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I I I I I I I I I I I I I I I I I I I Reach 1 - The 100-year flood plain and water surface profiles for Reach 1 are shown on the Flood Hazard Area Del ineation drawings, sheets 2 through 11. The downstream terminus of Reach 1 is Mclellan Reservoir. Low flows enter the reservoir V1a a g ft. x 12 ft. culvert under County Line Road; however, the culvert is inadequate to handle the maximum 100-year flood cons idered here so fl ood waters woul d al so spillover the roadway into the reservoir. The water depth would be appro x imately two feet above the low point in the roadway. The roadway embankment creates a backwater con- dition, but the backwater extends mostly parallel to the roadway in a westerly direction and does not exceed the normal channel banks upstream. The culvert at County Line Road was also checked for its abil ity to pass the 100-year flood under present day, undeveloped conditions. At a 0100 of 2,336 cfs the culvert was still found to be inadequate and flood waters would spill over the road surface with a maximum depth of about 1.25 feet at the low point in the roadway. and the fl ood waters wi 11 fl ow over the roadway wi th a max imum depth of about three feet at the low point in the roadway. Backwater cond it ions wh ich coul d be created by future culvert-type road crossings would cause the flood waters to rise outside the existing natural channel. By design, the flood waters will not overtop the roadway surfaces (the peak water surface elevation is kept three feet below the minimum roadway elevation); however, care must be taken to make sure that future development in these overbank areas will not be flooded by maintaining building pad elevations at least one foot above the peak 100-year flood water surface elevation. Future Reach 1 road crossings include Centennial Parkway (Future C470), Canal Road, Broadway, Dad Clark Drive/North High- lands Loop intersection and West Parker Road. Backwater conditions have been eval uated for four of these crossings and it was found the backwater would extend approximately 2,450 feet upstream from Canal Road all the way to the Broadway crossing, and an additional 1,800 feet upstream of Broadway due to its separate backwater effects. Backwater would also extend ap- proximately 800 feet upstream of the Dad Clark Drive/North Highlands Loop intersection and 1,300 feet upstream of the West Parker Road crossing. The Centennial Parkway crossing was not evaluated since it will be only a short distance downstream of the Canal Road crossing. The 100-year flood water surface elevation at the Highl ine Canal cross1ng is very close to that of the assumed low chord elevation of the canal superstructure. With an approach velocity of 7 to 10 fps and water depth of nearly eleven feet, the canal superstructure may sustain heavy damage during the 100-year flood event. The timber truss construction of this structure has a high debris collection potential which could also aggravate the possibil ity of structural failure during flooding periods. The flood- ing conditions at the Highl ine Canal crossing were also checked for the present day, undeveloped 0100 (2,336 cfs). Under these conditions the flume portion of the structure is a more comfortable height above the peak water surface and the water depth would be about 7.4 feet; however, the debris collection potential of the structure could still cause serious problems. The Dad Clark Drive/North Highlands Loop intersection 1S located within the flood plain 1 imits and further study of the retarding basin potential of this road crossing site has shown the site to be relatively ineffective due to insufficient storage volume. This site will therefore probably not be used as a retarding basin, thus allowing the gulch to be rechanneled as necessary to accommodate the road alignments. The 100-year flood event remains confined to the natural channel throughout Reach 1, although the flood plain becomes quite broad at the existing Highlands Ranch access road crossing. The culverts at the crossing are inadequate to handle the 100-year flood. The road crossing creates a backwater cond it ion wh ich extends approx imate 1 y one-quarter mi 1 e upstream Reach 2 - The 100-year flood plain and water surface profile for Reach 2 is shown on sheet 12 of the Flood Hazard Area Deli neat i on dr awi ng s . Reach 2 is very 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 this reach and the reach is not affected by future backwater conditions which could occur on Reach 1 downstream below the confluence with Reach 2. 10