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The south levee will extend east from proposed channel improvements downstream of the UPRR for about 1,500 feet to a dirt road just east of the limits of the 100-year floodplain of the South Platte River. The south levee will prohibit low flows from draining along their existing course around a 4,400 feet long oxbow formed by the existing channel; therefore, a new low flow <channel must be constructed to convey storm water between stations 16+00 and 72+00. lbis <channel will be trapezoidal in shape utilizing a 30 foot bottom width and 3: 1 sideslopes. The depth of the low flow channel will range from 2 feet at the point where it leaves the existing channel oxbow to about 9 feet where it must daylight back into Pawnee Creek. Channel improvements are also proposed from a point 100 feet upstream of HWY 6 to approximately 200 feet downstream of the UPRR The highway bridgt:s will need to be improved to accommodate the increase in discharges. As discussed earlier, the existing bridge for the northbound traffic is a wooden structure and CDOT plans to replace this bridge in the next 3 to 5 years. For the southbound lanes, four cells of 20' x 8' RCB would be installed just south of tht~ existing bridge. The culverts could be pre-cast which would minimize the time required to complet'~ the installation, thus limiting the impacts to existing traffic. Wing walls upstream and downstream, a~ well as riprap protection would be required. Conceptual cro:,s-sections of the charmel and bridge improvements at HWY 6 and the UPRR were prepared to aid in cost preparation. In order to construct the new box ';ulverts south of the existing bridge, the existing siphon along the Springdale Ditch which carries irrigation water under the railroad and highway may need 1to be extended to the south. There is also an existing concrete ditch diversion structure located just downstream of the UPRR which will have to be removed. This structure is presently abandoned and preliminary discussions with the owners indicated that they are agreeable to its removable. Based on the HEC-2 model prep2lred by the SCS, the existing charmel through the bridges falls approximately 1.9 feet from an elevation of 3980.0 upstream of HWY 6 to an elevation of 3978.1 downstream of the UPRR. In order to limit the extent of the bridge widening at HWY 6, the proposed channel through the bridges will be lowered about 1.5 feet. The proposed channel lowering will require a drop structure upstrearn of HWY 6. Ideally, the excess material generated from the major channel improvements in the vicinity of HWY 6 and the UPRR, in addition to the low flow chromel further doVYnstrearn, would be used to construct minor training dikes to help direct flows into the new bridges. However, the material must be tested to determine if its physical properties will achieve the goals of a flood control levee. In the summer of 1996 the UPRR eompleted the improvements to their bridge to meet the proposed 100-year flow conditions. However, the UPRR constructed the bridge over the existing channel cross- section so that the area under the new bridge is not increased until the channel improvements proposed in this study are completed. Costs have been included in the estimate for the channel improvements and for soil cement erosion protection under the new railroad bridge. This alternative includes the constmction of a conveyanc(: structure (box culverts) under the railroad embankment near the Town of Tovm of Atwood. This culvert will provide drainag(: directly to the South Platte River which is in close proximity at this location, This improvement is required since the right Pawnee Creek overbank flows are trapped in this area, resulting in high flood depths throughout Town of Atwood. The culverts will not be sized to accon:unodate the entire overbank flow in this area as this cost was determined to be prohibitively high. fII-23