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

conditions existing in the community
study. Maps and flood elevations
reflect future changes.

at the time of completion of this
will be amended periodically to

3.1 Hydrologic Analyses

Hydrologic analyses were carried out to establish peak discharge-
frequency relationships for each flooding source studied by
detailed methods affecting the communLty.

For the original study the SCS performed the hydrologic analyses of
Coal Creek and Boxelder Creek while compiling their Watershed Work
Plan (Reference 4). Precipitation values were taken from U.S.
Weather Bureau Technical Papers 40 and 49 (References 6 and 7,
respectively). Soil information was taken from the unpublished
detail soil survey completed by the SCS for Larimer County
(Reference 8). The SCS Program for Project Formulation, Hydrology
wa5 used to develop and route hydrographs (Reference 9).

At the time the original Flood Insurance Study was completed, only
two of the three reservoirs above Wellington had been constructed;
therefore, based on the information supplied by the SCS, the
storage for the third reservoir,. B-2, was not considered in the
discharge computation.

The discharges used for Coal Creek are based on
conditions. The unregulated discharges have been
account for the ",at er '~hich ",'i! 1 be intercepted by
Ditch, Located near the northern co~?or9.te L i:rni t s of
The Windsor Ditch diverts the 10-year flood; therefore,
shown for the 10-year flood in this report.

pre-projec:
reduced :'0
the Windsor
Wellington.
no data are

Hydrologic data used for the restudy of Boxelder Creek were
provided to the SCS by the CWCB. The original source of the
discharge-frequency values was the Simons ~ Li, & Associates, Inc..
study "Cooper Slough, Boxelder Creek Master Drainageway Planning
Study - 1981" {Reference 2). The data were reviewed and concurred
in by the CWCB and the SCS. The methodology Ior developing the
discharge data was the "Storm Water Management Model" (SWMM), as
developed by the Environmental Protection Agency, and revised by
the Missouri River Division, U.S. Army Corps of Engineers (COE),
1973 (Reference 10).

Hydrologic data were based on conditions that reflected final
completion of the BoxeLder Creek Watershed project. A general
assumption in the hydrologic data is that irrigation ditches and
canals that cross the basin do not intercept flood waters from the
drainages they cr05s. The logic behind this assumption is that the
ditches and canaLs come into the basin full from upstream runoff
and; therefore, have no capacity to intercept additional runoff.

Peak discharge-drainage a-rea -relationships for Boxelder Creek and
Coal Creek are shown in Table 1.

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