FLOOD09640

• County: Weld
• Basin: South Platte
• Title: Supplementing Flood Frequency Analysis in Eastern Colorado using Rainfall Modeling
• Date: 8/2/1999
• Prepared For: CWCB
• Floodplain - Doc Type: Flood Documentation Report

Station (a) is situated in a fairly straight and uniform reach of the main channel. The channel is
composed of sandy loam carved through badland formations. The main channel is well
developed and bordered by grassy slopes on either side. A bend approximately 300 feet down
stream separates the stations. The bend appears to be stable, carved out of the badland material
and having a vertical face extending approximately 20 feet above the channel bed.

The upstream cross-section had better preserved and more evident PSIs over that of the
downstream section. Small woody debris was found on either side of the main channel at similar
elevation at both cross-sections. This line of debris was well defined and relatively recent. This
was attributed to the July 29-30, 1997 storm that was widespread over the Pawnee National
Grassland with rainfall accumulations near 15.0 inches (CWCB, 1998). This served as an
additional point in the historic record used in the frequency analysis. Q~:__ ~,

The slopes on the left bank of station (a) had a well develop A orizon extending
approximately 0.50 foot. This level of soil development indicates . surface has not be
disturbed in thousands of years. The lack of any shallow fluvial deposits near the bank indicates
this to be a non-inundation surface. It is possible water may have been to the very edge of the
bank, but is more likely to have been slightly below this and the abrupt edge is a result of
sloughing. The probable range of the ~ surface was taken to be within a 0.50 foot range of
the top of the bank. AlthOUgh~' nee ,as not as strong at station (b), the defined range
coincided with a noted break in s pe a~ange in A horizon development.
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Geary Creek tributary PSIs indicate a slightly greater level of past flooding. Figure 8 illustrates
the cross-sections taken at this site.

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Figure 8 - Cross-section plots of the (a) upstream and (b) downstream stations at Geary Creek tributary.

This site had good PSIs at both cross-sections. The primary PSIs used at station (a) were the
damaged yucca plants near the gravelly bar. The damaged yucca labeled in Figure 8(a) is the
highest elevation damaged plant. This indicated the water surface had made it up to the abrupt
edge on the left bank with some shallow water moving on the grassy left slope. This estimate of
the water surface elevation corresponds well with a scour line observed on the right bank. The
lower bound was established assuming the yucca damage and abrupt edge were a result of run-up

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