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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 lOa-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 flooding potentials based on conditions existing in the community at the time of completion of this study. Maps and floods elevations will be amended periodically to reflect future changes. , . 3.1 Hydrologic Analyses Hydrologic analyses were carried discharge-frequency relationships for by detailed methods in the county. out to establish the peak each flooding source studied Hydrologic data from the various engineering reports di scussed in Section 6.0 have been used extensively for this revised study of Boulder County, including peak discharges and starting water-surface elevations. Peak discharge-drainage area relationships for all flooding sources studied by detailed methods are shown in Table 1. " 3.2 Hydraulic Analyses Analyses of the hydraulic characteristics of flooding sources studied were carried out to provide estimates elevations of floods of the selected recurrence intervals. from of the the Hydraulic analyses from the various engineering reports discussed in Section 6.0 have been incorporated into this revised study of Boulder County. Water-surface elevations of floods of the selected recurrence intervals on many of the streams studied by detai led methods were computed through use of the COE HEC-2 step backwater computer program (Reference 3). Cross sect ions for the backwater analyses were field surveyed at selected locations. Flood profiles were drawn showing computed water-surface elevations to an accuracy of 0.5 foot for floods of the selected recurrence intervals (Exhibit 1). All elevations are referenced to the National Geodetic Vertical Datum of 1929 (NGVD). Elevation reference marks used in this study are shown on the maps. The hydraulic analyses for the flooding sources studied by approximate methods were based upon data derived from the sources used to establish the peak discharges for these streams. .' , Locations of selected cross analyses are shown on the Flood sections used in the hydraulic Profiles (Exhibit 1). For stream 6