5/17/2010 12:52:32 PM
5/20/2008 2:29:02 PM
Probable Maximum Precipitation Study for Cherry Creek Reservoir - Questions and AWA Actions
Floodplain - Doc Type
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<br />4. HMR 55A used one-half of the ratio and excludes the first 1,000 feet of elevation <br />difference. Section 126.96.36.199 contains the following discussion for use of the 1,000 foot <br />exclusion: <br />HMR 51 made no 'adjustments for elevation east of the Mississippi River. <br />Variations between storm and transposed locations were generally small, less than <br />1,000 ft. ' <br />In the western plains, for area sizes larger than 1,000 me, a "gentle upslope" <br />reduction was applied, 6%-10% per 1,000 ft. Note: no exclusion of 1,000 ft was <br />made for the "gentle upslope" elevation adjustment. <br />In the present study (HMR 55A), the same procedure was adopted by making no <br />adjustment for the, changes in elevation of 1,000 ft or less. Note: The above <br />discussion made no mention of excluding 1,000 ft and when the reduction of 6%- <br />10% was made, there was no 1,000 ft exclusion. <br />HMR 57 uses the 1,000 foot exclusion based on judgment with a reference to the <br />above discussion from HMR 55A. <br />The WMO Manual states that some correction for storm elevation may be <br />appropriate. Some studies have not made an adjustment for storms than 300m. <br />This decision is based on the distance to the moisture source, the storm <br />characteristics and topography of the region. In the example provided for a storm <br />some distance from the moisture source on a broadly sloping plain, there is no <br />300m or 1,000 ft exemption applied. Neither is any exemption applied for <br />moisture adjustment by intervening barriers. <br />The Review Comrhittee for the Cherry Creek study was fully aware that A W A <br />did not use the 1,000 foot exemption and accepted the procedure. Several FERC <br />approved studies ih Maine and New York did not use the 1,000 foot exemption, <br />were reviewed by an FERC approved Board of Consultants and accepted by the <br />FERC. Approximately 8-10 site-specific PMP studies in Colorado have not used <br />the 1,000 foot exemption and have been accepted by the Colorado State Engineer <br />Office. There is both justification and precedence for not using the 1,000 foot <br />exemption. <br /> <br />The preface in HMR 55A mentions that HMR 55 used one-half the liquid water <br />variation observed: in the precipitable water tables. The authors changed this <br />adjustment in HMR 55A to conform to previous studies that allow for the full <br />moisture adjustmeiIt presented by the change in precipitable water. However, in <br />Section 188.8.131.52, it is stated that "In this study, we make a consensus decision to <br />adopt a vertical moisture adjustment one-half the traditional adjustment in an <br />attempt to control unrealistic maximization in general storms." Which was <br />actually done is un,known. <br />HMR 57 uses full moisture adjustment as presented in Equation 7-2 (Compare <br />with HMR 55A Equation 8-3). <br /> <br />A W A Action: The Corps requested that A W A revise the analysis to use lf2 the <br />ratio and the 1,000 feet immunity unless justification can be provided. <br />, <br />
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