FLOOD07906

• County: Summit
• Community: Frisco
• Stream Name: Tenmile Creek
• Basin: Colorado Mainstem
• Title: Flood Hazard Mitigation Tenmile Creek Frisco
• Date: 5/1/1996
• Prepared For: Frisco
• Floodplain - Doc Type: Floodplain Report/Masterplan

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was investigated to see if the levee system along the right bank could be reduced or
eliminated. Alternative 3 would be the most expensive alternative.

The three methods were evaluated based on the criteria of rmtlgation benefits,
construction methods and costs, and project feasibility. The recommended plan of
improvement is presented in section VIII.

VII. ENGINEERING ANALYSES

The lOO-year peak flows used for this study were obtained from the Town of Frisco Flood
Insurance Study (FEMA, 1994). A summary of the peak flows is as follows:

Location 100- Year Discharl!e (cfs)
Tenrnile Creek at Lake Dillon 1980
Tenmile Creek below North Tenmile Creek 1900

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The hydraulic analysis for this study was performed using the U.S. Army Corps of
Engineers HEC-2 step-backwater computer program. The base HEC-2 model used for
this study was prepared by Muller Engineering Company, Inc. under contract with FEMA
for the completion of hydraulic analyses for the Town of Frisco Flood Insurance Study
(1994). Three new HEC-2 models (study models) were created by the CWCB for the
purposes of this study. The first study model represents revised existing, and the second
and third study models represent the proposed conditions which include the alternative
mitigation improvements. A floodway analysis was not performed for this feasibility
study. The HEC-2 output files for the revised existing conditions and for the proposed
conditions (recommended plan only) are included in Appendix A.

Starting water surface elevations for the lOO-year event in the study models were
estimated using the slope-area option of HEC-2. Selected "n" values in the revised
existing and proposed conditions models were adjusted to improve consistency and to
better reflect actual conditions of the channel and overbank areas. A table of "n" values
for the effective, revised existing, and proposed conditions HEC-2 models is presented in
Appendix B. Three cross-sections were added to the study models based on field
surveyed cross-sections obtained by the CWCB. For simplicity, the fill material in the
proposed conditions HEC-2 runs were modeled by using X3 cards to block overflow
along the right channel bank.

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The revised existing conditions and proposed models eliminated the right bank split flow
that was modeled by Muller Engineering and shown on the effective FIRM panel. The
elimination of the split flow can be attributed mainly to the decrease in water surface
elevations resulting from the revised "n" values in the CWCB models. The split flow
elimination caused a higher discharge and slightly higher water surface elevations in the
downstream portion of the stream reach of the main channel from station 0+00 to 13+00.
However, there are no anticipated adverse impacts due to the water surface increases in

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