FLOOD05212

• County: Larimer
• Community: Fort Collins
• Stream Name: Boxelder Creek, Spring Creek, Dry Creek
• Basin: South Platte
• Title: Flood Control for Boxelder Creek at Fort Collins
• Date: 2/1/1993
• Prepared For: Fort Collins
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Floodplain Report/Masterplan

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channel diversion mayberequiredj however, the diversion will safely pass the flood
events. Velocities for the lD-and SD-yearevents. avenge 4 and 6.5 feet peT second,
respectively. Plate 8 shows the typical cross section ot'the diversion channel.
Mannings "n"valueforthechanncl diversion wasassumoo to be 0.035. Eleven 12-
fool by 6-foo\ by 195-foot-Iong concrete oox eulverts would convey the channel
diversion flows tltrough WiIlox Lane and the UPRR for the lOO-year plan, and four
12.foot by 6-fool concrete box culverts would be required for the 3D-year plan.
Design and construction requirements (or the conCrete box culverts under the
railroad were coordinated with the Union Pacific Railroad.

The concrete box culverts will be constructed in tv.'o phases. Willo)( Lane will
be temporarily detoured to the north of the construction during both phases of
construction. At the beginning of the rust phase, the railroad will he temporarily
relocated 10 the west by constructing a railroad shoefly. AU railroad track
construction and ballast construction, excludingeartllwork (cut-fill and grading),wiU
have to be accomplished by the Union Pacific Railroad (UPRR). The cost of this
work ....ill have to be paid to UPRR under a cost reimbursahle contract. After the
ftrst phase of the box culvert construction h completed, the rni1road will be
reconstructed in its original alignment over the new box culvert. The sc.;ond phase
of the box culvert construction will then be accomplished. Willox Lane will be
reconstructed after the SCC(Ind phase of construction iscompletcd.

Two alternate channel alignments were discussed with UPRR and were
investigated. The first option was to shift the ehanne1 alignment to the norJl'ooCSL
This would require three new bridges or three ncw box culvertS at Willox Lanc, thc
railroad line, and the road cast of the railroad. Thc second option would use the
original channcl alignment, construct a bridge on the railroad line at WilIox Lane,
con~ln.lCI lilt overpa.s~ on Willox Lane over the railroad line, and construct a bridgc

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on the road to the east of Ihe railroad. Both options were more expcnsive than the
selcc!ed plan due to added real estatc relocations, easements, and higher
oonstructioncosts.

All maintenance, repair, and replacement costs for the new box eulvens would
be the responsibility of the sponsor. Negotiating these agreements with the Union
Pacific Railroad will be the responsibility of the sponsor.

From WilIox Lane to the Cache La Poudre River, the slope of the diversion
channel wnuld be 0.004]3. Upstream from WilIox Lane, the slope would be 0.0034.
The diversion channel would be about 5,650 feet in length. A Tuas crossing would
be required across an existing road approximat::ly 1,200 feet from the downstream
end of the diversion channel. Earthen berms would be required, beginning
approximately 50 feet downstTcam from the Texas crossing for a length of
approximately 1,700 feel. These berms would be located in a low area of the
channel alignment and would prevent the applicable lOO-year or 3o.year flows from
flooding adjacent low areas. The berms would have a top width of 10 fee! and
IV on 6H sideslopes, and would average aboul 1.5 feet in height including] foot of
freeboard.

Riprap would beplaC<<l atlhe downstream end of all boxCIIlvertsand at the
outlet of the di~'ersion channel into the Cache La Poudre River. Riprap would al~
be placed upstream and dOllmstream from the Larimer and Weld Canal crossing and
atlhediversion weir.

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