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Mr. Bill DeGroot <br /> December 18, 1998 <br /> Page 2 <br /> wide at the present location of U.S. 36. Like any typical alluvial fan below a mountain <br /> canyon, floods conveyed and deposited sediment in the main stream bed and across the <br /> valley bottom, resulting In a stream that changed in location from time to time. Today, In <br /> many reaches, the western aide of the valley it lower then the eastern side of the valley <br /> where the mein South Boulder Creek currently runs. Overflows can occur upstream of <br /> Highway 93, Highway 39, and Arapahoe and travel to the went side of the valley. Migration <br /> of the mainstream to the west side could occur, but obviously would be and has been <br /> resisted by men. <br /> Flccds have and will cause overflow of significant velocities among many shallow routes. <br /> This overflow can also concentrate at certain locations In an urban setting, potentially <br /> increasing flood hazards. <br /> There is evidence that South Boulder Creek flooding has traveled along The path identified <br /> by Love. U.S. Geological Survey mapping reflects that the South Boulder Creek alluvial <br /> material extends into the existing neighborhoods north of U.S. 38 and along Foothills <br /> Parkway. Examination of historical and current topography shows that Dry Croak Ditch No. <br /> 2 and west volley conveyances cant' both local drainage and South Boulder Creak <br /> overflows. The relatively small flood of May 1989 flooded the west side of the valley and <br /> caused the closure of U.S. 38 for a period of Time near the South Boulder Road and Foothills <br /> Interchange. <br /> Love's study was the first to quantify the South Boulder Crack Spill to the west side of the <br /> valley at Highway 3e and Vials (Foothills Interchange!. Correction of the 100•year Federal <br /> Emergency Management Agency (FEMA) Flood Insurance Rate Maps (FIRM's) and the <br /> UDFCD FHAD floodplain for the west valley north of Highway 38 will likely result in the <br /> addition of a number of fingers of flow and areas of shallow flooding. A revised 500•year <br /> floodplain limit will probably result in major changes to the FIRM. <br /> The Flatirons Laves and the construction of U.S. 38 have impactad the South Boulder Creek <br /> floodplain, causing redirection and redistribution of flood waters. The original U.S. 36 <br /> highway construction created an obstruction across the South Boulder Valley which <br /> redistributed flood waters. The addition of the highway divider further obstructed and <br /> redistributed flood waters, which has worsonsd flood hazards To some parties. Vial& <br /> Channel improvements somewhat reduce these impacts, but have not fully mitigated the <br /> impacts of the highway and divider. <br /> If the existing levee around the Flatirons Pit is adequate, the 100-year floodplain would be <br /> as described herein. The upper and middle Flatirons Levee forces a narrow floodplain on the <br /> east side of the valley, approximately 1500 feet upstream of Highway 38. Flow spills to the <br /> west valley largely from the open space reach of 600 feet above Highway 38. The upper <br /> and middle Flatirone Levee reduces the spills leaving South Boulder Creek from that which <br />