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<br />4. ~QQ~~~_SlQUgb <br /> <br />The planned interchange is located immediately adjacent to <br />the east basin boundary of the Cooper Slough watershed. <br />Construction of the planned interchange will not: affect <br />Cooper Slough because of the relatively small drainage area <br />tributory to Cooper Slough in the vicinity of the planned <br />interchange. <br /> <br />5. I~I~E~HA~G~_DEAI~AGE_IMfEQY~M~~1S <br /> <br />Recommended drainage improvements for the planned <br />interchange are shown in Figure 6. <br /> <br />The general design strategy for the planned interchange is <br />to provide adequate drainage improvements to assure that <br />interchange construction will not increase flow depths <br />greater than half a foot during the lOa-year storm event or <br />significantly alter existing drainage patterns. <br /> <br />Construction of the planned interchange will reduce the <br />width of opening beneath the Larimer County Road 50 overpass <br />structure due to relocation of the existing frontage road. <br />The planned east to north ramp will prevent storm runoff <br />from overtopping north bound Interstate Highway 25. In <br />addition, the planned east to north ramp also interferes <br />with the existing cross culverts under north bound <br />Interstate Highway 25. <br /> <br />A schematic diagram of 100-year flows at the pli-lnned <br />interchange is shown in Figure 7. <br /> <br />A 11-14-11X6 triple concrete box culvert is recommended <br />under the Larimer County Road 50 overpass embankment. 'fhis <br />culvert would mitigate the effects of reducing the width of <br />opening under the Larimer County Road 50 overpass structure. <br />This culvert would be designed to convey 1434 cis during the <br />lOa-year storm event without increasing upstream flow depths <br />more than half a foot. <br /> <br />A 12-12X6 double concrete box culvert is recommended under <br />the east to north ramp. This culvert would be designed to <br />convey 550 cfs during the lOa-year storm event without <br />increasing upstream flow depths more than half a foot. <br /> <br />Invert elevations of the planned 12-12 X 6 double concrete <br />box culvert and the planned 11-14-11X6 triple concrete box <br />culvert would be set during final design to assure these <br />cuI vert flows dur ing the lOa-year storm event. prel iminary <br />calculations to demonstrate the hydraulic feasibility of <br /> <br />13 <br />