Laserfiche WebLink
<br />cloud bases, and a slightly tilted updraft stmcture contributed to a high <br />precipitation efficiency."] <br /> <br />"Intense rainfall began soon after 1900 MDT in the Big Thompson River and the <br />North Fork Cache la Poudre River basins. A cumulative rainfall curve <br />developed for Glen Comfort from radar data indicates that 7.5 inches of rain <br />fell during the period 1930-2040 MDT on July 31. In the central part ofthe <br />storm area west of Fort Collins, the heaviest rainfall began about 2200 MDT on <br />July 31 and continued until 0 I 00 MDT on August I." <br /> <br />Stanton (1990) studied in detail the rainfall production of the Big Thompson Canyon <br />storm relative to terrain elevation. He reported that over 70 percent of the rainfall in the <br />Big Thompson storm fell at elevations between 7,500 feet and 9,500 feet. The rainfall <br />which fell at elevations above 10,500 feet accounted fOJ' 0.5 percent ofthe siorm total <br />volnmetric rainfall of the Big Thompson Canyon event, This vain able inmhtjnto <br />the hi!!h altitude rainfall {'roduction of an excw.tional storm, re\:arded bY.lpany as <br />havinlr a 10,000 vear plus return frequency, helps in uHlderstandin!! the I!otential <br />upper limits for rainfall durin!! extreme precilill<ltion events. <br /> <br />3.5 Poudre River and Rist Canyon, Colorado; July 31 to August 1, 1976 <br /> <br />This event, although less well known, is closely associated with the General Convective <br />Storm event discussed in the preceding section, the Big Thompson Canyon flood of July <br />31,1976. It is discussed in a comprehensive report published by the U.S. Geological <br />Survey (McCain et ai, 1978). Much of what follows is based on this report. <br /> <br />Figure 7 shows the detailed location ofthc flooding associated with this event and with <br />the Big Thompson Canyon event. The North Fork of the Cache la Poudre River, the <br />Cache la Poudre River itself, and several small tributaries in the northwestern suburbs of <br />Fort Collins experienced severe flooding during this event. Topography of this flooded <br />area is characterized by narrow valleys and steep streambed gradients. In particular, <br />along the main stem of the Cache la Poudre system, elevations range from 5,700 feet near <br />the western limit of flooding at Poudre Park to 5,240 feet at the mouth ofRist Canyon. <br />The North Fork Cache la Poudre ranges from about 8,000 feet near the Wyoming border <br />to 5,360 feet near its mouth. This type ofrelicf is particularly prone to flash floods <br />produced by convective thunderstorms usually during the summer months when low- <br />level moisture from the Gulf of Mexico can travel great distances and impinge the <br />foothills canyon areas of Colorado. Runoff from these intense thunderstorms is rapidly <br />concentrated into nearby channels producing flash flooding further downstream. Henz <br />(1996a) describes the relationship between the Big Thompson Canyon event ,md the <br />Poudre River and Rist Canyon events. He states that the Big Thompson Canyon slorm <br /> <br />IHenz has constructed a physical model of the thunderstorms that produced this flooding. This model has a 2.9 km deep, warm <br />convective layer (temperatures above freezing) which, following the HMS Convective Stam1 Methodology, allows for a doubling of <br />precipitation production efficiency by warm coalescence processes. <br /> <br />14 <br />