Page - 1560 - in Book of Full Papers - Symposium Hydro Engineering

3.3 A ROLE FOR PROBABILITY? All of the above can and should be achieved in terms of deterministic analysis as there should be no uncertainty in how the designer intended the dam and its constitutive parts to function. This design intent raises the question as to whether or not there is a role for probabilistic assessment of the significance of operational and non-operational (failure) modes, noting that the definition of a failure mode as a sequence of events from initiation to complete failure is unrealistic as explained above. Given the deterministic nature of the process of setting out the operational functionalities of a dam system and for determining the failure cause – failure mode – failure effect relationships in terms of the intent of the design, any identifiable deviations from the design intent are also determined deterministically; specifically all not intended states are those that do not conform to the design intent. This means that any use of probability pertains to consideration as to whether or not the failure mode is considered to be “significant” or “critical” to safety. The Fault Tree Handbook [10] provides useful advice as follows: “In FMEA (and its variants) we can identify, with reasonable certainty, those component failures having "non-critical" effects, but the number of possible component failure modes that can realistically be considered is limited. Conservatism dictates that unspecified failure modes and questionable effects be deemed "critical". This observation points to a limited, if any, role if any for screening out failure modes on the basis of probability. 4. CONCLUSIONS On the basis of the above, we conclude that surveillance of the operational modes of a dam in the sense of visual inspections, monitoring, and testing provide the foundation of an effective dam safety management process. These activities can be carried out at various levels of refinement where the most fundamental elements are available to and implementable by all dam owners regardless of their institutional strength and sophistication. We observe that “screening out” of failure sequences on the basis of actual or perceived probability should not be attempted for a complex system such as a dam. This is because the number of possible combinations and interdependencies of functions and functional variances rises exponentially with the size of the system. The number of possible combinations of unfavourable events is correspondingly as large as the probability of any one of them occurring is small. The probability of at least one combination of pernicious events actually happening can be significant. 1560