The increased safety now required for thousands of dams may be very costly if reached only by structural improvements. Various low cost non structural measures may reduce or avoid structural expenses and may be implemented in a short lime. They are even more attractive in developing countries.
The Committee on costs of ICOLD has prepared the bulletin E 102 on the subject: " Non structural risk reduction measures: benefits and costs for dams".
Based upon many comments and data from industrialized countries it has been written by M.Smart (U.S. Committee). This bulletin is freely available on the Net through: www.icoldcigb.org (Publications. Abstract of recent publications, Bulletins, E 02).
Based upon many comments and data from industrialized countries it has been written by M.Smart (U.S. Committee). This bulletin is freely available on the Net through: www.icoldcigb.org (Publications. Abstract of recent publications, Bulletins, E 02).
The present paper refers to many findings of this bulletin and analyses the efficiency of measures according to dam types and height and for various circumstances (conditions like first filling, floods, ageing...). It studies also specific data and advantages for developing countries.
This paper focuses on human risk which is actually the main basis of most criteria and decisions. For instance a number of countries require now to design or uprate (upgrade) the spillways for the probable maximum flood if the dam failure may cause fatalities. Applying this worldwide could modify dozens of thousands of dams and costing hundreds of billons of $: this is unrealistic. But it seems possible to identify the true main risks, to reduce them on a number of dams by structural measures as far as they are cost effective and to insure a reasonable degree of safety by non structural measures applying to all dams at risk.
There are presently 45.000 large dams (39 000 being fill dams). Their failure would usually cause a (flood) wave flow of some thousands m3/s, and for 10.000 of them a flow over 10.000m3/s. Some past failures reached flows between 50 and 100.000 m3/s and possible failures could cause much higher flows. Moreover there are 100.000 small dams with storage between 0,1 and 3 millions m3 : their failure could cause waves in the range of 500 to 1.000 m3/s which may be dangerous locally in populated areas along small rivers.
As the world yearly rate of failure is in the range of 10-4 for the large dams and seems higher (possibly 5 x 10-4) for the small ones, some failures every year may have an impact on dozens
of km² and possibly over 100 km² : 50 or 100 may have an impact on few km² within a range of 10 or 20 km downstream of medium or small dams. The organization and cost of non structural measures may be different according to density of population (500/km² in most of Asia and 20/km² in most of America) and to failure hydro gram.
of km² and possibly over 100 km² : 50 or 100 may have an impact on few km² within a range of 10 or 20 km downstream of medium or small dams. The organization and cost of non structural measures may be different according to density of population (500/km² in most of Asia and 20/km² in most of America) and to failure hydro gram.
The efficiency of not structural measures may also vary according to dam type and failure circumstances because the time available for waming may be minutes or hours (or even days) and the percentage of fatalities in inundated areas may thus be less than 0,1 % or over 50 %. Night failures and cold water are evidently more dangerous.
Non structural measures include risk analysis, monitoring, training, maintenance, emergency planning and warning systems and modified operation.
I) COST OF NON STRUCTURAL MEASURES
For rather large dams in industrialized countries, typically costs range from 10.000 to 100.00 $ per dam but may be up to 500.000 $ in some cases. But studies made by a same team of engineers for many smaller dams, if focus on main risks, may require only two staff days for basic study and 10 or 20 additional staff days for some dams appearing most at risk from the basic study. Relevant costs may then be between 1.000 and 10.000 $ in industrialized countries, ten times less in countries such as China or India where cost of equally efficient staff in much lower.
Risk analysis should thus not be limited to few thousands of large hydroelectric schemes but should be adapted to dozens of thousands of water storage or irrigation dams: as relevant owners have low income and no technical knowledge, such rick analysis should be organized by authorities as well as advices and rules for structural or non structural measures. Risk analysis should take in account the fact that storage of usual floods in reservoir favours occupation of the river bed by people at risk for exceptional floods even when these floods do not endanger the dam.
The choice of reliable equipment with easy maintenance is more important than the value of investment or the theoretical precision of measures. For 80 % of dams at risk which are medium or small earth fill dams, low cost instrumentation based mainly upon simple piezometers and accurate leakage measurement may be reasonable. For all dams, visual inspection and simple measurements, if well organized and reported, are not costly and essential for safety : their cost is essentially bound with staff cost ; frequency of inspections may be reduced after some years for most dams in industrialized country but may be kept higher in low cost countries where standard of construction may have been lower thirty years ago.
In 1916, many people were drowned by a dam failure when they came seeing views of the inundated area : as often an artificial dam created by an embankment downstream of the main dam failed suddenly.
When a dam failure may endanger large cities, initial cost of emergency planning may he over 100.000 $ and the yearly cost in the range of 20.000 $ in industrialized countries, much less in developing countries as costs are essentially staff costs.
But most future failures shall be medium or small fill dams endangering some hundreds or thousands of people within 10 or 20 km downstream of the dam most often by flood failures. Simple emergency planning may be implemented at low cosy and based essentially on local organization and mobile phones. This may apply to dozen of thousands of dams at a yearly unit cost in the range of 1.000 $ in developing countries where is most of the risk.
