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| [ home / german_research / Article: Rettemeier, K., Nilkens, B., Falkenhagen, B., Köngeter, J. ] |
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Institute of Hydraulic Engineering and Water Resources Management
Aachen University of Technology, Aachen, Germany
ABSTRACT: The world wide discussion about dam safety leads to a change
in the approach towards risk related problems. Basically two opinions can be viewed,
the safety and the risk-oriented one. Traditionally dams are considered safe,
because they have been built according to high technical standards. Today people
become aware of the fact that all man made structures have a potential risk that
has to be evaluated, assessed and managed. Thus the risk of dams is no longer
neglected. This discussion about risk based dam safety leads to a new approach
for German dams as well. Because of the varying cultural and legal background
it is impossible to use a Risk Assessment procedure from another country without
adaptation. On the basis of known Risk Assessment procedures a new procedure was
developed for German dams taking the German background into account.
Like all technical buildings also dams hold a potential risk of failure.
One of the largest disasters in Germany was the failure of the Moehne Dam. It
was destroyed 1943 due to extensive bombing. The reservoir was almost full and
ran empty within few hours (RUHRVERBAND, 1988). The devastating extents of this
failure are represented in figure 1. The disaster cost 1200 human lives and led
to destruction of the down-stream settlement. Unlike dam failure in many other
countries this failure naturally did not effect the approach towards the risk
of dams in Germany since the awareness of the risk is small compared to other
war actions.
Figure 1: Möhne Dam Failure, World War II (RUHRVERBAND,
1988)
Different effects can lead to damage and cause the failure of a dam.
Some examples are flood, earthquake, landslides and piping or static reasons.
Operation incidents are important factors, which can be divided into incidents
due to technical and those due to human failure. The flow chart of a dam failure
is represented in figure 2.
Figure 2: Flow chart for dam failure
The failure of a dam results in a flood wave. The hydraulic characteristics
of this flood wave highly differ from a conventional flood wave in a river due
to high precipitation. The sudden failure results in a wave with large energy
and high flow rate, which suddenly floods the downstream settlement and causes
large devastation.
The resulting damage encloses far away sections of the downstream settlements
and can be very high for humans, economics and the environment.
The acceptance of the hazard potential of technical buildings is subject to change
in the society permanently. While in former times humans accepted such disasters,
today the awareness of risks is much higher and dams are regarded more critically.
Since risk management is executed in many other technical areas, the question
arises: how safe are our dams?
The present discussion about the safety evaluation of dams led world-wide,
covers two different view points. Here the conventional safety-oriented perspective
faces the risk-oriented perspective (figure 3). The safety-oriented perspective
assumes no risk of failure, since a dam is built according to high design criteria.
Consequently damage for humans, economics and the environment can be excluded
with a degree of probability verging on certainty unlikely. However an absolute
safety cannot be ensured technically. The risk-oriented view point takes a risk
of failure into account. Thus the residual risk has to be determined, evaluated
and man-aged even if failure seems unlikely.
Figure 3: View Point of a safety and risk oriented society
The safety-oriented perspective was represented in Germany so far.
But the world wide discussion about dam safety where dams are regarded more commonly
under aspects of risk led to a reorientation. Therefore the aspects of risk are
considered in the draft of the DIN 19700 section 11 dams (1999) - "the residual
risk in consequence of exceeding the BHQ2 (equivalent to a flood with recovery
period of 10.000 years) has to be evaluated - eventually considering the PMF.
It should be met by technical and/or organizational measures."
The estimation and evaluation of risk are known from other technical areas (e.g.
atomic industry) and the methods of risk management are often taken into account.
The process of Risk Management can be divided into the areas Risk Analysis, Risk
Assessment and Risk Management.
Figure 4 represents the context associated with Risk Assessment. The Risk Analysis
as the first step is part of the Process of Risk Assessment. While the Process
of Risk Assessment itself is part of the Process of Risk Management. The flow
chart gives an overview of the fundamental terms and represents both the relationship
of the individual steps (Risk Analysis, Risk Assessment, Risk Management) and
their integration into the entire process (Process of the Risk Assessment, Process
Risk Management).
