Dam safety and risk assessment

UNESCO - IHE Delft Dam Safety and Risk Assessment Why the need for dam safety evaluation Dams are, without doubt, among the safest structures constructed by man Dam engineers spare no effort in order to ensure that every dam is conceived, built and maintained according to the best experience, the most exacting criteria and the most advanced knowledge Efforts are, by and large, extremely successful However, no matter how well a dam is built or maintained, the risk of failure cannot be reduced to zero Dam failures are severe threats to life and property that fully justify the need for a better understanding of risks to the public posed by dams Society today, more than a relatively few years ago, demands that safety evaluations be carried out and documented for activities involving risks imposed on the public (as opposed to voluntary risks) Dam Safety and Risk Assessment Definition In the traditional approach risk is the likelihood or probability of adverse consequences In the field of dams: Risk = Probability of dam failure per year x consequences of realized failure For an identified scenario the probability of failure may be defined in terms of probability of load (external stress) times probability of adverse dam response (vulnerability to failure) to that load Risk = [Probability of Load]x[Probability of adverse]x[Consequences] response given load given responce Dam failure is the collapse or movement of part of a dam or part of its foundation, so that the dam cannot retain water Reliability is defined as the complement of risk, i.e the probability of non-failure If we use the variable R for resistance and the variable L for load, then we can define a failure as when the load exceeds the resistance and the consequent probability of failure as the probability of the loading exceeding the resistance, P(L>R) 2009 UNESCO - IHE Delft Dam Safety and Risk Assessment Why the need for dam safety evaluation Dams are, without doubt, among the safest structures constructed by man Dam engineers spare no effort in order to ensure that every dam is conceived, built and maintained according to the best experience, the most exacting criteria and the most advanced knowledge Efforts are, by and large, extremely successful However, no matter how well a dam is built or maintained, the risk of failure cannot be reduced to zero Dam failures are severe threats to life and property that fully justify the need for a better understanding of risks to the public posed by dams Society today, more than a relatively few years ago, demands that safety evaluations be carried out and documented for activities involving risks imposed on the public (as opposed to voluntary risks) Dam Safety and Risk Assessment Definition In the traditional approach risk is the likelihood or probability of adverse consequences In the field of dams: Risk = Probability of dam failure per year x consequences of realized failure For an identified scenario the probability of failure may be defined in terms of probability of load (external stress) times probability of adverse dam response (vulnerability to failure) to that load Risk = [Probability of Load]x[Probability of adverse]x[Consequences] response given load given responce Dam failure is the collapse or movement of part of a dam or part of its foundation, so that the dam cannot retain water Reliability is defined as the complement of risk, i.e the probability of non-failure If we use the variable R for resistance and the variable L for load, then we can define a failure as when the load exceeds the resistance and the consequent probability of failure as the probability of the loading exceeding the resistance, P(L>R) 2009 1 UNESCO - IHE Delft Dam Safety and Risk Assessment Definition Risk analysis process involves the scientific characterisation of what is known and what is uncertain about the present and future performance of the dam Risk management is a strategy applied to the tasks of analyzing, evaluating, controlling, and administration of risks witch threaten the well-being of life and natural and socio-economic environment The main purpose of carrying out a risk analysis is to provide decision support For many applications, the resulting numerical values not have to be accurate in absolute terms, but must be inherently consistent so they allow reliable relative comparisons among alter-natives The main issue stands with the existing dams More extensive floods records, and possibly changes in climatic conditions since the construction of a dam, necessitate updating of flood estimates and re-evaluation of dam safety Significant changes in physical conditions both upstream and downstream since the dam was built require a review of the risks involved Dam Safety and Risk Assessment Limits of Factor of Safety approach The assessment of safety in civil engineering works is traditionally obtained through a deterministic approach In order to take account of the many uncertainties and of the scatter in the data, and also to cover the fact that models are necessarily approximate, a "factor of safety" is introduced If L and R are well-established values as considered by the deterministic approach, a safety factor FS = R/L may be defined When FS > it will be no failure or damage and as a consequence there appears the idea of “complete safety " The external loadings (L) as well as the strength capacity of the dam-foundation system (R) are aleatory variables due to: reservoir level variation, aleatory character of seismic loadings, material parameter variation, foundation different mechanical characteristics, etc Though on the mean L0 it will be no failure or damage and as a consequence