FLOOD05425

• County: Delta
• Community: Delta
• Stream Name: Gunnison, Uncompahgre River
• Basin: Gunnison
• Title: City of Delta Confluence Park Project
• Date: 10/1/1989
• Prepared For: Delta
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Floodplain Report/Masterplan

PEAK FLOW DM'A
(in cubic feet per second)

Frequency of
Occurrence
(years)

Jncompahgre R.
at Delta

Gunnison R.
upstream of
Uncompahqre_:E-,-
(1983 FEMA)

*Gunnison R.
upstream of
Uncompahqre R.
(1989 COE)

10
50
100
500

3,100
4,400
5,000
6,600

13,500
18,700
21,200
26,700

15,000
23,000
27,000
36,000

* These flows are expected probable flows.

6. Hvdraulic Characteristics of the Rjyer~.

The Gunnis::m River in the pro:iect vicinity meanders,
exhibits bank erosion, and contains well-est:ablished
islands. The channel material appears to be a cohesive silt
with fine sand. No significant amount of coarse deposition
appears in the ~hannel along the study area. Much of the
time the Gunnis:m River appears muddy. The Manning's "n"
factor, or roughness, of the Gunnison River was estimated to
be between 0.032 to 0.05 in the channel. The average slope
of the channel is about 0.0017 fee1: per foot:. The mean
velocity in the study reach varies from about 5.4 to 7.2
feet per second.

The effect of the Aspinall unit: Reservoirs on the
Gunnison River streamflows has been to reduce the occurrence
of large peak flows as well as the occurrence of very low
flows. All Confluence Park projec1: features should benefit
from the reduction of the peak flows, and the boat launch
should benefit from avoidance of very low flows.

The revise:! draft water surface profiles for the 10-,
50-, 100-, and 500-year flows for the Gunnison River in the
project vicinitl' are provided on Plate 6-1. The profile for
the 100-year flow is about a half foot higher .than shown in
the 1983 FEMA flood study report for the arE~as more than
about 1,500 feet downstream of the Highway ~iO bridge. Areas
within 1,500 feet and for some distance upst:ream of the
bridge were found to have the 100-year flow profile lower
than shown in the 1983 study, in some places by over a
foot. The effect of the new flood plain delineation, based
on the revised water surface profile and even higher
profiles with the 1989 hydrology, should be considered in
the design elevation of Confluence Park feat:ures.

The Uncompahgre River in the project vicinity is a
straight channel that also shows evidence of ban]<;: erosion
and sandbar deposition. The banks consist of a cohesive
silt material with small amounts of fine sand. I1uch of the

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