PROJ02027

• Contract/PO #: FS0044FX
• Contractor Name: Larimer County, Board of County Commissioners
• Contract Type: Miscellaneous
• Water District: 0
• Loan Projects - Doc Type: Feasibility Study

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SECTIINFOUR

Peak fll. Dlverslln

Willox Lane was the only diversion location considered from the conceptual alternatives for
passing the peak flow from Dry Creek to the Cache la Poudre River. The diversion structure
would need to be designed to pass the peak developed 100-year design discharge (approximately
5,000 cfs) at the Larimer and Weld Canal. Preliminary evaluations indicated that this structure
would need to be 200 to 250 feet wide and would require conveyance structures beneath both
Willox Lane and the UPRR. These structures would be very expensive. In addition to the cost
of the structures there was a technical concern as to the feasibility of directing the entire 5,000
cfs flow into the road and railroad crossing structures.

Preliminary options evaluated for crossing of Will ox Lane and the UPRR have included 1) bored
and jacked siphon structures, 2) open trench excavation of culvert structures, 3) tunnel structures,
and 4) bridges.

Options for the diversion channels downstream of the UPRR crossing generally consist of
trapezoidal shaped earthen and grass lined channels with 2H: 1 V side slopes and from 4 to 6 feet
in depth. For a design discharge of5,000 cfs, the widths range from 200 to 250 feet. Figure 4-1
shows the proposed alignment of the 5,000 cfs channel.

The 5,000 cfs diversion was estimated to cost up to $6 million. The costs for this option are due
to the various conveyance structures for Willox Lane and the UPRR. The final selection of a
specific conveyance structure could not be selected with the information currently available.

The major technical concern for this alternative was how to capture the flood flows from the
wide Dry Creek floodplain and funnel the flows into the conveyance structure. Another
technical concern was the flood flow impact to the Cache la Poudre River. The concern was
adding an additional 5,000 cfs to the Cache la Poudre River and addressing the FEMA floodplain
Issues.

If there is an increase in flood heights on the Cache la Poudre River floodplain then mitigation or
flood easements would be required from each individual property owner. There are
approximately 255 structures within the Cache la Poudre River floodplain between the
confluence and the old confluence of the Cache la Poudre River that would need to be mitigated.

T:\Projects\6824660Dry CrkDiversionLarimerCounly\Sub_00\12,O Word ProcIFlood Control Plan.Rptdoc 041241OO{11 :14 AM) 4-1