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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 />Vi.4. Flood Probabilities. <br /> <br />The probability of a large flood occurring <br />should be expressed as an "Annual <br />Exceedance Probability (.4EP)" which is the <br />chance of a flood of that size, or greater, <br />occurring "in any year". <br /> <br />The use of "years" to indicate dam flood <br />probabilities for large and extreme floods <br />related to dam hydrologic safety is strongly <br />discouraged. It can give the community, and <br />non-technical ,managers, the misleading <br />impression that, for example, a "1000 year <br />flood" will only occur every 1000 years, rather <br />than a risk of I: 1000 of occurring, or being <br />exceeded, in any year (ARR87), <br /> <br />Further, for management and community <br />infonnation, it is more meaningful to express <br />flood risk relative to the assumed life of a dam. <br />An AFC assigned a I in 10000 AEP has <br />approximately a I in 100, or I % chance of <br />occurring over an assumed 100 year life of a <br />dam (refer Appendix 2, sub-section A2.1.2.), <br /> <br />Note also, it is not possible to estimate with <br />certainty absolute probabilities (or even <br />relative probabilities) for extreme flood or <br />other natural events. <br /> <br />2. CHANGES from 1986 GUIDELINES <br />on DESIGN FLOODS for DAMS. <br />(References to the relevant sections. of <br />these AFC Guidelines are shown in italics <br />in brackets) <br /> <br />The changes include: <br />. integration of a risk based approach to <br />assist in considering options for dam <br />flood capacity (Chap 6); as against the <br />simple prescriptive Recommended <br />Design Flood (RDF), Chapter 5, Table <br />5.1,1986; <br />. the new tenn Dam Crest Flood (DCF) <br />introduced as a more consistent <br />representative tenn in the review <br />process (sub-section 4.3), than the <br />1986 Imminent Failure Flood (IFF); <br />. scoping risk assessment (sub-section <br />6.1). to replace the 1986 Chapter 6 <br /> <br />arbitrary ranking ratio, IFFIRDF quick <br />check for inadequacy and <br />prioritisation; <br />. the new tenn, Spillway Design Flood <br />(SDF), for the hydraulic design phase <br />of the spillway structure under <br />operational conditions, (sub-section <br />4.4); , <br />. consideration of appropriate non-' <br />structural alternatives, including dam <br />break warning and evacuation plans, in <br />assessing risk reduction options, (sub- <br />section 7.3),- <br />. adoption of the expanded quantitative <br />basis for Incremental Flood Hazard <br />Categories (IFHC) in the ANCOLD <br />Guidelines on the Assessment of the <br />Consequences of Dam Failure, <br />superseding the simple 1986 <br />qualitative basis, Table 4,1; <br />. consideration of an alternative fallback <br />option in a phased study, or in cases <br />where a full risk study is not <br />considered cost effective, (Chap 8); <br />. sections added on Flood Management, <br />including gate operation impacts on <br />safety, and on Spillway Provisions for <br />Mining Dams, (Chap 9); <br /> <br />The related ANCOLD Guidelines are listed in <br />Appendix 2, sub-section, A2.2. <br /> <br />These AFC Guidelines do not repeat detailed <br />procedures from the related Guidelines. <br /> <br />3. FLOOD ESTIMATION. <br /> <br />3.1. Estimation Procedures. <br /> <br />The estimation of extreme floods and <br />associated assigned probabilities should be <br />based on the procedures in Book VI, <br />Estimation of Large to Extreme Floods <br />(Nathan & Weinmann, 1999) in Volume 1, <br />1999 (revised ARR87 Chapter 13) and the <br />current Australian Rainfall & Runoff, A <br />Guide to Flood Estimation (ARR99), for <br />other procedures. <br /> <br />ANCOLD Guidelines on Selection of an Acceptable flood Capacity for Dams 5 <br />