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<br />it <br />ilN <br />III <br />.. <br />I <br />~ <br />, <br />r <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />, <br />i <br /> <br />J. 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 spIirring out of the creek and draining to the no;::h <br />towards the City of Sterling, <br /> <br />4, Cross-section 2 1 was added in order to more accurately model tbe 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 />H\VY 6 and the l;PRR, 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 upstream of HWY 6 to 200 feet downstream of the <br />UPRR <br /> <br />6, Downstream of H\VY 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, TIle new mode! resulted in higher <br />water surface elevations with more widespread flooding, Theret,)[e, a system of floodwails and <br />levees were considered dO'WTIstream of the UPR.l\. bridge to coniine the t100d flows downstream to <br />the South Plane River 1 OO,year t1oodplain, <br /> <br />The revised model was used to compute water surface elevations for the lO-, 50-, and lOO-year <br />t1(),:,~ "vents under proposed project conditions, A I OO-year residud floocplain was delineated and <br />is presented on .\1ap Sheet 1. <br /> <br />The proposed Water surface elevations for the 100,ye:1I event were used to determine the size and <br />extent of the proposed t1oodwalls, levees, and bridge openings, The tops of the proposed <br />fJoodwalls and levees were set at 3 feet above the 1 OO-vear water surface elevation in order to meet <br />current FE;'vlA criteria, The bridge openings were SIzed assuming a trapezoidal channel section <br />with 2: 1 sideslopes, It was asswned that the 100-year discharge must be conveyed under the <br />proposed bridges at HV,iY 6 with a minimwn of j foot of freeboard, The low chord elevation of <br />the existing and proposed bridges at HWY 6 were assumed to be rhe same, Based on information <br />from UPRR, it was asswned that the proposed low chord of a new railroad bridge wouid be ~ feet <br />below rhe top of the exisring raiL It is important to note again that only the channel through the <br />proposed bridges was modeled, Final detailed hydraul.ic analysis of the proposed bridges wiiI be <br />compieted by others, The proposed improvements required to confine the IOO-year t100dplain and <br />eliminate overflows into the City of Sterling are presenred on Map Sheet 1 and are described in <br />more derail in the Plan Description, <br /> <br />31 <br />