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<br />Table 2 along with Standard Project Flood water surface elevations. Table 2 was takcn trom the <br />August 1968, Corps Design Memorandum No.2, General. Final levee crest elevations taken <br />from the Corps July 30, 1976, Plans for Construction of Levee and Appurtenant Facilities, are <br />shown Table 3. These plans are on file in the Colorado Water Conservation Board office in <br />Denver, Colorado. <br /> <br />Freeboard - Levee minimum freeboard is 4 feet from the beginning of the levee 6 miles <br />west of Las Animas down to the AT &SF Railroad bridge northeast of Las Animas. Additional <br />freeboard was provided upstream of the U.S. Highway 50 bridge across the Arkansas River north <br />of Las Animas and upstream of the AT &SF Railroad bridge northeast of Las Animas. Maximum <br />freeboard is 7.5 feet to provide for buildup of head at the two bridges. Freeboard varies from <br />2 feet just downstream of the AT &SF Railroad bridge to I foot at the end of the levee. <br /> <br />Bridge Clearance - The capacity of the U.S. Highway 50 is adequate to pass the design <br />110w of 140,000 c.f.s. with 2.4 feet of clearance. Th(: AT &SF Railroad bridge was raised and <br />lengthened to pass the design 110w with about 3 feet of clearance. <br /> <br />Floodplain Boundaries (Pre-Project) - Pre-project 1100dplain boundaries were prepared by <br />the Corps using the computed water surface profiles for the SPF. Pre-project floodplains for the <br />Arkansas River SPF and Purgatorie River SPF are sho'wn on Figure 2 (Plate 2 from the Corps' <br />August 1963 report). This figure also shows the pre-project floodplain boundary for the <br />maximum 1100d of record computed as 140,000 c.f.s. which is also the Project Design Flood. <br />The laO-year floodplain boundaries were not shown. <br /> <br />Floodplain Boundaries (Post-Project) - Post-project floodplain bound.aries were not <br />available in the Corps files including the 100-year flood post-project water surface elevations or <br />floodplain boundaries. An approximate post-project floodplain boundary map was prepared by <br />Board staff using Project Design Flood water surface: profile elevations from the August 22, 1968 <br />Corps Design Memorandum No.2. The Project Design Flood was estimated by the Corps as a <br />143-year flood. This information was used by Board staff in lieu of the :lOa-year flood <br />information. The approximate post-project floodplain boundaries are shown on Figure 2 for <br />quick comparison with the pre-project floodplain boundaries (the maximum flood ofrecord which <br />is also a 143-year flood). <br /> <br />Velocities - Average velocities in the charmel section would vary from '7 to 13 feet per <br />second for a discharge of 13,000 c.f.s. At the design discharge of 140,000 c.f.s. wlocities would <br />vary from 10 to 21 feet per second in the channel section. Average overbank velocities for the <br />design discharge would vary from 3 to 8 feet per s'~cond. <br /> <br />Increase in Depth - The modification of the pre-project floodplain by the Project levees <br />confines the flood to a smaller portion of the original floodplain thus increasing the post-project <br />depth of flow on the riverside of the levees. The increase in depth of 110w on the riverside of <br />the levee for the Project Design Flood (l43-year flood) varies from 0.2 feet at the west end, to <br />4.9 feet just downstream of the AT &SF bridge, to 0.3 feet near the east end. <br /> <br />9 <br />