FLOOD10617 (2) - Laserfiche WebLink

Home Browse Search

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 experiencing a rare flood increases when periods greater than 1 year are considered. For example, the risk of having a flood that 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); for any gO-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 tfme 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 establish peak discharge-frequency relationships for each flooding source studied by detailed methods affecting the community. Hydrologic data fram the various engineering reports discussed in Section 7.0 have been used extensively for the revised study of Boulder County, including peak discharges and starting water-surface elevations. Because no stream gage data are available for the study streams through the City of Boulder, a rainfall-runoff analysis was conducted on the watersheds to determine the flood discharges. This was accomplished by using the UDFCD CUBP-B rainfall-runoff computer program to develop the storm hydrographs (Reference 27) and the USACE HEC-1 computer program for the stream and reservoir routings (Reference 28). For the analysis, basin characteristics of the watershed, as well as rainfall amounts based on the selected recurrence intervals, are used to compute flood hydrographs for various design points in the basin. All stream and reservoir routings were accomplished using the Modified Puls Method. The SOa-year storm runoff values for various locations along each stream were extrapolated fram the discharge-frequency curves. A more detailed description of the input variables for the CUHP-B and the HEC-1 rainfall-runoff analysis, as well as the CUHP-B computer output and the summary of the final HEC-1 computer output, are located in a technical addendum to this Flood Insurance Study (Reference 29). Peak discharges for Gregory Creek, Bluebell Canyon Creek, Kings Gulch, Viele Channel, Anderson Channel, and Davids Draw were taken fram reports prepared by Wright-McLaughlin Engineers (References 17 and 30) as part of a plan to manage the city'S storm drainage. Discharges for James Creek and the downstream portion of Little James Creek through the Town of Jamestown were taken fram a USACE report (Reference 31). Technical Manual No.1, developed ~ the 33