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<br />I. No changes were made to the existing model from the study limits upstream of CR. 31 (cross- <br />section 3839) to just downstream of CR. 31 (cross-section 30.1) since no improvements are <br />proposed in this reach. <br /> <br />2. The model was revised to eliminate the existing flow splits in the north overbank dO\\'TIstream of <br />CR. 33 at cross-section 2728. This was accomplished by adding the flows which split out of <br />Pawnee Creek towards the City of Sterling back into the model starting at cross-section 26 and <br />continuing to cross-section 21. The discharges were also increased upstream of HWY 6 to account <br />for the flow splits draining from the Town of Town of Atwood and to eliminate the existing flows <br />which currently drain past the bridges at HWY 6 and on towards the City of Sterling. The amount <br />of storm water draining from th,e Town of Town of Atwood to Pawnee Creek on the northwest side <br />ofHWY 6 was determined by the Colorado Water Conservation Board based on field surveys and <br />utilizing a weir flow equation. <br /> <br />3. Cross-sections 27 and 29 were straightened out to reflect the actual :I1oodplain in the area where the <br />proposed bridge size increase will allow greater flows to follow the actual drainageway alignment, <br />and limit the flows from splitt:lng out of the cn:ek and draining to the north towards the City of <br />Sterling. <br /> <br />4. Cross-section 21 was added in order to more accurately model the proposed floodplain upstream of <br />HWY6. <br /> <br />5. New bridges at HWY 6 and the UPRR were modeled. Construction of the UPRR bridge was <br />completed during 1997, but downstream channel restrictions prevt:nted the bridge from using its <br />hydraulic capacity. <br /> <br />6. Downstream of HWY 6, an entirely new HEC-2 model was prepared and combined with the <br />revised model upstream in order to determine the affects of the greater flows reaching this location <br />as a result of the proposed levt:es and larger bridge openings. The: new model resulted in higher <br />water surface elevations with more widespread flooding. Therefore, a system of floodwalls and <br />levees were considered downstream of the UPRR bridge to confine the flood flows downstream to <br />the South Platte River 100-year floodplain. <br /> <br />7. The model of the Pawnee Crc:ek overflow area was IIsed unmodified except that flows were <br />increased to reflect the 5,000 cfs 100- Y ear event. <br /> <br />The revised model was used to compute water surface elevations for the 10-,50-, and 100-year <br />flood events under proposed prcject conditions. The proposed water surface elevations for the 100- <br />year event were used to detennine the size and extent of the proposed floodwalls, levees, and <br />bridge openings. The tops of the proposed flood walls and levees were set at 3 feet above the 100- <br />year water surface elevation in order to meet current FEMA criteria. The bridge: openings were <br />sized assuming a trapezoidal channel section with 3:1 sideslopes. It was assumed that the 100-year <br />discharge must be conveyed under the proposed bridges with a minimum of I foot of freeboard. <br />The low chord elevation of existing and proposed bridges at HWY 6 assumed to be the same. <br /> <br />I1I-2l <br />