FLOOD10696

• County: Larimer
• Community: Larimer County and Incorporated Areas
• Title: FIS - Larimer County and Incorporated Areas - Vol 1
• Date: 12/19/2006
• Prepared For: Larimer County and Incorporated Areas
• Prepared By: FEMA
• Floodplain - Doc Type: Historic FEMA Regulatory Floodplain Information

reduced. This water does not return to the main channel within the study reach, but is
accounted for in the concurrent Flood Insurance Study for Weld County, located
immediately downstream (Reference 40).

Each of these divided-flow paths were modeled separately using HEC-2.

For the revised area of Cache La Poudre River from Wood Street to Watson Lake, the
cross sections for the backwater analyses were obtained from photogrammetric topographic
maps provided by Larimer County. Below-water sections were obtained by field surveys
performed by Ayres Associates Inc. in 2005. Bridges and culverts were also surveyed by
Ayres Associates Inc., in 2005 to obtain elevation data and structural geometry.

Water-surface elevations on the Cache La Poudre River in Larimer County for floods of
the selected recurrence intervals were computed using the USACE HEC-RAS version 3.1.2
step-backwater computer program. Starting water-surface elevations for the Cache La
Poudre River in Larimer County were taken from the 1999 FIS.

Channel and overbank roughness factors (Manning's "n" values) used in the hydraulic
computations were chosen by engineering judgment and were based on field observations
of the stream and floodplain areas. No reliable calibration data was available for the Cache
La Poudre River. Horizontal variation was applied to the "n" values in the overbanks and
main channels throughout the study reaches. Overbank "n" values varied from 0.013 (wet
ponds) to 0.200(dense vegetation). At some cross sections the overbank "n" values were
artificially set higher than 0.200 in order to maintain reasonable continuity of flow in the
overbanks between adjacent cross sections. Main channel bank "n" values ranged from
0.013 (bike trails) to 0.080, and main channel bed "n" values ranged from 0.030 to 0.035.

For the lower reach of the Cache La Poudre River below the Timnath Reservoir Inlet ditch
diversion, a hydraulic model was developed for the 10-, 2-, and I-percent annual chance
floods and a separate model was developed for the 0.2-percent annual chance flood.

A model was developed to provide a I-percent annual chance flood water-surface profile
for the Cache La Poudre Lowflow channel between Sections 201202 and 205242 of the
main channel. When the river rises above a certain level, the majority of the flow breaks
out to the east and then travels through a low-gravel pit, bypassing the Lowflow channel.
The Lowflow channel HEC-2 model was created for the reach between Cross Sections
201202 and 204025 of the main channel. The gravel pit east of the river was blocked using
encroachments. The HEC-2 split-flow option calculated the discharge that breaks out of the
Lowflow channel to the east.

The split-flow computations showed that most of the I-percent annual chance flood
discharge bypasses the Lowflow channel, leaving approximately 10 percent of the 1-
percent annual chance flood discharge within the LowfIow channel. The discharge used to
compute the I-percent annual chance flood water-surface elevations along the gravel pit
was not reduced by the 10-percent flow conveyed in the Lowflow channel. A separate
profile was developed for the Cache La Poudre Lowflow channel.

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