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I FUTURE FLOODS Small areas within the flood boundaries may lie above the flood elevations and, therefore, not be subject to flooding; owing to limitations of the map scale, such areas are not shown. I I Flood Frequency and Discharge Encroachment on floodplains, such as artificial fill, reduces the flood-carrying capacity and increases flood heights, thus increasing flood hazards in areas beyond the encroachment itself. One aspect of floodplain management involves balancing the economic gain from floodplain development against the resulting increase in flood hazard. The concept of a floodway is used as a tool to assist local communi- ties in this aspect of floodplain management. Under this concept, the area of the 100-year flood plain is divided into a floodway and a floodway fringe. The floodway is the channel of a stream plus any adjacent floodplain areas that must be kept free of encroachment in order that the 100-year flood be carried without substantial increases in flood heights. The Urban Drainage and Flood Control District standards limit such increases in flood heights to 0.5 foot. Hazard- ous velocities at the edge of the floodway fringe are sometimes also considered on a case-by-case basis. For the Morrison Flood Insurance Study (Reference 5), the Federal Emergency Management Agency (FEMA) requested that a velocity above three feet per second was to be considered hazardous and this was adopted by the District for the Morrison area. The criteria adopted by Jefferson County limits en- croachment to areas under one foot or less of water. The topography along the re~ches studied is such that the Jefferson County criteria is the limiting constraint in the delineation of the floodways, and the increase in flood heights is insignificant in most cases. When a substantial encroachment is not possible, the floodway is assumed to be the same as the 100-year flood boundary. The results of the floodway computations are tabulated at selected cross-sections for each stream on which a floodway was established (see Tables 1 through 7). I Floods of the same and greater magnitude as those that have occurred in the past will occur in the future. With development occurring in the basin, flooding hazards will increase. The dis- charges reported in Tables 1 through 7 represent the 100-year flood event and are used to determine the extent and the impact of the flooding hazards. The information obtained is usable not only for floodplain regulation but also for planning, engineering and flood- plain management. I I I I The 100-year flood event has a one percent chance of being equaled or exceeded during anyone year and can be expected to occur at any time in a given area, based upon recorded historical precipitation and other valid data. The 100-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 which floodplains should be designated for regulatory and improvement purposes. In Colorado, the 100-year floodplain represents an area of state interest as defined in House Bill 1041. I I I Flood magnitudes greater than the 100-year magnitude can and will occur. Land improvements adjacent to the 100-year floodplain limits should also consider the chance for possible flood damage. I I Flooded Areas and Floodway Delineation I For each stream studied, the boundaries of the 100-year flood have been delineated using the flood elevations determined at each cross section. Between cross sections, the boundaries were interpolated using 1 inch equals 100 feet scale topographic maps with a contour interval of 2 feet, except for Bear Creek and Mount Vernon Creek at Morrison (Sheets 4 through 8), where 1 inch equals 200 feet scale topographic maps with a 2 foot contour interval were enlarged to a 1 inch equals 100 feet scale. I I I I -7- I