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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VII- HYDROLOGY A1"lD HYDRAULIC DESIG1;

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7-0l. General. Basic hydrologic and hydraulic da ta used in the develop-
ment of the definite proj ec t plan is contained in Design ller::oranduTiI. He. I)
liydrology, dated Septenber 1967. A summary of pertinent hydraulic data for
tlle Arkansas River flooc.hvay and the interior drainage system is given in
tables 1 through 4. Hater surface profiles for the Arkansas laver flood:\:Jay
are shOlm on plates 2 through 17.

7-02. Proiect Design Flood. The proposed levee system is designed to
form a floom,ay capable of safely passing a design flood of 140,000 c.f.s.
This flood is approximately 70 percent of the standard proj ect flood ane!
30 percent greater than the flood of record regulated by the authorized
Pueblo Dam. Frequency stuciies indicate that this. discharge, regulated by
Pueblo Dam, would occur once in about 143 years. Since Las Animas is
located near the confluence of the Arkansas and Purgatoire Rivers, the degree
of protection from flooding induced by the Purgatoire River should be compa1;"8lJle
to that afforded against Arkansas River flows. The Purgatoire f,iver, i:'lith
regulation by the authorized Trinidad Daw, would produce a peak of 102,000
" c. f. s. on the average 0= once in about 143 years.

7-03. The starting elevations for back~vater profiles presented in t~l~
project Jocument i;.Jere COI:lputed with a full irrigation pool in John Hartin
Reservoir, estimated seuinent ueposition in the reservoir projected 100 years,
and upstream channelconditions projected 100 years. Data developecl [or t:18
upstream ch3nnel conditions indicated that the Arkansas River Channel l:oul~
be aggraded upstream to the railroad bridge, would be s table between the rail-
road bridge and the highuay bridge, and would have deGraded above the high-
'Jay bridge. Subsequent studies conuucted for the hydrology me~oranduD indicated
that flood stage in the Arkansas River augI:lented \vith coincident flm;r from
the ,Purgatoire River Houlll create higher back'Vlater in the project area under
initial and future conditions than those considered in the project docu~ent.
Correlation studies of peak flOl'JS on the Arkansas r,iver "ith Dean daily flOl')s
on the Purgatoire River ,\.lere made to develop regression equations of coincident
flaY! for use in ir;ater surface dete.rminations.

7-04. /l.ltllOUgil the areal extent of the 1921 storm transposition (:;l.~:;:) \-Jas
not lar:;e enough to result in significant runoff from t;w PurGatoire River, it
is reasonable to assume that there 130uld be SOIT.e coincident flou in elC Pursatoire
r.iver. Conversely, it is likely that during the project design flo.,/ of 102,000
c. f.5. on the Purzatoire River there Hould be coincident flm.] on the Ar::':~nscJ.s
Kiver. Sihlple linear correlation of the logs of peak flm,] (43 events) on the
Arl:ansas r:.lvc.r coincident ~.vit:1 tbe logs of r.1car:_ daily flml on the. Pur2;.:1toirc
P.iver resulted in the regression equation, Log (Pursatoire) = l.046 (I,O(;
Arkansas) - l.OG7. Thus, "it:, the project desi:;n floH of 140,000 c.f .s. on
the Arl::.ansas River, the coincident flm'J on the PurGatoire "D.iv2r 'would he
19,350 c.f.s. Tile sane type of analysis for Pur8atoire River peal: fl(n
(137 p.vents) ^~':'r;~"nt with Arkansas River nean daily flm; resulte<l in LIe
reg~-ession equation, Lo;~ (t,"I"'k;:l_nsap.) = 1.031 (Log PurGatoire) - 1.263. ':;.'~tUS,
~:,ith t:-..e project Lcsibi. flmv of 102,000 e.r.s. on the ?ur~ator~e ~:ive1:, ~'L~
coinciu02ut flml" 0:1 t~1e Lrk2Dsas :'-:,iver 'i'lould be 7,960 c. f. s. For ruri1o:j<2.:~ of

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