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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SECTIINFIVE

Flood Detendon and Diversion

layout for Dam #5 is shown in Figure 5-5. The materials cost and land costs for Dam #5 were
estimated at $1.6 million. The total cost for the 2 Dam Alternative (not including the fuseplug)
$2.3 million.

5.2.3 1 Dam Alternative

The 1 Dam Alternative connects Dam #5 and Dam #6 using a large siphon structure under the
UPRR. The siphon structure allows the two separate storage areas to function as a single
reservoir. Dam #5 operates as the diversion dam and discharge control point. The old borrow
area for the S.H. 287 bypass was identified as an ideal off-line storage area and was labeled as
Dam #6. An embankment could be built on the east portion of the borrow pit. The allowable
storage will be approximately 450 acre-feet at an elevation of 5,040 feet. Dam #6 will have a
small outlet to aid with removing water from the bottom of the reservoir. A preliminary design
for the dam is shown in Figure 5-5.

The Full Equations Model (FEQ) was used to optimize the size of the siphon structure needed to
convey flows under the UPRR. The conveyance structure was determined to be four 6-foot
diameter siphons with an approximate length of 200 feet. The UPRR conveyance structure was
estimated to cost $400,000. The cost of Dam #6 including land costs was estimated at $1.0
million. The total cost for the 1 Dam Alternative (not including the fuseplug) $3.0 million.

5.3 RESIDUAL FLOW DNERSION AL TERNA TNES

5.3.1 Willox Lane Diversion

The width of the diversion channel varies between 20 and 40 feet. Figure 5-6 shows the
proposed layout for the 500 cfs channel. This diversion was estimated to cost $750,000
depending on the Willox Lane and UPRR conveyance structures.

T:\Projects\6824660Dry CrkDiversionlarimelCountylSub_00\12,OWord Proc:IFlood Control Pfan-Rptdoc 04124100(11:14 AM) 5-4

5.3.2 East Vine Diversion

This alternative utilizes the Larimer and Weld Canal to pass residual flood flows to a spill point
located about 0.5 miles east of North Lamay Avenue. The spilled flows would then be routed
downstream to the south, returning to Dry Creek at East Vine Drive. This plan eliminates the
concerns with the FrS on the Cache la Poudre River because spilled flows from Dry Creek will
enter the Cache la Poudre River at the historical confluence of Dry Creek and the Cache la
Poudre River.

The capacity of this diversion will be limited by the Larimer and Weld Canal. The current
design limits the flow in the Larimer and Weld Canal to approximately 500 cfs. A new channel
was designed to convey flows from the Larimer and Weld Canal to East Vine. A profile view of
the new canal is shown in Figure 5-7. Several energy dissipation structures were designed into
the new canal because of the steep grades. There were no structures designed for passing flows
under East Vine.

This alternative will require a SCADA system and a set of gates to empty the canal prior to flood
flows reaching the canal. A downstream set of gates in the bank of the Larimer and Weld Canal
will be installed at the upstream end of the new canal to release flood flows to the new canal.