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. . 3.0 . . Nissen Channel flows southeasterly through central Broomfield and is narrow and relatively shallow. The average slope is 56.9 feet per mile. For approximately 1,400 feet, the stream flows down East 7th Avenue between Ash and Birch Streets. Runoff from West Lake and Gay Reservoir basins, located in northeastern Broomfield, flows through shallow, poorly defined channels that have average slopes of 68,3 feet per mile. Flooding in Broomfield is generally caused by heavy local rainstorms. Flooding occurred in various parts of the city in June 1965, July 1965, and May 1973. Scenes of the May 1973 flood are shown in Figures 2 through 8. 2.4 Flood Protection Measures The City has purchased various parcels to be developed into parks. In 1975, zoning ordinances were passed to prohibit construction of structures within the floodplains and to prevent flood damage. Other flood protection measures include the construction of a levee and channel improvements along Gay Reservoir Channel. ENGINEERING METHODS For the flooding sources studied by detailed methods in 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 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 (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 flood elevations will be amended periodically to ~eflect future changes. 5