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

500-year floods were computed to be constant between the gages.
This indicates that attenuation and stream diversions
counterbalance tributary inflow between the stations.

For Minnesota Creek, Surface Creek, and Cedar Run, peak
discharges were computed by performing a regional floodflow-
frequency analysis for a mixed flow population of snowmelt and
rainfall (Reference 13). For the regional study, U.S. Geological
Survey stream-gage records for 15 gaging stations in
hydrologically similar watersheds were analyzed (Reference 9).
The station data were separated into rainfall and snowmelt peaks.
Frequency analyses were conducted on the rainfall and snowmel t
data (Reference 14). The results of the snowmelt and rainfall
frequency analysis were combined statistically to give a maximum
flow for both event s. Regional curves of drainage area versus
discharge were computed for the four frequency floods.

Peak discharge-drainage area relationships for Gunnison, North
Fork Gunnison, and Uncompahgre Rivers, and Minnesota and Surface
Creeks are shown in Table 1.

3.2 Hydraulic Analyses

Analyses of the hydraulic characteristics of the flooding sources
studied in the county were carried out to provide estimates of
the elevations of floods of the selected recurrence intervals
along each of these flooding sources.

Water-surface elevations for floods of the selected recurrence
intervals were computed through use of the U. S. Army Corps of
Engineers HEC-2 step-backwater computer program (Reference 15).

Cross sections used in the backwater analyses for Gunnison, North
Fork Gunnison, and Uncompahgre Rivers were obtained from
u.S. Army Corps of Engineers Flood Hazard Information reports for
these rivers (References 5 and 9). These cross sections were
developed from field survey data furnished to the U.S. Army Corps
of Engineers by the Colorado Water Conservation Board. Cross
sections used in the backwater analyses for Minnesota and Surface
Creeks, and Cedar Run were obtained by aerial photogrammetry
(Reference 16). The below-water sections of all cross sections
were obtained by field measurement. All bridges, dams, and
culverts were field surveyed to obtain elevation data and
structural geometry. Several small private driveway bridges
which were not considered sturdy enough to withstand peak floods
were assumed to be washed out. The results obtained from the
HEC-2 computer program were adjusted in the vicinity of bridges
to more accurately represent actual flooding conditions
(Reference 17).

Locations of selected cross sections used in the hydraulic
analyses are shown on the Flood Profiles (Exhibit 1). For stream
segments for which a floodway is computed (Section 4.2), selected

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