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<br />pae;e 3 <br /> <br />due to valley storage of the flood waters. The general rainstorm floods <br /> <br />result from rainfall Over areas of large areal extent. Rainfall from <br /> <br />this type of storm may occur over several day.,;. Because of this time <br /> <br />duration, the affected streams rise relatively slow, sometimes over a <br /> <br />period of several days. The duration of the flood period also is some- <br /> <br />times of considerable length. Because of the large areal extent of <br /> <br />rainfall, more tributaries contribute to the peak discharge. This results <br /> <br />in increasing discharge progressively downstream. However, downstream <br /> <br />from the contributing tributaries, the effects of valley storage cause <br /> <br />flood discharge attenuation. <br /> <br />Erosive effects can be considerable in the event of any flood, <br /> <br />regardless of its origin. Floods resulting from intense rainstorms <br /> <br />typically create severe erosion damages in a short period of time due <br /> <br /> <br />to the relatively large peak discharge and high velocity of the flood <br /> <br /> <br />waters. Erosion damages from snowmelt or general rainstorm floods usually <br /> <br /> <br />result from prolonged exposure to the flood water becaus,c of the long <br /> <br />duration typical of these floods. <br /> <br />Intense rainstorm floods as well as general rainstorm floods have <br /> <br />occurred on the South Platte River. Examples of the intense storms are <br /> <br />the 1965 events on Pawnee Creek and Bijou Creek and the 1976 event on <br /> <br />the Big Thompson River. <br /> <br />South Platte River <br />The South Platte River drains an area of approximately 18,000 square <br /> <br />miles upstream from Sterling. The western boundary of this basin is <br /> <br />formed by the Continental Divide with elevations up to about 14,:;00, <br /> <br />feet mean sea level (M.S.L.) and the Laramie Hange with elevations up <br /> <br />to about 10,000 feet M.S.L. The mountain area known as South Park is <br /> <br />located in the southwestern portion of the basin. This area is nearly <br />