Figure 4: Risk Management Process
The risk of the dam is determined within the Risk Analysis. The risk
is defined as the measure of the probability of failure and the severity level
of unfavorable effects (ICOLD, 1998). Thus the risk is determined as the product
of failure probability and extent of the damage (risk = probability of failure
* extent of damage).
The evaluation of the risk comprises the consideration of alternative mitigation
measures as well as the acceptance of risk. These aspects are formally referred
to as Risk Assessment.
Finally the Risk Management covers the decision making for the development and
conversion of a management plan as well as the monitoring of the implementation.
Germany has 311 dams which are listed in the ICOLD register of dams with
a height over 15 m (SIEBER, 2000). In Germany there are uniform technical standards
for dams, which are determined in the DIN 19700 (1986). The DIN 19700 (1986) classifies
five types of dams (reservoirs, flood control storage basin, weirs, pumped storage
reservoir, tailing dams), no matter the height or volume (table 1). According
to the draft of the DIN 19700 (1999) dams are classified according to their type
and size.
The building, operation and the monitoring of dams are regulated by the state
water law of the individual state of the Federal Republic of Germany and depend
on the state of the art. Most dams are subject to regulation of the individual
state. According to the state water law in North Rhine-Westphalia all dams with
more than 5 m height and a volume of at least 100.000 m³ are subject to regulation.
Especially in North Rhine-Westphalia the technical and legal demands depend on
the type of the dam (Table 1). RETTEMEIER & KÖNGETER (1998) provide an outline
of the classification and the legal and technical requirements of dams in Germany.
Table 1: Dams Subject to Regulation in Germany
The classification of dams limits the introduction of risk based dam
safety approaches in Germany. Other countries which classify dams according to
risk have a better understanding of the risk of their dams.
The German dams have been built and operated according to the DIN 19700 (1986)
with high request on the structural design and the maintenance. Since the failure
probability is assumed to be very small, dam failure has not been considered in
general. Consequently the possibility of failure has been neglected and flood
or emergency plans do not exist for any German dam. However inundation studies
are absolutely necessary for the regulation and the estimation of risk.
The world wide discussion about risk based dam safety leads to a new approach
for German dams as well. Be-cause of the cultural and legal difference it is impossible
to use a Risk Assessment procedure from another country directly. The Risk Assessment
procedure that is presented in figure 5 has been developed by the authors on the
bases of other known procedures as well as taking into account the German questions.
Risk Assessment is part of the Process of Risk Management. The Process of Risk
Assessment covers Risk Analysis and Risk Assessment. The risk which is determined
in the Risk Analysis is evaluated within the Risk Assessment. This context is
already represented in figure 4, and all Risk Management procedures have those
fundamentals in common.
Before a Risk Assessment can be executed the risk must be determined qualitatively
or quantitatively. There are many investigations for the quantitative determination
of risk, particularly with respect to failure mode analysis and their effects.
In addition, the qualitative estimation is quite usual, especially if the statistical
basis is insufficient. Here experts and risk analysts estimate the risk by experience.
While monitoring is part of the Risk Assessment (risk mitigation) in many countries,
it represents a part of the risk analysis in Germany and can be used likewise
for risk mitigation. In Germany the monitoring of dams is legally regulated, so
it can be regarded as an important constituent of dam safety.
After the risk was determined, it must be evaluated whether it is acceptable in
Risk Assessment. The risk acceptance depends on the hazard potential for humans,
economics and environment. Here many different aspects take influence, whereby
the most important point of discussion for German dams is acceptance with respect
to human lives. Engineers alone should not come to the decision. Rather an interdisciplinary
co-operation between engineers, sociologist, economist etc. is necessary.
If the risk is not acceptable, risk mitigation measures must be met. The risk
can be reduced by modification of probabilities or consequences. Possible measures
are e.g. extended monitoring, structural or operational changes, emergency planning.
The residual risk is evaluated with respect to the acceptance of risk and risk
mitigation measures. Since cost as well as human demands need to be considered
the risk evaluation should not only comprise cost-benefit analysis but also take
non economic values into account.
Figure 5: Risk Assessment procedure in Germany
The acceptance of risk as well as acceptance limits are not yet discussed in public,
documented or socially defined neither for the loss of assets nor for human lives,
in Germany. RIßLER (2000) provides a fair comparison of individual risk with risk
of dam failure. This approach can influence the acceptance of dam failure but
needs to be evaluated in more detail in the future.
