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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100- and SOD-year storm events. Water surface elevations (WSELs) were
computed using the U.S Army Corps of Engineers HEC-RAS step backwater
program (Reference II). Cross-sections for the backwater program were taken
from project topographic mapping. Hydraulic structures, such as bridges along
the project reach were field surveyed in 2003.

A map index for the floodplain boundary maps is shown on Plate 4. Floodplain
boundaries for the 100- and SOD-year flood events are delineated on Plates 5
through 12. The corresponding flood profiles on plates 13 through 22 present
the water surface elevations and water depths for the 10-, 50-, 100-, and 500-
year flood events relative to the river stream bed. Representative cross-
sections are shown on Plate 23.

4.0 FLOODPLAIN MANAGEMENT APPLICATIONS

4.1 Floodplain Boundaries

To provide a national standard without regional discrimination, the I-percent-
annual-chance (100-year) flood has been adopted by FEMA as the base flood
for floodplain management purposes. The O.2-percent-annual-chance (500-
year) flood is employed to indicate additional areas of flood risk in the
community. The 100-year floodplain boundaries have been delineated using
the flood elevations determined at each cross section. Between cross sections,
the boundaries were interpolated using the best available topographic mapping
at the time of this study.

4.2 Floodways

Encroachment on floodplains, such as structures and fill, reduces flood
carrying capacity, increases flood heights and velocities, and increases flood
hazards in areas beyond the encroachment itself. One aspect of floodplain
management involves balancing the economic gain from floodplain
development against the resulting increase in flood hazard. For purposes of the
National Flood Insurance Program (NFIP), a floodway is used as a tool to
assist local communities in this aspect of floodplain management. Under this
concept, the area of the I DO-year floodplain is divided into a floodway and a
floodway fringe. The floodway is the channel of a stream, plus any adjacent
floodplain areas, that must be kept free of encroachment so that the I DO-year
flood can be carried without substantial increases in flood heights. Minimum
federal standards limit such water surface increases to I,D foot, provided that
hazardous velocities are not produced. The floodways in this study are
presented as minimum standards that can be adopted directly or that can be
used as a basis for additional floodway studies.

ICON Engineering, Inc.
August 2004

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