FLOOD05064

• Designation Number: 205
• County: Larimer
• Community: Fort Collins
• Basin: South Platte
• Title: Master Drainageway Planning - Cooper Slough, Boxelder Creek, Fort Collins, Larimer County, Colorado
• Date: 8/1/1981
• Designation Date: 1/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

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dissipators, grade controls. bridges, culverts and flumes should be incor-
porated in the alternative development plans.

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Improvement Alternatives

Improvements in the study area are presented on a reach by reach basis.

The division of the study reaches were previously defined. Considering the

extent of the floodplain along the study reaches, different levels of improve-

ments are suggested. In particular, the floodplain along reaches 2 and 3 of

Boxelder Creek is fairly confined. For these reaches, two improvements are

suggested and are common for all improvement alternatives considered. Along

Reach 2, the channel under the Lake Canal crossing should be improved to

separate floodwaters from the ditch water. The second ilnprovement is along

Reach 3 and its purpose is to prevent flood flows from overtopping the

Larimer-Weld Canal.

More extensive flooding exists in Reaches 1 and 2 along Cooper Slough and

Reach 1 along Boxelder Creek. Three basic improvement plans were formulated

to alleviate flood damages in these three reaches. These plans are:

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3. Detention Storage: In many cases, detention of stormwater is a very

practical method of reducing peak flows. Planning of detention ponds should

consider use of detention ponds as parks or other recreational facilities.

Stormwater detention facilities in general should be capable of storing the

100-year runoff in excess of a desired release to reduce downstream flows to

meet the proposed or existing channel capacity.

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4. Irrigation Ditches: Larimer County drainage policy states that discharge

into canals should be avoided. Therefore, all irrigation ditch crossings

should be capable of passing the 100-year flood without combining irrigation

and stormwater, if possible. Special considerations should be given to irri-

gation ditches which have water rights on tributaries.

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1. Floodplain management through non-structural methods.

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5. Access to Improvements: A maintenance access road should be incorporated

into all master plan programs in order to allow the implementation of the

maintenance program.

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2. Channelization.

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3. Combination of channelization and detention.

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Alternative 1 represents the "do nothing" alternative which considers no

structural improvements to the existing drainage system.

Alternative 2 consists of improving channel, culvert and bridge capaci-

ties to the 10-, 50-, or 100-year design peak flows within the COoper Slough

basin. Grasslined channels were suggested along the study reaches except

along Reach 1 downstream of State Highway 14 where limited right of way dic-

tates the use of a concrete lined channel. Enlarged culverts or bridges are

necessary at all road crossings. A pond west of Cooper Slough at Highway 14

is drained to the channel upstream of the Highway. For the Boxelder Creek

basin, a grasslined channel is proposed with a 2,000 cfs capacity above

Prospect Street. Prospect Street bridge should be enlarged. Downstream of

Prospect Street the channel will remain in its natural state.

Alternative 3 utilizes detention storage to reduce peak flows in the

Cooper Slough basin. The 100-year flood is the basis for sizing the detention

ponds. Detention ponds are proposed at State Highway 14, the Colorado and

Southern Railroad crossing and at locations upstream of Vine Drive. Two

separate ponds are proposed at Highway 14. Larger culverts are necessary at

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6. Sedimentation and Erosion: Planning of drainage improvements should con-

sider sedimentation and erosion processes. The sediment supply changes during

urbanization. In general the sediment supply decreases following urbanization

but may be increased during development. If the sediment supply exceeds the

capacity of the drainage system, deposition will result which may reduce the

capacity of the system. When the sediment supply is less than the transport

capacity, channel erosion may occur resulting in lateral migration, head-

cutting, or bank failure. Hydraulic structures often change flow charac-

teristics resulting in scour or deposition.

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7. Non-Structural Measures: Non-structural measures are most suitable for
undeveloped areas where encroachment to the floodplains has not yet occurred.
Non-structural measures include floodplain zoning, subdivision regulations and
flood insurance.

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