FLOOD09485

• Designation Number: 206
• County: Larimer
• Community: Fort Collins
• Stream Name: Fossil Creek Drainage Basin
• Basin: South Platte
• Title: Master Drainageway Planning - Fossil Creek Drainage Basin, Fort Collins, Larimer County, Colorado
• Date: 8/1/1982
• Designation Date: 1/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

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8.1 Improvement Alternatives

Improvements along Fossil Creek are presented on a reach by reach basis.

The division of the study reaches was defined in Chapter II. The flood plain

along the study reaches was relatively narrow except where ponding occurred

behind road embankments. Based on the limited extent of flooding, four

general alternative improvement plans were evaluated.

The four alternative improvement plans are:

1. Flood plain management through nonstructural measures.

2. Channelization.

3. Flood plain management and enlargement of road crossings to prevent

embankment overtopping.

4. Flood plain management and raising of roads to prevent embankment over-

topping.

The south bank of Fossil Creek near Portner Reservoir has reached excessive

heights due to bank cutting and should be stabilized. The wingwall on the

south side of the upstream face of the Colorado & Southern Railroad on Fossil

Creek is shifting and should be stabilized.

The maintenance along the south bank of Fossil creek near Portner

Reservoir is recommended for Alternatives 2, 3 and 4 along with the stabiliza-

tion of the upstream wingwall at the C&S Railroad on Fossil Creek and

improving the headcut at the Union Pacific Railroad and Fossil Creek.

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8.2 Conceptual Designs

Conceptual designs for Alternatives 2, 3 and 4 were completed, based on

the considerations pres..nted earlier. The results of the design calculations

were used to estimate construction quantities and costs of the alternate

improvements.

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8.2.1 Alternative 1

Alternative 1 considers no improvements to the existing drainage system.

Future flood damage is avoided through flood plain zoning, flood proofing

individual structures, or flood insurance. The costs associated with these

measures such as administration and property values are generally not con-

sidered in an economic analysis and will not be considered for this study.

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8.2.2 Alternative 2

Channelization and enlarged road crossings were considered under

Alternative 2 as shown in Figure 30. Vegetated, trapezoidal channels with

concrete trickle channels were designed to carry the 10-, 50- or 100-year

developed flows. Alternatives 2a, 2b and 2c refer to the 10-, 50- and

lOa-year channel design frequencies, respectively.

Manning's equation, assuming uniform flow, was used to size the channels.

Design flow depths were limited to four feet, channel velocities were limited

to 7.5 ft/sec, side slopes ranged from 4h:lv to 5h;lv, and Manning's roughness

coefficient was equal to 0.035. For the design channel, one foot of freeboard

was allowed and a 10 foot access road for maintenance was included in the top

width Right-of-Way (ROW) requirement.

Table 27 presents a summary of the channel design data for Alternatives

2a (la-year), 2b (50-year) and 2c (lOa-year) along Fossil Creek.

Enlarged crossings are required at nine locations along Fossil Creek.

The dimensions of the box culverts and spans are listed in Table 28 for

Alternatives 2a (la-year), 2b (50-year) and 2c (100-year).

Velocities along the channels downstream of Highway 287 were less than

five ft/sec, which will limit erosion along the channel. Velocities along the

channels upstream of Highway 287 were between 6 and 7.5 ft/sec.

Enlarged openings at channel crossings along the tributaries were assumed

when computing design flows for Fossil Creek.

8.2.3 Alternative 3

Alternative 3 utilized the natural storage behind road embankments,

Figure 31. Where necessary to prevent overtopping the road during the

lOa-year recurrence interval flood for developed conditions, the culverts at

the channel crossing were enlarged. Development would not be allowed in the

flood plain. A vegetated trapezoidal channel between Fossil Creek Reservoir

and Union Pacific Railroad is recommended because of the limited storage

upstream of County Road 11. Enlarged channel crossings along Fossil Creek are

required at County Road 11, Trilby Road (County Road 34), Lemay Avenue (County

Road 13) and Shields street. Improvements along the tributaries were sized

for stanton Creek at County Road 32 and Lemay Avenue (County Road 13) and Lang

Gulch at Trilby Road (County Road 34) and Shields Street. These improvements

were not considered in the economic analysis.