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<br />. <br /> <br />. <br /> <br />. <br /> <br />Map became effective August 5, 1986. There have been no revisions to the Flood Insurance Rate <br />Map since that time. <br /> <br />Community Response to the CWCB Desil!nation and Approval Process <br />A letter from the CWCB was transmitted to Town of Walden regarding the Board's proposed <br />action for designation and approval of the Walden FIRM. Local officials confirmed that there have <br />been no flooding problems, significant development issues, or floodplain mapping needs for the <br />community. The Town was receptive to the Board's proposed designation action, and sent an <br />official letter requesting that the CWCB designate and approve the floodplain information currently <br />available to the community. A meeting between Town officials and CWCB staff was not held. A <br />copy of the request letter is included in the Appendix of this report. <br /> <br />ENGINEERING METHODS <br /> <br />Hvdrolol!ic Analvsis <br />Hydrologic information used for the FEMA floodplain delineations in Walden is not available or <br />was not documented by FEMA. However, acceptable hydrologic information was developed by <br />the CWCB using two separate methodologies for computing lOO-year flood-flow values. The first <br />method consisted of analyzing streamflow records from long-term gaging stations located on the <br />Michigan and Illinois Rivers near Walden. Twenty-five (25) years of annual peak flow values were <br />. available for the Michigan River, and 25 years of annual peak flow values were available for the <br />Illinois River. A flood frequency analysis was performed using the Corps of Engineers <br />computerized "Flood Frequency Analysis" program. A generalized skew coefficient of -OJ was <br />used. The results are presented in Table 2. <br /> <br />The second method involved the use of a publication by the U.S. Geological Survey (USGS) <br />entitled" Analysis of the Magnitude and Frequency of Floods in Colorado." The 100-year flow for <br />the Michigan and Illinois Rivers in Walden were estimated by utilizing the regional regression <br />equation for the Mountains Region. The Mountains Region equation is used for streams having <br />drainage watershed areas above an elevation 00,500 feet. The USGS regression equation is: <br /> <br />QIll" = 39.5(A)".706 (S+ 1.0)1.577 (For 5.5<A<988 and 0.081 <S<0.562) <br /> <br />The QIlM' is the 100-year peak flow in cubic feet per second, A is the upstream (contributing) <br />drainage area in square miles, and S is the mean drainage basin slope in feet per feet. The regression <br />equation is only valid for streams with drainage areas between 5.5 and 988 square miles, and mean <br />basin slopes between 0.081 and 0.562 feet per feet. <br /> <br />Table 2 presents 1 DO-year flow estimates, as computed using the Flood Frequency Analysis (FFA) <br />and the USGS regional regression equation (Regression), for the Michigan and Illinois Rivers at <br />Walden. <br /> <br />3 <br />