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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7-12. THO ditches that convey storm runoff for the city of Las Animas
empty dir'ectly into an abandoned gravel pit located just southeast of the U.S.
High"lay 50 bridge. The gravel pit has a surface area of approximately 6 acres
"ith a depth ranging froffi 0 to 12 feet. The capacity of the gravel pit is
approximately 36 acre-feet. FloH into the gravel pit comes from the north-
east section of Las Animas (urban area, infiltration rate, 0.50 inch pe~ hour)
and from fields and pastures (infiltration rate, 0.70 inch per hour) east of
U.S. Eig]way 50. The volume of runoff "as determined by using the I-hour,
25-year recurrence interval storm (2.3 inches) from U. S. Heather Bureau
Technical Paper ~lo. 40. . The urban area (63 acres) produced 9.4 acre-feet of
runoff; "hile the fields and pastures (54 acres) produced 7.2 acre-feet of
runoff for a total of 16.6 acre-feet.. The gravel pit has sufficient capacity
to contain a I:1uch larger flood or a sequence of floods.

7-13. The Hydrology memorandum made use of U. S. Heather Bureau Technical
Paper No. 25 to determinr rainfall. Technical Paper No. 40 "as used in the
computations of runoff for this memorandum. The volume of runoff "as computed
using I-hour, 25-year rainfall. Technical Paper No. 25 gave' .90 inches and
Technical Paper No. 40 gave 2.30 inches of rainfall. A compcrison of inflow
volume for each levee outlet is listed below.

Outlet
Designation Station

TP 110. 25
~acre-r:e~i...)

TP No. 40
(acre-feet)

South"es t
South
Southeast
North

107+50
290+00
473+00
352+00N

37
76
126
39

49
101
166
49

7-14. Chart D-l, EM 1110-2-1410, "Interior Drainage of Leveed U' ban
. .t\reas: Hydrolo3Y." Ivhieh relates recurrence interval to ponding stages and
elf'........'"'<=; of urban develo""'::"'<:>Tlt \Jas used to analize 8ravity outlet dischargE'
requirCffients. Pending stc6es are classified as A, B, and C (degree of damage
in ascending order of value) and urban developments are classified as I, II,
and III (degree of urban development in descending order of value). Detailed
topographic information was used to correlate elevations to the three ponding
stages. All of the ponding areas "ere desigl ated as class III (relatively
10\,,-valued urban) indicating that the normal design limit of stage C should
have a recurrence interval of not less than 25 years.

:-15. Studies indicate,] the 25-year rainfall of 2.30 inches "ith a
coincident 5-year flood on the Arl~aIlsas River 'l;vas critical for determiniIlb
requireu ponding capacities. Storage of the entire 'runoff resulting frOl:1 the
I-hour, I-year recurrence interval rainfall of 0.9 inch "as computed for each
ponding area ana detcrrc..ined. to be less critical. FlOi.vs in the Arkansas River
greater than the 5-year recurrence interval flood HoulJ. be required to C07fl.-
pletely block the outlet structures and necessitate ponding the entire volufile
of interior runoff.

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