Laserfiche WebLink
<br />000826 <br /> <br />The relevance of ~his repeated occurrence of heavy rain <br />events in Denver to Wetzel Creek is the underscoring of the <br />influences of topographic effects and rainfall cycles <br />on the occurrence frequency of rainfall events. It is our <br />opinion that the Wetzel and Badger drainage basins are <br />affected by storm tracks forming off hot spots of storm <br />development over Palmer Divide to the south, the Rockies to <br />the west and Cheyenne Ridge to the north of Fort Collins. <br />It is our opinion that rainfall events which exceed 4 <br />inches or more of rainfall over a three hour period or less <br />should be considered a 20 year event or less. The type of <br />merging and super-cell thunderstorms systems which produce <br />these heavy rain events commonly occur on the eastern plains <br />of Adams County and eastern Colorado. An additional <br />important consideration is the likely occurrence of copious <br />amounts of hail in swaths of 1/4 to 2 miles width and 1 to 10 <br />miles in length and 1-3 feet in depth. The combination of <br />heavy rain and hail can cause clogging of drainage systems <br />and spillways which must be considered in the design. <br /> <br />In conclusion it is our opinion that the Wetzel Creek flash <br />flood was caused by a merging thunderstorm complex that <br />remained relatively stationary over the basin for a two hour <br />period. Additional storm cells crossed portions of the basin <br />before and after the main body of the storm. Total rainfall <br />approached 8-11 inches across the basin. Our rainfall <br />calculations should be within +/- 10 percent of the <br />estimated values. In our opinion the occurrence of <br />thunderstorms which produce over 4 inches of rainfall in a 3 <br />hour period would constitute a 20 year event. <br />