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3.0 ENGINEERING METHODS For the flooding sources studied by detailed methods in the community, standard hydrologic and hydraulic study methods were used to determine the flood hazard data required for this study. Flood events of a magnitude which are expected to be equaled or exceeded once on the average during any 10-, 50-, 100-, and 500-year period (recurrence interval) have been selected as having special significance for floodplain management and for flood insurance rates. These events, commonly termed the 10-, 50-, 100-, and 500-year floods, have a 10-, 2-, 1-, and 0.2-percent chance, respectively, of being equaled or exceeded during any year. Although the recurrence interval represents the long-term average period between. floods of a specific magnitude, rare floods could occur at short interval s or even wi thin the same year. The risk of experiencing a rare flood increases when periods greater than one year are considered. For example, the risk of having a flood which equals or exceeds the 100-year flood (l-percent chance of annual exceedence) in any 50-year period is approximately 40 percent (4 in 10), and, for any 90-year period, the risk increases to approximately 60 percent (6 in 10). the analyses reported herein reflect flooding potentials based on conditions existing in the cOllllllunity at the time of completion of this study. Maps and flood elevations will be amended periodically to reflect future changes. 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish the peak discharge-frequency relationships for each flooding source studied by detailed methods affecting the community. The general rainstorm floods result from rainfall over areas of large areal extent. Rainfall from this type of storm may occur over several days. Because of this time duration, the affected streams rise relatively slowly. The duration of the flood period also is sometimes of considerable length. Because of the large areal extent of rainfall, many tributaries contribute to the peak discharge. This results in progressively increasing discharges downstream. However, downstream from the contributing tributaries, the effects of valley storage cause flood discharge attenuation. Discharge magnitudes for floods on the South Platte River were based upon data collected at u. S. Geological Survey stream gaging stations located at Denver, Henderson, Kersey, Weldona, Balzac, and Julesburg, all located in Colorado. Discharge values developed at Sterling were part of an overall South Platte River study. Peak discharge probability relationships were developed from these records. A basin model was developed to determine discharge probability relationships for locations other than at the stream locations. Peak flows from the hydrologic analysis were used as inputs to the COE HEC-2 water-surface profile computer model (Reference 2). This model provided estimates of the water-surface elevations for each 5