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In risk assessment of spatially distributed infrastructure, the probability of demand exceeding capacity is evaluated across the system. We describe and compare two levee system reliability analysis frameworks for seismic and high-water demands. The first approach considers spatial correlations and distributions of demand and capacity between “segments” (i.e., elemental levee lengths) through Monte Carlo simulation. We apply a capacity correlation model derived from seismic case histories in Japan. The seismic demand correlation model is based on global ground motion data, whereas the high-water correlation is taken as unity. The second approach examines the distribution and correlation of capacities and demands between physics-based “reaches” (i.e., length of levee having uniform statistical distributions of capacity and demand). Statistics and spatial correlation of the limit state function are computed using a first-order reliability method procedure. The probability of failure of the reach is then computed using level-crossing statistics. For implementation of level-crossing statistics, we replace Markov-type correlation functions for levee capacity with a Gaussian function. We illustrate both methods for a levee system subjected to realistic demand and capacity distributions and show that characteristic lengths (defined as lengths of levee that can be considered as statistically independent) are comparable for high-water and seismic demands. This outcome is specific to the considered failure mechanisms and is driven by use of similar capacity correlation models, whereas differences in demand correlation models have limited impact.

We synthesize engineering procedures for estimating the seismic performance of major flood-control levees as given in guidelines documents and design codes from Canada (British Columbia), China, Italy, Japan, New Zealand, and the United States. Some guidelines carry the weight of law whereas compliance is optional for others. Most procedures combine a probabilistic ground motion characterization with deterministic assessments of levee performance (uncoupled approach). Ground motions are typically described using peak accelerations for reference site conditions at return periods typically ranging from 100 to 2,475 years. Those motions are deterministically modified for soil conditions using numerical simulations or ergodic site factors. Accompanying hydrological conditions are either not specified or are taken at a frequently encountered water level (ranging from mean annual to a four-month flood event). These demands are used in combination with various soil properties to assess the potential for liquefaction, flow failure, and permanent shear deformations. Drawing upon best practices identified from this review, we recommend procedures for levee risk assessment at the section-level and for levee systems.

The Dutch government is in the process of revising its flood safety policy. The current safety standards for flood defenses in the Netherlands are largely based on the outcomes of cost-benefit analyses. Loss of life has not been considered separately in the choice for current standards. This article presents the results of a research project that evaluated the potential roles of two risk metrics, individual and societal risk, to support decision making about new flood safety standards. These risk metrics are already used in the Dutch major hazards policy for the evaluation of risks to the public. Individual risk concerns the annual probability of death of a person. Societal risk concerns the probability of an event with many fatalities. Technical aspects of the use of individual and societal risk metrics in flood risk assessments as well as policy implications are discussed. Preliminary estimates of nationwide levels of societal risk are presented. Societal risk levels appear relatively high in the southwestern part of the country where densely populated dike rings are threatened by a combination of river and coastal floods. It was found that cumulation, the simultaneous flooding of multiple dike rings during a single flood event, has significant impact on the national level of societal risk. Options for the application of the individual and societal risk in the new flood safety policy are presented and discussed. [ABSTRACT FROM AUTHOR]

Decisions about disaster preparedness are rarely informed by cost-benefit analyses. This paper presents an economic model to address the thorny question, 'how prepared is prepared enough?' Difficulties related to the use of cost-benefit analysis in the field of disaster management concern the tension between the large number of high-probability events that can be handled by a single emergency response unit and the small number of low-probability events that must be handled by a large number of them. A further special feature of disaster management concerns the opportunity for cooperation between different emergency response units. To account for these issues, we introduce a portfolio approach. Our analysis shows that it would be useful to define disaster preparedness not in terms of capacities, but in terms of the frequency with which response capacity is expected to fall short. [ABSTRACT FROM AUTHOR]

The Dutch government is in the process of revising its flood safety policy. The current safety standards for flood defenses in the Netherlands are largely based on the outcomes of cost-benefit analyses. Loss of life has not been considered separately in the choice for current standards. This article presents the results of a research project that evaluated the potential roles of two risk metrics, individual and societal risk, to support decision making about new flood safety standards. These risk metrics are already used in the Dutch major hazards policy for the evaluation of risks to the public. Individual risk concerns the annual probability of death of a person. Societal risk concerns the probability of an event with many fatalities. Technical aspects of the use of individual and societal risk metrics in flood risk assessments as well as policy implications are discussed. Preliminary estimates of nationwide levels of societal risk are presented. Societal risk levels appear relatively high in the southwestern part of the country where densely populated dike rings are threatened by a combination of river and coastal floods. It was found that cumulation, the simultaneous flooding of multiple dike rings during a single flood event, has significant impact on the national level of societal risk. Options for the application of the individual and societal risk in the new flood safety policy are presented and discussed., (© 2010 Society for Risk Analysis.)