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. Discharge-Probability Profiles. The rainfall, loss and runoff concepts and the modeling techniques discussed on pages 54 through 59 of this report were used to developed the discharge- probability profiles along Sand and Toll Gate Creeks shown on plates 46 through 57. One of the primary purposes of the studies in the Sand and Toll Gate Creek basins was to determine the effects of projected urbanization on flood peaks throughout the basin. EPA's surface water management model was used in this analysis since it is easily adaptable to changing land use patterns. MRO's diffusion routing model was used to combine and route the Sand Creek sub- basin hydrographs from near the Elbert-Arapahoe County line to the mouth. In absence of a gaged event, a Snyder's I-hour unit hydrograph was developed at the mouth of Sand Creek to compare with a model developed hydrograph with about 1 inch of runoff volume. This comparison is shown on plate 58. Development in the Sand and Toll Gate Creek basins was determined from 1975 aerial photographs and a field reconnaissance made the same year. The amount of impervious area associated with this development was based on the land use ratios presented in table 11. Future conditions were based on several projections of development. In the Toll Gate Creek basin consideration was given to total development by 1990 using a large lot residential con- dition and a normal residential condition, both with and without improved channel conditions. The Sand Creek analysis was limited to partial area development using normal residential and existing channel conditions. The major portion of the projected Sand Creek basin development was assumed to occur within the Toll Gate Creek basin. . 41