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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SECTII.SIX

AltlrnaUVls AnalVsls

The hydrologic model results indicate that a fuseplug at Douglas Dam will be effective in
reducing peak flood discharges downstream. Figure 6-1 provides hydrographs at Douglas Dam.
However, runoff from the watershed area below Douglas Dam creates a twin peaked hydrograph
at the Terry Lake Inlet and Larimer and Weld Canal. Figures 6-2 and 6-3 provides hydrograph
plots at the Terry Lake Inlet and Larimer and Weld Canal assuming varying flood storage in
Douglas Reservoir. Therefore, flood water storage at Douglas Reservoir is only effective in
reducing the peak discharge rates downstream to a specific point. Essentially, the peak: discharge
downstream at the Larimer and Weld Canal cannot be reduced below approximately 4,000 cfs
even if all of the runoff above Douglas Dam could be stored within the reservoir. The optimum
fuseplug design for Douglas Dam varies slightly with the incorporation of the downstream
storage alternatives. The fuseplug would store 2.5 to 3 feet of water during the 1 DO-year event.

Since improvements of Douglas Dam can not control the peak flow rates downstream, three
downstream storage alternatives were evaluated with the fuseplug. These three alternatives
cover both extremes and combination scenario. The 1 Dam Alternative provides all additional
storage at one site. The 4 Dam Alternative provides additional flood storage and protection
throughout the lower portion of the Dry Creek watershed. The 2 Dam Alternative is a
combination of the 1 and 4 Dam Alternatives. The hydrologic response from each alternative
attains a similar residual discharge downstream. The discharges for each alternative are shown
in Table 6-1.

Table 6-1
Hydrologic Model Summary of Alternatives

6,957
420
2,613

6,956
420
1,463

6,956
420
647

5,026
5,021

3,814
3,779

3,814

560*

2,669
546

1,111
520

* Flowrate Developed by the FEQ Model for the I Dam Alternative

The storage alternatives at Douglas Reservoir utilize stage-storage curves and spillway rating
curves previously prepared for the structure. Stage-storage curves and discharge rating curves
for Dam #4, Dam #5 and Dam #6 were based on 2 foot topographic information, dated 1999.
Dam sites #1-3 were estimated using USGS topographic information. These storage sites can be
integrated in several combinations to achieve a discharge of approximately 500 cfs at the
Larimer and Weld Canal. The HEC-l model input files are located in the Technical Appendix.

The modified storage and diversion schemes, which were studied for this report, provide a more
comprehensive alternative to flood control when compared to the original 5,000 cfs diversion
scheme. The new schemes involved flood attenuation alternatives and alternate diversion sizes
and locations.

T:\Projects1682466OOry CrkDiversionLarimerCountylSub_0ll\12.0Word ProcIFIood Control Plan-Rptdoc 04124100(11:14 AM) 6-1