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.; -~ ~ Mr. Dennis Maroney March 5, 1998 Page two over Fountain Creek; 2) the extension of the existing Dillon Drive frontage road from the Pueblo Mall north to State Highway 47 and then further north to the existing northern part of Dillon Drive; 3) the construction of an access road between North Drive and eastbound State Highway 47; 4) the construction of the new 40th Street Detention Basin to reduce flows from an unnamed west bank tributary to Fountain Creek near State Highway 47; and 5) the reconfiguration of the Dillon Drive frontage road south of the Pueblo Mall, to the new intersection with 29th Street and the Construction of a southerly extension of Dillon Drive south of 29th Street. In addition, the project would provide a new combination storm sewer/wetland channel system to convey flows from the unnamed west bank tributary near State Highway 47 from the west side of 1-25 southeast to Fountain Creek. There are some localized changes to the floodplain as a result of individual project features. In an overall sense, however, the post-project floodplain map does not look significantly different from the pre-project map. There will be some discussion in the remainder of this letter of some of the localized changes, but we do not have any major objections to the concept that has been developed. Revisions to the Pre-Proiect Floodolain Mao There are substantial differences between the CH2M HILL 1997 pre-project flood elevations and the 1986 FIS flood elevations. For much of the study stream reach the 1997 elevations are from 0 to 3 feet higher than the FIS elevations, when comoared on the same datum. When the two different survey data are used without adjustment, there is an elevation difference of 3 to 6 feet. For the upstream 1500 feet or so the 1997 elevations are much closer to the FIS elevations, ranging from approximately 2 feet higher to 0.5 foot lower than the 1986 elevations. Three reasons for these changes in flood elevations were given. The first reason has already been mentioned, the change in datum. Changing from NGVD '29 to NA VD '88 means an adjustment of 2.8 feet in flood elevations. The second reason was that there were significant changes in channel geometry between 1986 and 1997. Since the changes were almost all increases, it is presumed that there is an ongoing process of channel aggradation (deposition) in this reach of Fountain Creek. Given the dramatic changes in Fountain Creek upstream of Pueblo in Colorado Springs and Fountain, aggradation in Pueblo should not be surprising. The magnitude of the changes in channel geometry indicates a need for ongoing monitoring of the Fountain Creek channeL The "Comments" section of this letter will address that issue. The third reason given for changes was that bridge modeling information from the FIS was found to be inconsistent with recent field surveys of bridges. We presume that recent field surveys are more reflective of current conditions