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I I I I I I I I I I I I I I I I I I I The 100-year flood water surface elevation at the Highl ine Canal crossing is nearly five (4.97) feet, which is about one foot above the low-chord elevation of the canal super-structure. The timber construction of this structure may have a high debris collection potential which could aggravate the possibility of structural failure during flooding periods. not affected by future backwater conditions which could occur on Reach 1 downstream below the confl uence with Reach 2. A small checkdam at the extreme upper end of the reach will cause the water surface elevation to be raised above the natural banks for about 200 feet.:!:. upstream. The 100-year flood event remains confined to the natural channel through- out Reach 1, although the flood pl ain becomes quite broad at the existing "East Ranch Road" (see sheet 7 of 12 - Plan and Profiles) crossing. The culvert at the crossing is inadequate to handle the 100-year flood, and overtops the road with a maximum depth of about two (2) feet at the low point in the roadway. Reach 3 - The lOO-year flood plain and water surface profiles for Reach 3 are shown on the Flood Hazard Area Del ineation drawing, sheet 12. The downstream 1 imit of Reach 3 is midway along the Gulch between North High- lands Ranch Parkway and McLellan Drive. Reach 3 is also somewhat short (1000 feet .:!:.). The 100-year flood event is confined to the existing natural channel throughout its length. There are no existing future road crossings within this reach. However, the reach ties in with Reach 1 at a point just above a check dam, which causes a backwater condition for approximately 200 feet. Future road crossings in Reach 1 include Rangeline Road, North Highlands Ranch Parkway, McLell an Drive, South Highl ands Ranch Parkway, and the treatment plant access road. Backwater conditions have been evaluated for these cross i ngs and the fo 11 owi ng was concl uded: 1) at Rangel ine Road - maximum allowable backwater to elevation 5559.0 - this will back water approximately 1100 feet upstream of the pl anned al ignment; 2) at North Highlands Ranch Parkway - maximum allowable backwater to elevation 5627.0 - this will back water approximately 1300 feet upstream of the planned align- ment; 3) at McLellan Drive - maximum allowable backwater to elevation 5807.0 - this will back water approximately 500 feet upstream of the planned alignment; 4) at South Highlands Ranch Parkway - maximum allowable backwater to elevation 5753.0 - this will back water approximately 200 feet upstream of the pl anned al ignment; and 5) at the treatment pl ant access road - maximum allowable back water to elevation 5450.0 - this will back water approximately 150 feet upstream of the planned alignment. Reach 4 - The 100-year flood pl ain and water surface profiles for Reach 4 are shown on the Flood Hazard Area Del ineation drawings, sheets 3 and 4. 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, the existing Highline Canal intersects the canal at STA 611+00. For purposes of this report, a "worst possible type flooding" condition was considered at this point. The Highline Canal was considered to be flowing full, and as a consequence, all flood waters in Reach 4 were considered to overtop the west bank of the canal and continue toward Reach 1. For this reason, the flood waters spread over a somewhat wider than "normal" area until the west bank of the canal is breached at which point the overflow then stays within the confines of Reach 4 up to its confl uence with Reach 1. Water is backed up behind the Highl ine Canal for approximately 300 feet. Reach 2 - The 100-year flood plain and water surface profile for Reach 2 is shown on sheet 12 of the Flood Hazard Area Delineation drawings. Reach 2 is somewhat short (1300 feet .:!:.) and the 100-year flood event is confined to the existing natural channel throughout the majority of its length. There are no existing or future road crossings within this reach and the reach is Since it has been found that the 100-year flood event would be confined to the ex i s t i ng natural channel for a vet-y 1 arge part of all reaches (back- water conditions excepted), there are very few cross sections where 11