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I I I I I I I I I I I I I I I I I I I - FUTURE FLOODS - At Lowell Boulevard on Sheet 6, flood discharges spill side- ways across Hampden Avenue for a distance of approximately 400 feet upstream of the bridge. This flow spills into a backwater area created by Lowell Boulevard on the east and Hampden Avenue on the south. This backwater area also receives flood waters that flow along the north side of Hampden Avenue from the Bear Valley Shopping Center. A maximum flow of 150 cfs will spill from this backwater area across Lowell Boulevard and the Lowell/Hampden Intersection and continue downstream along Hampden Avenue. Flood Frequency and Discharge The lOa-year flood event based upon recorded historical pre- ciptation and other valid data has a one percent chance of being equalled or exceeded during anyone year. Floods of this magni- tude or greater have occurred on Bear Creek in the past and will occur in the future. lThe fully developed basin conditions\on Bear Creek below the dam will cause frequent high runoff during annual thunderstorm events. At Sheridan Boulevard on Sheet 9, very shallow overflows will cross the street into Bear Valley Shopping Center. Immediately upstream of the Sheridan Boulevard bridge, the calculated water surface elevation is below the minimum roadway elevation at which spill would occur. However, this calculated water surface elevation reflects high-velocity flow in the main channel. In the left overbank, low velocities indicate a higher water surface elevation. Therefore, the total energy was used to plot the flood- plain and to calculate overflows. The discharges reported 1n Figures I and II for the lOa-year flood frequency represent the relative impact of the flood event. This information is usuable not only for floodplain regulation, but also for planning, engineering and channel improvement. The estimated overflow discharge at this location 1S 250 cfs. Most of this overflow drains back into Bear Creek from the shopping center parking lot. About 100 cfs flows east from the parking lot and 1S conveyed along the north side of Hampden Avenue. The lOa-year flood event is considered by the Urban Drainage and Flood Control District, the Colorado Water Conservation Board and the Federal Emergency Management Agency as the flood magni- tude for regulation of floodplains and for the design of improvement facilities. In Colorado, the lOa-year floodplain represents an area of state interest as defined in HB 1041. Flooded Areas The floodplain immediately upstream of the Estes Street bridge 1S shown on Sheet 17. Between cross-sections 75 and 76, the 100- year floodplain is barely confined by a small ridge along the north side of the stream. In@ediately upstream of the bridge for about 100 feet, however, some waters will flow to the north and spi 11 across Estes Street during the lOa-year event. Water surface elevations in this overbank area will not corrcspond to those In the main channel. The spill discharge is approximately 300 c[s. The following section contains the Flood Hazard Area Delinea- tion Maps showing the limits of the lOa-year floodplain (Sheets 3 to 21). Water surface profiles are shown separately on Sheets 22 to 29. Table II presents the computed lOa-year flood elevations, along with data describing the floodway in the channel. The follow- ing paragraphs contain a general description of flooding throughout the study reach. South Platte River to South Federal Boulevard The floodplain limits for this reach, which extends from the confluence of the South Platte River to South Federal Boulevard _0)-