there appears the idea of “complete safety " The external loadings (L) as well as the strength capacity of the dam-foundation system (R) are aleatory variables due to: reservoir level variation, aleatory character of seismic loadings, material parameter variation, foundation different mechanical characteristics, etc Though on the mean L0 R, represents a measure of the probability of failure Pf If the distributions of L and R have large variances the failure probabilities are large and the safety factors lose its physical significance Safety analysis of a homogeneous rockfill dam: a- failure mechanism; b- probabilities FS Flattening the downstream slope FS increases but probability of failure has a lower bound – no complete safety exists Dam Safety and Risk Assessment Limits of Factor of Safety approach Since an absolute safety cannot be ensured technically, dam engineering field has to move from the conventional safety-oriented perspective towards the risk-oriented perspective The risk-oriented view point takes a risk of failure into account Thus the residual risk has to be determined, evaluated and managed even if failure seems unlikely Dams safe 2009 UNESCO - IHE Delft Dam Safety and Risk Assessment Limits of Factor of Safety approach The surface of the intersection domains, where L > R, represents a measure of the probability of failure Pf If the distributions of L and R have large variances the failure probabilities are large and the safety factors lose its physical significance Safety analysis of a homogeneous rockfill dam: a- failure mechanism; b- probabilities FS Flattening the downstream slope FS increases but probability of failure has a lower bound – no complete safety exists Dam Safety and Risk Assessment Limits of Factor of Safety approach Since an absolute safety cannot be ensured technically, dam engineering field has to move from the conventional safety-oriented perspective towards the risk-oriented perspective The risk-oriented view point takes a risk of failure into account Thus the residual risk has to be determined, evaluated and managed even if failure seems unlikely Dams safe 2009 UNESCO - IHE Delft Dam Safety and Risk Assessment Lessons from past failures St Francis dam St Francis Dam was a 60 m high concrete gravity-arch dam constructed by the City of Los Angeles between 1924 and 1926 Downstream steps 59 m River bed elevation Uplift relief wells 42 m Dam Safety and Risk Assessment Lessons from past failures St Francis dam Main cause of failure was the large deformation of the geologic formation in the left abutment and piping of its material along the fault A massive landslide of the dam’s eastern abutment initiated the failure The dam failed catastrophically on March 12-13, 1928, killing at least 420 people Just downstream of the dam the maximum depth of the flood was about 42 m The average velocity in this reach was about 28 km/hour 2009 UNESCO - IHE Delft Dam Safety and Risk Assessment Lessons from past failures St Francis dam St Francis Dam was a 60 m high concrete gravity-arch dam constructed by the City of Los Angeles between 1924 and 1926 Downstream steps 59 m River bed elevation Uplift relief wells 42 m Dam Safety and Risk Assessment Lessons from past failures St Francis dam Main cause of failure was the large deformation of the geologic formation in the left abutment and piping of its material along the fault A massive landslide of the dam’s eastern abutment initiated the failure The dam failed catastrophically on March 12-13, 1928, killing at least 420 people Just downstream of the dam the maximum depth of the flood was about 42 m The average velocity in this reach was about 28 km/hour 2009 UNESCO - IHE Delft Dam Safety and Risk Assessment Lessons from past failures Malpasset dam Malpasset dam was an arch dam with a height of 66 m and with 220 m long crest at its crown The dam was commissioned in 1954 and it was the thinnest arch dam of its size in the world when it was completed Dam Safety and Risk Assessment Lessons from past failures Malpasset dam The failure occurred as the arch ruptured when the left abutment gave away The dam body was left without support on the left abutment and failed in tension PLAN VIEW Removed rock block 2009 UNESCO - IHE Delft Dam Safety and Risk Assessment Lessons from past failures Malpasset dam Malpasset dam was an arch dam with a height of 66 m and with 220 m long crest at its crown The dam was commissioned in 1954 and it was the thinnest arch dam of its size in the world when it was completed Dam Safety and Risk Assessment Lessons from past failures Malpasset dam The failure occurred as the arch ruptured when the left abutment gave away The dam body was left without support on the left abutment and failed in tension PLAN VIEW Removed rock block 2009 UNESCO - IHE Delft Dam Safety and Risk Assessment Consequence assessment Flood routing Flood routing models the downstream propagation of the flood wave as it moves downstream of the dam The objective of these models is an accurate prediction of flow discharge, velocity and depth as a function of time and distance downstream after the formation of a breach Numerical solution to the St Venant equations provides the most accurate predictions Outflow m3/s At dam site In addition to uncertainties in the dam breach modelling, uncertainties also apply to flood routing It is conceptually straightforward to add parametric and model uncertainty to existing modelling approaches by probabilistic modelling of dam breach and downstream flood routing Near downstream Further downstream Far downstream Time Dam Safety and Risk Assessment Consequence assessment Flood routing Flood map for consequences