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I I I I I I I I I I I I I I I I I I I B_~.ac~ - The 100-year flood plain and water surface pt'ofiles for Reach 3 are shown on the Flood Hazard Area Del ineat ion drawings, sheets 13 through 16. The dO\\'IlstreaITI 1 imit of Reach 3 is assumed to be just upstream of existing Highlands Ranch Road where the flood flows coming from Reaches 1 and 3 would merge. DUt'ing low flows, hO\lever, water in these two channels may remain separate to a point downstream of Highl ands Ranch Road. The backwater effect from Highlands Ranch Road (as discussed above in Reach 1) extends a short distance upstream into the Reach 3 waterway al so. condition created by Broadway crossing Reach 1 would back up into the Reach 5 waterway for a distance of about 400 feet and would overtop the natural banks for that short distance. A backlvater condition would be created by the tentative culvert-type road crossing at North Highlands Loop. This road crossing \vould function the same as those discussed in Reach 1 and the flood waters would not overtop the roadway. nle backwater effects for this road crossing \vould extend about 900 feet upstream. The 100-year flood is confined to the existing natural channel throughout the remainder of Reach 3. Reach 6 - The 100-year flood plain and water surface profiles for Reach 6 are shown on the Flood Hazard Area Del incation drawings, sheets 21 through 23. Thet'e are no ex ist ing road crossings along this reach; however, Reach 6 may have future culvert-type i'oad Ct'ossings at North Highlands Loop and at Broadway. Since these two road crossings will be close together, only the Broadway (upstream) crossing was evaluated for backwater effects. It was found that the backwater from Broadway would extend about 1,100 feet upstream. Reach 6 would also experience backwater effects from the Canal Road crossing downstream on Reach 1 just below the confluence with Reach 6. This hackwater condition would extend approximately 1,350 feet upstream of the confl uence. Therefore, although Reach 6 will now carry the 100-year flood event within its ex isting natural channel, future road Ct'ossings could cause it to overtop the natural channel banks over a total distance of nearly one-half mile. ~each 4 - The 100-year flood plain and water surface profiles for Reach 4 are shown on the Flood Hazard Area Del ineation drawings, sheets 17 and 18. There are no existing or proposed road crossings within Reach 4 and the reach is not affected by the backwater created in Reach 3 downstream of its confluence with Reach 4. The 100-year flood is generally confined to the existing natural channel throughout its length; however, an existing retention dam adjacent to the Highlands Ranch headquarters could cause flood waters to spread over a somewhat wider area until the retention dam is breached by the flood waters. The \vater surface profiles given in this report are all based on the assumption that all existing retention dams such as this have been breached. Reach 5 - The 100-year flood plain and water surface profiles for Reach 5 are shown on the Flood Hazard Area Del ineation drawings, sheets ]9 and 20. Reach 5 is relatively short and the 100-year flood event is confined to the existing natural channel throughout its length. There are no existing or future road crossings within the reach; however, the tentative backwater Since it has been found that the 100-year flood event would be confined to the existing natural channel for a very large part of all reaches (back- water conditions excepted), there are very few cross sections where mean- ingful floodway data could be tabulated in Tables 3 and 4. We have there- fore chosen to show f100dway data only at those few cross sections where encroachments into the overbank areas could have an effect on the water surface elevation. Floodway data for all other cross sections are assumed to be identical to the 100-year flood plain data. Floodway data within those subreaches where backwater conditions have been evaluated would not be meaningful since the backwater being considered has already been set at its maximum allowable elevation. C. FLOOD VELOCITIES AND EROSION POTENTIAL Average flow velocities are given on the Flood Hazard Area Del ineation drawings for specific cross sections which have been detailed. The range 11