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Van Bibber Tributary - Ramstetter Reservoir, which is located at the upstream end of the area studied, near State Highway 93, pro- vides minimal routing effect to the peak discharges. 3.0 ENGINEERING METHODS For the flooding sources studied in detail in the cOllllllunity, standard hydrologic and hydrauhc 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 se- lected as having special significance for flood plain management and for flood insurance rates. These events, cOllllllonly 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 intervals or even within the same year. The risk of exper1encing 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 (1 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 flood1ng potentials based on conditions existing in the cOllllllunity at the time of completion of this study. Maps and flood elevations w1ll be amended periodically to reflect future changes. 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish the peak dis- charge-frequency relationships for each flood1ng source studied 1n detail affecting the cOllllllunity. Detailed discussions of the hydrologic analyses for the streams studied by the study contractor were presented in previous hydrology reports. The hydrologic analyses for Cold Spring, Kerr, Swede, and Switzer Gulches, and the SJCD 6200 North Tributary were discussed in a 1982 hydrology report (Reference II). The remainder of the 8 treams were discussed in a four-part technica 1 addendum prepared in 1979 (Reference 12). Discharges for streams with drainage areas greater than 15 square miles were calculated using a log-Pearson Type III frequency anal- ysis (Reference 13). As is typical in Colorado, flood events in the study area comprise two distinct and generally independent populations (i.e., rain and snowmelt floods). For this reason, it was necessary to analyze the rain and snowmelt events separately. Each type of flood event was assumed to follow log-Pearson Type 9