FLOOD09878

• County: Logan
• Community: Logan County
• Stream Name: Pawnee Creek
• Basin: South Platte
• Title: Floodplain Management Study
• Date: 4/1/1992
• Prepared For: Logan County
• Prepared By: USDA Soil Conservation Service
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Fig 2 ~~~N~E CR ~LDOD PLAIN MA~~GEMEN~ S~UDY

TR2li'l S::HE"ATIC

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A table showing peak discharge, location, and frequency was
developed for this study. The procedures used in an earlier
study on Lonetree Creek (similar drainage to the west) was used.

Two procedures were tested to see which produced the more
critical results. Procedure (1) involved the use of the SCS TR-
20 Model in conjunction with rainfall depth data from NOAA Atlas
for Colorado, areal adjustment of rainfall using NOAA Hydro~et
51, and the areal distribution of rainfall to the entire basin
using an elliptical storm pattern in NOAA Hydromet 52.
Procedure (2) involved applying a local storm pattern exclusively
to individual sub-basins in the lower part of the basin. The
elliptical storm pattern was not used for this local storm
procedure, but rainfall was assumed to be uniformly distributed
and adjusted for areal extent using the procedure in NOAA Atlas.
The peak discharges developed by the local storm were used to
support the concept that the basin peak discharge for a given
frequency cannot be less than that produced by an individual sub-
basin. WhiChever storm pattern produced the greater peak
discharge should be used for planning or design purposes.
The SCS 24 hour Type II rainfall distribution was used to apply a
time distribution to rainfall depths for both procedures
discussed previously.
Hydrologic parameters necessary for the TR-20 Model were
developed from available 50ils and land use maps. Time of
concentration values were based on standard SCS procedures.
Precipitation values from NOAA Atlas for Colorado were considered
the best reference for depth-frequency data, hO\<l~v~!' its <ue"l
adjustment and distribution procedure does not seem appropriate
for large drainage areas such as involved in the general basin
wide storm analysls for this study. Therefore Hydromet 51 and 52
were used for th~t purpose, except for the loc~l storm procedure
The Modified Att-kin reach routing method was used by the model
to move hydrographs through stream re~ches.
Figure 2 and inCluded Table 1 show a schematic view of the basin
and values of basic parameters used in the TR-20 Model.
Hydraulic analyses were used along with TR-20 to determine the
carrying capacity of Pawnee Creek itself and to help define where
and how much out-of-channel flow occurs. It was determined that
significant out-of-bank division of flow occurs at several
loc~tions along the study reach.

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