MALPASSET AND VAJONT DAMS: West Seti Dam Safety

By DR. AB THAPA

At present our country is heading for the implementation of the West Seti storage dam project. This project will have a concrete faced rockfill dam which is going to be the highest dam of this type in the whole world. The dam will impound a vast storage reservoir. It will be about 1,500 million cubic meters in volume. Needless to explain that the failure of the West Seti dam would result to a colossal loss of life and property.

Malpasset and Vajont dam disasters struck because ample attention was not paid to dam safety. We are going to handover the West Seti dam project to a private developer to implement. It is unfortunate that our government is already determined to implement this highly sensitive project virtually without examining the technical aspects of this project with the help of competent foreign experts.

West Seti Dam Safety

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 view.

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 deformations 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 behavior although none has been satisfactory. Now underwater inspections are in progress to try to identify any factor, which could explain the observed behavior.

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.

Competence of Government Institutions

Nepal ’s laissez-faire hydropower development policy could have adverse effect on dam safety. Until now our government’s performance in handling private developers of hydropower projects has been extremely shocking. Government endorsed whatever the private developers proposed. There is nobody from the government side to check the works of the private developers at the site. As a result, in case of the Bhotekosi Project the concerned Department and the Ministry came to know that the installed capacity of that hydropower project had been raised from 35 MW to 45 MW completely defying the power purchase agreement only after the completion of the construction works. Government learnt about it only after a claimed was lodged by the private developer with the government to compel the NEA to buy extra power to be generated entirely during the monsoon season which is going to be totally wasted.

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. Institutions like the Electricity Department, Ministry of Water Resources and National Planning Commission have yet been seen to be competent at ensuring that the storage dam projects have been planned, constructed and operated in accordance with internationally accepted norms and practices.

Dam Failures

Insensitivity to advice 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 2 nd 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 Vajont Dam in Italy. The Vajont 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 ‘over-topping’ 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.

In Conclusion

Nepal ’s 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.

(Dr. Thapa writes on water resources)