Hydrologic Issues for Dams - Association of State Dam Safety Officials

Decision BasisThe third feature of the decision process that drives dam owner’s research needs is thedecision basis which includes the parameters required to make the decision. The risk tothe maximally exposed individual is one such parameter. People who are exposed torisks are becoming increasingly interested in the nature and degree of the risk and oftenwish to be in the position to make choices concerning their exposure. To address thisimportant public concern and for the purpose of simplicity at the moment, it can beassumed that the maximally exposed individual (MEI) has no means of escape andspends his/her entire life immediately downstream of the dam. Hence, it can be assumedthat the risk to the MEI is equal to the probability of dam failure. Again thissimplification is conceptually sound for the purposes of this proposal because itoverestimates the risk to any individual downstream of the dam. Further, a more realisticcharacterization of the risk to individuals downstream of dams is now available (Assafand Hartford 2001), and this more advanced approach can be used if required.Other parameters include the various measures of societal risk, including the risk to thepopulation downstream of the dam, property damage, environmental degradation, andbroader socio-economic activity impacts.To be meaningful for decision-making, it is important that the measure of risk to theindividual include a full characterization of the uncertainty in the estimate of risk. Themeasure of uncertainty can be presented as a mean value with an associated probabilitydistribution, or as a single value obtained by integrating over the probability distribution.This measure of uncertainty in the risk estimate (Figure 3) is something that is usually notfound in contemporary applications of risk assessment, although some might argue that itis done in some cases where Monte Carlo simulation is used. The need to fullycharacterize the uncertainty on the risk estimate and the associated need to be able todemonstrate that the estimate of risk and associated uncertainties have been made in ascientifically valid and legally defensible way provide a basis for determining theresearch needs of dam owners.Associated UncertaintyMean ValueFigure 3 Full Characterization of the RiskThe risk estimate and associated uncertainty can then be incorporated in a decisionframework such as that of the Health and Safety Executive approach to judging thetolerability of risk (HSE, ibid.) is of the form illustrated in Figure 4.91Paper 14 - Hartford