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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ponds. The outlets, emhankments and pond areas must be maintainl'..!d. It is

also necessary to maintain the channel system. Detention ponds were not con-

sidered along Boxelder Creek because the floodplain is narrow and the flooding

is confined to the valley channel. Advantages of this alternative-are the

dual use of the ponds as open space if properly designed, the narrow channel

right of way, the reduced capacity of openings at channel crossings along

Cooper Slough as compared to Alternative 2 and the sequential manner in which

it is possible to implement the improvements as development occurs.

The recommended plan is Alternative 3. The recommended level of protec-

tion is the 100-year flood. Along reaches 2 and 3 of Boxelder Creek, it is

recommended to leave the channel as it is and implement land-use restrictions

for the floodway. Also, the openings at the Larimer-Weld Ditch and Lake Canal

should be enlarged. Along reach 1 of Boxelder Creek, a grass lined trapezoidal

channel should be constructed upstream of Prospect Street. Downstream of

Prospect, proposed access roads for the sludge application area will serve as

berms and the channel will remain in its natural state. The bridge at

Prospect Street should be enlarged to pass the 100-year flow and culverts at

private drains should carry the ten-year flow. A drop control structure and

five erosion control checks are required. Alternative 3 is recommended along

reach 2 of the Cooper Slough. This will require ponds upstream of Vine Drive

with a combined storage volume of 205 acre-feet, an enlarged opening at Vine

Drive and Cooper Slough with a capacity of 300 cfs, a detention pond at the

Colorado and Southern Railroad with a storage volume of 108 acre-feet and an

enlarged opening at the railroad crossing with a capacity of 350 cfs. A chan-

nel capacity of 350 cfs along the reach between the railroad crossing and

Highway 14 is required. A detention pond upstream of Highway 14 with a

storage volume of 112 acre-feet is also necessary. The capacity of the

culvert at Highway 14 is adequate with upstream improvements. A detention

pond west of Cooper Slough near Highway 14 with a capacity of 7B acre-feet is

required. The volume will be drained towards the pond at Cooper Slough and

Highway 14. This volume is drained through the existing culvert under Highway

14. Downstream of Highway 14, an improved concrete-lined channel with a capa-

city of 300 cfs is required. An improved crossing at Sherry Drive is

required. Downstream of here, a grasslined channel will run parallel to Lake

Canal. Near the confluence of Boxelder Creek, an undercrossing will convey

the flow under Lake Canal to Boxelder Creek.

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Preliminary Design

The preliminary design for the recommended plan is based on the 100-year

recurrence interval flows. The design data was presented earlier for both

Alternatives 2 and 3. This information was used in determining the cost data.

Plate 26 is the index to the plates showing the preliminary design for the

recommended improvements. Plates 27 through 33 show the preliminary design

requirements for Boxelder Creek and Cooper Slough based on Alternative 3.

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Plan Implementation

Implementation of the recommended plan should be carried out in a sequen-

tial manner that is coordinated with new development. The detention ponds at

Highway 14 and Colorado and Southern Railroad along Cooper Slough should be

constructed first. This will protect the area downstream of the railroad for

the 50-year recurrence interval flood under existing conditions. The overflow

at Highway 14 is eliminated, and thus the commercial area downstream of

Highway 14 is also protected. With these ponds in place, a peak flow of 630

cfs will occur downstream of the railroad during the 100-year reCurrence

interval flood under existing conditions. This is approximately the existing

channel capacity. The second phase of the plan requires the channel improve-

ments along reach 1 to be implemented. The capacity of the existing culvert

at Highway 14 is the design capacity of the proposed channel system.

The third phase of the plan would implement the improvements along

Boxelder Creek. Improvements along Boxelder Creek are necessary to prevent

floodwater from overflowing the Lake Canal and Larimer-Weld Ditch. The chan-

nel improvements are necessary to prevent water from leaving the channel at

Prospect Street. DownRtream of Prospect Street improvements should occur at

an earlier time due to the impending development of the proposed sludge appli-

cation area. No improvements or analyses were made to alleviate the flow

split at 1-25 and Boxelder. If improvements are made or considered to return

this water to the Boxelder Creek system, the proposed channel improvements and

this analysis should be modified.

The final phase of the implementation would require detention ponds

upstream of Vine Drive as development occurs. Here, further analysis should

be made to locate the suggested storage detention and outlet capacity of 205

acre-feet and 300 cfs, respectively.

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