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I I I I I I I I I I I I I I I I I I I !leach 3 - The IOO-year flood plain and water sUI'face profiles for Reach 3 are shol'in on the Flood Hazard Area Del ineation drawings, sheets 13 through 16. The dOl'instream 1 imit of Reach 3 is assumed to be just downstream of existing Highlands Ranch Road where the small channels coming from the two road culverts Inerge. During low flows, water in these two channels may remain sepal-ate down to this point; hO\'iever, during the laO-year flood event the flows would actually combine some distance upstream of Highlands Ranch Road. For this reason the first 1,000 feet of the Reach 3 flood plain and profile is a duplication of that same portion of Reach 1. The backwater effect from Highlands Ranch Road (as discussed above in Reach 1) extends a short distance upstream into the Reach 3 waterway also. or future road crossings within the reach; however, the tentative backwater 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. Reach 6 - The IOO-year flood pI ain and water surface profiles for Reach 6 are shown on the Flood Hazard Area Del ineation drawings, sheets 21 through 23. There are no existing road crossings along this reach; however, Reach 6 may have future culvert-type road crossings at North Highlands Loop and at Broadway. Since these two road crossings will be close together, only the Broadl,ay (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 backwater condition would extend approximately 1,350 feet upstream of the confluence. Therefore, although Reach 6 will now carry the IOO-year flood event within its existing natural channel, future road crossings could cause it to overtop the natural channel banks over a total distance of nearly one-half mile. A backwater condition would be created by the tentative culvert-type road crossing at North Highlands Loop. This road crossing would function the same as those discussed in Reach I and the flood waters would not overtop the roadway. The backwater effects for this road crossing would extend about 900 feet upstream. The lOa-year flood is confined to the existing natural channel throughout the remainder of Reach 3. B-e.acl!..'!. - The 100-year flood pI ain 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 lOa-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 I,aters to spread over a somewhat wider area unt i 1 the retent ion dam is breached by the flood waters. The water surface profiles given in this report are all based on the assumption that all existing retention dams such as this have been breached. Since it has been found that the IOO,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 tabul ated in Tables 3 and 4. We have there- fore chosen to show floodway data only at those few cross sect ions 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 lOa-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. Reach 5 - The lOa-year flood plain and water surface profiles for Reach 5 are shown on the Flood Hazard Area Del ineation drawings, sheets 19 and 20. Reach 5 is relatively short and the lOa-year flood event is confined to the existing natural channel throughout its length. There are no existing C. FLOOD VELOCITIES AND EROSION POTENTIAL Average flow velocities are given on the Flood Hazard Area Delineation drawings for specific cross sections which have been detailed. The range 11