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<br />~ ;;.:; <br /> <br />Page 2 <br />Mr. William Horak, Jr. <br /> <br />estimated by Mr. Carver to have a 12-year flood frequency. The information in the Corps letter <br />was quite surprising to our staff. <br /> <br />Due to the low variability of Colorado's mountain streamflows (which are predominantly driven <br />by snowmelt runoff as you know) the addition of 400 cfs to the gaged flow makes a significant <br />difference in the estimated flood frequency of the flood event. <br /> <br />Attached for your review are two tables that summarize hydrology data for the Elk River near <br />Milner. As you can see from the tables, the source of the flood-frequency values and the <br />variability in the June 3 peak flow estimate make a big difference in the estimated flood <br />frequency for the subject event. We would appreciate input from one of your staff members <br />regarding the information shown in the attached table. <br /> <br />In conclusion, the CWCB would like a written explanation regarding the difference in the initial <br />flow estimate of 6,600 cfs vs. the recent flow estimate of 5,370 cfs. Additionally, the CWCB <br />feels that it is important that the unrecorded overbank flow be thoroughly investigated using <br />detailed surveyed cross-sections, high water marks, and other data. The overbank flow should <br />definitely be added to the recorded streamflow in order to determine a more realistic peak flow <br />value for June 3, 1997. <br /> <br />If you have any questions or need additional information, please call me at (303) 866-3441. <br />Thank you very much for your consideration on this matter. <br /> <br /> <br />~ <br />Thomas W. Browning, P,E. <br />Flood Control and Floodplain Management Section <br /> <br />cc: Bob Jarrett <br /> <br />TWB/twb <br />Attachment <br /> <br />C:Isecalfloodlelk-rivlhorak Itr.doc <br />