FLOOD00413

• Designation Number: 339
• County: Delta
• Community: Unincorporated Delta County
• Title: FIS - Delta County, Colorado, Unincorporated Areas
• Date: 8/1/1991
• Designation Date: 3/1/1993
• Floodplain - Doc Type: Floodplain Report/Masterplan

This section has been added to
significant revisions made since the
printed. Future revisions may be
republishing of the Flood .Insurance
user is aware of all revisions,
community repository.

provide information regarding
original Flood Insurance Study was
made that do not result in the
Study report. To ensure that any
it is advisable to contact the

9.0 REVISIONS DESCRIPTIONS

9.1

First Revision

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This study was revised on August 19, 1991, to include the restudy
conducted on a portion of Gunnison River for the City of Delta
and Delta County and to include the most up-to-date corporate
limits for these two communities.

Gunnison River was restudied from its confluence with Uncompahgre
River in the City of Delta to approximately 0.9 mile upstream of
the U.S. Highway 50 bridge in Delta County. The hydrologic and
hydraulic analyses for' Gunnison River were prepared by the U.S.
Army Corps of Engineers, Sacramento District, for the Federal
Emergency Management Agency, under Interagency Agreement EMW-88-
2768. This restudy was completed in May 1989.

The hydrologic analyses developed by the Corps of Engineers were
based on the 1983 Flood Insurance Study for Delta County
(Reference 34). The 1988 hydrologic data for Gunnison River
upstream from Delta County were updated for a Water Control
Manual for Blue Mesa Dam and ReseTvoir on the Gunnison River
(Reference 33). The new data were. only slightly higher than data
in the 1983 Flood Insurance Study and the 1980 Flood Hazard
Information report. Since the new flow' estimates fell within the
90-percent confidence interval of the existing published flow
values, the original flow values were used for this restudy.

The revised HEC-2 hydraulic analyses for this reach of Gunnison
River utilized new sections between the mouth of the Uncompahgre
River and the U.S. Highway 50 bridge, based on 1988 topography.
For upstream of the U.S. Highway 50 bridge, cross section data
from the previously effective Flood Insurance Study were used.

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The starting water-surface elevations for the revised study were
taken from the previously effective Flood Insurance Study, which
used the slope-area method of determination.

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Roughness coefficients (Manning's "n") used in the hydraulic
computations for the revised study were chosen by engineering
judgment, based on field observations of the stream and
floodplain areas. The roughness coefficients used for the main
channel of Gunnison River varied from 0.032 to 0.036, and the
value used for the overbank was 0.070.

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