hand, resilience but also vulnerability have been defined alongside reliability as performance indicators for water resources systems [43] and further, a method computing all three concepts using FORM also exists [54]. Thus, bringing viability and reliability methods together may improve the definition and computation of other related concepts such as resilience and vulnerability.
7. Conclusion
Stochastic viability and reliability have the same broad goal of computing the probability for a system to not violate its con- straints. This work used stochastic viability to propose new concepts and methods in time-variant reliability. Conceptually, similarities between stochastic viability and reliability led to the definition of the reliability kernel to deal with designproblems: this is an analogof the stochasticviability kernel. Viability being a branch of control theory, the notion of reliability kernel has been extended to cases where both the design and maintenance of the system have to be considered together. This kernel then regroups the possible designs such that the system can be maintained in the survival set with a high probability and for a sufficient amount of time. Besides, its definition relies on a formulation that is independent from the assumption of a mono- tonic decrease in performance over time, even though the applica- tion shows that the framework is well-suited to tackle these cases as well. As for the method, dynamic programming is applicable to time- variant reliability in the specific cases in which a design and maintenance problem is also a stochastic viability problem, namely when the uncertainty and stochasticity of the system can be summarized at discrete dates by a vector called the state vector, and when the search of the design is related to that of the initial state. If the uncertainty can be expressed by independent random variables, then dynamic programming yields both the reliable designs and the adequate maintenance strategy for these designs.
Acknowledgements
This work has been supported by a Région Auvergne's scholar- ship. The authors also want to thank an anonymous reviewer, whose constructive remarks on earlier versions of this manuscript considerably improved its content.