IV. EXISTING ROCKFILL DAMS
There are 2.000 large rock fill dams, most of them higher than 30 m. Waterproofing can be clay core, bituminous screen or concrete facing. Risks of piping or earthquakes are lower than for earth fill dams and risk analysis should focus on overtopping as one per cent of rock fill dams so failed during construction or operation, along the 20th century.
Failure breach is caused by overtopping depth in the range of 1 m over the crest ; the breach may widen more quickly and extensively than with a cohesive fill ; as relevant storage is often large, emergency planning and warning systems are essential.
V. TAILING DAMS
Water storage is small but worldwide fill volumes is globally larger than for all other fill dams.
Risks may be very high (sliding, piping, earthquake).
The risk of overtopping is usually not caused by floods but by wrong operation. Failures may be sudden and very dangerous for close populations and detrimental to environment in large areas.
Risk analysis and remedial measures are very specific and are not studied in this paper.
VI. EXISTING CONCRETE DAMS
The probability of failure in operation has been lower then for fill dams (in the range of 2 x 10-5 ?) but failures breaches may be sudden and wide (over 4 times the breach depth) and failure of a 15 m high dam may cause a flow of thousands m3/s.
Monitoring and particularly instrumentation have avoided many failures and justified structural improvements or decommissioning of a number of arch dams.
As many concrete dams are older than 40 years ageing may become a serious problem for thin structures. Earthquake risk may be serious for buttress dams or multiarches dams.
The risk of sliding on foundations such as shale's may be much increased during floods for low gravity dams. An increase of the reservoir depth by few meters and of the downstream level which creates up lift reduces considerably the margin of stability : failure may happen before or after overtopping of the crest.
VII. EXISTING MASONRY DAMS
There are over 1.000 large masonry dams. They are more dangerous than concrete dams:
 Many have been built before 1930, often with thin profile or poor foundation.
Basic strength of masonry is much lower and it is more difficult to avoid poor workmanship. The failure may happen in dam body where cracks due to various reasons may extend downstream.
Ageing due to leakage reduces strength and density.
Monitoring is not always easy. Structural improvements are often justified. Warning systems, at least during floods, may be justified even for rather small dams.
Will full damage should not be overlooked, a well as for a number of thin concrete structures.
VIII. GATES
A number of gates accidents, at first filling or during operation or due to ageing has been reported.
Failures are sudden and may be very dangerous as the flow may raise suddenly from Nil to 1.000 m3/s.
Risk analysis and maintenance are essential.
IX. FUTURE DAMS
When these dams will be in operation, the opportunities of failure will usually be the sane as for existing dams. But designs based upon improved experience and criteria will reduce the relevant probability. The risks analysed hereunder are thus 3 specific risks during construction and first filling (excluding resettlement problem)
IX - a First filing failures
Beyond China, 42 such failures of large dams have been reported in the 20th century : 34 from 10.000 built before 1970 and 8 from the 11.000 built later : they were two failures of masonry dams, 8 of buttress dams or multi arches (1,6%) 3 from arch dams (0,5%),26 from fill dams (0,2%) and 3 from concrete gravit y dams (0,07%).
Many old failure were due to a general lack of knowledge about dams or foundations behaviour but most failures and specially recent ones were caused by human behaviour : lack of experience of designers or contractors or over confidence of competent ones, reduced expenses for foundation studies, lack of communication or ill defined responsibilities, unclear specifications, control looking to many details and not focusing on key points. A number of failures were caused by poor workmanship for instance in masonry ; poor quality of a full lift of an earth fill or RCC dam cannot be excluded and care of embedded pipes in earth fill is difficult.
Risk assessment could refer to these human problems as much as to physical data: foundation failures are not caused by geology : they are caused by a lack of knowledge or ill adapted design or treatment.
First filling may happen earlier than foreseen by flood during construction and many piping failures happened some time after first filling. Many failures have been bound with foundations : this increases the risk of very long dams when foundation is not homogeneous.
Sudden and wide breaches of concrete or masonry dams were more dangerous then progressive piping failures of fill dams.
