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are expected to be equaled or exceeded once on the average during any 10-, 50-, 100-, or SOO-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 SOO-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 intervals or even within the same year. The risk of experiencing a rare flood increases when periods greater than 1 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 percernt (6 in 10). The analyses reported herein reflects flooding potentials based on conditions existing in the community 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 peak discharge- frequency relationships for each flooding source studied in detail affecting the community. Since there were no streamgage records on Sand Creek or its tributaries, synthetically derived hydrographs were computed to determine the potential flood magnitudes for tributary streams with relatively small drainage areas. These hydrographs reflect the effects of precipitation characteristics of the basin, ground cover and densi ty, geology and soils, slope, drainage area, and other physical characteristics (References 1 and 10). To ensure that the synthetic data were correct, recorded peak discharge data i~ adjacent areas were also analyzed and a graph of 100-year discharge versus drainage area was plotted. Rainfall data for the synthetic analysis were taken from the Denver Regional Council of Governments' Urban Storm Drainage Criteria Manual (USDCM) (Reference 9). Synthetic hydrograph procedures used in the study included the Colorado Urban Hydrograph Procedure, outlined in the Denver Regional Council of Governments' USDCM (Reference 9), and the COE HEC-l computer hydrograph package (Reference 10). The peak discharge-frequency relationships for the South Platte River through Commerce City in the 1977 Flood Insurance Study were provided by the COE, Omaha District. The discharge profiles were keyed to the 100-year flood values com- puted from the Denver and Henderson gaging records of the South Platte River, which were adjusted for the regulating effects of 11