2.2 Method used for overview
Cemagref was asked to inventory all dams recently falling under Environmental Ministry jurisdiction. From 1990-94, 361 dams ranging from 12-80 m in height were rapidly analyzed (Degoutte 1992) as follows:
-brief analysis of possible consequences from dam breakage to classify the dam in terms of "possible public security risk". This analysis was based on a simplified formula to calculate peak flow due to the rupture and on 1/25000 scale maps of downstream valleys to evaluate the vulnerability;
-verification of design flood based on overall regional formulas (Duband et al. 1988). The peak flow Q (m0.72/s) of the 1000 years return period flood is estimated by :
Q = a S0.72
where a is a regional coefficient and S is the catchment surface in km²;
-verification of spillway capacity using simple hydraulic formulas and rapid analysis of risks of incidents in gate operating;
-expertise to evaluate overall dam condition and stability based on on-site visit and rapid examination of dam blueprints;
-evaluation of monitoring equipment, based on dam type, and evaluation of monitoring measurement analysis;
-evaluation of safety control tasks for the Regulatory Service in charge of organizing required annual and decennial visits.
This inventory was performed by a team of six engineers, using the same methodology and criteria. Work was done in close collaboration with local authorities, who where asked to check the list of dams under their jurisdiction and to prepare documents.
Dams operated by skilled organizations and well known by the authority of control were very quickly overviewed. At the opposite side, diagnosis was also very quick for old dams poorly documented and inspected, going immediately to the conclusion that indepth studies were necessary. So the overview was principally focused on intermediate cases.
Finally this inventory required an average of one engineer expert day per dam (not taking in account the time spent by local authority engineers). The cost was 1000 € per dam, supported by the combined Agricultural and Environmental Mini stries.
2.3 Main results of the inventory
This inventory (Royet 1995) 'helped identify a certain number of dams which were not living up to current safety standards:
-16 undersized spillways or gated spillways with insufficient operation safety;
-37 cases where hydrology and hydraulics should undergo an in-depth verification;
-27 dams with high or medium-high safety risks based on size, conception or general condition;
-16 poorly documented dams which should undergo further in depth analysis
-some tens of dams which needed further analysis to conclude as for their impact on public safety.
In terms of dam surveillance, 40% of the sites had no or inadequate monitoring equipment and frequent lapses occurred in monitoring measurements and periodicity. Finally, less than half of the dams were regularly checked by the Administration.
2.4 Inventory consequences
Reactions rapidly occurred in most cases. Studies or work where initiated on most of the dams not meeting all safety requirements. One dam was demolished, because of its general state and the lack of use (Royet 1997).
Surveillance and monitoring devices were improved through installation of new equipment and initiation of several year contracts between owners and specialized Consulting Engineers for on site technical assistance.
Wnen necessary, the control was reinstated with annual and decennial visits by the Regulatory Authority. The Environmental Ministry assigned Cemagref the permanent task of helping safety services for the following aspects:
-on the job training of administrative engineers locally in charge of dam safety;
-follow-up studies during decennial visits to analyze dam condition, security and monitoring. Such analysis require an average of ten engineer expert day per dam and go far more into details than the first inventory described here above;
-development and updating of a descriptive and historical data base for all dams.
3 LOW COST SAFETY lMPROVEMENTS
3.1 Improvement of structural safety.
Some simple and low cost measures can be taken to rapidly improve structural safety of dams identified as not fitting modem standards.
3.1.1 Gravity dams
As said here before, stability of small gravity dams may be endangered during floods corresponding to higher than expected water levels. But structural safety may also decrease with age, due to progressive weakening of masonry or increase of uplift pressures under dam body.
3.1.1.1 Lowering reservoir level
If there are some doubts on the stability of a dam, the first and lowest cost measure to consider consists in a provisional lowering of reservoir water level, at least during time for in-depth analysis. This can usually be done by opening gates or valves, even if this measure may be insufficient in case of high floods.
If the studies confirm the inadequate structural safety of the dam, the lowering of reservoir level can be a definitive measure, of course with a loss of useful water volume. This disadvantage is sometimes of low importance for old dams, whose uses may have been decreasing since the period of construction. The definitive lowering of reservoir level is achieved by digging a breach in the upper part of the dam body, whose dimension is given by hydrology.
3.l.l.2 Dealing with uplift pressure
Very few old small masonry dams where equipped with gallery and drains in foundation; so there is no means to control uplift pressure. In connection with in-depth investigations of dam body and foundations, it is often necessary to drill boreholes from the crest and from the downstream toe of the dam, in order to get masonry and rock sam pies. We recommend to equip boreholes from the crest in piezometers and boreholes from the downstream toe in drains. 80 we have a means to reduce and measure uplift pressure, at a very low additional cost. In the recent decades this was done on a lot of French masonry dams.
