FLOOD10349

• County: Gunnison
• Community: Marble and Vicinity
• Stream Name: Crystal River
• Title: Floodplain Information Report - Crystal River - Marble and Vicinity
• Date: 5/1/2004
• Prepared For: Marble and CWCB
• Prepared By: ICON Engineering, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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3.0 ENGINEERING METHODS

3.1 Hydrologic Analyses

Hydrologic analyses were carried out to establish the peak discharge-
frequency relationships for the Crystal River in the vicinity of the Town of
Marble, Colorado.

Discharges along the Crystal River were calculated using the PEAKFQ
computer program (Reference 2) which performs statistical flood
frequency analyses of annual peak discharges following procedures
recommended in Bulletin 17-B of the Interagency Advisory Committee on
Water Data (1982). The analysis for the Crystal River incorporates USGS
gage information with over 46 years of record, along the Crystal River
(1956 to 2001). Peak discharges were determined using annual peak
discharges from the Placita and Avalanche Creek gages, located just
downstream of the Town of Marble. The calculated peak discharges were
adjusted at various locations along the study reach based on the guidelines
provided in the Manual for Estimating Characteristics of Natural-Flow
Streams in Colorado. Technical Manual No.1 (Reference 5),

Peak discharge-drainage area relationships for the Crystal River are shown
in Table 2.

Table 2: Summary of Discharges for the Crystal River

Flooding Source and Location
Along Crystal River
Confluence with Rapid Creek
Confluence with Milton Creek
Town of Marble
Upstream Limit of Study

Drainage Area
(Square Miles)
93.05
83.65
73,59
53.32

10-Year
1,994
1,833
1,657
1,284

Peak Discharge (cfs)
50-Year 100- Year
2,445 2,625
2,248 2,413
2,031 2,181
1,575 1,691

500- Year
3,028
2,783
2,515
1,950

3.2 Hydraulic Analysis

Analysis of the hydraulic characteristics of flooding from the Crystal
River were carried out to provide estimates of the elevations of floods for
the 10-, 50-, 100- and 500-year storm events, Water surface elevations
(WSELs) were computed using the U. S Army Corps of Engineers HEC-
RAS step backwater program (Reference 6), Cross-sections for the
backwater program were taken from project topographic mappmg.
Hydraulic structures, such as bridges along the project reach were field
surveyed in 2003.

ICON Engineering, Inc.
May 2004

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