Laserfiche WebLink
<br />r <br />..,1 <br />.. <br /> <br /> <br />J <br />rJ <br />J <br />J <br />, J <br />r <br />1 <br />t' <br />.. <br /> <br /> <br /> <br />l <br /> <br />.. <br /> <br />l <br />.' <br /> <br />l <br />- <br /> <br />l.. <br />~ <br />~ <br />\ <br />~ <br />t <br />1 <br />t <br />1 <br /> <br />._---_._.~_. . <br /> <br />3. Cross-sections 27 and 29 were straightened out to reflect the actual floodplain in the area where the <br />proposed levee will prevent the flows from splitting out of the creek and draining to the nonh <br />towards the City of Sterling. <br /> <br />4. Cross-section 2 I was added in order to more accurately model the proposed floodplain upstream of <br />HV/Y 6. <br /> <br />5. At the direction of the project sponsors, an open channel was modeled in lieu of new bridges at <br />HWY 6 and the UPRR. This was completed in order to determine the required channel dimensions <br />and flooding depths at this location. This information will be used by others to perform more <br />detailed hydraulic analysis for sizing proposed bridge improvements. The proposed channel <br />improvements were considered from 100 feet upstremn of H\VY 6 to 200 feel downstream of the <br />UPRR. <br /> <br />6. Dov>l1stream 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 levees 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 1 DO-year floodplain. <br /> <br />The revised model was used to compute water surface elevations for the 10-, 50.., and 1 DO-year <br />flood events under proposed project conditions. A 1 DO-year residual floodplain was delineated and <br />is presented on Map Sheet I. <br /> <br />The proposed water surface elevations for the 1 DO-year event were used to determine the size and <br />extent of the proposed floodwalls, levees, and bridge openings. The tops of the proposed <br />flood walls and levees were set at 3 feet above the 1 DO-year water surface elevation in order to meet <br />current FEMA criteria. The bridge openings were sized assuming a trapezoidal channel section <br />with 2:1 sideslopes. It was assumed that the 1 DO-year discharge must be conveyed under the <br />proposed bridges at HWY 6 with a minimum of 1 foot of freeboard. The low chord elevation of <br />the existing and proposed bridges at HWY 6 were asswned to be the same. Based on information <br />from UPRR, it was assumed that the proposed low chord of a new railroad bridge would be 4 feet <br />below the top of the existing rail. It is important to note again that only the channel through the <br />proposed bridges was modeled. Final detailed hydraulic analysis of the proposed bridges will be <br />completed by others. The proposed improvements required to confine the 1 DO-year floodplain and <br />eliminate overflows into the City of Sterling are presented on Map Sheet I and are described in <br />more detail in the Plan Description. <br /> <br />31 <br />