F. LEMPERIERE
Hydrocoop - France
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).
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 circonstances (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 the spillways for the probable maximum flood if the dam failure may cause fatalities. Applying this worldwide could modify dozen of thousands of dams and cost hundreds 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 wave flow of some thousands m3/s, and for 10.000 ofthem 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.
The efficiency of not structural measures may also vary according to dam type and failure circonstances 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
 Risk analysis reviews possible risks with rough evaluation of relevant probability and consequences (hazard) and may study various solutions to mitigate these risks.
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 purpose of monitoring, beyond checking design data, is to give informations in order to take measures avoiding failures, or at least to give time for efficiency of warning systems. Monitoring includes instrumentation, visual inspections and periodic assessment performance. Cost of instrumentation may be high, in the range of one per cent of the dam cost but may be fully justified, particulary for arch dams or difficult foundations. However its efficiency is very questionable if the operating staff is not well trained.
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.
For emergency planning and early warning system, timing and quality of warning are the keys of success, it is necessary to win as much time as necessary but "the clarity of warning messages in term of who, what, when and where is an important condition of successful waming" .
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.
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.
Training may apply to all safety problems or focus on some key points such as spillway gates operation or visual inspections or warning systems. Training efficiency may be much improved by computer based simulators
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.
Maintenance and more specially gates maintenance is an essential point. Budgets may not be high but informations about the many details to check and about past accidents are very important: addition of several minor defects has caused past disasters.
Modified operation may include lowering of the reservoir level. This may be permanent before improvement of a structural defect or limited to the flood season in order to reduce the probability of flood failure. In this later solution a human risk is often overlooked : when a part of the flood is stored before spilling the downstream flow at the time of spilling may raise much more quickly than with the natural flood. This is true either by gates operations of for many free flow spillways for with the time to peak of the downstream flow became in the range of :
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 refered to hereabove. < 1 2 3 > Next
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