Two special risks may be important for some large reservoirs and difficult to assess exactly : sudden slope sliding in the reservoir and induced earthquakes which may be more dangerous for neighbouring than for the dam.
Risk analysis for first filling is thus very specific and difficult : probability of failure is low but cannot be excluded entirely because there are many possible reasons. Monitoring and instrumentation are essential and have avoided many accidents. Warning systems are advisable for most large dams at least during the few monthes which are usually necessary.
IX - b Floods during construction Few masonry or concrete dams failed during construction and their failure was usually not due to the special conditions during construction. Failure would have probably happened at first filling. (Tigra in India in 1917).
Few construction failures of fill dams lower than 30 m. have been reported because construction in the river may often be completed in a dry season or because the failure caused little damage for an embankment of reduced height at failure time. But from the fill dams higher than 30 m., three per cent ofthose built before 1930 and 0,5 % of the recent ones failed by floods during construction. Failure was caused by floods exceeding the capacity of temporary diversion structures or by delays in construction. Consequences of Panshet (1961 in India) and Sempor (1967 in Indonesia) failures were heavy. Over half of failures were rock fill dams; possibly they were wrongly supposed to withstand limited overtopping.
Risk analysis is specific but can be rather precise. Risk may be reduced by increased temporary flood control facilities (including auxiliary tunnel higher than the basic diversion tunnel) and close analysis of delays consequences. It should be too expensive to avoid completely the probability of failure but it is essential to study and implement during few months warning systems which may be very cost effective. Over 100.000 people at risk were evacuated for Oros dam in Brazil in 1961. Such failures may cause downstream dams failures.
Fill dams higher than 30 m. may represent over 20 % of all future large dams. This risk is consequently a serious one. Failure of high fill cofferdams is a similar risk.
IX - c Work accidents
Work accidents during construction are a main cause of fatalities bound with dams. And, according to ICOLD Bulletin n° 80 : "Dam construction sites: accident prevention" the corresponding direct and indirect cost has been in the range of three per cent of construction cost, i. e. more than the cost of all failures combined. Most future dams in Asia or Africa will be built with heavy plant but the number of workers shall be kept higher than in industrialised countries because of their low cost. These construction sites associating heavy equipment with many workers may cause more victims of accidents than past entirely hand built dams. In a number of large schemes over some years, up to one per cent of workers were killed by accidents and as average five per cent were absent from work through injuries.
In industrialised countries, rates of accidents have been divided by about 3 in 15 years. This may be obtained if devoting 0,2 to 0,5 of construction cost to safety measures and beyond human target, this may reduce by 2% the cost of construction.
Efficiency of safety measures is the responsibility of the contractor and is essentially bound with the site organization and management. However, owners (and laws) may very usefully impose at the tender time safety rules to be applied by contractors and control them during construction. Statistics have shown that the frequency rate of accidents was higher for medium size dams than for very large ones, probably because greater care of safety was taken on largest sites. Consequently efforts on safety should not be limited to the largest dams.
Relevant risk analysis and suggested measures for improvement are detailed in ICOLD Bulletin 80 "Dam construction sites: accident prevention" which is easily available.
CONCLUSION
Over 100 large dams and 1 000 small ones failed worldwide since 30 years. Similar rates of
accidents may be avoided in the future if associating structural expenses where cost effective with not structural measures. : cost of these measures may be low if they are well adapted to various problems and extremely low in many developing countries where cost of efficient corresponding staff is low. Consequences of failures may also be greatly reduced.
It is also possible to reduce the human risks bound with all exceptional floods not
endangering the dams and the workers accidents during construction: there two risks are
more important than human risks from failures and are often overlooked.
Implementation of these measures can be made in few years and is more a problem of
organization and clear responsibilities than a problem of cost. ln many countries most dam
owners have little technical knowledge and the authorities should organize these improvements.
BIBLIOGRAPHY
ICOLD :
*Lessons from dams incidents (1975)
*Bulletins n° 99,109,111,117.
*International symposium on earthquakes (Beijing 1987)
Symposium on floods :
*Huangshan China
1992
*Beijing China
1999
*Unusual storm events : India
1993
*Rehabilitation of dams: India
1998
*Safety of dams: India
1999
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