If the dam has been equipped with drains in the past, we can very often observe problems of clogging, due to mineral or bacterial deposits. That leads to progressive increase of uplift pressure. Avery simple means to restore drainage efficiency consist in periodically cleaning boreholes with a high pressure water jet. This is now currently done on French gravity dams.
3.1.2 Earthfill dams
If we except risks due to overtopping, the main ageing phenomenon to fear with earthfill dams is internal erosion.
3.1.2.1 Risk evaluation of internal erosion
Analysis of dam design and surveillance are the two ways to evaluate risk of internal erosion. Regarding modem standards, a lot of old dams were designed and built without or with inadequate drainage system: no proper zoning of materials in the embankment, no vertical drain, no or inadequate filters. Problems of internal erosion have been encountered on such dams, sometimes more than a century after their construction. If such an inadequate drainage system has been identified, that must lead to a reinforcement of surveillance.
For dams in operation, when internal erosion occurs, it is at the beginning a slow process whose speed increases with time. So there are very few cases where the phenomenon can not be identified by surveillance.
Visual surveillance must focus on downstream face and toe of the dam, paying attention to wet zones and hydrophilic vegetation.
Monitoring is the second aspect of surveillance.Priory must be given to measurement of leakage, with observation of possible fine material deposits. For medium size dams measurement of piezometry of downstream dam slope and base is a good complement.
3.1.2.2 Reducing risk of internal erosion
As for stability problems with gravity dams, risk of internal erosion in embankment dams can be reduced at very low cost by lowering reservoir water level. That will directly reduce hydraulic gradient which is the "engine" of internal erosion. But this measure can only be considered as provisional.
Definite repair can be achieved only by structural, works, that can be divided in two categories: works acting on watertightness, such as an upstream facing or a slurry curtain and works to controlling erosion, such as a toe embankment.
Works of the first category are usually costly. So if the loss of water due to leakage is not a constraint, the most economic solution consist in placing a filter and a drain on downstream slope of the embankment and building a fill over this drainage complex. This solution has been adopted in the recent years on some old embankment dams in France, Torcy-Vieux for example.
3.2 Safety improvement against floods
As worldwide, 2/3 of European failures which happened after first filling were caused by floods (yearly risk close to 0,5 x 10-4). Some were caused by wrong operation but most by underdimension of spillways.
3.2.1 Gated spillways
30% of European spillways are gated. Risk of gates jamming or wrong operation may be much reduced by proper maintenance, review of operating criteria and training of operators. French Committee on Large Dams has published in June 1998 "Practical guidelines for improvement of dam safety during floods" which is based upon analysis of accidents, incidents and operation of hundreds of dams and applies also to ungated spillways.
Operation of free flow spillways is safer, provided that two actual risks, often underestimated, are avoided : many small and medium spillways were in France (unofficially) partly closed by sandbags, boards, or even masonry; this could be easily controlled and avoided, at least during flood season. Another risk is due to floating debris and trees, often much more important than anticipated, which may block free flow spillways (Palagnedra, 1978), for instance against bridge piers when free width is less than about 10 m.
Most failures were caused by underdimensioned spillways. For many European dams the design flood was estimated for a yearly probability between 10-2 and 10-3, often actually close to 10-2 as floods were often underestimated. ln faet the average yearly actual failure probability is less than 10-4 thanks to the freeboard which divides by 10 or more the risk . But this impact of freeboard varies greatly and is less important for gated spillways. Thus an efficient risk analysis should not focus upon a conventional design flood but upon a check flood close to the real risk of fill dam overtopping.
To identify quickly dams most at risk in one climatic area, it may be better to estimate all extreme floods by the same simple regional formula based upon data easily known (catchment area and shape, yearly rain) rather than to use sophisticated formulae based upon a great number of data difficult to collect and check.
3.2.2 Ungated spil/ways:
Spillway capacity increase, if necessary, is easier for ungated spillways. Majority of these 3000 European spillways have a design flood between 30 and 200 m3/s and a nape depth for the design flood between 1 and 2 m and a freeboard allowing an extra depth in the range of 1 m before endangering the dam. Typical solutions are presented hereunder to increase the capacity of a spillway having a flood design of 100 m3/s. Its length is about 25 m , the design flood depth is 1,5 m and freeboard allows a nappe depth up to 2,5 m before really endangering the dam (for a flood of about 200 m3/s). It is for instance easy to increase these flood values by about 100 m3/s if lowering the spillway sill by 1 m and placing fuse elements to keep the same normal operating level; some difference between their characteristies will cause tilting or bending for different nape depth over elements; for instance 1,20 m for the first tilting element (for a flood of 70 m3/s) and 1,80 m for the last one (for a flood of 200 m3/s) with no risk for the dam for a flood under 300 m3/s.
