FLOOD10353

• County: Pitkin
• Community: Redstone
• Stream Name: Crystal River
• Title: An Analysis of the Levee System at Redstone Colorado
• Date: 5/1/1992
• Prepared For: CWCB
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

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Redstone

FLOW (cfs)
10-YR 50-YR 100-YR

500-YR

Crystal River above Coal Creek
Crystal River below Coal Creek

2,600
3,000

3,650
4,200

6,000
6,800

3,200
3,700

HYDRAULIC CONDITIONS

Water-surface elevations were computed using the latest
version of the Corps HEC-2 step-backwater computer program
(Version 4.6.0, dated February 1991). Basic input was that used
in the FEMA study and obtained through FEMA headquarters in
Washington, D.C. These data contained cross sections upstream
and downstream of the recently surveyed cross sections (as well
as basic information such as reach lengths and other variables).
The cross-section locations of the recent survey coincide with
those of the 1988 FEMA study and the 1979 county/state study.
The cross section locations (and flood plains from the two
studies) are shown on Attachments 2 and 3. The hydraulic
analysis consisted of the following:

- Develop computer model.
- Update and run the model using current information.
- Vary "n" values in the model to determine sensitivity.
- Revise the model to reflect potential project conditions.

The original FEMA information was used to create an
executable computer file. All of the data were entered exactly
as originally written, including the old cross sections, flows,
etc. The file was then run and debugged until it compared very
closely with the original output for the 100-year water-surface
profile. The few remaining differences were due to changes made
to the HEC-2 program itself (the original FEMA model being run
with a March 1982 version of the program). The water-surface
profile for the run is shown on the first page of Appendix A
(Crystal River Water Surface profiles). The HEC-2 file,
identified as "OLDRS.OUT," is included in Appendix B -(HEC-2 Input
and Output Files).

The computer model was revised to include current
information on (1) revised peak flows, (2) the newly surveyed
cross sections, and (3) revised Manning's "n" values. The peak
flows used are those identified in the Hydrology section above.
Although the flows used are slightly higher than those used in
the 1984 FEMA HEC-2 model, they were believed appropriate since
they were identified in the three study reports and would also
yield a more conservative water-surface profile. Because the
flows were slightly higher than those in the old model, the
resulting water-surface elevations were just a little higher, as

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