Opinion (Spotlight Weekly)

WHO CHECKED DESIGN? West Seti Dam Disaster

— Dr. AB Thapa

Nepal might be heading for a big dam disaster. Our government is on the verge of granting permission to a private developer to build 195 m high concrete faced rock filled dam (CFRD) which would be the highest in the world among this type of dams built so far. Needless to say that it is cheap to build this type of dam. Unfortunately our government has not cared to constitute a panel of internationally known competent experts to verify the reliability of the design of the proposed dam. It is very clear that tens of thousands of people would be killed if the West Seti Dam collapsed due to faulty design. Vast area of lands in Nepal and India would be laid waste with sand deposits. Those of us involved in the process of granting permission to the private developer to build the West Seti Dam without verifying the technical design should take this matter very seriously before it is too late. This brief article is published to draw the attention of the National Planning Commission and its Honorable Vice-Chairman and Members, Ministry of Water Resources, Ministry of Environment etc. to realize the gravity of the situation. Certainly institutions like the ICIMOD set up to resolve mountain development problems can not distance itself from West Seti Dam Disaster issue that could trigger unprecedented water induced catastrophe in our whole region.

German Government Ruled Out CFRD

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 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 20 years earlier in 1970s while planning the Kankai High Dam by the German consulting firm SALZGITTER. CFRD was one of the alternatives. 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 CFRD. 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 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 ‘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.

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.

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 Nepal.

(Dr. Thapa writes on water resources)