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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expected. The profile is shown on the second page of Appendix A.
The HEC-2 file is identified as "OLDRSNWQ.OUT" in Appendix B.

The model was then revised (January 1991) to include newly
surveyed cross sections (87 to 91.5 on Attachment 2) to reflect
existing conditions adjacent to Redstone. (See Appendix E.) As
mentioned, the location of the cross sections correspond to those
of the previous studies with the exception of one additional
cross section (cross section 91.5) which was added just upstream
of the existing levee. A minor discrepancy was noted in that the
channel invert at cross section 86 (old survey) was slightly
higher than the next upstream cross section .(new survey).
Adjacent highway elevations at both sections were checked to
confirm that the cross sections had been taken at the same
locations indicated on the mapping. Pitkin County, which is
responsible for stream maintenance, indicated that some channel
clearing had been done in this reach and also that the sediment
regime fluctuates greatly on the Crystal River. To compensate
for the discrepancy, the downstream cross section was lowered
slightly (to be just lower than the upstream section). Also, a
few minor code changes were made to the input deck as recommended
by the Corps Hydrologic Engineering Center.

Manning's "n" values were raised slightly at some cross
sections to reflect present conditions. To aid in this
evaluation, a series of photographs and video recordings were
taken at each cross section. A general "n" value determination
was also made for the channel using the methodology described in
a U.S Geological Survey publication entitled "Determination of
Roughness Coefficients for Streams in Colorado" and a technical
paper by Robert Jarrett entitled "Hydraulics of High-Gradient
Streams" published in the November 1984 Journal of Hydraulic
Engineering. This procedure resulted in a general "n" value of
0.05 for the channel. Slight variations were incorporated where
physical differences (dense brush, etc.) in the channel were
identified. Overbank "n" values were based on the classification
chart in "Open-Channel Hydraulics" by Ven Te Chow (and also
included in the above references).

The resulting water-surface profile is shown on the third
page of Appendix A. The HEC-2 file was identified as "NEWRS.OUT"
and is included in Appendix B. Cross sections of the channel and
overbank areas showing the resulting 100- and 500- year water-
surface elevations were subsequently plotted. These are included
in Appendix C (Flood Plain Cross sections).

The sensitivity of the water-surface profiles was checked by
varying the Manning's "n" values. A channel "n" of 0.6 and
overbank "n's" of 1.0 were inserted into the model. These values
were higher than all previous flood studies and are believed the

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