FLOOD07806

• County: Larimer
• Community: Fort Collins
• Stream Name: South Platte Tributaries
• Basin: South Platte
• Title: Hazard Mitigation 404 Grant Application Early Warning System Expansion
• Date: 9/1/1999
• Prepared For: Larimer County
• Prepared By: Fort Collins
• Floodplain - Doc Type: Miscellaneous

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however has a difficult time predicting flooding in basins that respond in time scales
of less than 2 hours. It is estimated that more than 70 percent of flash flood warnings
have less than one-hour lead time; while more than 50 percent of flash flood
warnings have no lead time (NWS, ]982). The City of Fort Collins contains thirteen
stormwater basins within the City boundary that are considered to be "flashy" in
nature (they respond in Jess than I hour). The quick response of these basins is a
function ofland development within the basin and the nature of the meteorology of
the north-east Colorado front range.

There is a common perception that severe (often flood producing) weather events
occur frequently during the first three weeks of June and again in late July and early
August along the Colorado and Wyoming Front Range. The Big Thompson flood
that killed 139 people occurred on Ju]y 31. ]976. The Fort Collins flood of! 997 that
killed 5 people occurred on July 27 and 28. The Fort Collins hail storm that killed
one person occurred on July 30, 1979. These severe weather events have been
documented (McKee and Doesken, 1997) and are summarized in Table 2 below.

Table 2
Documented Extreme Storm Events
(Reproduced from McKee and Doesken, 1997)

Storm Name

Date

Maximum
Rainfall

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LivermorelBoxelder
PueblolPenrose
Savageton, WY
Cherry Creek/Hale
N. Colo. Front Range
Rye (S. Ft. Range)
Gibson Dam, MT
Plum Creek (and others)
Big Elk Meadows
Rapid City, SD
Big Thompson
Frijole Creek
Fort Collins
Pawnee Creek

May 20-21, 1904
June 2-6, 1921
Sep 27-29, ]923
May30-3], ]935
Sept. 2-3, 1938
May 18-20, 1955
June 6-8, 1964
June 16-] 7, ]965
May 4-8, ] 969
June 9, ]972
July 3], ]976
July 2-3, ] 98]
July 27-28, ] 997
July 29-30, 1997

8" (or more)
6-12"
17"
12-24"
6-10"
6-13"
16"
14-16"
6-14"
15"
12"
8-]6"
14.5"
15.1"

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Note that the table includes only the largest storm events (most of which had
associated fata]ities) and includes all known storms from the Fort Collins region that
exceeded] 0 inches of rainfa]1. The table does not include the many storms of the 4-8
inch category that locally may exceed the 100-year storm event. Weaver and
Doesken performed an analysis in order to statistically support the hypothesis that
severe weather frequently occurs during the first three weeks of June and again in
late July and early August (Weaver and Doesken, 1990). The results of their analysis
confirmed this hypothesis. A total of 28 out of the 30 years studied experienced

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