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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />specific to hydrologic safety have been <br />highlighted. <br /> <br />Appendix 2, Commentary, provides <br />background material, and discusswn on <br />matters in the guidelines that are subject to <br />development, review and differing opinions. <br /> <br />Some generic risk procedures and examples <br />are included in the text and in Appendix 3, <br />Risk Supplement, for guidance, with <br />reference to the current ANCOLD risk <br />guidelines on procedures and terminology. <br /> <br />1.3. Risk-Based Approach & Dam <br />Hydrologic Safety. <br /> <br />1.3.1. Risk Assessment Process. <br /> <br />The risk assessment process provides a <br />comprehensive input to assist in the overall <br />decisions, and should not be used as the sole <br />decision maker. <br /> <br />Risk assessment is seen as an enhancement to <br />the traditional approach rather than as an <br />alternative or the sole basis for decision <br />making, noting risk procedures are still <br />evolving. <br /> <br />For assessment of eXlstmg dams the risk <br />based approach provides a comprehensive <br />basis to: <br />. identify the critical failure conditions <br />over the whole range ofIoads; <br />. help prioritise detailed reviews of <br />existing dams; <br />. consider risk reduction options and <br />costs, <br /> <br />For proposed dams a risk approach can be <br />used to consider effective arrangements. <br /> <br />The information from a risk study should be <br />considered by the decisionmaker against the <br />owner business risk criteria, including <br />financial and legal liabilities, provision of <br />services and safety responsibilities to the <br />community and environment, <br /> <br />The probabilities estimated in a risk study are <br />only an assessment of the degree of <br />confidence in an outcome, given the evidence <br /> <br />and knowledge available, within 1m: <br />uncertainties in data, methodologies and <br />applicability of probability theory to reality, <br />Risk probabilities should not be taken as the <br />actual likelihood of an event. <br /> <br />1.3.2. Total Load Basis. <br /> <br />A risk study that encompasses all loads is <br />the preferred approach for identifying the <br />critical load failure scenarios, and priorities <br />for detailed reviews and works in regard to <br />dam safety. <br /> <br />Dam hydrologic safety had been given a <br />priority in dam safety reviews because <br />historical data identified inadequate spillway <br />capacity as a major cause of dam failure, <br />When the 1986 guideline procedures were <br />applied to existing dams it was found that <br />many had seriously inadequate spillways, <br />particularly with respect to the deterministic <br />PMF standard that was generally adopted <br />where dams place lives at risk. <br /> <br />The advantage of risk assessment for safety <br />and economic effectiveness is that a total load <br />approach is taken to assess the priority dam <br />safety inadequacies and scenarios. This may <br />identify spillway inadequacy is not the critical <br />safety scenario, <br /> <br />The study may also indicate that the critical <br />flood failure scenario is not the extreme flood <br />case, but spillway malfunction, piping or <br />severe scour, at a lesser flood. <br /> <br />1.3.3. Preliminary Risk Study. <br /> <br />A preliminary overall risk study is <br />recommended to identify the potential critical <br />scenarios and the likely ell:tent of further <br />detailed studies. It may indicate a <br />straightforward spillway option without the <br />need for further detailed risk assessment. <br /> <br />It may also indicate that within the total <br />probability offailure, the component for flood <br />loading is not a critical factor in the total risk <br />to life with the possibility of considering a <br />higher flood risk than the PMP design flood <br />or PMF (Appendix 3, sub-section A3.5). <br /> <br />, <br /> <br />~ l\TrnT.D {;uidelines on Selection of an Acceptable flood Capacity for Dams <br />