FLOOD00037

• County: Logan
• Community: Sterling
• Stream Name: South Platte River
• Title: Flood Mitigation Measures for the South Platte at Sterling
• Date: 9/1/1989
• Prepared For: Sterling
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Floodplain Report/Masterplan

3.1.2 Comparison with Flood Insurance study. A comparison
of the results'of the existing conditions water surface profiles calculated
for this study with a preliminary copy of the Flood Insurance Study showed
that all of the profiles except the 100-year were in good agreement. The
100-year water surface profile calculated for this s1:udy is approximately
one to two feet higher than for the lrlood Insurance Study. A11:hough the
Flood Insurance Study's HEC-2 model ~'las !Clot available for comparison of
model input and output, the difference in water surface profilE~s may be
explained by this study's assumption to have the 100-year discharge confined
to the channel through the bridges as "hown by historic data.

3.2 Existing Conditions Floodway. Once the exi"ting conditi.ons model
was finalized a 100-year floodway was computed. The initial floodway runs
were ran with the method four encroachmen1: option for HEC-2. This option
computes encroachment stations for the specified on" foot rise in water
surface with an equal loss of conve~'ancE~ on each overbank. Because of
irregularities in the cross sections and flood plain these encroachments
were then smoothed to provide a smooth conS3istent floodway outlinE! as shown
on Plates 3 and 4. Encroachment met;hod one was us"d to accomplish this
task. A floodway data table for the E~xis1:ing conditions is shown in Table
Three.

3.3 Proposed City Levee Option. In attachment 1, the City of Sterling
presented a possible levee alignment for flood mitigat:ion. This alignment
was then modeled in the HEC-2 existing conditions floCldway model. Whenever
the levee was located inside of the exi.sting conditions floodway t.h" method
one encroachment station was changed to meE~t the levee! station. The levee
floodway was then run to determine t.he \'later surface increase for this
option and is shown in Table Four. The levee encroachment into the floodway
caused water surface increases by as much as 0.9 feet above the standard 1.0
foot floodway rise. This option may requi:r-e negotiat,ions for compensation
with land owners whose land would be susceptible to increa,sed flood
elevations.
A 100-year levee constructed for this option should be constructed at
least as high as the floodway water SUrfaCE! elevations in Table Four with an
additional three feet of freeboard. The additional freeboard should meet
FEMA criteria for levee adequacy and. allow a factor of safE~ty for the
design.

3.4 Channel Clearing Option. The city also requested hydraulic data for
possible clearing of the South Platte chann,.l. This was modeled by reducing
the existing conditions channel "n" values. Where the channel "n" value was
0.035, it was reduced to 0.030. When the channel "n" value was higher in
portions to reflect trees or shrubs it was reduced to an "n" value of 0.035
to reflect the clearing measures. The ca.lculated water surface profiles
were then plotted against existing conditions water surface profiles and are
shown on Plate 2. Next a cleared floodway was run and the results shown in

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