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<br />~ <br /> <br /> <br />DEPARTMENT OF THE ARMY <br />CORPS OF ENGINEERS. OMAHA DISTRICT <br />215 NORTH 17TH STREET <br />OMAHA. NEBRASKA 68102-4978 <br />September 27, 1990 <br /> <br />REPLY TO <br />ATTENTION OF <br /> <br />Hydrologic Engineering Branch <br /> <br />Mr. Larry Lang <br />Chief, Flood Control and Floodplain <br />Management section <br />Colorado Water Conservation Board <br />721 State Centennial Building <br />1313 Sherman Street <br />Denver, Colorado 80203 <br /> <br />Dear Mr. Lang: <br /> <br />Thank you for your letter dated June 13, 1990 regarding <br />the Hydrologic Analysis for the Clear Creek Flood Insurance <br />Study. In order to complete the original study under a <br />limited budget, several shortcuts were taken in the analysis. <br />Your comments on that study pointed out serious problems with <br />those shortcuts. Therefore, we have revised the analysis to <br />address your comments and eliminate the shortcuts taken in the <br />earlier study. A copy of the revised report is enclosed. Our <br />responses to your specific comments are provided as follows: <br /> <br />CWCB Comment #1. <br /> <br />The approach of cOmbining gaged snowmelt data with <br />synthetic rainfall data seems unusual. At the beginning of <br />the METHODOLOGY section of your report it was stated that <br />historical records consist "primarily of snowmelt floods" and <br />that "the potential for infrequent rainfall floods to <br />occur. . . . cannot be estimated" from those records. Even with <br />that justification, it still strikes us as an unusual <br />procedure. Does the Corps of Engineers feel comfortable about <br />combining such difference sources of information to arrive at <br />a single answer? <br /> <br />Corps Response #1. <br /> <br />Due to the relatively high infiltration rates and low <br />intensity rainfall experienced in the mountain areas, it is <br />difficult to extrapolate infrequent rainfall flood discharges <br />from a frequency relationship derived from discharges <br />resulting from relatively low intensity rainfall events. Our <br />analysis indicates that it takes approximately a 10-year <br />