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

3.2 Hydraulic Analyses

Analyses of the hydraulic characteristics of flooding
sources studied were carried out to provide estimates
elevations of floods of the selected recurrence intervals.

from
of

the
the

For the original study, water-surface elevations were computed for
the 10-, 50-, 100-, and 500-year floods using the COE HEC-2
computer program (Reference 11).

For Coal Creek, a culvert at the southern
County Road 62, was used as a control,
elevation at its inlet was used as the
elevation.

corporate
and the
starting

1 imi t s, under
water-surface
water-surface

Cross section data for streams in the area were obtained from
topographic maps at a scale of 1:2,400, with a contour interval of
2 feet (Reference 12). All bridges and culverts were surveyed to
obtain elevation data and structural geometry.

The COE HEC-2 computer program was also used to perform water-
surface profile computations for the restudy of Boxelder Creek.
Numerous bridges and culverts exist along the channel through the
study reach. Those that created significant backwater effect were
handled in the HEC-2 analysis with appropriate bridge routines. A
few bridges and culverts are only minor segments of the total
floodplain, and, therefore, were considered in the Mannings
retardance factor rather than in separate bridge analyses.

Cross section data and reach length information were obtained from
photogrammetric maps prepared especially for the SCS study
(Reference 1). Hydraulic roughness coefficients ("n" values) were
determined from field inspection.

Locations of selected cross sections used in the hydraulic analyses
are shown on the Flood Profiles (Exhibit 1) and on the FLood
Insurance Rate Map.

The hydraulic analyses for this study were based on unobstructed
flow. The flood elevations shown on the profiles are thus
considered valid only if hydraulic structures remain unobstructed,
operate properly, and do not fail.

All elevations are referenced to the National Geodetic Vertical
Datum of 1929 (NGVD). Elevation reference marks and the
descriptions of the marks used in this study are shown on the maps.

4.0 FLOODPLAIN MANAGEMENT APPLICATIONS

The NFIP encourages State and local governments to adopt sound
floodplain management programs. Therefore, each Flood Insurance Study
provides lOO-year flood elevations and delineations of the 100- and 500-
year floodplain boundaries and 100-year floodway to assist communities
in developing floodplain management measures.

7