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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 />In determining the DCF the information <br />required is the Annual Exceedance Probability <br />(AEP) of the defined still water reservoir level <br />at the crest of the dam. In practice it is usual to <br />relate the specified "still WlIter reservoir level" <br />to the corresponding outflow discharge and to <br />determine the AEP of this outflow discharge, <br />based on the envelope outflow flood frequency <br />curve. <br /> <br />The specified reservoir level is related to the <br />outflow discharge estimated from the <br />appropriate spillway rating curve and outlet <br />discharge curve for the specific dam. The <br />detailed procedure is given in Appendix I. <br /> <br />4.3.4. Contexts <br /> <br />The DCF can be used in two contexts: <br />(i) for existing and proposed dams, <br />the initial assessment of spillway <br />adequacy, and potential failure <br />consequences; <br />(ii) for concept development of <br />remedial work for existing dams. <br /> <br />4.3.5 Pre-flood Reservoir level. <br /> <br />For risk assessment, consideration can be <br />given to a joint probabili(v assessment of <br />reservoir level, storms and winds, <br />overtopping, and gate operation, relevant to <br />an extreme storm andflood situation. <br /> <br />(i) When routing the flood through the <br />reservoir to determine the flood surcharge <br />level, the traditional engineering approach has <br />been to assume the reservoir initially at Full <br />Supply Level, although some countries assume <br />a spilling condition (UK, South Africa) or <br />preceding flood (USBR). <br /> <br />(ii) For a risk based approach, refer ARR, <br />Book VI (Nathan & Weinmann, 1999), for <br />procedures for joint probability assessment and <br />the current ANCOLD Guidelines on Risk <br />Assessment (Refer also sub-section 9.2) <br /> <br />{iii} As with all risk assessments, a sanity <br />check is desirable to assess the applicability <br />and sensitivity of the estimate to the <br />assumptions on pre-flood level. <br /> <br />(iv) For gated spillways, assumptions are <br />required on gate operation for routing the <br />flood, and probabilities of gate misoperation or <br />malfunction (Refer sub-section 9,1). <br /> <br />With a deterministic approach it is usual to <br />assume at least one gate, or a proportion if <br />there are a large number of gates, are <br />inoperable during an extreme event. <br /> <br />For a risk approach, consideration should be <br />given to the human factors, which historical <br />evidence indicates are responsible for many <br />gated dam failures. The effectiveness of <br />regular maintenance and wet or dry run <br />checking of gates could give an indication of <br />potential problems under extreme conditions. <br /> <br />4.3.6 Imminent Failure Flood <br />(Refer Appendix 2, sub-section A2.3) <br /> <br />This term has been superseded, as It IS not <br />feasible to assess definitively the imminent <br />failure, or the alternate specific safety level. If <br />it is required to estimate a failure flood <br />condition for a specific dam, then this could be <br />assessed, with the normal assumptions for <br />deterministic design, or with a system <br />response for a risk study. {refer Appendix 3, <br />sub-section A3.6,}. <br /> <br />I <br />I <br />~ <br />I <br />I <br /> <br />4.4. Spillway Design Flood (SDF) and <br />Spillway Provisions. <br /> <br />4.4.1. Definition <br /> <br />The SDF is the flow stage selected from site <br />and economic considerations for the <br />hydraulic design of the spillway structure, <br />chute and dissipator, under operational <br />conditions, as distinctfrom potential, but low <br />probability extreme floods selected for overall <br />dam safety. <br /> <br />The SDF is not an indicator of dam safety but <br />a basis for selection and design of spillway <br />structures. The S D F concept can be used in the <br />economic assessment of optimum spillway <br />provisions to provide the required AFC. <br /> <br />8 <br /> <br />ANCOLD Guidelines on Selection of an Acceptable flood Capacity for Dams <br />