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<br />Nissen Channel flows southeasterly through central Broomfield and <br />is narrow and relatively shallow. The average slope is 56.9 feet <br />per mile. For approximately 1,400 feet, the stream flows down East <br />7th Avenue between Ash and Birch Streets. Runoff from West Lake <br />and Gay Reservoir basins, located in northeastern Broomfield, flows <br />through shallow, poorly defined channels that have average slopes <br />of 68.3 feet per mile. <br /> <br />Flooding in Broomfield is generally caused by heavy local <br />rainstorms. Flooding occurred in various parts of the city in <br />June 1965, July 1965, and May 1973. Scenes of the May 1973 flood <br />are shown in Figures 2 through 8. <br /> <br />2.4 Flood Protection Measures <br /> <br />The City has purchased various parcels to be developed into parks. <br />In 1975, zoning ordinances were passed to prohibit construction of <br />structures within the floodplains and to prevent flood damage. <br /> <br />Other flood protection measures include the construction of a levee <br />and channel improvements along Gay Reservoir Channel. <br /> <br />3.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied by detailed methods in the communi ty, <br />standard hydrologic and hydraulic study methods were used to determine <br />the flood hazard data required for this study. Flood events of a <br />magni tude which are expected to be equaled or exceeded once on the <br />average during any 10-, 50-, 100-, or 500- year period (recurrence <br />interval) have been selected as having special significance for flood- <br />plain management and for flood insurance rates. These events. commonly <br />termed the 10-, 50-, 100-, and 500-year floods, have a 10-, 2-, 1-, and <br />0.2-percent chance, respectively, of being equaled or exceeded during <br />any year. Although the recurrence interval represents the long-term <br />avera2e period between floods of a specific magnitude, rare floods could <br />occur at short interval s or even wi thin the same year. The ri sk of <br />experiencing a rare flood increases when periods greater than 1 year are <br />considered. For example, the risk of having a flood which equals or <br />exceeds the 100-year flood (l percent chance of annual exceedence) in <br />any 50-year period is approximately 40 percent (4 in 10), and, for any <br />90-year period, the risk increases to approximately 60 percent (6 in <br />10). The analyses reported herein reflect flooding potentials based on <br />conditions existing in the community at the time of completion of this <br />study. Maps and flood elevations will be amended periodically to <br />reflect future changes. <br /> <br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish the peak <br />discharge-frequency relationships for each flooding source studied <br />by detailed methods affecting the community. <br /> <br />5 <br />