COMMISSION INTERNATIONALE
DES GRANDS BARRAGES
------DAMS AND RESERVOIR FOR
MULTIPLE PURPOSES
Brasilia, MAY 2009
-------
DAM SAFETY - THE MAIN ELEMENT FOR ANY SUSTAINABLE STORAGE
PROJECT(*)
Martin WIELAND
Dam and Earthquake Expert, Dipl. Ing. ETH, Dr. sc. techn.
Pöyry Energy Ltd., Zurich, SWITZERLAND
R. Peter BRENNER
Dam Consultant, Dipl. Ing. ETH, PhD
Weinfelden, SWITZERLAND
1.
INTRODUCTION
Modern water storage projects are evaluated on the basis of sustainability
criteria. However, among the different parties and stakeholders involved in a
large dam project there is often no general consensus on what exactly means
‘sustainable’. The term ‘sustainable’ is widely used also in other areas of man’s
development activities. In general, a development is considered sustainable if it
satisfies the needs of today’s generation without jeopardizing the possibilities of
future generations to satisfy their needs at their time. What does this mean for
water storage projects? The project must be conceived in such a way that it
satisfies the purposes it has been designed for, e.g. energy production, water
supply, irrigation, flood control, etc., but it must not later negatively affect future
generations’ benefits needed at that distant time. The project must not lead to a
situation where later some or all of the benefits required by the population can no
longer be provided.
Sustainability of a storage project is closely related to dam safety and thus
to the ageing and service life of the project structures. All parts of a storage
-------------------------------------(*)
Sécurité des barrages – L’élément principal pour n’importe quel ouvrage de retenue
1
scheme (civil, mechanical and electro-mechanical) are subject to ageing. To
some extent ageing can be mitigated by maintenance activities or rehabilitation
work. Dams with an age of over 100 years are now not uncommon. These dams
and their appurtenances have been looked after systematically by the owner and
inspected regularly for possible deficiencies by dam specialists, usually from a
government organization. Records of their performance are available since their
commissioning.
Ageing not only affects the durability of the storage structures but also their
functionality which in turn determines the safety of the storage facility. Safety is a
key factor in the sustainability of a storage scheme. If the safety of, for example
the dam, is no longer guaranteed, the dam becomes a risk and remedial works
have to be initiated. The benefits of such a dam will decrease or even vanish.
The dam may have to be abandoned or de-commissioned.
This paper discusses the importance of dam safety in maintaining the
sustainability of a storage scheme, looking at such factors as ageing,
maintenance, rehabilitation and upgrading.
2.
SUSTAINABILITY OF DAMS
Many factors govern the sustainability of a storage facility, such as: the
natural environment, including river characteristics, site conditions and natural
hazards, the existing infrastructure and social network, the design, quality of
construction and equipment, and economic benefits. In simple terms, sustainable
development has four pillars: the main and most important pillar is dam safety,
followed by environmental, economic and social aspects (Fig. 1). A storage dam
is basically an element of the infrastructure, like a road, a tunnel, or a canal. It
has to provide benefits to the population (food production, electricity, water
supply, flood protection, aquaculture, recreation, etc.). Environmental
sustainability means that the new infrastructure must not cause uncontrolled and
inacceptable and irreversible damage. Nature must be able to adjust to the new
infrastructure without permanent negative impacts. Economic sustainability
means that the infrastructure should provide sufficient revenue that it can be
properly maintained and managed and that the owner obtains a fair return for his
investment. Finally, social sustainability means that possible resettlements are
handled with adequate understanding for people’s basic needs, fair
compensation and improved living conditions. The reservoir should promote
outdoor activities and recreation and as well fishing opportunities. Potential
water-born diseases must be mitigated by proper education of the residents and
reservoir management.
