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Table 7 entitled "Flood Plain Refe~en<.:e Data - South Platte River"presents dlS<':harge datil on the South Platte Rive r. DIscharges for the I O-yea~, 50-yea~, 100-year and SOO-year floods on Pa~nee Creek at the U.S. Hlgh~ay 6 crossing about 5 miles south~est of Sted ing are 7,000, 26,000, 41,000 and 90,000 cubk feet per second ~espectively. fREQUENCY Sterling area would result in the Inundation of and subsequent damage to agrkultural, residential, coomerClal, and Industrial p~operties as well as public utilities and publ ic roadways. Deep flOOdwater flowing at high velocity and carrying floating debris would create conditions hazardous to persons and vehicles attempt- Ing to cross flooded areas. In general, floodwater 3 orrrore feet deep and flowing at a velocity of 3 or rro~e feet por ,econd could easily sweep adult persons off their feet, thus creating definite danger of injury or dro...nlng. Rapidly ~lsln9 and s...1 ftly f 10...1 ng floodwater may trap persons In homes that are ultimately destroyed or In vehicles that are ultimately Submerged or tloated. Isolation of areas by floodwaters <.:ou+d create hazards in terms of medical, fire, or law enforcement emergencIes. The 500-ye<lr f 100<.1 Is not th", la~gest flood that <.:..n occur, but the probability of larger floods Is ren>ote. As can be seen from the stream gaging records tor the South Platte RIver, discharges smaller than either the 100-ye1>r or SOD-year floods 1>re much mo~e cO/llllOn. Large f loads, however, can happen; thl s was clearly demonstr1>ted by the Denver area floods of 1965; the June 1972 flood at Rapid City, South Dakota; and the July 1976 flood In the BIg Thompson canyon. fIOOdedareaSandflooddam1l!les-Watersurfaceproflles for the floods studied In this report were developed using the back.water computer program HEC-2, "Water Surhce Profiles", devel- oped by the U.S. A~my Corps of Engineers. The computations were b~sed on Channel and flood plain conditions as represented by $ur- veyd<Jtagathered rnD"<.:ambe~ 1976 and as supplamented by later tield Investigations. flooding can occur with reletlvely little diSCharge In the event of channel blockllgecaus"dbyrlebrlso....lce. Inthese S 1 tuat Ions, wete.... surfilce e I eVilt ion~ rl se unt II re II ef Is i1ch I eyed by flows over the flood plilln. Historical datd, ho...ev"r, Indicate thllt obstructions due to Ice ,,~e not likely to occur In the study reach. The lo~mation of obstructions at bridges In the study ~eilch Is, however, ~ult" ~n~~ihle - especial!y due to deb....!s accumulation. The profiles on plates 1:5 and 14 present ...ater surface ",Ievat!o~s '~d w"ter depth of the !O-yc"~, 50-yoJr, 1()()-'fO~~, iln~ 500-year floods rel1>tlve to the streamboo. Inthlsparticul"r <.:"se 1>nd throughout the study, the stred~bed Is dCtudlly the chdnnel 10... flow ...ater surface elevatIon since the surveys were conducted by aerlill photogrammctrlc methods. The wilter surfilce elevations of the 100-ye1>~ and 500-yellr floods ~ere ~sed to deteNIIlne flood limits on the cross sections and topographic maps. The results "re shown on the flooded Are"s, pllltes 3 through 12. An Index to thO flooded HAZARDS OF LARGE FLOODS Th.. extent of damage caused by any flood depends on th<l topography of tho a~e~ flooded, depth and duration of flooding, velocity of flow, rate of rise, developments on the flood plain, and amount of debris In the floodwater. A 100-year flOOd or 11 500-year flood on the South Platte River and Pawnee C....eck In the n '29