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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 />A25.6, Decision. <br /> <br />The decision on freeboard should be site <br />specific and based on considerations of <br />design, construction, site restrictions, and <br />incremental costs and consequences of <br />failure, within a business risk assessment. <br /> <br />A2.5.7, References. <br /> <br />Useful References on detenninistic <br />assessment of freeboard include: <br />ICE (1996), UK Table I: gives detailed <br />provisions for each dam category relative <br />to initial reservoir level, concurrent wind <br />speed as well as a minimum wave <br />surcharge allowance. <br />SAN COLD (1990) gives interim freeboard <br />guidelines for South African Dams. <br />SANCOLD (1991). suggests consideration <br />of zero freeboard with the Safety <br />Evaluation Flood (equivalent to DCF), <br />SANCOLD (1991), also conservatively <br />suggests that with gated spillways, it <br />should be assumed either one gate, or 15% <br />of a number of'gates, whichever is greater, <br />are not operative during a flood. <br />CDA (1999), Canada, states sufficient <br />freeboard shall be provided under all <br />operating conditions, including extreme <br />floods or winds, and subsequent to <br />earthquakes, with the percentage of <br />overtopping waves limited to that not <br />leading to failure, The maximum reservoir <br />level is also to be at or below the top of <br />the impervious core of embankment dams. <br /> <br />For embankment dams the freeboard <br />should be sufficient to avoid overtopping <br />for 95% of the waves created under the <br />specified wind conditions. If greater <br />overtopping is permitted, the volume <br />should not endanger the dam. The dam <br />crest is normally set at a level to satisfY <br />the following: <br /> <br />Wave conditions and set-up due to a <br />I : 1000 AEP wind with reservoir at <br />maximum normal level, or using a "wind <br />speed-total duration" relationship over the <br />expected life of the dam.. <br /> <br />Wave conditions and set-up due to the <br />most severe reasonable wind conditions <br />for the reservoir at maximum extreme <br />level based on the selected design flood, <br />For large reservoirs and basins the mean <br />maximum annual wind is normally used. <br />For small reservoirs and basins the I: 100 <br />AEP maximum annual wind is normally <br />used. Cases in between to be selected <br />between those limits, <br /> <br />For concrete dams with abutments <br />resistant to erosion or low consequence <br />dams, freeboard can be based on an <br />economic analysis of damages. <br /> <br />Resistance of concrete dams ' to <br />overtopping may be considered with due <br />consideration of abutment and ancillary <br />structure vulnerability and other <br />downstream consequences. <br /> <br />Additional freeboard or provlSlon for <br />overtopping may be required for reservoirs <br />subject to landslide-induced waves. <br /> <br />A2.6. WARNING AND EVACUATION <br />PLANS. <br />(Refer to sub- section 7.3. warning & <br />evacuation plans & non-structural <br />options). <br /> <br />The effectiveness of warning and <br />evacuation plans will relate to the <br />following, including human reactions in <br />an emergency (panic) situation. <br /> <br />The availability of rain and flood data, and <br />practicality of operators recognising a <br />potential extreme flood and dam break <br />situation when operational experience and <br />data is only for floods from say I :20 to <br />1:100 AEP. <br />In most cases it will not be possible to <br />readily assess from rain data and inflow <br />hydrographs, if available, or reservoir <br />level rise, whether the flood will build up <br />to even a I: 1000 AEP flood, let alone a <br />dambreak situation. <br />The order of early flooding for huger <br />catchments is likely to be catastrophic, <br />with emergency authorities stretched, and <br /> <br />ANCOLD Guidelines on Selection of an Acceptable flood Capacity for Dams A2-9 <br />