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4.2 Floodways Encroachment on flood plains, 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 flood plain management involves balancing the economic gain from flood plain development against the resulting increase in flood hazard. For purposes of the National Flood Insurance Program, the concept of a floodway is used as a tool to .assist .local communities in this aspect of flood plain management. Onder this concept, the area of the lOa-year flood is divided into a floodway and a floodway fringe. The floodway is the channel of a stream, plus any adjacent flood plain areas, that must be kept free of encroachment in order that the lOa-year flood be carried without substantial increases in flood heights. As minimum standards, the Federal Insurance Administration limits such increases in flood heights to 1.0 foot, provided that hazardous velocities are not produced. The floodways for the streams in this study were determined using both the criteria of flood plain encroachment and conveyance reduction, as well as consideration of hazardous velocities and depths within the flood plain. The smaller tributary streams to the South Platte River are generally steep and severely encroached upon by urban development. As a result, further encroachment or conveyance ability reduction would result in increased velocities above already hazardous velocity conditions. Hazardous velocities are considered to be average overbank velocities in excess of 2 feet to 3 feet per second. The floodway limits for each of the streams have been plotted at the cross section locations, between the cross sections, the floodway boundaries have been drawn to represent streamlined limits characteristic of floodway limits. Many factors affect the determination of the encroachment limits on the Littleton reach of the Platte River includ- ing sand and gravel mining operations, overtopped bridges, present development within the flood plain, and divided flow conditions. To determine the final floodway limit at each cross section, characteristic velocities were considered along with encroachment limits which cause no more than the maximum one; foot rise in the flood height. Where appropriate, an equal degree of encroach- ment was permitted on each side of the main channel. The degree of conveyance reduction on each side of the channel was determined by hydraulic limitations. 17