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<br />attracted more National Weather Service attention. The Fort Collins stonn was small in <br />area extent and was not accompanied by unusually tall clouds or by excessive lightning. <br />Fortunately the WSR-88D radar display algorithms successfully detected the large <br />rainfall accumulations associated with this nearly stationary stonn, Although the radar <br />underestimated stonn rainfall over Fort Collins by nearly 50 percent, it did alert <br />forecasters to a potential problem and contributed to the issuance of a Flash Flood <br />Warning for the city at 9:40 PM while the stonn was still in progress and before most of <br />the worst damage and fatalities had occurred. A report on this will be issued by the <br />National Weather Service team working on the stonn. <br /> <br />In summary, 10 to 14.5 inches of rain fell over an approximately 30 hour period in a band <br />extending along the base of the foothills from southwest Fort Collins northward to <br />northwest of Laporte. Rainfall of this intensity is rare, but stonns of similar magnitude <br />like the Big Thompson flood of July 1976 have been observed roughly once every ten to <br />twenty years somewhere in Colorado with a distinct preference toward being in or near <br />the eastern fuOthills of the Rocky Mountains (McKee and Doesken, 1997), During the <br />recorded history of Colorado, the Fort Collins stonn produced the heaviest rains ever <br />documented to have fallen over an urbanized area in this state, <br /> <br />The Runoff <br /> <br />While the stonn was remarkable, the runoffwas at least as dramatic, and some peak <br />discharges exceeded estimated 100-year flows by a factors of ten. <br /> <br />Hydraulically, the two main flow paths that caused greatest damage during the flood were <br />along Spring Creek and through the Colorado' State University campus (Figure 1). Spring <br />Creek begins in the foothills west ofFt, Collins. When Horsetooth Reservoir was <br />constructed in the early 1950's, it intercepted about half of the Spring Creek drainage <br />basin. Now, the urban portion of Spring Creek begins below Spring Canyon Dam at <br />Horsetooth Reservoir, then flows generally from west to east through Ft. Collins. Spring <br />Creek terminates at the Poudre River to the east off of the map. The total drainage area <br />from below Horsetooth Reservoir to the confluence with the Poudre River is <br />approximately 12 square miles. Generally, Spring Creek flows with a small discharge <br />representing snowmelt, small spills from irrigation canals, groundwater seepage, and <br />local stormwater. As Spring Creek nears College Avenue, it flows through a large <br />detention area behind the Burlington Northern Railroad embankment and just upstream <br />of the trailer park. In 1989, numerous improvements were made to the stormwater <br />system in this area. A 14' X 12' box culvert leading to the trailer park area was plugged <br />with compacted fill and Spring Creek was rerouted to the south to pass through three 84" <br />pipes, Also the Burlington Northern Railroad embankment was reinforced and stabilized <br />and designed to withstand the 500-year flood. <br /> <br />Table 2 shows estimated discharges as compared to previously-established 100-year and <br />500-year flow estimates, Note that all along Spring Creek estimated flows exceeded the <br />previously-established levels by factors of two or more. The most dramatic flow <br />measurement was the "combined flow below the Canal Importation Channel" on Spring <br /> <br />5 <br />