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, '" Mr. Dennis Maroney March 5, 1998 Page three than are 10-year old or 12-year old computer runs. incorporated in the "Corrected Effective" analysis are explanation offered for those changes. Although the net changes substantial, we accept the FloodDlain Chane:es Attributable to the Hil!:hwav Proiect The changes to the flood elevations that are a direct result of the highway project are generally On the order of 4 inches or less, There is only one exception. The reconfiguration of the intersection of Dillon Drive and 29" Street includes some encroachment into the FIS floodway. The result is an increase of 0.75 foo~ (9 inches) in the water surface. While we understand the physical constraints that necessitated this particular project feature, we still wish there were a different way to addresst\1e traffic situation at that intersection. For the remainder of the project, increases in flood elevations are not a significant concern. The post-project floodplain map does not look very different from the pre-project map. The primary changes are that new roads or road features have converted some areas formerly subject to 100-year flooding into areas subject to 500-year flooding. The ground would be elevated at those locations as part of the project, and it would take more water to inundate them. The overall difference in the maps is slight. Comments The combination of I) updating the floodplain mapping (as performed by CH2M HILL) and 2) the proposed construction of the highway project for 1-25, U.S. Highway 50 and State Highway 47 would result in 100-year flood elevations that are higher than those shown in the 1986 FIS (generally 0-3 feet higher, with the datum adjustment). Most of those changes are attributable to the "natural" channel geometry changes, but a small part of the changes is attributable to the highway project. The changes in the floodplain delineation would be minor. The work by CH2M HILL indicates a strong likelihood that significant geologic processes are underway on that reach of Fountain Creek. We recommend that the city, perhaps in cooperation with the Colorado Department of Transportation, initiate a monitoring system for this stream reach. A simple start would be to identify appropriate locations for transects across the creek that would then be resurveyed on a regular basis to see what is happening to channel geometry. The "Corrected Effective" channel geometry could easily be compared to the FIS channel geometry to preliminarily determine critical locations where significant geologic changes are going on. It is conceivable that such a monitoring program should be integrated with the ongoing efforts of the Fountain Creek Project housed at the Pikes Peak Area Council of Governments in Colorado Springs.