FLOOD01452

• County: Larimer
• Community: Fort Collins
• Stream Name: Fossil Creek Drainageway Basin
• Title: Master Drainageway Planning Study
• Date: 8/1/1982
• Prepared For: Larimer County
• Prepared By: Simons Li & Associates Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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The analysis was performed on a reach-by-reach basis. The results are

presented as benefit-cost ratios and net benefits for each reach for each

improvement plan. The range of the benefit-cost ratio was from 0.04 to 0.85.

All ratios are less than one since there is little existing damage potential

to economically justify the cost of improvements. The average benefit-cost

ratio for Alternative 2 was 0.10 as compared to 0.45 for Alternative 3 and

0.38 for Alternative 4. These ratios differ from the range of ratios because

all benefits and costs are considered to affect the entire basin whereas on a

reach-by-reach basis only cost and benefits for each reach are considered.

Total costs for Alternatives 2, 3 and 4 are $21,117,900, $3,355,300, and

$3,814,300 for the 100-year design flow. Total annual cost is $1,632,200 for

Alternative 2, $276,900 for Alternative 3, and $316,100 for Alternative 4.

The annual costs are based on a 50-year life and a discount rate of 7-3/8 per-

cent. The discount rate is from federal guidelines. Alternative 3 was the

selected improvement plan based on the highest benefit cost ratio.

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stability Analysis of Channel Banks

A stability analysis was performed to evaluate bank stability and deter-

mine equilibrium slopes for existing and developed hydrology and hydrology

corresponding to the selected improvement plan. Bank stability along Fossil

Creek and the tributaries ranged from extremely stable to ~xtremely unstable.

Unstable banks were generally on the outside of meander bends. Major bank

instability problems are produced by general degradation of the streambed.

The equi1ibriunl slope analysis determined aggradation and/or degradation

potential of reaches along the channel syste~ for three flow regimes.

Recommended improvements to limit degradation and stabilize banks in these

reaches were based on the flow regime corresponding to Alternative 3.

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Plan Formulation

The selected improvement plan was based on enlarging culverts and main-

taining three feet of freeboard behind embankments which pond water. The

suggested improvements are guidelines for the local communities. The improve-

ments are based on the 100-year flood frequency for developed conditions.

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The plan calls for a vegetated channel from Fossil Creek Reservoir to the

Union Pacific Railroad. The crossing at Timberline Road (County Road 11)

should be enlarged. A headWall should be placed at the upstream end of the

Union Pacific Railroad. The road at Trilby Road (County Road 34) should be

raised and the culvert enlarged. The channel crossing at Lemay Avenue (County

Road 13) should be enlarged. The reach between Mail Creek and the Colorado &

Southern Railroad requires riprap along the south bank near Portner Reservoir

in order to stabilize the bank. The upstream wingwa11 on the south side of

the C&S Railroad should be stabilized. It appears to be failing due to

embankment sloughing. The downstream embankment at Fossil Creek Drive should

be adequately protected to handle one foot of overtopping. Currently there is

downstream protection and the stability during a 100-year storm should be

checked. The channel crossing at Shields Street should be enlarged to prevent

overtopping. The channel at the downstream side of all culverts and channel

crossings should be riprapped.

Also, based on the proposed improvements, the equilibrium slope analysis

determined locations for erosion control structures along Fossil Creek. Five-

feet of degradation should be allowed for downstream of Lemay Avenue (County

Road 13). The reach upstream of Lemay Avenue will degrade slightly but will

stabilize due to the aggradation occurring in the reach upstream of the Mail

Creek confluence. Between Highway 287 and the C&S Railroad about 8.7 feet of

degradation is expected due to high velocities. Two erosion control struc-

tures located at the railroad crossing and the center of this channel length

are recommended. Another Fossil Creek channel reach that will tend to degrade

is between the Burns Tributary Confluence and Shields Street. Here, an ero-

sion control structure that allows for three feet of degradation should be

placed at the center portion of the reach and another 3.3 feet of degradation

should be allowed at Shields Street.

Slight aggradation is expected along Smith Creek and Burns Tributary to

achieve an equilibrium slope and no improvements are recommended. Both

Stanton Creek and Lang Gulch require improvement to limit degradation.