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Three adjacent detention ponds on EImers Twomile Creek were constructed by the City to control runoff. These ponds are located just south of Iris Avenue behind the K-Mart store. These ponds will attenuate the 100-year peak flood discharge from 615 to 510 cfs. A small detention/siltation pond on Twomile Creek just upstream of Linden Avenue will control moderate flows; however, it does not attenuate the 100-year peak floodflow. Flood protection measures along Skunk Creek consist of several detention/siltation ponds on the U.S. Bureau of Standards property west of Broadway and the channelization of the stream between Morehead Avenue and U. S. Highway 36. The detention ponds are small and silted in and, therefore, were not modeled. \" .' The improved Bear Canyon Creek channel along Table Mesa Drive from Broadway to Gillespie Street (upstream Limit of Detailed Study) will efficiently convey moderate floodflows downstream; however, major storms will continue to cause flooding problems. The channel improvements in Martin Park completed under a recent UDFCD maintenance project were not designed to contain the 100-year floodflow in the channel (Reference B). Flood protection measures along Goose Creek include channelization of North and South Goose Creek. Flood protection measures along Dry Creek include channelization and construction of a new bridge at Flatirons Parkway (Reference 20). Flood protection measures along Fourmile Canyon Creek channelization through the Palo Park Subdivision (Reference include 18). 3.0 ENGINEERING METHODS For the flooding sources studied by detailed methods in the community, standard hydrol,ogic and hydraulic study methods were used to determine the flood hazard data required for this study. Flood events of a magni tude 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 flood- plain 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 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 100-year flood (I-percent chance of annual exceedence) in /') ,,.., 10