FLOOD09227

• Designation Number: 530
• County: Adams
• Community: Unincorporated Adams County
• Basin: South Platte
• Title: FIS - Adams County and Incorporated Areas Volume I of III
• Date: 8/1/1995
• Designation Date: 9/1/2000
• Floodplain - Doc Type: Floodplain Report/Masterplan

(Reference 36). The 500-year discharge on all streams was obtained
from a straight-line plot on log probability paper.

The lO-, 50-, and lOO-year values for Grange Hall Creek, South Fork
Grange Hall Creek, and Grange Hall Creek Tributary were provided by
the Denver Region UDFCD (Reference 37).

The lO- and lOO-year values for Big Dry
the Denver Region UDFCD (Reference 20).
available.

Creek were also provided by
The 50-year values are not

The lO-, 50-, and lOO-year values for Basin 4100 and Brantner Gulch
were computed by the study contractor.

Big Drv Creek (Above Interstate 25) and Tanglewood Creek

Since no stream gage data were available for the streams studied, a
rainfall-runoff analysis was conducted on the watersheds to
determine the flood discharges. This was accomplished by using the
UDFCD CUHP rainfall-runoff computer program to develop the storm
hydrographs (Reference 31) and the USACE HEC-l flood hydrograph
package computer program for the stream and reservoir routings
(Reference 38). For this analysis, basin characteristics which
define the size, shape and runoff characteristics of the watershed
as well as rainfall amounts based on the selected recurrence
intervals (obtained from the National Oceanic and Atmospheric
Administration atlas of precipitation - Reference 32) are used to
compute flood hydrographs for various design points in the basin.
All stream and reservoir routings were accomplished using the
Modified Puls Method.

Since there was a lack of 500-year precipitation data, the 500-year
frequency storm runoff values at each design point were calculated.
The logarithmic values of the lO-, 50-, and lOO-year peak
discharges were fit to a regression line by method of least
squares. The 500-year discharges were analytically extrapolated
from the regression line based upon a log-normal probability
relationship.

Peak discharge-drainage area relationships for streams studied by
detailed methods are shown in Table 2.

3.2 Hydraulic Analyses

Analyses of the hydraulic characteristics of flooding
sources studied were carried Qut to provide estimates
elevations of floods of the selected recurrence intervals.

from
of

the
the

For all streams studied
elevations for floods of
computed using the USACE
(Reference 39).

by detailed methods, the water-surface
the selected recurrence intervals were
HEC-2 step-backwater computer program

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