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.
 Beyond China there are 17.000 large fill dams. Before 1970, 7000 were built and 100 failures reported. Since 1970, 10.000 more were built and 44 failures reported ; 7 flood failures during construction, 7 piping failures at first filing and 30 in operation including 22 by floods 6 by piping and 2 by earthquake. Rate of reported failures in operation after 1970 was thus 0,8 x 10-4 about the same for old or new dams. But all failures were not reported and the rate has been much higher within small dams. In China 20.000 large fill dams and 60.000 small ones (over 100.000 m3 storage) were built after 1950. 3% failed (including 2% flood failures) but the rate of failures after 1980 was reduced (in the range of 3 x 10-4 ?)
Beyond China there are some hundreds of masonry dams, often very old : 18 failed before 1970 and 2 later.
There are 4500 large concrete dams beyond China: most built before 1970. 12 failures have been reported before 1970 and 3 later. Most failures happened at first filling and were bound with foundations.
Sudden failure are much more dangerous than progressive ones : beyond China 10% of the fill dams failures and 40% of the masonry or concrete dams failures caused more than 100 fatalities each : half of the relevant reservoirs stored less then 20 millions m3 and over half of these dams were lower than 30 meters.
III. EXISTING EARTHFILL DAMS
Over 80% of large dams and 90% of small dams are earthfill dams.
III - 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 :
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 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.
III - b / Eartfill 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.
Warning systems in case offailure have been very efficient for large or small reservoirs.
III - 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.
III -d /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.
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