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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 Flood Insurance Program, the concept of a floodway is used as a tool to assist local communities in this aspect of flood plain management. Under 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 Emergency Management Agency limits such increases in flood heights to 1.0 foot, provided that hazardous velocities are not produced. The floodway in this report is presented to local agencies as a minimum standard that can be adopted or that can be used as a basis for additional studies. The floodway for the Rio Grande at Alamosa was computed for maximum use of the existing levee system. Floodway adjustments were made in the reach below cross section C for manrnade obstructions in the flood plain. From cross section C to cross section 0, the right overbank levee was assumed as the right bank floodway limit. The left bank floodway limit was adjusted to use the left overbank levee as the floodway limit. Therefore, the left overbank levee is, with few exceptions, the left floodway limit. Above cross section 0, the right bank floodway limit was assumed in accordance with the city's development plans; the left overbank curvilinear flood plain, without restrictions, is the left floodway limit. The results of the floodway computations are tabulated at selected cross sections for the Rio Grande (Table 1). As shown on the Flood Boundary Floodway Map, the floodway boundaries were determined at cross sections; between cross sections, the boundaries were determined photogrammetrically. In cases where the lOa-year flood and the floodway boundaries are close together, only the floodway boundaries have been shown. The area between the floodway and the boundary of the lOa-year flood is termed the floodway fringe. The floodway fringe thus encompasses the portion of the flood plain that could be completely obstructed without increasing the water-surface elevation of the lOa-year flood more than 1.0 foot at any point. Typical relationships between the floodway and the floodway fringe and their significance to flood plain development are shown in Figure 7. 11