FLOOD10780 ‎(3)‎

• County: Delta
• Community: Delta County and Unincorporated Areas
• Title: FIS - Delta County and Unincorporated Areas
• Date: 3/15/1984
• Prepared For: Delta County
• Prepared By: FEMA
• Floodplain - Doc Type: Current FEMA Regulatory Floodplain Information

greater than 1 year are considered. For example, the risk of having a
flood which equals or exceeds the lOO-year flood (1 percent chance of
annual occurrence) in any SO-year period is approximately 40 percent (4
in lQ); for any 90-year period. the risk increases to approximately 60
percent {6 in IO}. The analyses reported here reflect flooding
potentials based on conditions existing in the county at the time of
completion of this study. Maps and flood eievations will be amended
periodically to reflect future changes.

3.1 Hydrologic Analyses

Hydrologic analyses were carried aut to establish the peak
discharge-frequency relationships for floods of the selected
recurrence intervals for each flooding source studied by detailed
methods affecting the county.

For Gunnison, North Fork Gunni son, and Uncompahgre Rivers, peak
discharges for floods of the 10-, 50-, 100-, and 500-year
recurrence in terval s were computed by the U . S. Army Corps of
Engineers (Reference 10).. For the U.S. Army Corps of Engineers
determination, a regional floodflow-frequency analysis of gaging
station records was made to obtain generalized relationships of
the mean, standard deviation and skew coefficients to develop
floodflow-frequency relationships. This analysis included a
separate determination of rainflood and snowmelt peaks.

The floodflaw-frequency curves for Gunnison and North Fork
Gunnison Rivers were based primarily on streamflow data collected
at u.s. Geological Survey gages located near Somerset, near Grand
Junction, and below the Gunnison Tunnel, and secondarily on
values adopted from other gages in the area. The peak flows
recorded since the regulation of flow by Paonia Reservoir and
Blue Mesa Reservoir were also used in the determina.tion of the
peak flows for both rivers. The present condition peak-flow
frequency curves for the ungaged sites ~n North Fork Gunnison and
Gunnison Rivers we~e developed by applying the difference between
the pre-regulated and present conditions for the gage located
near Somerset and the gage located below the Gunnison Tunnel to
the pre-regulated peak-flow frequency curves for the ungaged
sites.

For Uncompahgre River, the U.S. Army Corps of Engineers developed
frequency curves for rainfall and snowmelt events at the Delta
and Colona u.s. Geological Survey stream gages (Reference 10).
The Del ta gage is located approximately 20 mi les downs tream of
Olathe, and the Colona gage mea.sures flow on Uncompa.hgre Ri ver
approximately 20 miles upstream of Olathe. These frequency
curves were computed by statistical analyses of stream-gage data
for 58 years of record at the Delta gage and 57 years of record
at the Colona gage. The independent frequency curves for
rainfall and snowmelt events were combined statistically to get
"all eventl1 frequency curves at the gage locations. Because the
two all event frequency curves were nearly identical at the two
gage locations, the peak discharges for the 10-, 50-, 100-, and

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