FLOOD10704 ‎(2)‎

• County: Delta
• Community: Delta
• Title: FIS - Delta
• Date: 8/19/1991
• Prepared For: Delta
• Prepared By: FEMA
• Floodplain - Doc Type: Current FEMA Regulatory Floodplain Information

Locations of selected cross sections used in the hydraulic analyses
are shown on the Flood Profiles (Exhibit 1). For stream segments
for which a f100dway was computed <Section 4.2), selected cross
section locations are also shown on the Flood Inauranc:e Rate Map
(Exhibit 2).

A levee has been constructed along the left bank downstream of the
u.s. Highway 50 bridge. The flood of 1984 (25,500 cfs) was similar
in magnitude to the lOa-year flood, and the site was not flooded.
However, the levee that surrounds the 1aaoons between the Gunnison
and Uncompahgre Rivers was inspected by the U.S. Army Corps of
Engineers and found to have openings and insufficient freeboard to
provide certifiable protection from the 109-year flGod; therefore,
for the purpose of this study, this levee is assume4 to fail.

Roughness factors (Manning's "n") used in the hydraulic computation
for the study streams were chosen by engin.eerin.g jWQ.glBent and based
on field observations of the flooding sources and floodplain areas.
The roughness values for the strea.AlS studied in this report are
tabulated below.

Stream

Roughness Coeffi.cients
Main Channel Overbank

Gunnison River
Uncompahgre River

0.032 to 0.036
0.035

0.070
0.04 to 0.06

For the Gunnison and Uncompahgre Rivers, the water-surface
elevations of floods of the selected recurrence intervals were
computed through use of the U. S. Army Corp-s of Engineers HEC-2
step-backwater computer program (Reference 11).

Starting water-surface elevations for the Gunnison and Uncompahgre
Rivers were determined by the slope~area method.

The hydraulic analyses for this study were based on unobstructed
flow. The flood elevations shown on the profiles are thus
considered valid only if hydraulic structures remain unobstructed,
operate properly, and do not fail.

All elevations are referenced to the National Geodetic Vertical
Datum of 1929 (NGVD). Elevation reference marks used in this study
are shown on the maps; the descriptions of the marks are presented
in Elevation Reference Marks (Exhibit 3).

4.0 FLOODPLAIN MANAGEMENT APPLICATIONS

The NFIP encourages State and local governments to adopt sound
floodplain management programs. Therefore, each Flood Insurance Study
provides lOO-year flood elevations and delineations of th,e 100- and
500-year floodplain boundaries and lOO-year floodway to assist
communities in developing floodplain management measures.

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