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FLOOD SITUATION SOURCES OF DATA AND RECORDS Information on past floods was obtained from Corps of Engineers flood records, the city of Longmont, Bo~lder County, and Weld County. Cross section data and topographic mapping having a contour interval of 2 feet and a scale of 1 Inch equals 200 feet were established by photogrammetrlc methods from flights made In June 1976. These data were furnished by Boulder County in coop- eration with the Colorado Water Conservation Board. U.S. Geological Survey 7.5-minute quadrangle maps with 10-foot contour Intervals at a scale of 1 inch equals 2,000 feet were also used in the study. Bridge and roadway survey data were provided by the Colorado Water Conservation Board. FLOOD SEASON AND FLOOD CHARACTERISTICS The Dry Creek basin is located within an area which can receive very intense rainfall of cloudburst magnitude. Floods have resulted from storms covering large areas receiving heavy general rainfall as well as from storms covering sma I I areas but producing extremely Intense rainfal I. Floods generally occur In the period from May through September but could occur during anytime of the year. FACTORS AFFECTING FLOODING AND ITS IMPACT Obstructions to floodflows - There are 20 roadways which cross the Dry Creek flood plain. Those roadways which present potential for floodflow obstruction are North 63rd Street, Colorado Highway 119, the Colorado and Southern Railway, County Road at reference point 30, Colorado Highway 52, Monarch Road, North 79th Street, Niwot Road, North 83rd Street, North 107th Street, and North 119th Street. 10 Manmade objects, such as buildings, cars, and fences, as well as trees, brush, and other vegetation located on the flood plain, can also cause floodflow obstructions. These items tend to create higher stages on the flood plain because of reduced flow area and flow blockage and they may also create higher localized velocities as the floodwaters flow around them. Flood stages may be lowered In a local ized area If obstructions are dislodged. The debris may then accumulate at locations farther downstream, thus compounding' problems. This floating debris commonly lodges against bridge piers and bridge decks; the flow area is then reduced or the channel is completely blocked. The blockage can produce severe stage and velocity increases, often accompanied by bridge damage or complete failure. Flood damage reduction measures - Irrigation reservoirs and structures are numerous throughout the entire Dry Creek basin. Those structures considered pertinent to the results of thIs study are Boulder Reservoir, Six Mile Reservoir, and Gaynor Lakes. Boulder Reservoir was built in 1955 by the city of Boulder. The main function of the reservoir Is to provIde water for munIcipal use by the city of Boulder and storage of irrigation water for the Northern Colorado Water Conservancy District. It Is also used for recreation. Flood control in the reservoir is sufficient to make the area upstream from the reservoir noncontributing to peak dis- charges In the study reach. Six Mile Reservoir was built about 1895. Its use Is pri- marIly storage of Irrigation water. Because the emergency spillway outlets Into Boulder Reservoir, there Is no contribution to peak discharges downstream from Six Mile Reservoir. 11