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 DellnUon Ind Dlvlrslon

It is envisioned that some reaches of the stream may need to be upgraded and that 9 road cross
culverts will need to be enlarged to convey 75 cfs. Typical upgrades to the cross culverts are
shown in Figure 5-2. Also, potential increased erosion of the waterway will need to be
evaluated. The upgrades for the 9 cross culverts were estimated to cost $200,000.

The outflow hydrograph resulting from the installation of the segmented fuseplug is shown in
Figure 6-3. While the fuseplug does not significantly reduce the peak flow at the Larimar and
Weld Canal, it does significantly reduce the storage needed in the downstream detention
alternatives.

5.2 DOWNSTREAM STORAGE ALTERNATIVES

The hydrologic analysis showed that Douglas Dam provides control for the upper portion of the
watershed. However, even with installation ofa fuse plug spillway, the lower portion of the
watershed produces a peak discharge exceeding the proposed 500 cfs diversion. The
downstream watershed contributes a peak of approximately 4,000 cfs. Existing and new
structures were considered when evaluating potential locations for additional storage. The 6
potential new dam sites were located to reduce the peak discharge from the lower portion of the
watershed. These 6 sites can be combined in numerous combinations to achieve the desired peak
discharge when combined with projects at Douglas Dam. The 6 sites are not effective without
Douglas Dam providing detention or storage.

T:\Projects\6824660Dry CrkDiversionLarimetCountylSub_00\12.0 Word ProcIFlood Control P1an.Rptdoc 04124100(11 :14 AM) 5-3

5.2.1 4 Dam Alternative

The 4 Dam Alternative is a combination Dams #1-4. Dams #1-3 were located on tributaries to
the west of Dry Creek. These sites were selected based on topographic information from USGS
quad maps and the outputs of the HEC-1 model. These dams were designed to have a storage
volume of 150-200 acre-feet each and a peak discharge of 50-70cfs. The storage at each site
appears to be present, however the height of the embankments is not defined due to the lack of
topographic information.

Dam #3 may be able to be combined with the current County Road 19. Minor improvements to
the culvert passing through the embankment or a minor raise of the road may be needed to
achieve the correct discharge and storage requirements. The conceptual cross-section for Dam
Nos. 1-3 is shown in Figure 5-3. Dam #1,2 and 3 were estimated to have a cost of $800,000,
$700,000 and $700,000 respectively.

Dam #4 is located upstream of the proposed larger Dam #5. This dam site was selected because
a smaller embankment can be constructed and the area inundated will be less than Dam #5. This
dam is designed to release a peak discharge of 500 cfs at the peak stage of the 100-year event.
The storage produced by this embankment is approximately 184 acre-feet at elevation 5,048 feet.
Figure 5-4 shows the conceptual design for Dam #4. Dam #4 was estimated to cost $1.3 million.
The total cost for the 4 Dam Alternative (not including the fuseplug) $3.5 million.

5.2.2 2 Dam Alternative

The 2 Dam Alternative uses Dams #3 and #5. Dam # 3 was discussed above in the 4 Dam
Alternative. Dam #5 storage is approximately 425 acre-feet at an elevation of 5,040 feet. The
peak discharge from the dam without being overtopped is approximately 500 cfs. A preliminary