FLOOD01451

• Designation Number: 321
• County: Weld
• Community: Windsor
• Title: Flood Insurance Study - Windsor, Colorado, Weld County
• Date: 9/1/1991
• Designation Date: 5/1/1992
• Floodplain - Doc Type: Floodplain Report/Masterplan

Peak discharge-drainage area relationships for Cache I~a }JolJllrp
River are shown in Table 1.

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

Cross-sectional data, including overbanks, for the backwater
analyses were obtained by field survey. All bridges were surveyed
to obtain elevation data and structural geometry. The land-use and
hydraulic roughness data were also obtained by field surveys.

Water-surface elevations for floods of the selected recurrence
intervals were computed using the CDE HEC-2 step-backwater program
(Reference 8). Starting water-surface elevations for the Cache La
Poudre River were determined by the slope/area method.

Channel and overbank hydraulic roughness factors (Manning's "n")
used in the hydraulic computations were determined using
engineering judgment and were based on field observations of the
stream and floodplain areas and the descriptions presented in
standard engineering references (Reference 9). The channel "n"
value for the Cache La Poudre River was estimated as 0.035
throughout the study reach, and the overbank "n" values ranged from
0.035 to 0.110.

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 Ceodetic Vertical
Datum of 1929 (NGVD). Elevation reference marks and their
descriptions used in this study are shown on the maps.

4.0 FLODDPLAIN MANAGEMENT APPLICATIDNS

The NFIP encourages State and local governments to adopt sound
floodplain management programs. Therefore, each Flood Insurance Study
provides 100-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.

4.1 Floodplain 80undaries

To provide a national standard without regional discrimination, the
1 percent annual chance (lOO-year) flood has been adopted by FEMA
as the base flood for floodplain management purposes. The 0.2
percent annual chance (500-year) flood is employed to indicate
addi tional areas of flood risk in the community. For each st ream
studied by detailed methods, the 100- and 500-year floodplain

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