assessment 19 2009 19 UNESCO - IHE Delft Dam Safety and Risk Assessment 20 Consequence assessment Evaluation of consequences Loss of life Characterising the number of potential fatalities due to a dam failure is complicated by the large number of factors that influence the death rate within a population potentially exposed to inundation The most important factors are: (1)the number of people occupying the dam failure flood plain; (2)the amount of warning provided to the people exposed; and (3)the severity of flooding Economic consequences The principal categories are direct consequences and indirect consequences Direct consequences are the immediate impacts of the failure or being in contact with floodwaters (loss of the capital investment in the dam and appurtenant structures; destruction of downstream property: buildings, equipment, land improvements; and loss of seasonal crops) Environmental consequences Including environmental consequences within risk analyses for dam safety raises a number of challenges: how comprehensively to account for environmental consequences since many of which are subtle; how to economically value those impacts in the absence of markets? Dam Safety and Risk Assessment RISK MANAGEMENT Risk management process SAFETY ANALYSIS YES 2009 20 UNESCO - IHE Delft Dam Safety and Risk Assessment Consequence assessment Evaluation of consequences Loss of life Characterising the number of potential fatalities due to a dam failure is complicated by the large number of factors that influence the death rate within a population potentially exposed to inundation The most important factors are: (1)the number of people occupying the dam failure flood plain; (2)the amount of warning provided to the people exposed; and (3)the severity of flooding Economic consequences The principal categories are direct consequences and indirect consequences Direct consequences are the immediate impacts of the failure or being in contact with floodwaters (loss of the capital investment in the dam and appurtenant structures; destruction of downstream property: buildings, equipment, land improvements; and loss of seasonal crops) Environmental consequences Including environmental consequences within risk analyses for dam safety raises a number of challenges: how comprehensively to account for environmental consequences since many of which are subtle; how to economically value those impacts in the absence of markets? Dam Safety and Risk Assessment RISK MANAGEMENT Risk management process SAFETY ANALYSIS YES 20 2009 20 UNESCO - IHE Delft 21 Dam Safety and Risk Assessment RISK MANAGEMENT Effective risk management Conduct continual surveillance Periodic dam safety reviews and Management system audit Every to 10 years Does the dam meet all safety requirements No Dam safety assessment Traditional good practice Risk assessment Yes Yes Operate normally and Maintain emergency preparedness Is the dam safe enough? No Improve dam safety or Reduce consequences Dam Safety and Risk Assessment RISK MANAGEMENT Tolerability and acceptance of risk The most complex, controversial and sensitive issue that matters to society is the relation between classical engineering safety as guided by experience and codes on the one hand and the philosophies about acceptable risk levels Calculation of the probability of failure and the consequences of failure lead inevitably to the question: "what is acceptable or what is tolerable?" Differences between public and engineering perception of risk The individual risk, tolerable societal risk, economical optimisation and financial capacity of the dam owner all have to be combined in assessing the, acceptable level of safety of a dam 2009 21 UNESCO - IHE Delft Dam Safety and Risk Assessment RISK MANAGEMENT Effective risk management Conduct continual surveillance Periodic dam safety reviews and Management system audit Every to 10 years Does the dam meet all safety requirements No Dam safety assessment Traditional good practice Risk assessment Yes Yes Operate normally and Maintain emergency preparedness Is the dam safe enough? No Improve dam safety or Reduce consequences Dam Safety and Risk Assessment RISK MANAGEMENT Tolerability and acceptance of risk The most complex, controversial and sensitive issue that matters to society is the relation between classical engineering safety as guided by experience and codes on the one hand and the philosophies about acceptable risk levels Calculation of the probability of failure and the consequences of failure lead inevitably to the question: "what is acceptable or what is tolerable?" Differences between public and engineering perception of risk The individual risk, tolerable societal risk, economical optimisation and financial capacity of the dam owner all have to be combined in assessing the, acceptable level of safety of a dam 21 2009 21 UNESCO - IHE Delft Dam Safety and Risk Assessment 22 RISK MANAGEMENT Tolerability and acceptance of risk Based on statistics of fatalities from human activities, the concept of socially acceptable risk (SAR) was developed (a means to assign value to life without resorting to monetary units) SAR is generally depicted in so-called F/N diagrams, with F generally the cumulative failure probabilities of scenarios with loss of life and N the number of lives lost in these scenarios Dam Safety and Risk Assessment RISK MANAGEMENT Tolerability and acceptance of risk It should be noted that the criterion of limiting the expected value of damages to $10,000 per year is not an economic criteria; instead it is related to an insurance concept Risk for selected engineering projects The standards of safety should be changed as societal values and