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<br />In cases where the lOO- and 500-year flood boundaries are close <br />together, only the laO-year flood boundary has been shown. <br /> <br />Flood boundaries in a:reas studied by approximate methods were <br />either taken from Flood Hazard Information reports for the area <br />(References 2, 3, and 4) or established according to the professional <br />judgment of engineers familiar with the region, taking into account <br />flood elevations from historical flood data, minor hydrologic and <br />hydraulic analyses, correlations with similar stream basins studied <br />in detail, field observations, and available topographic data (Re- <br />ference 13). In addi"tion, areas of approximate study along portions <br />of the Gunnison and Dolores Rivers were taken from the Federal <br />Insurance Administration's Flood Hazard Boundary Maps (Reference l4). <br /> <br />., <br /> <br />Flood boundaries are indicated on the Flood Insurance Rate Map (Ex- <br />hibit 3). On this m~p, the lOa-year flood boundary corresponds to <br />the boundary of the areas of special flood hazards (Zones AO, AI, A2, <br />A4-A7, A9, and Al4); and the SOO-year flood boundary corresponds to <br />the boundary of the areas of moderate flood hazards (Zone B). <br /> <br />Small areas within the flood boundaries may lie above the flood <br />elevations and, therefore, not be subject to flooding; owing to <br />limitations of the map scale, such areas are not shown. <br /> <br />Some areas in Mesa County are subject to broad, shallow, overland <br />flooding that is generally less than 3 feet deep and characterized <br />by unpredictable flow paths. The water-surface elevations of <br />flooding in these areas are essentially independent of those along <br />adjacent streamways and are affected principally by barriers to <br />flow in the flooded areas. <br /> <br />4.2 Floodways <br /> <br />Encroachment on flood plains, such as artificial fill, reduces the <br />flood-carrying capacity and increases flood heights, thus increasing <br />flood hazards in areas beyond the encroachment itself. One aspect <br />of flood plain management involves balancing the economic gain from <br />flood plain development against the resulting increase in flood <br />hazard. For purposes of the Flood Insurance Program, the concept <br />of a floodway is used as a tool to assist local communities in this <br />aspect of flood plain management. Under this concept, the area of <br />the lOa-year flood is divided into a floodway and a floodway fringe. <br />The floodway is the channel of a stream, plus any adjacent flood <br />plain areas, that must be kept free of encroachment in order that <br />the lOa-year flood be carried without substantial increases in <br />flood heights. As minimum standards, the Federal Insurance Admin- <br />istration limits such increases in flood heights to 1.0 foot, <br />provided that hazardous velocities are not produced. <br /> <br />s <br /> <br />20 <br />