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<br />channel brns toward the north to pass through the railroad bridgc. Some of the flow can spill <br />across a road embankment and 110ws downstream south () f the railroad embankment while tbe <br />main cha1nel and floodplain are north ofthe railroad (~mbankment. This split flow rejoins the <br />main flow just upstream of 35th Avenue <br />The amount of the split 110w was computed using Ihc split flow routine in tbe HEC-2 <br />program. This routine computes the split flow and subtracts it from the main channel flow. The <br />program ]-eturns the computed s:Jlit flow to the main flow at the appropriate location. The <br />alllount of split flow was deterrr.ined by the geometry of the ground or road surface at the <br />overflow locations. This geometry was taken from the topographic mapping. <br /> <br />6.0 WA1LERSURFACEPROFILES <br />6.11 Computed Wat{Ir Surface Profile Results <br />In certain areas, spoil banks on the riverbanks act as levees. In the areas where these <br />spoil banks are present, floodwaters would be confined bec:\'.;een the banks unti I the banks <br />overtop or fail and admit floodwater to the overbanks. Depending on the height and condition of <br />the spoil banks, the maximum flood water surface elevations within the spoil banks may occur <br />when the banks overtop. If the hanks do not immediately fail, a water surface elevation <br />surcharge may exist riverward of the banks slightly above the top-or.bank elevations. <br />The failed spoil bank condition was used for this study to compute flood water surface <br />elevation!! that reflect the conditions in the overbank areas landward of the banks. Because the <br />spoil banks have irregular profiks, detailed. bank profile surveys would be needed to determine <br />the maxinum flood water elevations that could be obtairu~d rivcrward of the spoil banks where <br />they exist. <br />The computed flood water surface profiles for l:1e : 0-, 50-, 100-, and 500-year events for <br />the Cache La Poudre River maiL channel are plottcd or Plates I through 4. The water surface <br />profiles for the split flow reache;; are plotted on Plate 5 and Platc 6. Because the split flows may <br />not occur at the I a-year or 50-year floocllevels, these prorJes arc nol. shown for thc split flow <br />reaches. The computed water suface elevations for the I Co, 50.., 100-, and 5(lO-year floods at <br />selected cross sections are given in Table 2 <br /> <br />6 <br />