Three typical solutions are presented hereunder. In all solutions, it is advisable (and easy) to keep proper aeration under the overtopping nappe.
3.2.2.1 Fuseplates (fig. 1 b)
Hundreds or thousands of spillways, in U.S.A., have used since 50 or 100 years flashboards which are often wooden boards supported by vertical steel pipes embedded in the sill concrete. Their height is usually in the range of 1 m They may be dismantled by hand before the flood season or are overtopped by small floods and bend for a preestimated upstream level, that is for a medium flood. The present low cost of steel offers the possibility to apply the same principle with steel plates instead of wooden boards or even to use self resisting steel plates. In the present example self resisting steel plates thickness should be in the range of 12 mm and total necessary steel weigh about 4 tons to increase acceptable flood by 100 m3/s. Total cost, including removal of 30 m3 of sill concrete should be in the range of 20.000 US$ (or Euros) (or 200 US$ to increase flow by 1 m3/s).
Design, construction and control are very simple. It is also easy to quickly replace by spare elements some elements which should be bent by exceptional floods.
3.2.2.2 Fuseplugs (fig. 1 c)
A concrete block simply laid on a sill withstands a water load up to a certain level which may be estimated (and easily checked by hydraulic model) as far as the uplift under the element is determined.
This uplift varies with the position of waterproofing., Some possible cross section is given in fig. 1c. To increase flow by 100 m3/s in the present example would require 20 or 30 m3 of ordinary concrete or 10 m3 of reinforced concrete.
Steel may also be used and favours prefabrication: steel quantity should be about 4 tons but unit cost higher than with fuseplates, this may however be interesting if standardized for a number of spillways.
3.2,2.3 Fusegates (fig. 1d)
Devices 1 and 2 are very simple but the water level corresponding to bending or tilting is not very precise. A better precision is obtained by modifying device 2 and creating uplift under the element for a precise upstream level. This may be obtained by a well placed on each element.
This solution (Hydroplus system) has been used to increase spillway capacity in France, Great Britain, Switzerland and South Africa and in a dozen of countries to increase storage . The structure may be the same as for fuseplugs but wells and more complex studies increase the total cost. It may be mainly interesting for the most important spillways. It may also be used with a more complex labyrinth shape which increases by over 50% the overtopping flow before tilting.
There are other solutions beyond fuse elements:
3.2.2.4 Labyrinth walls
For the same example as above, it is also possible to lower the existing sill by about 2.5 m and to build labyrinth walls, thus increasing the flow by 50%. The labyrinth walls have been used for some dozen of spillways, in Europe or Africa, usually for flows between 100 and 1000 m3/s. To increase flow by 100 m3/s, the total area of walls shall be here about 150 m² with an average thickness of 0,25 m, i.e. about 30 m3 of reinforced concrete. Total cost, including removal of 100 m3 of concrete is in the range of 50.000 US$ (or Euros) to increase flow by 100 m3/s. Maintenance and control are even simpler than with fuse elements.
3.2.2.5 Mixed solution
Associating labyrinth walls along half of the spillway length with fuse elements along other part may be a very attractive solution, mainly for medium or large spillways. Such solution has already been used in France and South Africa.
CONCLUSION
A lot of dams around the word are owned or operated by organizations with limited financial means. As presented in this paper, there are ways to check and, if necessary, improve safety of those dams at low cost.
It is recommended to periodically review the safety of a large dam portfolio with a reduced team of experts working on the same criteria. This is the best way to identify dams more risky and then undergo in-depth verifications.
Low cost solutions are proposed to improve temporally or more definitely structural safety as well as flood safety.
 
REFERENCES
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DEGOUTTE G. 1992. Guide pour le diagnostic rapide des barrages anciens. « Etudes» du Cemagref, série Hydraulique Agricole n° 13. Antony: Cemagref
DUBAND D et al. 1988. Evaluation des crues extrêmes et de la crue de projet par la méthode du Gradex XVI th Congress of ICOLD San Francisco USA; Q63, R60, 1008-1047.
ROYET P., MERIAUX P., D. POULAIN, F. PAREDES. 1995. Evaluation de la sécurité des barrages autorisés. IngénierieEAT N° 13 : 37-44.Antony: Cemagref.
ROYET P. 1997. Démolition du barrage de Kernansquillec et gestion des sédiments de la retenue. XIX th Congress of ICOLD Florence Italie; Q74, R20, Vol V 469-476. Previous < 1 2 >
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