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FLOOD02718
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Last modified
1/25/2010 6:25:16 PM
Creation date
10/4/2006 11:05:49 PM
Metadata
Fields
Template:
Floodplain Documents
County
Logan
Community
Logan County
Stream Name
South Platte River, Pawnee Creek
Basin
South Platte
Title
Floodplain Information South Platte, Pawnee
Date
9/1/1978
Prepared For
Logan County
Prepared By
US Army Corps of Engineers
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />data and topog..aphicmapplng having a scale of I inch equals 200 <br />f69t and a contou.. inte..val of 2 feet we..e established by photo- <br />grammetrlc methods from photographs taken 1n December 1976. These <br />data were furnished by Logan County In coope..ation with the COlorado <br />Water Conse..vation Board. Additional c..oss section data was furniShed <br />by the Co..ps of Engineers. U.S. Geological Su..vey 7.5-mlnute quad- <br />..angle maps wIth Io-foot contou.. Intervals and a scala of I Inch equals <br />2,000 fe..twe.... a!so used In the study. Bridgeandroadwaysu..vey <br />data we..e fu..nlshed by the Colorado wate.. COnse..vatlon Boa..d. <br /> <br />The general rainstorm floods ..esult from rainfall ove.. <br />a..eas of large areal extent. Rainfall from this type of storm may <br />occur ove.. sev....al days. Because of this time duration, the <br />affect..d st....ams rIse relatIvely slow, sometimes over a pe..iod of <br />several days. TheduratlonofthBfloodperlOdalsolssometlmEls <br />of considerable length. Because of the large a..eal extent of rain- <br />fall, more tributaries contribute to the peak discharge. This <br />results In Increasing dIscharge prog..esslvely downst..eam. However, <br />downstream from the contributing tributaries, the effects of valley <br />storage cause flood discharge attenuation. <br /> <br />FLOOD SEASON AND FLOOD CHARACTERISTICS <br />Flcodlng along the South Platte RIver and Pawnee C..eek In <br />the Sterling area normally occu..s from March tbrough September with <br />thorrostfroquentf!oodlngoccu..rlng during JUM. Annual pcakdls- <br />charges, however, have been recorded througtxlut the year. Floods In <br />the Sterling area normally occu.. unde.. non Ice-affected conditions. <br />Ice jams, however, ere possIble On the South Platte Riv..r even though <br />their historical frequencyofoccurrencelsrara. Irrlqationworks <br />also Influence floodIng in the !:terl ing a..ea ~y dlvart1nq floodwatars, <br />by leveo failure, or by cornblnatlonsof both. <br /> <br />ErosIve effeets can be conslder~ble In the event of ~ny <br />flood reg~rdless of Its origIn. Floods resul tlng from Intense <br />r~lnstorms typically create sever.. erosIon damages In a sho..t <br />period of tIme dUB to the rel~tlvely large peak dIscharge and high <br />velocIty of the flood~ter. Erosion dameges from snowmelt or <br />genera! ..alnstorm floods usually result from prolonged exposure to <br />the flood~ater because of the long duratIon typIcal of these floods. <br /> <br />Floodwaters on the South PlatTe RIver and Pawnee Creek In <br />the vicinity of Sterling can ..esult from Intense locallwd rainfall <br /> <br />Int..nse rainstorm flOOdS as well as gen....al rainstorm <br />floods have occur..ed on the South Platte River, Examples of the <br />Intense storms ere the 1965 events on Pawnee Creek and SIJou Creek <br />and the 1976 event on the 81g Thompson River. Examples of the <br />larger more generalIzed storms are th.. 1969 and 197J events pro- <br />ducing flooding on the South Platte River. <br /> <br />or general rains, either of which ~y be augmented by snowmelt. <br />The Intense rainstorm floods ..esult from Int0nse rainfall over areaS <br />of relatively s~ll areal extent. These storms can produceextre<<1ely <br />rapid concent..atlons of floodwaters dnd ensu6nt rapId rises In strkam <br />levels. Because of lIttle wil..n1ng time p..io.. to st..eams attainIng <br />flood stagos, this typo of storm can produce floods which are extremely <br />hazardous. This type of flOOd produces high peak dlscha..ge on trib- <br />utaries Inthagoneral viclnltyofthe..alnsto..m. Thedisch"..ge, how- <br />eve.., attenuates rilpidly on the South Platte R1v"r downstream from <br />the ~ntrlbutlnq t..ibutarles duo to valley stor3ge at the floOd~atGr. <br /> <br />FACTORS AFFECTING FLOODING AND ITS IMPACT <br />Obstructions to floodflows - There a..e two crossings over <br />the South Platte River flood plain as descrlbad In this report. <br />These are the 8urllngton No..thern ..allroad and U.S. Highway 6. 60th <br />of these create f1oodflow obstructions. <br /> <br />10 <br /> <br />II <br />
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