expectations (culture) change, or if the society wants more safety at higher costs or less expenditure with correspondingly higher risks 2009 22 UNESCO - IHE Delft Dam Safety and Risk Assessment RISK MANAGEMENT Tolerability and acceptance of risk Based on statistics of fatalities from human activities, the concept of socially acceptable risk (SAR) was developed (a means to assign value to life without resorting to monetary units) SAR is generally depicted in so-called F/N diagrams, with F generally the cumulative failure probabilities of scenarios with loss of life and N the number of lives lost in these scenarios Dam Safety and Risk Assessment RISK MANAGEMENT Tolerability and acceptance of risk It should be noted that the criterion of limiting the expected value of damages to $10,000 per year is not an economic criteria; instead it is related to an insurance concept Risk for selected engineering projects The standards of safety should be changed as societal values and expectations (culture) change, or if the society wants more safety at higher costs or less expenditure with correspondingly higher risks 22 2009 22 UNESCO - IHE Delft Dam Safety and Risk Assessment 23 RISK MANAGEMENT Risk reduction The reducing of risk associated to a certain dam is made both by increasing of its safety and by reducing the potential consequences in case of its failure Dam safety includes structural safety - that may be achieved through appropriate design - and monitoring of the dam behaviour under operation (acting on the first factor of the risk, namely the probability of failure) Risk reduction also includes the emergency preparedness that aims to mitigate the consequences if the dam failure happens Dam Safety and Risk Assessment RISK MANAGEMENT Dam surveillance concept 2009 23 UNESCO - IHE Delft Dam Safety and Risk Assessment RISK MANAGEMENT Risk reduction The reducing of risk associated to a certain dam is made both by increasing of its safety and by reducing the potential consequences in case of its failure Dam safety includes structural safety - that may be achieved through appropriate design - and monitoring of the dam behaviour under operation (acting on the first factor of the risk, namely the probability of failure) Risk reduction also includes the emergency preparedness that aims to mitigate the consequences if the dam failure happens Dam Safety and Risk Assessment RISK MANAGEMENT Dam surveillance concept 23 2009 23 UNESCO - IHE Delft Dam Safety and Risk Assessment 24 RISK MANAGEMENT Dam surveillance Organizational levels of the dam monitoring system Dam Safety and Risk Assessment RISK MANAGEMENT Dam surveillance A complete assessment of the condition and of the behaviour may be achieve by visual checks, measurements on the instrumentation and operating tests of gates and valves as well as of emergency power unit Incidents that can be relived by visual inspection of an earth dam Visual checks are needed because changes in condition and certain behaviours cannot be assessed by measure-ments or only with much delay 2009 24 UNESCO - IHE Delft Dam Safety and Risk Assessment RISK MANAGEMENT Dam surveillance Organizational levels of the dam monitoring system Dam Safety and Risk Assessment RISK MANAGEMENT Dam surveillance A complete assessment of the condition and of the behaviour may be achieve by visual checks, measurements on the instrumentation and operating tests of gates and valves as well as of emergency power unit Incidents that can be relived by visual inspection of an earth dam Visual checks are needed because changes in condition and certain behaviours cannot be assessed by measure-ments or only with much delay 24 2009 24 UNESCO - IHE Delft Dam Safety and Risk Assessment 25 RISK MANAGEMENT Instrumentation Concrete dams ARCH DAM GRAVITY DAM LEGEND Dam Safety and Risk Assessment RISK MANAGEMENT Instrumentation LEGEND: LEGEND: 2009 25 UNESCO - IHE Delft Dam Safety and Risk Assessment RISK MANAGEMENT Instrumentation Concrete dams ARCH DAM GRAVITY DAM LEGEND Dam Safety and Risk Assessment RISK MANAGEMENT Instrumentation LEGEND: LEGEND: 25 2009 25 UNESCO - IHE Delft Dam Safety and Risk Assessment 26 RISK MANAGEMENT Emergency concept/ strategy Dam Safety and Risk Assessment RISK MANAGEMENT Alarm system The emergency strategy specifies that the evacuation of the people in the potentially submerged area must be ordered This is done in most of the cases with sirens within the localities and with mobile alarming teams out-side of the localities The message flows from the owner to the relevant authorities which must act accordingly 2009 26 UNESCO - IHE Delft Dam Safety and Risk Assessment RISK MANAGEMENT Emergency concept/ strategy Dam Safety and Risk Assessment RISK MANAGEMENT Alarm system The emergency strategy specifies that the evacuation of the people in the potentially submerged area must be ordered This is done in most of the cases with sirens within the localities and with mobile alarming teams out-side of the localities The message flows from the owner to the relevant authorities which must act accordingly 26 2009 26 ... markets? Dam Safety and Risk Assessment RISK MANAGEMENT Risk management process SAFETY ANALYSIS YES 20 2009 20 UNESCO - IHE Delft 21 Dam Safety and Risk Assessment RISK MANAGEMENT Effective risk. .. UNESCO - IHE Delft Dam Safety and Risk Assessment 24 RISK MANAGEMENT Dam surveillance Organizational levels of the dam monitoring system Dam Safety and Risk Assessment RISK MANAGEMENT Dam surveillance... Concrete dams ARCH DAM GRAVITY DAM LEGEND Dam Safety and Risk Assessment RISK MANAGEMENT Instrumentation LEGEND: LEGEND: 25 2009 25 UNESCO - IHE Delft Dam Safety and Risk Assessment 26 RISK MANAGEMENT