FLOOD07584

• Designation Number: 486
• County: Larimer
• Community: Wellington
• Basin: South Platte
• Title: Flood Insurance Study - Wellington, CO, Larimer County
• Date: 10/1/1998
• Designation Date: 3/1/1999
• Floodplain - Doc Type: Floodplain Report/Masterplan

The Boxelder Creek watershed, which includes Coal Creek, is
approximately 32 miles long, with an average width of 8 miles.
Altitudes in the watershed range from 4,860 feet at the confluence
with the Cache La Poudre River to 7,720 feet. Geologic formations
that are exposed within the watershed consist of rocks and
sediments ranging from Precambrian to Quaternary in age. Most of
the watershed is covered by irrigated croplands and prairie grass
(Reference 4).

The climate in Wellington is typical of the eastern foothills of
the Rocky Mountains. The mean annual temperature is 48.0oF, with
the warmest month being July at 70.8'F and the coldest month being
January at 26.8"F. The average annual precipitation in Wellington
is 14.94 inches (Reference 5).

2.3 Principal Flood Problems

Floods have been recorded in the Boxelder Creek watershed on 13
occasions since 1900. On August 1, 1961, a storm with a frequency
of 50- to 100-years caused an estimated $76,150 damage in the
Wellington vicinity. In May and June 1967, two overlapping 25-year
storms caused an estimated $46,100 damage and took four lives in
the Wellington vicinity (Reference 4). The most recent flood
occurred in June 1967.

2.4 Flood Protection Measures

The SCS has constructed four detention structures in the watersheds
above Wellington to reduce the flood discharges on Boxelder Creek
and Coal Creek. The reservoirs retain runoff from a combined area
of 17 5.5 square miles or 70 percent of the drainage basin. The
level of flood protection decreases; however, as the distance below
the structures Increases.

3.0 ENGINEERING METHODS

For the flooding sources studied by detailed methods in the community,
standard hydrologic and hydraulic study methods were used to determine
the flood hazard data required for this study. Flood events of a
magnitude which are expected to be equaled or exceeded once on the
average during any 10-, 50-, 100-, or SOO-year period (recurrence
interval) have been selected as having special significance for
floodplain management and for flood insurance rates. These events,
commonly termed the 10-, 50-, 100-, and SOD-year floods, have a 10, 2, 1
and 0.2 percent chance, respectively, of being equaled or exceeded
during any year. Although the recurrence interval represents the long-
term, average period between floods of a specific magnitude, rare floods
could occur at short intervals or even within the same year. The risk of
experiencing a rare flood increases when periods greater than 1 year are
considered. For example, the risk of having a flood which equal s or
exceeds the 100-year flood (l percent chance of annual exceedence) in
any 50-year period is approximately 40 percent (4 in 10); for any
90-year period, the risk increases to approximately 60 percent (6 in
10). The analyses reported herein reflect flooding potentials based on

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