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I I I I I I I I I I I I I I I I I I I 7 2. Impervious Land Density The amount of impervious area within each basin was estimated for two conditions, namely; (1) existing development, and (2) future development with the proposed Denver Airport. Existing development within the study area is predominantly agricultural. The purpose of this study is to develop hydrolo- gic informations for existing and ultimate basin conditions within the Airport site. The future imperviousness was estimated by reviewing the Airport faci- lity map and the land use map for the City of Aurora (and assigning values of impervious area to each land use category). Table 2 presents the amount of impervious area for each land use cate- gory. Figure 3 shows the land uses with the proposed Airport. 3. Basin Characteristics Subcatchments were delineated using USGS quadrangle maps with a scale of 1" = 2000' and 10-foot contour interval. Subcatchment boundaries were established based on physical drainage boundaries, expected future development and logical design points. Subcatchment areas were measured using AutoCAD and the CUHPCAD data input program. The measurement process util izes a digitizer--'and a personal com- puter. Subcatchment length, and length along the flow path ~ the centroid of the area were al so measured using the AutoCAD and CUHPCAD system. Subcatchment delineation is given in Figures 4 and 5. Subcatchment impervious areas were obtained by overl ayi ng the I and use maps on the subcatchment delineation and proportioning the appropriate imper- vious percentages. The CUHP model requires adjusting the slope of the basin to account for slope variations. The procedure is described in the CUHPE-PC manual. CUHP is recommended for bas i n areas greater than gO acres. A time of concentration is required for basins smaller than gO acres. Time of con- centrations for small basins were calculated using the procedures contained in the CUHPE-PC manual. Impervious area surface storage and pervious area surface storage were estimated to be 0.1 inch and 0.35 inch, respectively. These values are con- sistent with recommendations found in the USDCM.