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<br />most economical size that would provide a high degree of protection. <br /> <br />Coincident Flow Study - Based on analysis of data, the Corps concluded it was reasonable <br />that with the discharge of the Project Design Flood in the Arkansas River there would be some <br />coincident flow in the Purgatorie River and vice-versa. The Project Design Flow on the Arkansas <br />River was 140,000 c.f.s. with a coincident flow in the Purgatorie River of 20,000 c.f.s. The <br />Project Design Flow on the Purgatorie River was 102,000 c.f.s. with a coincident now in the <br />Arkansas River of 8,000 c.f.s. <br /> <br />Hvdraulic Analvsis <br /> <br />The Corps carried out detailed hydraulic studies in support of the Las Animas Flood <br />Control Project. These studies meet the Board's requirements for detailed floodplain information. <br />Hydraulic information is summarized in the following paragraphs. <br /> <br />Roughness Coefficients - The following Malmings "n" roughness coefficients were <br />a.ssigned to the floodplain channel: <br /> <br />Channel areas <br />Overbank areas <br /> <br />n = 0.020, <br />n = 0.06 and 0.07 <br /> <br />Overbank Clearing - An "n" value of 0.035 was assigned to the overbaruc an,a between <br />the U.S. Highway 50 bridge and the AT&SF Railroad bridge (3700 feet). The Corps August 22, <br />1968 Design Memorandum No.2 states, ..."Overbank clearing would be provided and maintained <br />to provide a "n" value of 0.035 between the railway and highway bridges. With this clearing, <br />there will be a cleared flow width of 830 feet at the highway bridge and 858 feet at the railway <br />bridge. The width of clearing would be transitioned to natural conditions upstream of the <br />highway bridge and downstream from the railway bridge.".... The importance of the overbank <br />clearing assumption in the design cannot be over emphasized because this 3700 foot section is <br />also within the narrowest 1 mile reach of the floodway. <br /> <br />Project Design Flood - The PDF on the Arkansas River was 140,000 c.f.s. with a <br />coincident flow in the Purgatorie River of 20,000 c.f.s. The PDF on the Purgatorie River was <br />102,000 c.f.s. with a coincident flow in the Arkansas Hjver of 8,000 c.f.s. <br /> <br />Flood Routing Criteria - The Successive-Average-Lag method was used to route floods. <br /> <br />Water Surface Profiles - Pre-project and post-'project water surface elevations and profiles <br />were computed for the PDF and the SPF by the standard step method using Manning's equation. <br />The hydraulic data for the pre-project and post-project Project Design Floocl water surface <br />elevations and initial levee crest elevations for design of the levee are shown on Figure 4. The <br />post-project water surface profile for the Project Design Flood is shown on Figures 3 and 4. <br />Figures 3 and 4 were taken from the September 1967 Corps Design Memorandum No. I. <br />Revised Project Design Flood water surface elevations and levee crest elevatiom, are shown on <br /> <br />8 <br />