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Meetings were held by the Colorado Water Conservation Board with the Colorado Deparunent of Transportation (CDOT) to discuss the feasibiliry of widening the existing highway bridges. Because ' the northbound bridge is an existing wood strucrure 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 Deparunent of Transportation (eDOT) currently has plans to replace the timber bridge within the next 3 to 5 vears. For the southbound lanes, the existing concrete bridge would be ~ ~ ...... widened to convey the 1 OO,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 detennined that four 20' x 8' concrete box culverts would be required in addition to the existing bridges to convey the 100,year peak discharge, The culverts would be placed adjacent to and south of the existing bridge srrucrure, Alternative 3 investigated utilizing an existing railroad bridge crossing strucrure which is located approximately 1.500 feet north of the Pawnee Creek crossing, The highway crossing at this location has two existing concrete box cui 'lens (3' x 10') which carry some local flows under the highway and through an existing railroad bridge. The railroad bridge opening is fairly large (approximately 5' x 113 ') and would carry a larger flow than the highway crossing, Alternative 3 would require initial flood flows on Pav,nee Creek to be conveyed through the existing highway and railroad bridges along the main channel. Once the capaciry of these structures is reached, oven lows would be conveyed by a system of levees to the crossing 1.500 feet nonh. At this location, HWY 6 would be depressed to act as an ove:1:10w weir and the flood flows would overtop the highway and be conveyed through the existing railroad bridge, A system of additional levees would be :-equired dov,nsrream to confine the flood Hows to the South Plane River I OO'ye:rr 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 (4.100 cfs) could be conveyed through the existing Pawnee Creek bridges: therefore, overflows to the north would be on the order of about 4, I 00 cfs, Depressing HVlY 6 to pass the flows would require a length of about 1.200 to 1.400 feet in order to meet CDOT design criteria, With a weir length this long, flows ovenopping the highway would be very wide and could not get back into the existing channel in order to be conveyed through the railroad bridge, Additionally. because of the adiacent 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 Pa""nee Creek Overflow would be difficult. Based on the cursory review of Alternative 3, it was not studied further. Plan Description 100- Year Residual Floodplain: The results of the hydraulic analysis show that by eliminating the split flows at channel station 224-'-00, the 1 OO-ye:rr 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 :rrea to the south and flood a low lying area between channel stations] 68-;-00 and 224-00, There may also be additional shallow flooding to the south of Pawnee Creek between stations 14.1+00 and 168-;-00, However. based the existing mapping (scale of I" = 800' and a 5 foot comour interval) it is difficult to be certain that this flooding will occur, ") ~-