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<br />5.5.11 Site specific hydrometeorologic analysis may be used to determine the appropriate <br />extreme precipitation event for the determination of the IDF. The procedures used shall be <br />consistent with procedures used by the National Weather Service to transpose large <br />historical storms to the drainage basin under consideration to determine the Probable <br />Maximum Precipitation. Snowmelt conditions shall be considered as base flow when <br />appropriate. Site specific evaluations are subject to approval by the State Engineer. The <br />percentage reduction of the PMP as shown in Rules 5.5.6, 5.5.7 and 6.1.4 are not applicable <br />or allowed in the determination of site specific extreme precipitation event or PMP values <br />determined by the procedures and analysis provided for in this Rule for all High Hazard <br />dams and Large Significant Hazard dams. <br /> <br />5.5.12 For any dam whose spiIIway is not designed to pass the inflow design flood <br />requirements defined in Rule 5.5.6, 5.5.7 and 6.1.4, the engineer may as an alternative, <br />provide documentation ofthe analysis that overtopping of the dam by floods which exceed <br />the spiIIway capacity up to the design flood wiII not cause failure of the dam. Otherwise, <br />overtopping protection shall be provided. <br /> <br />5.5.13 The minimum freeboard requirements for new or enlarged dams shall be based upon <br />the dam height required to prevent overtopping by wave action, or the sum of the inflow <br />design flood maximum water surface level plus one foot ofresidual freeboard, but not less <br />than five feet unless the State Engineer approves a lesser amount. <br /> <br />5.6 Incremental Damage Analysis - An Incremental Damage Analysis (IDA) used to justify an <br />Inflow Design Flood (IDF) less than the requirements of Rule 5 or Rule 6, shall be based on a <br />comparison of two floods: first, a base flow flood ofthe minimum magnitude which exceeds the <br />capacity of all spiIIways, resulting in overtopping of the dam routed through the downstream <br />floodway assuming no dam is in place; and second, the dam failure flood which occurs due to <br />overtopping, and is routed downstream with the base flow flood. The spiIIway capacity and IDF <br />will be acceptable where it can be shown that the dam failure flood would cause no additional loss <br />of life nor additional significant property damages downstream within the zone between the two <br />floods. <br /> <br />5.6.1 No loss oflife or significant damage is expected to occur if the increased depth of <br />flow is two feet or less and the product of the flood flow velocity in the incremental zone <br />and the depth of flow at critical locations along the floodway is less than seven. <br /> <br />5.6.2 Documentation for the IDA shall include but not be limited to: a plot showing both the <br />base flow and dam break flood on topographic maps ofthe affected areas; cross-sections of <br />the downstream channel showing flood stages, velocities, and discharges for the two floods <br />at the critical locations; incremental damage and loss oflife determinations; and a summary <br />of all assumed hydraulic parameters. A table summarizing the results of the IDA at the <br />various downstream cross sections showing that the criteria in Rule 5.6.1 has been satisfied <br />shall be included in the IDA study report. Documentation shall also include, if deemed <br />necessary by the State Engineer, channel profiles with the various flood stages, aerial <br /> <br />t7 <br />