FLOOD09797

• Designation Number: 137
• County: Weld
• Community: Pierce
• Basin: South Platte
• Title: FIS - Pierce
• Date: 5/1/1974
• Designation Date: 5/1/1979
• Floodplain - Doc Type: Floodplain Report/Masterplan

3.0 ENGINEERING METHODS

For flooding sour~es studied in detail in the community, standard hydrologic
and hydraulic study methods were used to determine the flood hazard data
required for this study. Floods having recurrence intervals of 10, 50,
100, and 500 years have been selected as having special significance for
flood plain management and for flood insurance premium rates. The analyses
reported here reflect current conditions in the watersheds of the flooding
sources.

3.1 Hydrologic Analyses

Hydrologic analyses were carried out to establish the peak discharge-
frequency relationships for floods of the selected recurrence inter-
vals for each stream studied in detail in the community.

There are no streamflow gages on Spring Creek. A search of records
of existing gaged streams in Colorado indicated that there are no
basins of either similar size and physiographic characteristics or
with recorded rainfall distributions from which comparisons could be
made for a model of the basin hydrology. Therefore, synthetic
hydro graph analysis was used to develop storm discharges.

Rainfall ~ata and distributions used in the development of peak
disGharges for Spring Creek were obtained from the National Oceanic
and Atmospheric Administration Atlas for Colorado (Reference 5).
The U.S. Soil Conservation Service Method, as described in the
National Enqineerinq Handbook (Reference 6), was selected as the
method which would best represent the runoff characteristics of the
Spring Creek basin. Weighted average curve numbers were computed
using soil and land use maps for Weld County (References 7 and 8) .
The times of concentration were determined using stream hydraulics
of approximate cross sections taken from U.S. Geological Survey
topographic maps at a scale of 1:24,000, with a contour interval of
10 feet (Reference 9). Discharges were computed for the 10- and
100-year flood events and plotted on log-probability paper. The 50-
and 500-year discharges were obtained by straight-line extrapolation
of the 10- and 100-year values.

Peak discharge-drainage area relationships for the main channel of
Spring Creek and Spring Creek Overflow are shown in Table 1.

The discharges in Table 1 represent computations of split flow which
occurs at the intersection of Rowe Avenue and U.S. Highway 85, as
well as distribution of weir flow along the Union Pacific Railroad
tracks.

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