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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Chapter 6 <br />PRELIMINARY DESIGN AND ESTIMATED COSTS <br /> <br />This chapter presents the preliminary design and estimated <br />costs for the selected flood control measures for Basin 6100 <br />and the North Tributary to Basin 6200. The types of flood <br />control measures selected include channel regrading and reshaping, <br />storage ponds, drops, berms, inverted siphons and drainageway <br />crossing replacement. The Kiewit Area and Direct Flow Basin 6100.5 <br />are covered in Chapters 7 and 8. However, the following <br />discussion on design criteria applies to all four basins. <br /> <br />DESIGN CRITERIA <br /> <br />The preliminary designs of the flood control measures in the <br />study are based on a 100-year frequency flood. The difference <br />in sizing of facilities for a 10-year flood compared to a <br />100-year flood was found to be insignificant for the study <br />area, while the damages from the 100-year flood were substantially <br />greater. Thus, the lower construction cost to design for a <br />10-year flood was not considered justified in terms of the <br />apparent large damage difference between the 10-year and <br />100-year floods. <br /> <br />The design criteria for the study area were adopted from the <br />Urban Storm Drainage Criteria Manual. State agencies, Jefferson <br />County and Arapahoe County and irrigation districts were also <br />contacted to develop design criteria. The design discharge <br />for inverted siphons at Platte Canyon Ditch crossings is 25 <br />cfs and at Nevada Ditch crossings is 45 cfs as recommended by <br />the ditch companies. <br /> <br />Gabion drops have been recommended in this study, although <br />concrete structures are a possible alternative. Gabions are <br />less expensive and just as effective as concrete but may be <br />more susceptible to vandalism. If concrete structures are <br />decided on for the final design, the costs given in this study <br />would have to be changed accordingly. <br /> <br />PRELIMINARY DESIGN <br /> <br />BASIN 6100 <br /> <br />. Construct 250-foot berm with asphalt pavement over- <br />flow section at north side of Webster Pond in vicinity <br />of Webster Avenue drainageway crossing. <br /> <br />. Replace existing three 48-inch CMP's at Lamar Street <br />with two 4 foot x 8 foot concrete box culverts. <br /> <br />6-1 <br />