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Mr. Kevin Houck, Senior Engineer <br />Page Two <br />Nlarch 30, 2007 <br />I think that the discharges in the 2001 US~-CE study are the most reliable. The flows are lower <br />than the 1967 USACE study because a different methodology was used and because a <br />significantly longer period of record was a~~ailable. Each study uses the hypothetical impact of <br />Pueblo Dam for pre-1975 flows, however, ~this is more reasonable than ignoring the flow records <br />prior to 1975. Figure 3 shows a compariscrn (tabular and graphical) of each of the analyses. <br />This study produced virtually identical results to the USACE 2001 study for all recurrence <br />interval flows. <br />Loss of Channel Capacitv. Each of the po:st-1967 studies indicated loss of channel capacity <br />due to aggradation, Tamarisk, and floodw~~y encroachment due to development. As an <br />indication of the loss of capacity, I reviewed the stage versus discharge data from peak flows <br />records at La Junta (USGS Gage 071230C~0) and Las Animas (USGS Gage 07124000). These <br />data are plotted in Figures 4 and 5 and indicate up to 6 feet of lost flow capacity since the 1960s <br />and that loss of capacity continues. <br />Although the 1967 USACE report indicate:; that the levee design for Las Animas was for <br />140,000 cfs with 4 feet of freeboard, the le~rees certainly do not meet that standard today. The <br />~1SGS 2006 report indicates that the levee capacity is 80,000 cfs, which is greater than the 200- <br />year flood, but this value apparently does riot include any freeboard. The USGS 2006 study <br />was based on estimating channel capacity using the WSPRO model. Although this is an <br />acceptable model, they indicated that therE; were computational issues with one cross section <br />due to constriction. I recommend using the; HEC-RAS model because ineffective flow areas <br />(important at constrictions) are easily incorporated, interpolated cross sections are easy to <br />include and there are more options for evaluating bridge losses. <br />Please feel free to contact me if you have ~~ny questions or comments on this review. <br />Sincerely, <br />Ayres Associates Inc <br />\~~~O~~p,40 R 0G, <br />Lyle W. Zevenbergen, PhD, PE ~~o~a•'2~V~~vE <br />Manager - River Engineering ~ ~ ~ ~ ~ <br />= 9~ :.~ <br />LWZ:sP ~ ~T~~~ <br />Enclosure ~ i/ / °"'ooe~o+ <br />'%~ ssiotvat <br />HOUCK3L DOC <br />(32-C987 Oa) <br />