But most future failures shall be medium or small fill dams endangering some hundreds or thousands of people within 10 or 20 km downstream of the dam most often by flood failures. Simple emergency planning may be implemented at low cosy and based essentially on local organization and mobile phones. This may apply to dozen of thousands of dams at a yearly unit cost in the range of 1.000 $ in developing countries where is most of the risk.
The need of permanent watching in based upon the number of people in the inundated area and the failure probability which may vary considerably along the year.
Beyond floods, the average daily risk of failure of a large dam is less than 10-7. During first
filling and during 10 or 20 days per years when weather forecasting foresees a risk of heavy rains, this average probability may be about 10-5. For floods during construction or during some days after an earthquake it may be close to 10-3, during 10 or 20 days per year the daily probability of an exceptional flood not endangering the dam but inundating large areas downstream may be 10-3 and the impact of dams on such flood is not always favourable. Permanent watching may thus be justified for thousands of dams and temporary watching for dozens of thousands.
Beyond floods, the average daily risk of failure of a large dam is less than 10-7. During first
filling and during 10 or 20 days per years when weather forecasting foresees a risk of heavy rains, this average probability may be about 10-5. For floods during construction or during some days after an earthquake it may be close to 10-3, during 10 or 20 days per year the daily probability of an exceptional flood not endangering the dam but inundating large areas downstream may be 10-3 and the impact of dams on such flood is not always favourable. Permanent watching may thus be justified for thousands of dams and temporary watching for dozens of thousands.
It is advisable to organize the training for many dams and operators : training time of an operator may request few days and unit cost is low specially in developing countries. Quality of training is the key of success.
T (in hours) = 200 s / L
"s" being the reservoir area in km² and "L" the free spillway length in m. for dozens of thousands of dams, "s" is between 0,05 and 1 km² and the spillway length between 20 and 100 m and the downstream flow may raise from nil to hundreds of m3/s in less than one hour : this risk may be substantial for thousands of dams even for not exceptional floods.
Many more data about costs are available in the ICOLD bulletin referred to hereabove.
II) FAILURES OF DAMS
Risk analysis is mainly based upon reported past failures. It should mainly refer to recent ones, for instance since 1970. moreover many failures have not be reported for small dams or when failures caused few or no fatalities.
III) EXISTING EARTHFILL DAMS
Over 80% of large dams and 90% of small dams are earthfill dams.
a) Flood failures
The main failure risk worldwide is their overtopping by floods. It may be mitigated by low cost structural or not structural measures.
"Most floods failures have been caused by under-dimensioned spillways : risk assessment for this aspects is quite easy and effective if it is focused on the real problem :
Which dams are most at risk and what is the probability of actual failure, corresponding to imminent failure flow (IFF), and not merely of exceeding a regulatory high water level ? Because of their free-board, many ungated dams where the 500 year flood exceeds the regulatory high water level may well withstand the PMF, but some large gated dams with a 1.000 year design flood have little safety margin. The catchment area of the great majority of dams is less than 500 km², and simple regional flow formula will apply to most reservoirs in a given climatic area. For instance, the 10.000 year peak discharge, Q, can be evaluated simply by the formula Q = K S0,75, S being the catchment area and K a regional coefficient. Some simple adjustments, taking account of the shape of the catchment area and yearly local rainfall, can be made. Comparing the imminent failure flow with this calculated 10.000 year discharge can help do two things : it can identify the dams most at risk, and it can estimate a range of failure probabilities. All the factors used are easily determined. The impact that storage has on peak flow should be taken into account ; it may be important if the reservoir area of ungated dams is more than 1 or 2 per cent of the catchment area".
Scientific evaluation of the area inundated by a dam failure is difficult and precision of the result far from guaranteed.
But it is possible to give a range of value of the peak flow by a formula such as:
Q (flow in m3/s) = k v 0,5 h1,5 , v being the stored volume at failure time in m3, h the breach depth in m, and k a coefficient between 0,01 for cohesive dam body and 0,03 if uncohesive. It is then possible to know the range of number of people at risk and of value of possible damages and to have elements justifying structural or not structural measures.
Increasing the capacity of spillway is economically justified if its cost is lower than the actualised value of the risk of damages. If q (in m3/s) is the flow initiating overtopping of the dam crest, 1/T the yearly probability of such flow, D the amount of damages and c the cost for increasing the spillage capacity by 1 m3/s the improvement is justified as far as :
Q (flow in m3/s) = k v 0,5 h1,5 , v being the stored volume at failure time in m3, h the breach depth in m, and k a coefficient between 0,01 for cohesive dam body and 0,03 if uncohesive. It is then possible to know the range of number of people at risk and of value of possible damages and to have elements justifying structural or not structural measures.
