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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-, or 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-, 5D-. 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, avera~e 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 I year are considered. For example, the risk of having a flood which equals or exceeds the lOO-year flood (I-percent chance of annual exceedence) in any 50-year period is approximately 40 percent (4 in 10); for any 9O-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 community at lhe time of completion of lhis 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 by detailed methods affecting the community. Four sources of information were used to estimate peak discharge-frequency relationships for lhe streams studied in detail: (1) hydrology studies designated by the State of Colorado for local use in floodplain regulation; (2) material adopted from a 1976 USACE hydrologic report for EI Paso County (Reference 16); (3) new material generated from olher existing information and reports; and (4) new hydrologic data developed by Camp Dresser and McKee, Inc. The hydrologic analyses used for Sand Creek and Sand Creek's tributaries, as well as Jimmy Camp Creek and associated tributaries upstream of Peaceful Valley Road, were obtained from U.S. Department of Agriculture, Soil Conservation Service (SCS), Flood Hazard Analysis reports (References 17 and 18). The SCS report used SCS hydrologic melhods to develop peak discharges for the 10-, 5D-, and lOO-year recurrence intervals. The 500-year peak: discharge was obtained by extrapolating the 10-, 50-, and lOO-year peak discharges on log-probability paper. Because of inadequate streamflow data on these streams, a synlhetic rainfall-runoff evaluation procedure was used to verify these studies. The SCS studies have been designated by the CWCB as complying with state statutory requirements. For Jimmy Camp Creek below Peaceful Valley Road, the peak flow rates were developed by lhe USACE and obtained from the December 1976 Report on Hydroio~ic Investie:ations (Reference 16). Synthetic unit hydrographs and rainfall data from the NOAA report, Precipitation-Frequency Atlas of the Western United States (Reference 19), were used to develop the peak discharges. Flow rates for portions of Fountain Creek, Upper Fountain Creek, and Monument Creek downstream of the U.S. Air Force Academy were adopted from a USACE 1976 hydrology report (Reference 16) and USACE Flood Plain Information (FPI) reports (References 9 and 20). This hydrologic method consists of gage station analysis, whereby stream gaging data have been analyzed to estimate peak flows for the various recurrence intervals. 19