FLOOD04805

• Designation Number: 356.02
• County: Bent
• Community: Las Animas
• Stream Name: Arkansas River
• Basin: Arkansas
• Title: Floodplain Information Report - Las Animas
• Date: 7/1/1994
• Designation Date: 7/1/1994
• Floodplain - Doc Type: Floodplain Report/Masterplan

most economical size that would provide a high degree of protection.

Coincident Flow Study - Based on analysis of data, the Corps concluded it was reasonable
that with the discharge of the Project Design Flood in the Arkansas River there would be some
coincident flow in the Purgatorie River and vice-versa. The Project Design Flow on the Arkansas
River was 140,000 c.f.s. with a coincident flow in the Purgatorie River of 20,000 c,f.s, The
Project Design Flow on the Purgatorie River was 102,000 c.f.s. with a coincident now in the
Arkansas River of 8,000 c.f.s.

Hvdraulic Analvsis

The Corps carried out detailed hydraulic studies in support of the Las Animas Flood
Control Project. These studies meet the Board's requirements for detailed floodplain information.
Hydraulic information is summarized in the following paragraphs,

Roughness Coefficients - The following Malmings "n" roughness coefficients were
assigned to the floodplain channel:

Channel areas
Overbank areas

n = 0.020,
n = 0.06 and 0.07

Overbank Clearing - An "n" value of 0.035 was assigned to the overbank an,a between
the U.S. Highway 50 bridge and the AT&SF Railroad bridge (3700 feet). The Corps August 22,
1968 Design Memorandum No.2 states, ..."Overbank clearing would be provided and maintained
to provide a "n" value of 0.035 between the railway and highway bridges. With this clearing,
there will be a cleared flow width of 830 feet at the highway bridge and 858 feet at the railway
bridge. The width of clearing would be transitioned to natural conditions upstream of the
highway bridge and downstream from the railway bridge."..,. The importance of the overbank
clearing assumption in the design cannot be over emphasized because this 3700 foot section is
also within the narrowest 1 mile reach of the floodway.

Project Design Flood - The PDF on the Arkansas River was 140,000 c.f.s, with a
coincident flow in the Purgatorie River of 20,000 c.f.s. The PDF on the Purgatorie River was
102,000 c.f.s. with a coincident flow in the Arkansas Hjver of 8,000 c.f.s.

Flood Routing Criteria - The Successive-Average-Lag method was used to route floods.

Water Surface Profiles.. Pre-project and post-project water surface elevations and profiles
were computed for the PDF and the SPF by the standard step method using Manning's equation.
The hydraulic data for the pre-project and post-project Project Design Flood water surface
elevations and initial levee crest elevations for design of the levee are shown on Figure 4, The
post-project water surface profile for the Project Design Flood is shown on Figures 3 and 4.
Figures 3 and 4 were taken from the September 1967 Corps Design Memorandum No.1,
Revised Project Design Flood water surface elevations and levee crest elevatiom, are shown on

8