Increasing the capacity of spillway is economically justified if its cost is lower than the actualised value of the risk of damages. If q (in m3/s) is the flow initiating overtopping of the dam crest, 1/T the yearly probability of such flow, D the amount of damages and c the cost for increasing the spillage capacity by 1 m3/s the improvement is justified as far as :
Cq<k D/T
Value of k varies with local and financial conditions but is most often in the range of 50.
Various low cost structural solutions may be used for increasing spillage capacity : parapet walls, lowering free flow spillways and placing fuse devices, downstream slope RCC lining : c may be in the range of 100 $ in not industrialized countries and 500 $ in industrialized countries.
It may be justified to improve the capacity of small spillways for instance in a developing country, if T = 1.000, c = 100 and q = 200, the improvement by 50 % costs 10.000 $ and is justified if the amount of damages is higher than 400.000 $.
It may be justified to improve the capacity of small spillways for instance in a developing country, if T = 1.000, c = 100 and q = 200, the improvement by 50 % costs 10.000 $ and is justified if the amount of damages is higher than 400.000 $.
It may be unjustified to improve the capacity of a large one. For instance, in an industrial country, if T = 5.000, q = 2.000, c = 500, improvement by 50 % costs 500.000 $ and is not justified if the cost of damages D is under 100 millions $.
Emergency planning and warning systems are usually much less costly than structural measures and may be implemented before or beyond them. They have also the key advantage of being used also for exceptional floods not endangering the dam. Floods of yearly probability 10-2 or 10-4 inundate areas in the range of 10 to 50 % of the areas inundated by a dam failure and this overall human risk may be higher than the failure risk. Downstream level may raise quickly ; specially if a part of the flood is stored in the reservoir and river bed may have been occupied since the dam construction. Usefulness of warning systems may thus be important for all exceptional floods.
Another important risk is jamming of all gates. Relevant risk analysis, proper maintenance, training of operators for emergency conditions, redundancy of operating devices are efficient measures.
Analysis of overtopping risk should include for a number of sites the problems of upstream dam failures in construction or operation, possible breaches of natural reservoirs and landslides in the reservoir.
b) Earthfill dams piping
Piping has been the main cause of fill dams failures at first filling and is the second main cause in operation. The failure flow is usually substantially less than flood failure flow as reservoir level and volume are lower and there is no incoming flow.
The relevant yearly probability of failure seems to be in average between 10-4 and 10-5 but varies considerably and is difficult to assess for each dam. It is higher for old dams which have been built without proper drainage or filters systems and for long dams where foundation conditions may not be well known. A serious risk is also bound with embedded pipes and junction to concrete structures.
But the probability of failure is closely linked with quality of inspection and monitoring. "For dams in operation, when internal erosion occurs, it is at the beginning a slow process whose speed increases with time. Most often the phenomenon can be identified by surveillance.
Visual surveillance must focus on downstream face and toe of the dam, paying attention to wet zones and hydrophilic vegetation."
Instrumentation is the second aspect of surveillance. Priority must be given to measurement of leakage, with observation of possible fine materiel deposits. Measurement of piezometry of downstream dam slope and base is a good complement.
c) Earthquakes
Few relevant failures have been reported for large dams and the average yearly probability of failures appears lower than 10-5. Actually this risk is quite nil for most dams and rather high for some of them. The risk is evidently bound with the probability of earthquake in the area but even more with the nature of dam body and/or foundation. Some fill dams, and particulary those built by hydraulic fill, are subject to liquefaction and may be partly or completely destroyed in few minutes; the failure is then very dangerous even for rather small reservoirs. Efficiency of warning systems is not evident. Hundreds of small dams have been destroyed by earthquakes and the failure of the 40 m high Van Norman dam in U.S. in 1971 was close to a disaster.
Risk analysis should then focus mainly on dams subject to liquefaction or sometimes to sliding due to their construction methods and materials. The wide experience of China, mainly in Huanghe basin and of Japan, may be very useful Some dams should be decommissioned.
Beyond sudden failures earthquakes have caused settlement and cracks of many dams. These cracks may extend within few days, specially for old dams not properly equipped with drains and filters ; monitoring and warning systems are thus essential after an earthquake.
c) War
Breaching the dykes has been a military tool in China for over 1.000 years and Huanghe river banks breaches in 1938 caused the losses of 800.000 lives.
No complete failure of large fill dam due to war has been reported but in fact the number of
large dams x years during past wars is limited worldwide to few thousands. The war risk is also increased by the fact that the main targets may be the highest dams and largest reservoirs of which the failure probability during wars may be high. War risk analysis is then very specific, may anyway study possible breach waves, efficiency of warning systems and impact on safety of partial lowering of reservoir.
large dams x years during past wars is limited worldwide to few thousands. The war risk is also increased by the fact that the main targets may be the highest dams and largest reservoirs of which the failure probability during wars may be high. War risk analysis is then very specific, may anyway study possible breach waves, efficiency of warning systems and impact on safety of partial lowering of reservoir.
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.
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)
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.
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.
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
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