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Report in Support of the Recertification of the University of Colorado's Flood Control Levee Page 5 upstream end of the Levee. The 2007 version had a clear location (at about station 147 +50) where the flood came into contact with the Levee. The 2008 version also comes in contact with the Levee at this same location, but it approaches at a narrower angle. How the flood plain limit line approaches the Levee is important because the elevations of the flood immediately to the south and west of station 147 +50 are steeper than the elevations of the main flood (which approaches directly from the south), and they are steeper than the grade of the Levee at this location. The reason why the flood elevations are steeper to the south and west is that shallow overland flow approaches from Colorado Highway 93 (to the west), and the ground in between is steep. This flow is returning after having been separated from the main flood plain at the Marshall Road crossing, about one mile upstream. The situation was discussed in detail at the January 10, 2008 meeting of interested parties mentioned above. Based on that meeting and on the reported feedback from FEMA after that meeting, it was concluded that the Levee did not have to relate to these steep shallow areas of returning flow; these areas are "away" from the Levee. The Levee only had to relate to the main flood where it was in direct contact with the Levee (from station 147 +50 and on downstream). The 2007 version of the flood plain includes the shallow overland flow areas south and west of the upstream end of the Levee. See Figures 12 and 13. The 2008 version of the flood plain does not connect the flood plain at Colorado Highway 93 with the main flood plain at the upstream end of the Levee; it only includes those areas where the depth of flow is greater than 0.5 feet. The resulting flood -plain delineation would seem to indicate that the main flood plain backs up into the area adjacent to the Levee. This is not in fact the case, and both hydraulic studies show that some of this area has no depth of flow at all. To clarify exactly where the Levee exists in this area, LRE indicated the toe of the Levee on Figures 12 and 13 based on analyses of the one -foot topography, aerial photographs, and field inspection. We also indicated how the South Boulder and Bear Creek Ditch flows through this area. It approaches the Levee from the south near station 147 +50, and then it flows to the west, paralleling the Levee. The ditch's channel was excavated into the natural ground, and the whole channel is "away" from the toe of the Levee. The main flood plain does back up into this portion of the ditch, and the 2008 version of the flood plain follows the right (north) bank of the ditch. This report assumes that the main flood comes into contact with the Levee at station 147 +50, and the freeboard for that portion of the Levee to the west (station 147 +50 to 150 +70) is based on the main flood elevation at station 147 +50. VI. THREE REACHES OF LEVEE The Levee is approximately 7,540 feet long and it extends from high ground on the upstream (southern) end to the on -ramp from south -bound Foothills Parkway to east -bound US Highway 36 on its northern end. Figure 9 shows how the Levee is divided into its three reaches, which are described below. The Levee stationing included in this report was established by SCA in its 1998 surveys (Appendix B -8). The stationing starts low at 75 +50 at its northern end, and it increases along the Levee as it heads south and west in the upstream direction. The upstream end of the levee is at station 150 +70. © Leonard Rice Engineers, Inc. June 2009 — 207FIP05