For a first approach a F-N diagram was developed which can be used for German
dams (figure 6). This diagram does not consider the individual risk. The main
objective of this diagram was to provide a measure of acceptance for a validation
of the procedure described in chapter 3.2 (RETTEMEIER, et al. 2000). The diagram
is flexible and should be adapted for each dam.
Figure 6: F-N diagram for Germany
First of all it can be noticed that a Risk is only acceptable if no
one dies. Taking into account the insurance measure for catastrophe vs. misfortune,
a failure with less than 20 dead persons is acceptable within general limits.
All other Risks are unacceptable. This does not take into account the probability
of failure (vertical limits). Only within the area of tolerable but not acceptable
Risk the probability of failure is considered. In this case emergency planing
is essential and should be used for risk mitigation. The range of the probability
of failure is limited to 10-3 because of the design flood (required capacity of
the spillway), according to DIN 19700 (1986).
The limit of tolerability curve is subject to change for individual dams. The
range of the number of fatalities depends on the number of endangered population
and should be increased if necessary. The trend of the "limit of tolerability"
curve can only vary in the value where the slope changes. This value depends for
example on the number of people that can be evacuated in a limited time. The horizontal
line is fixed to 10-6 according to IDEL (1986) for the - totally unlikely - risk.
The fundamental difference of the German Risk Assessment procedure compared
to other procedures results from the approach to the question of dam safety under
risk criteria. In many countries, where the Risk Assessment is applied, dams are
classified according to their risk. Therefore Risk Analysis is an established
factor for the design of dams. However dams are classified according to their
type in Germany. So the risk of a German dam is unknown so far.
High request on the structural design led to the assumption that German dams have
a very small risk of failure. It seems to be reasonable that dams in Germany according
to IDEL (1988) and RIßLER (1998) have a probability of failure of 10-5 at the
most. Especially for the load case flood the technical demands require a design
"flood" for the spillway of a 1.000 year recovery period. Additionally
a free-board is demanded that is large enough to ensure that a dam will not be
spilled in case of PMF (DIN 19700, 1986).
Presumably a detailed Risk Analysis will not result in a wide change in the probability
of failure for German dams. Still the risk cannot be neglected. The density of
population and the industry downstream of dams result in a high potential of damage.
The presented procedure takes the German standards in dam design into account.
It allows to prove the very high level of safety with respect to probability of
failure (Risk Analysis). At the same time the emphasis is not drawn towards Risk
Analysis with respect to the quantification the probability of failure. Instead
failure modes are identified and their effects are determined. This information
is essential for Risk Assessment.
While monitoring and surveillance is part of Risk Assessment in many countries,
the German approach takes it into account in Risk Analysis. Germany has very high
legal and technical standards in surveillance which directly contribute to the
low probability of failure.
The draft of the DIN 19700 (1999) demands the evaluation of risk with respect
to organizational and/or technical mitigation measures. The Risk Assessment is
well suited to fulfill these requirements. The presented procedure assures the
mitigation of risk (Risk Assessment) taking the German question into account.
Since failure modes and effects have been identified within Risk Analysis, the
Risk Assessment can focus on optimal mitigation measures. Thus the risk mitigation
in the German Risk Assessment procedure comprises new failure mode and effects
analysis which do not result in a new Risk Analysis.
Since the individual aspects (Risk Analysis, Risk Assessment, Risk Management)
are regarded separately, a Risk Assessment can be executed without a detailed
Risk Analysis e.g. if only the worst case complete failure is considered. Naturally
different failure modes and inundation studies must be regarded, in particular
for the risk mitigation, thus fundamental elements of the Risk Analysis have to
be considered within a Risk Assessment.
The aim of this procedure, in particular the separation of the Risk Analysis and
the Risk Assessment should be to encourage the forthcoming of risk based dam safety
in Germany. Especially for existing dams a complete, detailed and quantitative
Risk Analysis is very difficult if not impossible. Thus in engineering practice
an approach which does not emphasis on the probability of failure determination
but focus on the damage with respect to failure modes and effects can provide
a very good approach and lead to a reduction in risk. Consequently the demands
of the DIN 19700 can be met on the level of engineering judgement.
It is our responsibility to minimize the hazard of technical buildings.
