FLOOD00956

• County: Logan
• Community: Crook
• Stream Name: South Platte
• Title: Hydraulic Analysis for Levee Improvements Along the South Platte at Crook
• Date: 2/17/1987
• Prepared For: State of Colorado
• Prepared By: Muller Engineering Company, Inc.
• Floodplain - Doc Type: Project

L. La ng
February 17, 1987
Page 2

the slope of both the main channel and the railroad. Section 19
was added approximately 3,300 feet upstream of SE!ction 18 and is
based primarily on contours shown on the USGS Crook Quadrangle.
Elevations at the main channel and the ran road were interpolated.
Figure 1 presents a map showing the location of the cross sections
as well as the proposed levee and ,jetty .improvements.

The only revi sion to the COOH model ot.her theln addi ng cross
sections was raising the elevation of the railroad by 0,7 feet at
Section 14. This was to make the rai lroad prof'ile conform more
closely to a straight interpolation between adjacent USGS contours
crossing the railroad. This interpolation matches the .Information
shown on the detailed COOH mapping. The results of the expanded
basel i ne model are very simi 1 ar to the IBM-.PC-XT vers.j on of the
COOH model, differing by two to three tenths of a foot at
Sections 1 and 2 and by approximately olle tenth of a foot or less
at all of the other' sections. This HEC-.2 run demonstrates general
agreement with the original COOH results while being expanded to
provide additional information for the upstream and downstream
ends of the levee.

Levee Model

The proposed 1 evee was coded into the basel i ne model along the
alignment presented as Alternative A on Plate 4 of the CWCB report
entitled "Reconnaissance Report for a nood and Erosion Control
Project on the South Platte River at Crook, Colorado". This
al ignment was selected by the CWCEl and the Town of Crook based on
the reconnai ssance report and input from 1 oca 1 resi dents, The
results of the HEC-2 model with the 1 evee in pl ace, in the
laO-year flood, show an increase in the water surface of
approximately 1.5 feet and in the main channel velocity of one to
two feet per second (fps) near the bridges. No significant
changes in the water surface or mai n channl~l vel oci ty are apparent
upstream of Section 14. Table 1 compares the water surface
elevations and main channel velocHies between the baseline model
and the model with the levee in place for both the 50- and
lOO-year floods. Figure 2 shows the 100-year water surface
profiles along the levee alignment.

Cl ear Cut ~10del

As specified in the Agreement, the HEC-2 model with the levee in
pl ace was revised to reflect a hypoth'B,tic:al 200-foot wide "clear
cut" zone on the south overbank, The new Manning's roughness
value used for this area is 0.035. The results of this mOdE!l