FLOOD00588

• County: Delta
• Community: Orchard City
• Title: FIS - Orchard City
• Date: 1/16/1981
• Prepared For: Delta County
• Prepared By: FEMA
• Floodplain - Doc Type: Current FEMA Regulatory Floodplain Information

Table 1. Summary of Discharges (Cont'd)

Flooding Source
and Location

Drainage Area
(Square Miles)

Peak Discharges
(Cubic Feet Per Second)
10-Year 50-Year 100-Year 500-Year

Cedar Run
At Upstream Study Limit,
Southwest Corner,
Section 6 2.47 90 138 179 280
At Eckert Cemetery 3.14 110 163 220 325
At Orchard City Post Office 3.43 113 175 222 345
At Half Section Line of
Section 26 3.96 126 193 245 380
At Downstream Study Limit 4.68 142 217 275 430
3.2 Hydraulic Analyses

Analyses of the hydraulic characteristics of streams in the com-
munity were carried out to provide estimates of the elevations
of floods of the selected recurrence intervals along each stream
studied in the community.

For Surface Creek and Cedar Run, the water-surface elevations
of 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 9). The results obtained from the HEC-2 computer
program were adjusted in the vicinity of bridges to more accurately
represent actual flooding conditions (Reference 10).

Cross sections for the backwater analyses of Surface Creek and
Cedar Run were obtained by aerial photogrammetry (Reference 11).
The below-water sections were obtained by field measurement.
All bridges, dams, and culverts were field surveyed to obtain
elevation data and structural geometry. On both streams, several
small private driveway bridges, which were not considered sturdy
enough to withstand peak floods, were assumed to be washed out
during any major flood.

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 cross
section locations are also shown on the Flood Boundary and Floodway
Map (Exhibit 2).

Roughness factors (Manning's "n") used in the hydraulic comput-
ations for the study streams were chosen by engineering judgment
and based on field observations of the flooding sources and flood
plain areas. Roughness values for the main channel of Surface
Creek and Cedar Run ranged from 0.017 to 0.040, and flood plain
roughness values ranged from 0.035 to 0.070.

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