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/~I'RE5 ASSt~C1ATES March 30, 2007 Mr. Kevin Houck, Senior Engineer Colorado Water Conservation Board 1313 Sherman Street, Suite 721 Denver, Colorado 80203 Re: Arkansas River at Las Animas, Hydrology Review Dear Kevin, Here are the results for the review of hydre-logy and channel capacity for the Arkansas River at Las Animas. We are in agreement with th~: more recent USACE flow frequency values and recommend that loss in channel and levee capacity be reviewed more closely. Hvdroloav. There are three documents th~~t include independent hydrology analyses. The earliest is the 1967 USACE Hydrology Mernorandum. The analysis uses a period of record from 1921 through 1965 (all pre-Pueblo D~~m, which began operation in 1975) in whic9~ the impact of Pueblo Dam on reducing peak flc~ws is estimated. Table 13 of that report shows the peak flows with and without Pueblo Dam. The report includes an exceedence frequency plot showing the results of the analysis, which is purported to be based on "Statistical Methods in Hydrology, 1962." This document is in the CSU library but I did not review the procedure. The 100-year peak discharge from the 1967 U;iACE study is 113,000 cfs. The next evaluation of hydrology was in 15175 by Hydro Engineering. The method was fitting a log-normal curve to data from 1939 to 197:2 and applying an undescribed adjustment fior the effects of Pueblo Dam. The 100-year pealc discharge from the 1975 Hydro Engineering study is 47,000 cfs. The third evaluation of hydrology was concfucted in 2001, also by the Corps of Engineers. They do not provide the actual data used in the :~nalysis but indicate that the data from the 1967 report was used and that the record extends from 1921 through 1999. The method was a Log- Pearson Type III analysis. The 100-year peak discharge from the 2001 USACE study is 53,800 cfs. { performed a Log-Pearson Type III analysis using the flow data from the 1967 USACE report including their estimates of the effects of Pueblo Dam. Had this approach been used in the 1967 study, they would have estimated a 100-year peak flow of 71,400 cfs rather than 113,000 cfs. Figure 1 shows the flow frequency plat for this analysis. I then performed a Log-Pearson Type III analysis using this same data but extending it through 2005. For flow peaks from 1966 to 1975, I used the direct gage readings at the Las Animas Gage (07124000) without adjustment. Thi;s is because the highest peak recorded flow in this period was only 6,600 cfs. Flows after 197'5 were also direct gage readings because they would include Pueblo Dam effects. The 100-year peak flow from this analysis is 53,200 cfs, which is nearly identical to the USACE 2001 study. Figure 2 shows the flow frequency plot for this analysis. 3H65 JI=K Parkway Building 2 Suite 200 Forl Collins, CO 80525 970.223.5556 ; ww~v.AyresAssociaYe~s.com ~~.OUC~(3_ COC 132-0987 pn..;