FLOOD10184

• Designation Number: 430
• County: Larimer
• Community: Fort Collins
• Basin: South Platte
• Title: FIS - Fort Collins, CO
• Date: 3/1/1996
• Designation Date: 7/1/1996
• Floodplain - Doc Type: Floodplain Report/Masterplan

For the lower reach below the Timnath Reservoir Inlet ditch diversion, a hydraulic model
was developed for the 10-, 50-, and lOO-year floods and a separate model was developed
for the 5oo-year flood.

A model was developed to provide a lOG-year 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.

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

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The BNRR embankment divides the floodplain downstream of the City of Fort Collins'
Wastewater Treatment Plant No.2. For the 10-, 50-, and lOO-year floods, only the area
of the right bank north of the railroad embankment was considered effective for flow.
To develop water-surface elevations south of the railroad, a model was developed to
consider the results of railroad-embankment failure. For the 500-year flood, the split-
flow option was used to calculate the flows that overtop the railroad embankment. The
loo-year water-surface elevations south and north of the railroad embankment are shown
on the same profile panels.

There are two major flow diversions in the lower reach of the Cache La Poudre River.
Between Cross Sections AE and AF (Sections 211017 and 212317 in the HEC-2 model)
on the main channel, flow spills to the left bank gravel-pit area and flows along a separate
flow path (referenced as Cache La Poudre LP A TH) before rejoining the main flow path
just below the Fossil Creek Ditch diversion. The amount of flow divened to LP A TH was
computed using the split-flow routine from the 1989 study.

The other major flow diversion in the lower reach is a divided flow through gravel pits
on the right bank from just below the Timnath Reservoir Canal diversion to just upstream
of Prospect Road (referenced as Cache La Poudre RPATH). The division of flow
between the main channel and RP A TH was determined by balancing the computed energy
loss between the models for the 100- and 500-year floods in the lower reach below the
Timnath Reservoir Inlet ditch diversion and the model for RPATH.

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In the reach above the Timnath Reservoir Ditch diversion, the main flow of the Cache
La Poudre River was modeled using two separate HEC-2 models. One model was for
the 10-, 50-, and 100-year floods and the other was for the 5OG-year flood. In addition,
there are several divided flows. Between Mulberry Street and Lincoln Avenue, the 500-
year flood ovenops Lemay Avenue on the left bank. This flows enters the drainage basin
for Dry Creek and returns to the Cache La Poudre River below the Timnath Reservoir
Ditch diversion.

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