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~ ~,. ~ ~~ 1wII ,;11 ,.. 1~ ~" ~. ~II 'III ... ~~ Irll tr" 'r~ frJ' ~, Meetings were held by the Colorado Water Conservation Board with the Colorado Department of Transportation (CDOT) to discuss the feasibility of widening the existing highway bridges. Because the northbound bridge is an existing wood structure it cannot be widened; therefore, total replacement would be necessary. Three alternatives were investigated to resolve the problem of the undersized bridges at HWY 6. Alternative 1 involves the removal and replacement of existing timber bridge along the northbound lanes. The Colorado Department of Transportation (CDOT) currently has plans to replace the timb'~r bridge within the next 3 to 5 years. For the southbound lanes, the existing concrete bridge would be widened to convey the I DO-year peak discharge. Alternative 2 involves adding box culverts just south of the existing bridges in lieu of widening or replacing them. Based on a cursory evaluation, it was determined that four 20' x 8' concrete box culverts would be required in addition to the existing bridges to convey the 1 DO-year peak discharge. The culverts would be placed adjacent to and south of the existing blidge structure. Alternative 3 investigated utilizing an existing railroad bridge crossing Structure which is located approximately 1,500 feet north of the Pawnee Creek crossing. The: highway crossing at this location has two existing concrete box culverts (3' x lO') which carry some local flows under the highway and through an existing railroad blidge. The railroad bridge opening is fairly large (approximately 5' x 1 13') and would carry a larger flow than we highway crossing. Alternative 3 would require initial flood flows 011 Pawnee Creek to be conveyed through the existing highway and railroad bridges along the main channel. Once the capacity of these structures is reached, overflows would be conveyed by a system of levees to the crossing 1,500 feet north. At this location, HWY 6 would be depressed to ac: as an overflow weir and the flood flows would overtop the highway and be conveyed through the existing railroad blidge. A system of additional levees would be required dO"'l1stream to confine the flood flows to the South Platte River 1 DO-year floodplain. Cursory evaluations completed for this alternative proved it not to be feasible. Evaluations showed that only about half of the 100-year flood event (4,lOO cfs) could be cOll\'eyed through the existing Pa\\l1ee Creek bridges; therefore, overflows to the north would be on the order of about 4, 1 00 cfs. Depressing HWY 6 to pass the flows would require a length of about l,200 to 1,400 feet in order to meet CDOT design criteria. With a weir letlgth this long. flows overtopping the highway would be very wide and could not get back into the existing charUlel in order to be conveyed through the railroad bridge. Additionally, because of the adjacent topography and elevation of the highway. the flood control levees cannot be constructed high enough to provide 3 feet of freeboard. Therefore, FEMA criteria could not be met and removal of the flood prone areas along the Pawnee Creek Overflow would be difficult. Based on the cursory review of Altemative 3, it was not studied filrther. Plan Description lOO-Year Residual Floodplain: The results of the hydraulic analysis show that by eliminating the split flows at channel station 224+00, the I DO-year water surface elevations will increase by I to :2 feet from channel station 125+00 to channel station 214+00. This increase in the water surface elevation will cause the floodplain to inundate a wider area to the south and flood a low lying area between channel stations 168+00 and 224+00. There may also be additional shallow Hooding to the south of Pav,l1ee Creek between stations 144+00 and 168+00. Bowever, based the existing mapping (scale of I" = 800' and a 5 foot contour interval) it is difficult to be certain that this flooding will occur. ,~ ~L