FLOOD04664

• County: Logan
• Community: Sterling
• Stream Name: South Platte River
• Basin: South Platte
• Title: Flood Mitigation Measures for South Platte at Sterling
• Date: 9/1/1979
• Prepared For: Sterling
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Floodplain Report/Masterplan

3.1.2 Comparielon with Flood Insura.nce 81:udy. A comparison
of 1:he result~s 'of the existing conditions ,.rateJ:' surface profiles calculated
for this study with 8. proliminary copy of the Flood Insurance Study showed
tha1: all of the profiles except the 100-yc'ar "ere in good agreement. The
lOOhyear wat.er surface profilH calculated for this study is approximately
one to two feet higher than for the Flood. InsurancE~ Study. Although the
Flood Insura,nce Study's HEC-2 model was no1:. available for comparison of
modEll input and out.Fut, the differE~nce In water surface profile~3 may be
explained by this s't;udy's assumption t;o have the 100-'year discharge confined
to t:he channel through the bridges as sh~\<n by historic data.

3.2 Existing Conditi.ons f'lood\>lay. OnCE! t.he exi.sting conditicms model
was finalized a IOO-year floodway was computed. ThE~ initial floodway runs
were' ran with the method four enCl:oachtnl3Il't clpt.ion for HEC-2. This option
computes encroachment stations tor t.he erpecified on.e foot rifle in water
surface with an equal 10s8 af C'()nv€~ya).lcle on each overbank~ Because of
irregularities in the crOBS section8 and flood. plain these encroachments
were then smoothed to provide a srt100th con:sistent floodway outline as shown
on Plates 3 and 4. Encroachment: me,thocl one "'as used to accomplish this
task. A floodway data table for t;he e;cLel'ting conditions is shown in Table
Three.

3.3 Proposed Ci.ty Levee Option. In Cl't.tachment 1, the City of Sterling
presented a pt:>ssible 1(3vee alignment for flood mitigation. This alignment
was then modeled in thl~ HEC-2 existing cond LticJns floodway model. Whenever
the levee was located inside of the~ exi.stinq conditions floodway the method
one encroachml3nt stat ion was changed to meE!'C the levee station. The levee
floodway was then ,t"un to determine thE~ \''later surface increa.se for this
opti::m and is shown in Table FOUl:. The levl~l~ encroachment int.o the floodway
caused water !;l:urface increases by as much cu3 0.9 feet abovE~ the standard 1.0
foot floodway rise. ~~hiB option may rE!quiJ:e neqotia.tions for compensation
with land O\'iners whose land would bE! !3usceptibl,:::! 1:0 increased flood
elevations.
A IOO-year levee constructed for thi.s option should be constructed at
least. as high as the L_oodway water sucfa.cE:! elevation:::;; in Table Four with an
addi'~ional three feet of freeboard. Thl2"' addit;i..cmal freeboard should meet
FEMA criteria for llHvee adequacy and a,l:.ow a facto:~~ of safety for the
desiqn,

.3.4 Channel Clearing Option. The city also 1.:eque!3tecl hydraulic data for
poss.i.ble clearing of 1:he South Platte channEll. This was modeled by reducing
the existing c:onditioI1E1 channel "n" vah.1E;;s. Where thE~ channel "n" value was
O.03!;, it wa~1 reduced to 0.030. When 1:h~:! channel "n" value was higher in
port:Lons to rElflect trE!eS or shrubS it was x'educed to an "n" value of 0.035
to l:eflect t:he cleari.ng measures~ The ca.] culated water surface profiles
\olere then plot~ted against €~xist.Lng condi1:ions water surface prof .Lie:;; and are
shown on Plate! 2. NEnl:t a cleared f.loodway was run and t.he results shown in

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