The artificial lakes created by dams must be properly managed and the
dams need continuous maintenance. Reservoirs are very often a valuable source
of drinking water. Although at present most large reservoirs are used for energy
2
production, the generation of hydropower may decrease with time. Water,
however, will always remain an indispensable commodity and so will there be a
need for flood protection. Hence, the need for dams and reservoirs will persist.
The requirements for sustainability and safety will remain applicable for all dams,
regardless of their particular use.
Bridle [2] introduced the concept of an Engineered Sustainable
Infrastructure Cycle (ESIC). The idea is first to identify the social needs and then
find the infrastructure (e.g. a storage facility) that best can satisfy these needs.
The infrastructure must be able to generate sufficient income to support its
management and maintenance. Then the question is asked whether the
environment can sustain this infrastructure. Additional input required for the
model includes: the lifetime of this infrastructure and what are its emissions and
how they can be handled, and what are the resources to construct it. Each step
must be optimized.
The actual lifespan of a dam project is usually not taken into account in a
feasibility study, only the period of concession. Private owners and developers of
infrastructure projects use the concession period as a guideline for the design life
of their project. For hydropower projects in Switzerland, for example, the
concession period is 80 years, in less developed countries, such as in the Lao
PDR, it may be as short as 30 years. When the concession period in these
countries has expired, the ownership is usually transferred to the government.
Costs for the de-commissioning of a dam are also ignored. A serious process
affecting the service life of a reservoir is sedimentation. Proper sediment
management strategies, which must be based on detailed studies of the river
flow, sediment transport and watershed characteristics, can make reservoir
siltation sustainable. Such sediment handling techniques are also an important
investment in the safety of the facility (Palmieri [6]).
3.
THE ROLE OF AGEING
There is substantial experience in handling the ageing issue. Hundreds of
existing large dams were built before 1900 and around 5000 before 1950
(Lempérière [5]). Ageing processes affect the dam body, the foundation and the
appurtenant works. Dam body and foundation form a dynamic system subject to
variable loads, especially caused by temperature fluctuations and hydraulic
gradients. In addition to the ageing of the civil structures, mechanical and electromechanical parts (gates, valves, cranes, and the power supplies) are subject to
physical (wear, erosion, frost etc.), chemical (corrosion, AAR, etc.), and biological
processes. Maintenance and rehabilitation of vital components deteriorated by
ageing must therefore have a high priority in dam safety management. In
general, the budget for maintenance and repair is a small percentage of the
original investment in the storage project.
Penman & Milligan [7] concluded that the modern embankment dam has
marked advantages, both from the point of view of economy and environment. It
3
can be constructed on almost any foundation and is an extremely economical
form to construct. When complete, it can be maintained practically indefinitely,
although the reservoir may limit the time period of operation.
4.
SAFETY OF DAMS
The safety concept for storage dams is based on the following pillars
(Biedermann [1]): (i) structural safety, (ii) dam safety monitoring, and (iii)
emergency preparedness (see Fig. 1). In addition to these pillars, a fourth pillar,
which accounts for the operational safety and maintenance must also be
included.
Fig. 1
Relationship between dam safety, sustainability and ageing
Relation entre sécurité des barrages, durabilité et vieillissement
4
4.1. STRUCTURAL SAFETY
Structural safety is the main prerequisite for the safe operation of a storage
facility. The foundation for structural safety is laid mainly during design, as given
by the design criteria. The design must be carried out assuming that the dam
may become exposed to the worst possible condition during a natural hazardous
event, i.e. mainly floods and earthquakes. Older dams are often not designed
according to today’s design standards. One reason is that since their conception
more data have become available which allow a more realistic prediction of
extreme events. Such storage projects may have inadequate spillway capacity or
the dam structures were designed with inadequate seismic loading.
Inadequate spillway capacity can be handled by constructing an additional
spillway, or where feasible, by a rising of the dam crest.
