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<br />3.0
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<br />ENGINEERING METHODS
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<br />For the flooding sources studied by detailed methods in the community, standard hydrologic and
<br />hydraulic study methods were used to determine the flood hazard data required for this study, Flood
<br />events of a magnitude which are expected to be equaled or exceeded once on the average during any
<br />10-, 50-, 100-, or 500-year period (recurrence interval) have been selected as having special
<br />significance for floodplain management and for flood insurance rates, These events, commonly
<br />termed the 10-, 50-, 100-, and 500-year floods, have a 10-. 2-, 1- and O,2-percent chance,
<br />respectively, of being equaled or exceeded during any year, Although the recurrence interval
<br />represents the long-term, avera~e period between floods of a specific magnitude, rare floods could
<br />occur at short intervals or even within the same year, The risk of experiencing a rare flood increases
<br />when periods greater than I year are considered, For example, the risk of having a flood which
<br />equals or exceeds the lOO-year flood (I-percent chance of annual exceedence) in any 50-year period
<br />is approximately 40 percent (4 in 10); for any 9O-year period, the risk increases to approximately 60
<br />percent (6 in 10), The analyses reported herein reflect flooding potentials based on conditions existing
<br />in the community at 1he time of completion of 1his study, Maps and flood elevations will be amended
<br />periodically to reflect future changes,
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<br />3,1 Hydrologic Analyses
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<br />Hydrologic analyses were carried out to establish peak discharge-frequency relationships for
<br />each flooding source studied by detailed methods affecting the community,
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<br />Four sources of information were used to estimate peak discharge-frequency relationships for
<br />1he streams studied in detail: (1) hydrology studies designated by the State of Colorado for
<br />local use in floodplain regulation; (2) material adopted from a 1976 USACE hydrologic report
<br />for EI Paso County (Reference 16); (3) new material generated from o1her existing
<br />information and reports; and (4) new hydrologic data developed by Camp Dresser and
<br />McKee, Inc,
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<br />The hydrologic analyses used for Sand Creek and Sand Creek's tributaries, as well as Jimmy
<br />Camp Creek and associated tributaries upstream of Peaceful Valley Road, were obtained
<br />from U ,S, Department of Agriculture, Soil Conservation Service (SCS), Flood Hazard
<br />Analysis reports (References 17 and 18), The SCS report used SCS hydrologic me1hods to
<br />develop peak discharges for the 10-, 50-, and lOO-year recurrence intervals, The 500-year
<br />peak: discharge was obtained by extrapolating the 10-, 50-, and lOO-year peak discharges on
<br />log-probability paper. Because of inadequate streamflow data on these streams, a syn1hetic
<br />rainfall-runoff evaluation procedure was used to verify these studies, The SCS studies have
<br />been designated by the CWCB as complying with state statutory requirements,
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<br />For Jimmy Camp Creek below Peaceful Valley Road, the peak flow rates were developed
<br />by 1he USACE and obtained from the December 1976 Report on Hydrolo~ic Investie:ations
<br />(Reference 16). Synthetic unit hydrographs and rainfall data from the NOAA report,
<br />Precipitation-Frequency Atlas of the Western United States (Reference 19), were used to
<br />develop the peak discharges,
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<br />Flow rates for portions of Fountain Creek, Upper Fountain Creek, and Monument Creek
<br />downstream of the U,S. Air Force Academy were adopted from a USACE 1976 hydrology
<br />report (Reference 16) and USACE Flood Plain Information (FPI) reports (References 9 and
<br />20), This hydrologic method consists of gage station analysis, whereby stream gaging data
<br />have been analyzed to estimate peak flows for the various recurrence intervals,
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<br />19
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