Germany is constantly improving its design standards to meet public demands on
dam safety. Still risk based approaches have not been considered so far..
The world-wide discussion on Risk Assessment as a tool for dam safety leads to
new developments in Germany. The Institute for Hydraulic Engineering and Water
Resources Management, Aachen University of Technology developed a Risk Assessment
procedure, which is applicable to German dams. Hereby the new requests which result
from the draft of the DIN 19700 (1999) can be met
A special feature of this Risk Assessment procedure is the context of Risk Analysis
and Risk Assessment. The process of Risk Assessment contains Risk Analysis and
Risk Assessment, which build on one another, but are final in themselves. So they
can be treated separately from each other. The estimated risk, e.g. of the largest
damage, can be considered for the Risk Assessment in the line with the demands
of DIN 19700 (1999).
With Risk Assessment a tool is available, which allows a safety evaluation of
dams with respect to risk on a practicably and economically meaningful basis.
A implementation for German dams is technically possible. For the future work
detailed inundation studies as basis of the damage assessment have to be carried
out. The consideration of failure modes with appropriate load case combinations
is an important issue that has to be considered for the forthcoming of Risk Assessment
in Germany as well.
DIN 19700 (1986): Deutsch Norm Stauanlagen, Teil 10 bis Teil 15.
DIN 19700 (1999): Deutsch Norm Stauanlagen, Entwurf Teil 11 Talsperren.
FALKENHAGEN, B. (1999): Risk Assessment - Untersuchungen zur Anwendung auf die
Rurtalsperre. Master Thesis. Institute of Hydraulic Engineering and Water Resources
Management, University of Technology Aachen. Unpublished.
ICOLD (INTERNATIONAL COMMITTEE ON LARGE DAMS) (1998): ICOLD Guidelines on Risk
Assessment for Dams (Attachment by: Williams, A. (ICOLD International Committee
on Dam Safety, AWT Director) (1998): ICOLD Chair man's 1997/98 Progress Report
for New Delhi Meeting, No vember, 1998 (Informationletter)), pp. 1-28.
IDEL, K.H. (1986): Sicherheitsuntersuchungen auf probal istischer Grundlage für
Staudämme. Final Report, Anwendungsband. Untersuchungen für einen Refernzstaudamm.
/ ed. by Deutsche Gesellschaft für Erd-und Grundbau im Auftrag des Bundesministers
für Forschung und Technologie. Essen.
RETTEMEIER, K. & KÖNGETER J. (1998): Dam Safety Management: Overview of the
State of the Art in Germany compared to other European Countries, Proceedings
of the International Symposium on New Trends and Guidlines on Dam Safety, Barcelona,
17-19 June 1998. / ed. by L. Berga, Vol. 1. Rotterdam u.a.: Balkema, pp. 55-62.
ISBN 90-5410-974-2.
RETTEMEIER, K.; FALKENHAGEN, B.; KÖNGETER, J. (2000): Q. 76 - R. 41: Risk Assessment
- New Trends in Germany. In: Twentieth Congress on Large Dams = Vingtième congrès
des Grands Barrages, 19-22 September 2000, Beijing, China. Transactions = Compes
rendus, Vol. I: Question No. 76. Paris: Commission International des Grands Barrages
(ICOLD), pp. 625-641. - ISSN 0254-0703.
RIßLER, P. (1998): Risk Assessment für Talsperren - Internationale Entwicklungen.
In: Wasserwirtschaftliche Systeme - Konzepte, Konflikte, Kompromisse: 28. IWASA;
Aachen 1998 / Internationales Wasserbau-Symposium. Aachen: (Technische Hochschule
Aachen / Lehrstuhl und Institut für Wasserbau und Wasserwirtschaft: Mitteilungen;
115), pp. 310-337. ISBN 3-89653-615-x.
RIßLER, P. (2000): Die Hochwasserbemessung als Teil eines in sich schlüssigen
Sicherheitskonzeptes für Talsperren / KA: Wasserwirtschaft, Abwasser, Abfall,
Jg. 47, H. 11, pp. 1689-1699. ISSN 0341-1540.
RUHRVERBAND (1988): Die Möhnekatastrophe. Essen: (reprint from: 75 Jahre im Dienst
für die Ruhr: 1913 - 1988 / Ruhrverband. Essen, 1988).
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