The most severe loading conditions for dam structures originate from
earthquakes. With older dams, earthquake loading was commonly taken into
account by means of a seismic coefficient. At most dam sites a seismic
coefficient of 0.1 was used. Seismic hazard analyses, however have revealed
that also in regions of low to moderate seismicity, such as Central Europe, the
possibility of a strong shock , say in the order of M=6.5 does exist, although with
a very low probability. This means that the earthquake safety of many of these
older dams is not known and some may be structurally deficient.
The 106 m high Sefid Rud buttress dam in Iran was constructed in the
1960s in a region of high seismicity. The design assumed a seismic coefficient of
0.25. In 1990 it was struck by a mb7.3 (Ms7.7) earthquake (i.e. a magnitude close
to the maximum credible value) with its epicenter very close to the dam site
(Indermaur et al. [4]). The reservoir was nearly full. Buttress dams are known as
rather delicate structures but the concrete dam body showed a surprising
resilience against cracking. Cracks developed almost exclusively along horizontal
lift joints in the transition zone of web and head of the central buttresses (Fig. 2).
Relative movements between the buttresses also gave rise to concrete spalling
along the vertical construction joints. Other structures in the vicinity of the dam
suffered heavy damage to full collapse. Back-calculations showed (no functioning
strong motion instruments were located in the dam area) that the peak ground
acceleration was about 0.7 g. The dam was (and still is) a well-maintained
structure with a full record of its performance in the past. It remained basically
functional, although comprehensive repair work (grouting of the weakened joints
and cracks, and post-tensioned rock anchors were installed in all buttresses).
The main purpose of the dam is irrigation and water supply. Sediment problems
are severe at this site but managed by regular flushing. The sustainability of the
5
facility remained preserved, also after the earthquake, because the wellconstructed dam had a sufficient margin of safety.
The 158 m high Zipingpu concrete face rockfill dam (CFRD) in Sichuan
Province, China, was 17 km from the epicenter of the May 12, 2008 M8.0
Wenchuan earthquake. This was the first time a large CFRD has been subjected
to very strong earthquake ground shaking. The dam suffered some damage,
mainly at the joints of the face slabs and in the crest region but at no time was
the dam in a critical condition. The reservoir level was relatively low at the time of
the earthquake and was drawn down for safety reasons.
The ICOLD Bulletin 72 (ICOLD, 1989), which is being revised, presents
guidelines for the design of dams against earthquake loading. In order to achieve
a sustainable design with respect to structural performance, dam safety
authorities should check whether their dams can satisfy the new guidelines. In
case of deficiencies dam owners should be urged to upgrade their dams
accordingly.
Fig 2
Critical crack which developed at the kink (slope change) and along the
working joint at the head-web interface
Fissure critique qui s’est développée le long d’un joint de construction au
contact tête/âme du contrefort
1
2
3
4
Buttress head
Buttress web
Lift joint
Crack
1
2
3
4
Tête du contrefort
Ame du contrefort
Joint de reprise
Fissure
There are many small embankment dams used mainly for irrigation and
water supply purposes which were not designed for extreme loadings and where
site investigations may have been kept to a minimum. Such dams when
6
subjected to a strong earthquake can be damaged by either slope failure,
spreading due to foundation liquefaction, or multiple cracking. Although the
reservoirs retained by these dams are usually not large, the uncontrolled release
of the stored water can still cause severe damage and enhance the already
critical conditions of the infrastructure caused by the earthquake itself.
Wang [9] presented several examples of damaged embankment dams in
China. These were constructed with inadequate and only lightly compacted
materials (silts, loam, sands) in regions which had been classified as being of low
seismic intensity (≤MM VI), but then were hit by a major earthquake. Small dams
do generally not satisfy sustainability criteria and water resources agencies must
make every effort to rectify this deficit through upgrading.
4.2. MONITORING AND SURVEILLANCE
Dam safety monitoring is a key activity in dam safety management. It
consists essentially of two components, namely:
(1)
(2)
A field check or visual inspection of the entire dam and its appurtenances. It
also includes checking the functioning of the flood control elements, i.e.
spillway gates (if any) and the valves for the various outlets, and as well the
emergency power supply.
