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<br /> <br />Table 2 <br />Dry Creek Basin Drainage Areas <br /> <br />Location <br /> <br />Drainage <br />Area <br />(sq. mi.) <br /> <br />Boulder Reservoir <br /> <br />Upstream from Reference <br />Point IIA <br /> <br />Downstream from Reference <br />Point IIA <br /> <br />Reference Point 32 <br /> <br />14.0 <br /> <br />14.2 <br /> <br />Reference Point 73 <br />Reference Point 97 <br />Reference Point 107 <br />Referenee Point 114 <br />Reference Point 123A <br /> <br />16.8 <br />17.9 <br />21.4 <br /> <br />23.2 <br />25.1 <br />25.8 <br />30.2 <br /> <br />Confluence of Dry Creek <br />and St. Vrain Creek <br /> <br />34.0 <br /> <br />FREOUENCY <br /> <br />Although remote, floods larger than the 500-year flood <br />can occur. Floods smaller than the 100-year are much more common. <br />Large floods, however, can happen; this was clear,ly demonstrated <br />by the Denver area floods of 1965; the June 1972 flood at Rapid <br />City, South Dakota; and the July 1976 flood In the Big Thompson <br />canyon. <br /> <br />. <br /> <br />Flooding can occur wIth relatively little discharge In <br />the event of channel blockage caused by debris or Ice. In'these <br />situations, water surface elevations rise until relief is achieved <br />by flows over the flood plain. Historical data, however, Indicate <br />that obstructions, due to ice, are not likely to occur In the study <br />reach. The formation of obstructions at bridges in the study reach <br />is, however, quite possible - especially due to debris accumulation. <br /> <br />20 <br /> <br /> <br />HAZARDS OF LARGE FLOODS <br />The extent of damage caused by any flood depends on the <br />topography of the area flooded, depth and duration of flooding, <br />velocity of flow, rate of rise, developments on the flood plain, <br />amount of debris in the floodwater, and Ice conditions - If ice is <br />a problem In that particular area. A 100-year flood or a 500-year <br />flood on Dry Creek In the study reach would result In the Inunda- <br />tion of and subsequent damage to primarily agricultural properties <br />and assoctated developments as well as public utilities and public <br />roadways. Deep floodwater flowing at high velocity and carrying <br />floating debris would create conditions hazardous to persons and <br />vehicles attempting to cross flooded areas. In general, floodwater <br />3 or more feet deep and flowing at a velocity of 3 or more feet per <br />second could easily sweep adult persons off their feet, thus creat- <br />ing definite danger of injury or drowning. Rapidly rising and <br />swiftly flowing floodwater may trap persons in homes that are ulti- <br />mately destroyed or In vehicles that are ultimately submerged or <br />floated. Isolation of areas by floodwaters could create hazards In <br />terms of medical, fire, or law enforcement emergencies. <br /> <br />Flooded areas and flood damages - Water surface profiles <br />for the floods studied in this report were developed using the back- <br />water computer program HEC-2, "Water Surface Profiles," developed by <br />the U.S. Army Corps of Engineers. The computations were based on <br />channel and flood plain conditions as represented by survey data <br />gathered In June 1976 and as supplemented by later field Investiga- <br />tions. <br /> <br />The profiles on plates 14 through 27 show the water sur- <br />face elevations and water depth of the 10-year, 50-year, 100-year, <br />and 500-year floods relative to the streambed. In this particular <br />case and throughout the study, the streambed Is actually the channel <br /> <br />21 <br />