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LAND USE e Land use is a key factor in the analysis. It is used as the primary indicator of develop- ment status and analytical methods key on it to forecast the hydrologic. economic. and environmental consequences of existing and alternative future development patterns. The development and use of a reasonable set of land use categories received special attention because of the critical land use focus of the techniques. The criteria applied to determine a rational set of land use categories included compatibility with local agency classification systems, potential for automatic classification by use of remote sensing technology, and responsiveness to technical requirements needed to perform hydrologic. economic. and environmental analysis. Table 1 defines the describes the characteristics of each of the ten categories adopted for the test studies. The pilot study and consequent studies have made use of as many as 22 categories. IdeaJly, the existing and alternative future land use patterns will be avail- able from local planning agencies and used for analysis. In most instances, however. future patterns will be developed by the local agency and the study staff working together. Figure 2 displays computer-printer plots of the Trail Creek existing 1975 land use and a selected alternative future 1990 projected land use patterns. The distribution of land use within Trail Creek for these land use patterns is tabulated in Table 1. , . fLOOD HAZARD EVALUATION The objective of the flood hazard evaluation that is the usual product of flood plain infonnation studies are the spatial and elevation description of specific hydrologic events such as the 100-year exceedance interval flood. The analysis normally consists of hydro- logic studies to define flows and exceedance frequencies, hydraulic studies to relate flow to stage and thus develop water surface elevation profiles, and mapping studies to relate the water surface elevation profiles to flooded area, usu"ally presented as outlines of flooded area. The methods of analysis developed for the pilot study are consistent wilh the objec- tives and the methods that have been used in past studies, but several tasks associated with obtaining basic hydrologic modeling data have been substantially automated so that large scale, relatively rapid analysis of future land use patterns is possible. The 'flood hazard' portion of Figure I depicts the overall flood hazard evaluation process developed for the pilot study. e Hydr%gic Studies The basic strategy is to generate hydrologic simulation program HEC-l model param- eters automatically from the grid cell data bank, develop routing criteria from a special utility program, and execute HEC-I for a range of synthetic events (such as the 100-year event) using the generated model parameters to develop flow frequency curves, and con- vert selected flows at selected control points to stage and area flooded by conventional analysis using stream profile program HEC-2, and topographic map analysis to delineate flooded areas. An HEC-I model requires precipitation, loss rates, unit hydrographs and routing cri- teria to be developed. The rainfall-runoff computation methods of the U.S. Soil Con- servation Service (SCS) (National Engineering Handbook, U.S. Soil Conservation Service) . " 6 e