2011-06-30_REVISION - M1981302 (12)

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Flood Control Levee August 2, 1999 Page During the 1 00-year flood of Veile Gulch, it is anticipated f-bat back up mill occur behind the triple 60-inch culverts. The tributary area used to generate the design flog for Vile Gulch channel included the area which is now behind the levee. The overall drainage pattern for the site has not been altered by the gravel reining nor the levee; all of the site runoff` normally ends up in Veile Charnel. The channel was constructed in the mid 1970s. Base Flood Plain Needs Revision Oren though the G&O study does not indicate that the levee will be overtopped by the base flood of South Boulder Creep at any point along its lengths both the Love and Taggart studies indicate that it will be overtopped in its downstreaM reach. It is also apparent that during the 1 -year flood the overbank flooding that is not forced back to the South Boulder Creek channel by the levee will back up behind US Highway 36 and its crash barrier. The resulting backwater will be forced back into a previously mined gravel pit area which is north of the present ruining site. The backed -up water will co- mingle with the Veile phannel flows, the interior drainage water, and the levee -oy rtop flows. The resulting flooded area is shown on Figure 6 oft the Love study (Appendix 6). Under certain head conditions, the South Boulder Creek oerbank flooding can also cause the flow through the triple 60-inch culverts to reverse and thus contribute to the co-mingled area. During the peak of the South Boulder Creek 100-year flood, the outfall for the water backed-up behind Highway 36 water is not as shown in the G &o study (which is over the highway embarkment and Mien northeast back to South Boulder Creek). The outfall for the backed-up flood water is over the top of the highway's crash bam*er at a location further to the northwest) and into the Keewayden Meadows area west of Foothills Parkway. This "flood - escaping' problem was first mentioned in the 1973 R.W. Beck study (although the limits of flooding were not mapped at that time), and it was recently confined and quantified in the Love and Taggart studies. while it is clear that this portion of the base flood plain is not correct, the discrepancy does not change the validity of the base flood plain and base flood profile adjacent to the upstream and middle reaches of the levee. After the major South Boulder Creep flood recedes, the outlet for the backed -up flood water in the excavated area behind the highway and the levee will be through the channel and culverts of Voile channel and ultimately back to South Boulder Creek. Several parties have indicated their intention to cooperate and conduct a master-planning study to der quantify the flooding problem at LAS Highway 36 and identify a range of solutions. The parties are: 0 the Urban ]drainage and Flood Control District; • the City of Boulder; a Boulder County; and 0 the University of Colorado. LEONARD FIDE CONSULTING WATER ENGINEERS* INC,