 and caused the death of 1,500 people downstream. In this case, the malfunctioning of equipment contributed to the failure, as the spillway gates could not be opened in time. The failure of spillways to function properly also led to the near failure of the 140 m high Tarbela dam in Pakistan in 1975-6. It is reported that in this case design errors and possible poor construction materials were also involved.

Nepalís laissez-faire hydropower development policy could have adverse effect on dam safety. It is difficult to visualize how we can be assured that the private developers would be sincerely interested in maintaining the high standard of workmanship and superior quality of materials which are fundamental prerequisite for dam safety that would invariably lead to higher cost. What is the guarantee that the private developers would not be tempted to cut a lot of corners in order to make hefty profit that could be disastrous in long run.

In Conclusion

At present anybody even with the slightest knowledge of water resources would be eagerly waiting to get befitting answers to the problems briefly described in this short paper from the management of the Water Resources Ministry. We all should remember that what is now at issue is the safety of life and property of tens of thousands of people living in the Western Terai and the risk of loosing the enormously large downstream benefits accruable from the regulated flow of the West Seti river.