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<br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br /> <br />Since the Cherry Creek basin is oriented roughly north-south as shown in Figure <br />2, the extreme storm events that align with the basin orientation have southerly cloud <br />layer winds and southeast to east sub-cloud layer winds. By comparison the NWS study <br />specifies northerly sub-cloud winds. For northerly sub-cloud wind directions, Figure 2 <br />should be rotated 45-90 degrees to the left to provide a "physically possible" scenario. <br />This rotates the rainfall pattern from its most stressing position aligned with the basin to a <br />cross-basin pattern that reduces the volume of rain within the basin boundaries <br />dramatically. The combination of the NWS study rainfall pattern aligned with the basin <br />with northerly sub-cloud winds is not consistent with Colorado extreme storms. The <br />A W A team study also aligns the rainfall pattern with the basin but relies on southeast to <br />east sub-cloud winds. <br /> <br />The A W A team approach to determine site-specific PMP values for the Cherry <br />Creek basin was to use explicit storm maximization and transposition procedures for <br />historic extreme rainfall storms that potentially could have occurred over the Cherry <br />Creek basin, It is recognized that rainfall centers within some of the historic storms' <br />rainfall patterns were associated with orographic enhancement, i.e. the underlying <br />topography produced more rainfall than would have been produced over flat or down <br />sloping terrain. This study made no attempt to decrease these rainfall amounts when <br />transpositioning storm rainfall patterns to the Cherry Creek location where orographic <br />enhancements are not present for extreme rainfall storms. The result is that more rainfall <br />is produced with some of the transpositioned historic storm rainfall patterns than would <br />occur had the orographic effects been removed. This procedure is conservative and <br />appropriate when validated procedures are not available to justifY lower rainfall amounts. <br />The NWS study modified rainfall amounts using a "k-factor" to account for orographic <br />influences, The rainfall increases within the Cherry Creek basin are contrary to observed <br />topographic effects on rainfall within the basin and inconsistent with Colorado storm <br />observations. <br /> <br />The key study elements in the A W A team site-specific PMP study are as follows: <br />I. Use only Colorado extreme rainfall storms that occurred east of the foothills <br />2. Maximize and transposition storms using standard procedures <br />3. Incorporate atmosphere moisture depletion by upwind barriers <br />4. Critically center and orient the rainfall patterns to put the maximum amount of <br />rainfall within the Cherry Creek basin boundaries <br />5, Keep orographically enhanced rainfall amounts within the transpositioned rainfall <br />patterns over the Cherry Creek basin <br />6. Do not increase rainfall amounts within the Cherry Creek Basin because of <br />orographic influences <br />7. Do not lower rainfall amounts based on downslope winds within the basin <br />8, Use the most critical rain rates developed from historic storm analyses <br /> <br />The A W A team site-specific PMP study produced approximately 25% less <br /> <br />xv <br />