Measurements of physical quantities (mainly pressures, deformation, flow
volumes and sometimes also temperature) describing the status of the dam
and its foundation. Also included here are data analysis and their
interpretation. Monitoring provides a rational insight into the safety of the
dam-foundation system. With modern automatic data acquisition systems
real-time monitoring becomes possible and rapidly changing conditions can
be recorded.
Instrumental monitoring, if systematically performed, can detect a
developing deficiency at an early stage, however, only at locations where
corresponding instruments have been installed, e.g. piezometers, seepage weir,
settlement point, etc. In other locations only visual inspection can detect whether
something is wrong or unusual, such as the development of a wet spot on the
downstream slope of an embankment dam.
Unfortunately, there are still many older large dams (say between 15 and
about 30 m in height) which have inadequate monitoring facilities (i.e. parts of the
installed instruments are out of order and earlier records are missing) or there is
no instrumental monitoring at all. Even today, certain water resources authorities
are difficult to convince that their new dam project requires a minimum of
instrumentation, especially with embankment dams placed on alluvial foundations
or residual soils. Instrumental monitoring also requires a strict data management
and a graphical display of the measurements to enable the rapid identification of
irregularities caused by deficiencies or also by faulty measurements or deficient
equipment. An important concept in monitoring is redundancy. It means that a
7
few very important quantities should be measured simultaneously, but
independently, by two instruments at the same location. The instruments can be
of the same type but preferably they are of a different type, e.g. a vibrating wire
piezometer together with a pneumatic type.
4.3. EMERGENCY CONCEPTS
A dam is not supposed to fail, and dam failures have in fact decreased
substantially in the second half of the 20th century (Schnitter [8]), but zero risk is
virtually impossible to achieve. Lempérière [5] puts the present overall risk of
failure of existing large dams at about 10-4 per year. The main risk is the
overtopping of embankment dams during large floods. Hence, upgrading of
spillways will reduce this risk further. In addition, all dams with a large reservoir
volume should be provided with a bottom outlet, such that the reservoir can be
drawn down in an emergency situation, especially after a strong earthquake.
The Swiss emergency strategy distinguishes three levels of danger,
depending on the severity of the situation (Biedermann [1]). Level 3 is the worst
case, i.e. failure of the dam can probably not be avoided. People in all the
potentially submerged areas have to be evacuated before the disaster strikes the
area. In order to define the appropriate emergency level, the dam specialist must
be able to recognize the causes of the distress or of the hazard and how it will
develop with time. Monitoring with increased frequency may have to be initiated,
especially at levels 1 and 2. For example, seepage collection may be done in
zones (left abutment, right abutment, dam foot, etc.) to enable a better
identification of the source of leakage. The main objective of any emergency
preparedness is to avoid the loss of lives.
5.
CONCLUSIONS
Water storage projects are components of the infrastructure in the area for
which they should provide benefits. Infrastructure projects must be sustainable
such that they can serve the needs of the people for a very long time. Water
storage projects, however, cannot be considered sustainable if their safety
according to modern standards is not assured. A failure of a large storage dam
would cause tremendous damages to infrastructure, environment and the social
network and produce a set back in the development of the region. Dam safety is
therefore a foremost prerequisite for a sustainable development of a storage
project or for the sustainable operation of an existing storage facility. As a matter
of fact, a technology, which is unsafe has no future, and the history of dam
construction, spanning over 2000 years, has shown that safety is the basis of
sustainability.
8
Keeping the dam in a safe condition requires proper management of all the
safety-relevant elements and issues, in particular maintenance of the various
structures, rehabilitation of identified deficiencies and aged components,
upgrading and replacing instrumentation where necessary, and a periodic review
of the emergency concept.
