Laserfiche WebLink
<br />5.A.(5)(c) The Incremental Damage Analysis (IDA) used to <br />justify an Inflow Design Flood (IDF) less than the minimum <br />requirements of Rule 5.A.(5)(b) or Rule 6.A.(4), shall be based upon a <br />comparison of two floods: first, a base flow flood of a magnitude <br />which will cause overtopping failure of a dam and routing it down- <br />stream assuming no dam is in place and second, the dam failure flood <br />due to overtopping, imposed upon the base flow flood. The spillway <br />capacity and IDF will be acceptable where it can be shown that the dam <br />failure flood would cause no expected additional loss of life and <br />would not cause significant incremental flood damages downstream. <br /> <br />5.A.(5)(c)(I) For comparison of the two types of floods, no <br />additional loss of life nor "significant" damage is expected in the <br />incremental zone if the incremental increased depth of flow is two <br />feet or less and the product of the average floodplain flow velocity <br />(in feet per second) and the incremental depth of flood (in feet) is <br />less than seven. <br /> <br />5.A.(5)(c)(II) Documentation for the IDA shall include but <br />not be limited to: a dam break flood plotted on topographic maps of <br />the affected areas; hydraulically appropriate cross-sections of the <br />downstream channel showing flood stages with velocities and discharges <br />for the two floods; incremental damage and loss of life determina- <br />tions; and a summary of all hydraulic parameters. Documentation shall <br />also include, if deemed necessary by the State Engineer, channel <br />profiles with the various flood stages; aerial photographs of the <br />affected areas; and computer printouts showing flood discharges, <br />stage, and velocities with respect to time. <br /> <br />5.A.(5)(d) The Inflow Design Flood (IDF) shall range between <br />the Probable Maximum Flood and the 25-year flood, with the Inflow <br />Design Flood determined by the following methods: <br /> <br />5.A.(5)(d)(I) When the 100-year flood event is the <br />appropriate Inflow Design Flood, the probable future flow shall be <br />determined by one of the following methods: <br /> <br />5.A.(5)(d)(I)(A) Whenever the records basic to a deter- <br />mination of probable future water flows extend for a period of one <br />hundred or more years at reliable gaging stations, the calculation of <br />the IDF based upon those records shall be deemed conclusive. The <br />records however, must be adjusted for historic diversions and the <br />effects of upstream storage. If such records do not extend for a <br />period of 100 or more years, then the determination shall be made by <br />interpolation and correlation to a full 100 years of records by <br />relating them to known records of known basins as similar as <br />reasonably possible to the basin under consideration. A stream gaging <br /> <br />- 16 - <br />