FLOOD06287

• Designation Number: 488
• County: Larimer
• Community: Loveland
• Basin: South Platte
• Title: Flood Insurance Study - Loveland, Larimer County, Colorado
• Date: 3/23/1999
• Designation Date: 3/1/1999
• Floodplain - Doc Type: Floodplain Report/Masterplan

The U.S, Army Corps of Engineers, Omaha District, provided the
data for the 10-, 50, and 100-year discharges for the Big
Thompson River through Loveland (Reference 2). The SOO-year
discharge was estimated by a straight-line plot on log probability
paper. The SOO-year discharge was approximately the same as that
of the Standard Project Flood computed by the U,S. Arl1\Y Corps of
Engineers; therefore, the Standard Project Flood discharge was
used as the SOO-year discharge.

Resource Consultants, Inc. reviewed the discharges for the Big
Thompson River to verify the consistency of discharges used in
other flood-plain studies conducted for this reach and the
adjacent reaches of the Big Thompson River (References 2, 9, 10
and 11). The discharges used in this study are consistent with
those used in previous studies of the Big Thompson River.

A summary of drainage area-peak discharge relationships for
the Bi g Thorr,pso n Ri ver is shown in Ta D 1 e 1.

TABLE 1 - SUMMAR Y OF DI SCH.~RGES

FLOODING SDURCE DRAINAGE AREA PEAK DISCHARGES (cfs)
AND LDCA TI ON (Sq. Miles) 10-Year 50-Year 100-Year SOO-Year
-,-
Big Thompson River
Ra il road Avenue 515 4,700 12,300 19,000 44,000
3,2 flydraulic Analyses

Analyses of the hydraulic characteristics of the flOOding
sources studied in detail in the communi ty were carried out
to provide estimates of the elevations of floods of the
selected recurrence intervals along each of these flood
sources,

Cross sections for the Big Thompson River were initially
obtained by photogrammetric and field survey methods in August
1976, This survey information and base maps were then updated
by the same methods to reflect May 1981 conditions (Reference
12) . All bri dges and culverts were surveyed to obta i n e 1 eva ti on
data and structural geometry.

Locations of selected cross sections used in the hydraul ic
analyses are shown on the Flood Profiles. For stream seqments
for which a floodway was computed (Section 4.2), selected
cross-section locations are also shown on the Flood Boundary
and Floodway Map.

Water-surface profiles were developed using a HEC-2 step-backwater

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