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channel bank. It is also recommended that the residents adjacent to the 100-year floodplain be notified annually of the potential flood hazards. Commentary on Sheet 6 Plans for the development upstream of old Wadsworth are in the concep- tual stages. A sedimentatin pond would be constructed upstream of Colorado State Highway 121 to catch the sediment carried by the first flush of storm runoffs. The sedimentation pond was designed for 2- year, I-hour storm runoff without emergency spillway and embankment overflow. A 60 foot wide grouted riprap spillway was provided to pass the 100-year storm runoff without overtopping the embankment. A 4- foot diameter riser outlet was provided to drain the top of the pond in about l-l~ hours and a 12-inch orifice to provide a longer residence time needed to settle out the finer silt particles. ESTIMATED CONSTRUCTION COSTS Item Cost I I I I I I I I I I I I I I I I I I I Sedimentation Pond - 13.0 acre feet $ Grass-lined channel - 1,140 feet 84-inch RCP (jacked) with Head Wall and Wind Wall - 154 feet Concrete-lined Channel - 310 feet 30-inch x 120-inch Concrete Box Culvert - 60 feet 48-inch Diameter CMP Drain with 48-inch Diameter Concrete Riser and 12-inch orifice - 75 feet Relocated 2-inch Gas - 50 feet A grass-lined channel, with trickle channel, was provided between the pond to Colorado State Highway 121. The eXisting 84-inch CMP at Colorado State Highway 121 has a capacity of only 450 cubic feet per second, which is equivalent to as-year stonn. An additional 84-inch RCP would be constructed parallel to the existing pipe so that a 100- year storm runoff woul d not overtop the hi ghway. Total Construction Costs $ A concrete-lined channel with sloped bottom would be constructed bet- ween Colorado State Highway 121 and old Wadsworth (the Marston Sump). Since this area is planned for residential development, a side slope of 3:1 and security fencing along both sides of the channel were recommended. A concrete box culvert would be constructed at old Wadsworth to pass 5-year stonn runoff. Major stonns would overtop old Wadsworth as it has happened in the past. Engineering and Contingencies (35% of Construction Costs) $ Fiscal, legal and Administrative (5% of Construction Costs) land Acquisition costs - 4.5 acres Total Costs Annual Maintenance Cost A grass-lined channel, with trickle channel, would be constructed downstream of old Wadsworth. All channels would carry 100-year stonn runoff. The improvements of the channel and sedimentation pond should be the developer's responsibility. Runoff from the 100-year flood would be contained in the improved channel. It is recommended that the site of the future development in the vicinity of the channel be graded to an elevation at least that of the sedimentation pond embankment and/or 82,100 92,700 218,900 79,700 15,000 3,600 3,500 495,500 173,400 24,800 135,000 $ 828,700 $ 1,000