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The areas studied by detailed methods were selected with priority liven to all known flood hazard areas and areas of projected development or proposed construction through March 1990. 2.2 Community Description The majority of Broomfield is located in Boulder County, although portions are situated in Adams and Jefferson Counties. Broomfield is located in north-central Colorado, on the eastern slope of the Rocky Mountains, approximately 13 miles north of Denver. In 1970, the population of Broomfield was 7,161; at the end of 1980, the population was 20,730 (Reference 1). Continued growth is expected for the City of Broomfield. The City Park Channel, Nissen Reservoir, Gay Reservoir, and the West Lake Reservoir drain a total area of approximately 8.4 square miles and are bounded on the northwest by a rise at an elevation of approximately 5,400 feet. The streams generally flow southeasterly, emptying into Big Dry Creek at an approximate elevation of 5,200 feet. The drainage boundaries of the individual basins are indistinct, being located at the crests of the east-west rolls of the high plains. The creeks are generally shallow, with no distinct channels. The climate of the area has characteristic features of low relative hUlllidity, low rainfall, moderate to high winds, and a large daily range in temperature. Temperatures range from a high of I020F to a low of -26oF at RoCkb Flats. The average maximum and minimum temperatures are 60.2 F and 39.1oF, respectively. The average annual precipitation at Rocky Flats is 15.14 inches. Sixty-nine percent of the precipitation occurs between April and September. Summer precipitation is largely from thunderstorm activity and is sometimes extremely heavy. Typical area soils are composed mostly of the Nunn, Platner, and Renshill series, as defined by the U.S. Soil Conservation Service. These soils are generally well drained, nearly level to moderately sloping formations of loamy alluvial material. Slopes vary from 0 to 9 percent, and the natural vegetation is grass, although most undeveloped areas are cultivated. The permeability of these soils is moderate to slow (Reference 2). Land use within the drainage basins consists of a mixture of commercial and residential development with several parks and open space areas. 2.3 Principal Flood Problems The City Park Channel, located in southwestern Broomfield, is well defined and narrow until it crosses West l20th Avenue, where it becomes very shallow with low banks. The slope of this channel averages 47.6 feet per mile. 4