REFERENCES
[1]
BIEDERMANN, R., Safety concept for dams: Development of the Swiss
concept since 1980. wasser, energie, luft – eau, énergie, air, 89(3/4):5563, 1997.
[2]
BRIDLE, R., Engineering sustainable dams. 14th German Dam
Symposium, Freising, Bericht Nr. 115 des Lehrstuhls und der
Versuchsanstalt für Wasserbau und Wasserwirtschaft der TU München,
P. Rutschmann ed., pp. 46-50, 2007.
[3]
ICOLD, Selecting Seismic Parameters for Large Dams, Guidelines.
Bulletin 72, Committee on Seismic Aspects of Dam Design, International
Commission on Large Dams, Paris, 1989.
[4]
INDERMAUR, W., BRENNER, R.P. & ARASTEH, T., The Effects of the
1990 Manjil earthquake on Sefid Rud buttress dam. Dam Engineering,
2(4):275-305, 1991.
[5]
LEMPÉRIÈRE, F., The role of dams in the XXI century: Achieving a
sustainable development target. Hydropower & Dams, 13(3):99-108,
2006.
[6]
PALMIERI, A., Sustainability of dams: Reservoir sedimentation
management and safety implications. World Bank Paper,
http://tc.iaea.org/tcweb/abouttc/strategy/thematic/pdf/presentations/dam_s
afety/dam_sustainability_world_bank_paper.pdf, 1998.
[7]
PENMAN, A.D.M., & MILLIGAN, V., Longevity of embankment dams – a
critical review. Int. Workshop on Dam Safety Evaluation, 1:33-51, 1993.
[8]
SCHNITTER, N.J., Contribution to Q. 49 Deterioration or failure of dams.
Trans. 13th Int. Congress on Large Dams, New Delhi, 5:488-493, 1979.
[9]
WANG, WENSHAO, Lessons from earthquake damages of earth dams in
China. Proc. Int. Symp. Earthquake and Dams, Beijing, 1:243-257, 1984.
9
SUMMARY
The paper discusses the role of dam safety in the sustainable operation of
water storage facilities. Developers and dam owners often do not realize that
safety is actually the dominant factor in the sustainable performance of water
retaining structures. Dam safety is based on an integral concept of three pillars:
structural safety, monitoring (surveillance), and emergency concept. Dams are
subject to ageing and have therefore a limited life span. Practically, the life span
of a dam is as long as its safety can be guaranteed. Maintenance and
rehabilitation can prolong the life span. A dam which is not safe can no longer be
considered a sustainable infrastructure. Failure of a dam can cause immense
damage to infrastructure, environment and population, Comprehensive dam
safety management enables the owner to know the condition of the dam at any
time. The sustainability of the structure can thus be assured.
RÉSUMÉ
L’article traite le rôle de la sécurité des barrages concernant l’opération
durable des ouvrages de retenue. Souvent, les investisseurs et les propriétaires
des barrages ne se rendent pas compte que la sécurité est actuellement le
facteur dominant dans la durabilité des structures des ouvrages de retenue. La
sécurité des barrages est fondée sur un concept intégral de trois piliers : la
sécurité structurale, la surveillance (auscultation) et le concept cas d’urgence.
Les barrages sont soumis au vieillissement et par conséquent ils ont une durée
de vie limitée. Pratiquement, la durée de vie est aussi longue que la sécurité peut
être garantie. L’entretien et la rénovation peuvent prolonger la durée de vie. La
rupture de barrage peut endommager immensément l’infrastructure,
l’environnement et la population. La gestion compréhensive de la sécurité des
barrages permet de mettre le propriétaire au courant sur la condition du barrage
à n’importe quel temps. Ainsi, la durabilité de la structure peut rester assurée.
Keywords:
ageing
vieillissement
benefits of dams
bienfaits des barrages
maintenance
entretien
monitoring
auscultation
safety of dams
sécurité des barrages
10