FLOOD09193

• Designation Number: 579
• County: Gunnison
• Community: Marble and Vicinity
• Stream Name: Crystal River
• Basin: Gunnison
• Title: Floodplain Information Report - Crystal River - Marble and Vicinity
• Date: 8/1/2004
• 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.t 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 flood
discharges were determined using annual peak flow data from the Placita and
Avalanche Creek gages, located just downstream of the Town of Marble. The
calculated peak flood 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. I
(Reference 10).

Peak discharge-drainage area relationships for the Crystal River are shown in
Table 3.

Table 3.' Summary of Discharges for the Crystal River

Flooding Source and Location Drainage Area Peak Discharge (cfs)
Along Crystal River (Square Miles) to-Year 50-Year tOO-Year 500- Year
Confluence with Rapid Creek 93.05 1,994 2,445 2,625 3,028
Confluence with Milton Creek 83.65 1,833 2,248 2,413 2,783
Town of Marble 73.59 1,657 2,031 2,181 2,515
Upstream Limit of Study 53.32 1,284 1,575 1,691 1,950

Peak flood discharges presented in Table 3 compare weIl with those presented
in the 1979 Floodplain Information Report for the Crystal River and Coal
Creek, prepared for Pitkin County by Wright Water Engineers, Inc. This 1979
report indicates a 100-year peak discharge of 2,400-cfs at the Town of Marble.
This compares within 10% of the peak flood discharge presented in Table 3 at
the Town of Marble (2,181-cfs). The hydrologic analysis for this study
incorporates an additional 22 years of gage records beyond the 1979 study.

3.2 Hydraulic Analysis

Analysis of the hydraulic characteristics of flooding from the Crystal River
were carried out to provide estimates of the flood elevations for the 10-, 50-,

ICON Engineering, Inc.
August 2004

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