Laserfiche WebLink
<br />.., I "'~ h <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 ofthe 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 elements in the A W A team site-specific PMP study are as follows: <br />1. 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 orographic ally 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 site-specific PMP study produced approximately 25% less rainfall over <br />Cherry Creek drainage basin compared to the site-specific PMP values from the NWS <br />study. Determination of the change in the probable maximum flood (PMF) using these <br />results is beyond the scope ofthis PMP study. <br /> <br />Xl <br />