consideration of the predicted dam failure hydrograph embracing a
sequence of impact assessments embracing likely loss of life and third-
party damage. Overall probability of failure and consequence are then
considered in combination to determine the ‘tolerability of risk’, with the
latter assessed in terms of the predicted likely annual loss of life, con-
sequence, and probability of failure. The outcome is then subject to com-
parative review to determine whether a more detailed and exhaustive risk
assessment is to be recommended.
Viewed in the context of dam surveillance QRA methodologies have
been criticized on a number of grounds, notably their dependence upon
the estimated probabilities of failure associated with specific threats to the
integrity of a dam of particular type. A feature of quantitative methodolo-
gies is therefore their dependence upon access to historical data identify-
ing incidents, where the integrity of a dam has been compromised, and
lesser occurrences have threatened fitness for purpose. Historical data
must, by it’s nature, be less than complete and may be of questionable reli-
ability and accuracy. National data will, in addition, reflect the national
perspective as regards the composition and nature of the dam population
and the prevailing legislative provision for dam safety. In the interests of
developing a verifiable UK database of incidents and lesser events imping-
ing upon fitness for purpose it has been suggested, for the UK, that a
national incident and occurrence reporting system should be introduced
(Moffat, 2001).
The complexity of QRA and the superficial precision implicit in the
output probabilities have also been the subject of criticism. More
fundamentally, in the specific context of dam risk assessment and manage-
ment, where each dam within a population is quite unique, reservations
have been expressed with regard to the operational utility of the outcome,
whether quantitative (Brown and Gosden, 2004) or semi-quantitative
(Hugheset al., 2000). The role of risk assessment in safety management is
discussed in Charles, Tedd and Skinner (1998).
7.5.2 Dam-break analysis, flood wave propagation and inundation mapping
The extension of hazard analysis to include dam-break contingency plan-
ning is a sensitive issue. It is, however, a logical and prudent step when
applied selectively to reservoirs commanding a populous and vulnerable
floodplain. Rigorous dam-break analysis involves a balanced consideration
of hydrological, hydraulic, environmental, and geotechnical and structural
parameters applicable to the dam and floodplain being studied. An empiri-
cal first approximation for dam-breach formation characteristics for
embankments has been suggested in MacDonald and Langridge-Monopolis
(1984); an alternative initial approximation is presented in Dewey
312 DAM SAFETY: INSTRUMENTATION AND SURVEILLANCE
