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Floodwaters on the South Platte River, Pawnee Creek Overflow, and Sand Creek in the vicinity of Sterling can result from intense localized rainfall or general rains, either of which may be augmented by snowmel t. The intense rainstorm floods result from intense rainfall over areas of relatively small area extent. These storms can produce extremely rapid concentrations of floodwaters and resultant rapid rises in stream levels. Because of little warning time prior to streams attaining flood stages, this type of storm can produce floods which are extremely hazardous. This type of flood produces high peak discharges on tributaries in the general vicinity of the rainstorm. The discharge, however, attenuates rapidly on the South Platte River downstream from the contributing tributaries due to valley storage of the floodwater. Erosive effects can be considerable in the event of any flood regardless of its origin. Floods resulting from intense rainstorms typically create severe erosion damages in a short period of. time due to the relatively large peak discharge and high velocity of the floodwater. Erosion damages from snowmelt or general rainstorm floods usually result from prolonged exposure to the floodwater because of the long duration typical of these floods. The Pawnee Creek Overflow floodplain in the Sterling area is traversed by U. S. Highway 6, State Highways 14 and 138, and the combined Union Pacific and Burlington Northern Railroads. Their embankments divert runoff out of natural channels, while undersized bridges and culverts severely restrict floodflows. Hence, much of the existing flood hazard in Sterling would be reduced simply by replacing inadequate crossings with larger hydraulic structures. The most severe constrictions are the U.S. Highway 6 and Union Pacific Railroad bridge at Pawnee Creek south of Sterling, the Burlington Northern Railroad bridge at pioneer Ditch, the State Highway 14 bridge at Pioneer Ditch, and the State Highway 138/Burlington Northern/Union Pacific Railroad bridge at Pioneer Ditch. During periods of high flood stages on Pawnee Creek, floodwater can escape the Pawnee Creek channel to flow overland toward the South Platte River. However, these floodwaters are intercepted by highway and railroad embankments, which redirect the floodwater toward Sterling. This overflow is referred to as the "Pawnee Creek Overflow" in this report. This redirected floodwater flows northerly along the west side of State Highway 138 and the Union Pacific Railroad, past the Riverside Cemetery, and then through the City of Sterling. Ice effects hav"e, historically, not been a problem on the South Platte River or Pawnee Creek Overflow in the Sterling area. A remote chance of ice jams exists on the South Platte River. 2.4 Flood Protection Measures The City of Sterling presently has no flood protection measures within the corporate limits. 4