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3.0 ENGINEERING METHODS For the flooding sources studied by detailed methods 1n 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-, 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 specifi" magnitude, rare floods could occur at short intervals or even within the same year. The risk of experiencing a rare f1'ood increases when periods greater than 1 year are considered. For example, the risk of h~ving 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 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 establ ish discharge-frequency relationships for floods of the recurrence intervals for each stream studied by methods in the community. the peak selected detailed The U.S. Army Corps of Engineers (COE), Omaha District, provided the flood discharge characteristics of Sand Creek, determined as part of a Flood Control District (Reference 18). The hydrologic analysis of Sand Creek was based on the development of a surface runoff model of the Sand Creek basin using the runoff block of the Environmental Protection Agency's Storm Water Management Program (Reference 19). Toll Gate Creek drains approxilll8tely 40 square miles at its confluence with Sand Creek and receives runoff from two major tributaries. The first tributary, West Toll Gate Creek, originates south of Quincy Dam, approximately three miles east of Cherry Creek Reservoir. The second tributary, East Toll Gate Creek, extends into Arapahoe County southeast of Buckley Air National Guard Base. Runoff calculations for the 10-, 25-, and 100-year and Standard Project Floods were made by the COE, Omaha District, for both major tributaries. The intermediate regional or lOO-year flood discharges, which were computed by the COE, are given in the June 1973 Special Flood Hazard Information Report for Upper Toll Gate Creek and Tributaries (Reference 13). Discharges for the other 12