FLOOD01986

• County: Bent
• Community: Las Animas
• Stream Name: Arkansas River
• Title: Las Animas Local Protection Project Design Memorandum
• Date: 8/22/1968
• Prepared For: Las Animas
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Project

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slope and 1.42 for the lancside slope at station 394+50 and 1.73 for the
riverside slope and 1.44 for the lands ide slope at station 389+00. ThP
safety factors at station 389+00 were calculated using the infinite slope
equation. Factors of safety for assumed failure arcs that penetrated the
fine grained foundation layer ranged from 2.3 to 4,2. Critical factors of
safety for the drawdown condition were 1.05 at station 394+50 and 1.26 at
station 389+00. Post construction condition was evaluated at statiun 354+00
North levee and station 389+00 on the South levee. Factors of safety for the
critical arcs were 1.46 and 1.53 respectively. Analyses are not presented
for post construction condition~

8-16, Seepage and seepage control. Foundation materials for the river
levees consist generally of a surface blanket of relatively fine-grained
overburden, underlain by granular river alluvium which, in turn, is under-
lain by shale bedrock at depths ranging from 18 to 30 feet below ground surface.
Thickness of the surface blanket of fine-grained overburden is variable,
ranging from 1 to 10 feet. Levee fill materials would be the generally fine-
grained surficial materials available from the borrow areas and required
excavation. Flm, net and seepage analyses of the levee and foundatiol' were
made using a stratification ratio of 9 horizontal to one vertical for the
foundation materials, Permeabilities of foundation and levee ~aterials were
estimated from gradation curves of the materials, Seepage studies used
permeaoilities of 100 x 10-~ feet per minute for granular foundation materials
and 1 x 10-4 feet per minute for levee TIlaterials, The natural blanket of
fine-grainea over burden landward of the levee and the underlying shale bed-
rock were considered to be iIT~ervious.

8-17. River levees would be subjected to a maximum head of about 17
feet during the design flood flo;, of 140,000 c.Ls. Time-seepage studies
indicate that, during the design flood duration of 42 hours, through levee
.seepa8" condition would not develop. Although only partial saturation of
the levee section 1;>Tould occur durins the design flood duration, landuard
seepage through the more pervious granular foundation materials \\7ould cause
excessive uplift at the landward toe of the levee where the natural blanket
of fin"'-grained overburden is thin. Flm, net analyses were made along the
levee alinement to define areas where critical gradients were probable.
based on this analysis, the areas and type of treatn:ent required to relieve
or control the uplift pressure were determined.

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8.-18. T,;o methods 1muld bc usecl to relieve or control the uplift pressures.
The first method utilizes the proposecJ surface drainage ditch in the areas '">There
the invert of the c1itch extends through the impervious layer. Flow net analysis
indicates that pressures at ti,e toe "ould be reduced to acceptable limits by
the drain c1ir~", Areas "hp~A t',e surface layer is relatively thin are supple-
mented by using the excavated materials from the ditch as a uncot:ll'acted fill
layer vetueen the levee toe and the edge of the drainage di ten. A surcharge
load of 3 feet "ill be provided from the excavated materials. 1n1ere the
draina,;e ditch does not penetrate the surface layer. a toe trench uill be

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