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TABLE IV-2 CONTINUED ROCKY FLATS PLANT SITE 6-HR STORM DISTRIBUTION CUHP Model Lorah and Wright developed the original CUHP in 1967 for the Denver Regional Council of Governments. CUHP was based on research on runoff patterns of urban drainage basins in Denver, Illinois, Massachusetts, and Maryland using the early procedural work on unit hydrographs of Snyder of USACE, who is supportive of the CUHP procedures. TIME 2YR 5YA 10YA 25YR 50YR 100YA (MIN) (IN) (IN) (IN) (IN) (IN) (IN) 185 0.009 0.008 0.011 0.012 0.013 0.015 190 0.007 0.006 0.011 0.012 0.013 0.015 195 0.007 0.006 0.011 0.012 0.013 0.015 200 0.007 0.006 0.011 0.012 0.013 0.015 205 0.007 0.006 0.011 0.012 0.013 0.015 210 0.007 0.006 0.011 0.012 0.013 0.015 215 0.007 0.006 0.011 0.012 0.013 0.015 220 0.007 0.006 0.011 0.012 0.013 0.015 225 0.007 0.006 0.011 0.012 0.013 0.015 230 0.007 0.006 0.011 0.012 0.013 0.015 235 0.007 0.006 0.011 0.012 0.013 0.015 240 0.007 0.006 0.011 0.012 0.013 0.015 245 0.007 0.006 0.011 0.012 0.013 0.015 250 0.007 0.006 0.011 0.012 0.013 0.015 255 0.007 0.006 0.011 0.012 0.013 0.015 260 0.007 0.006 0.011 0.012 0.013 0.015 265 0.007 0.006 0.011 0.012 0.013 0.015 270 0.007 0.006 0.011 0.012 0.013 0.015 275 0.007 0.006 0.011 0.012 0.013 0.015 280 0.007 0.006 0.011 0.012 0.013 0.015 285 0.007 0.006 0.011 0.012 0.013 0.015 290 0.007 0.006 0.011 0.012 0.013 0.015 295 0.007 0.006 0.008 0.012 0.013 0.015 300 0.007 0.006 0.008 0.012 0.013 0.015 305 0.007 0.006 0.008 0.010 0.013 0.015 310 0.007 0.006 0.008 0.010 0.013 0.015 315 0.007 0.006 0.008 0.010 0.013 0.015 320 0.007 0.006 0.008 0.010 0.013 0.015 325 0.007 0.006 0.008 0.010 0.013 0.015 330 0.007 0.006 0.008 0.010 0.013 0.015 335 0.007 0.006 0.008 0.010 0.013 0.015 340 0.007 0.006 0.008 0.010 0.013 0.015 345 0.007 0.006 0.008 0.010 0.013 0.015 350 0.007 0.006 0.008 0.010 0.013 0.015 355 0.007 0.006 0.008 0.010 0.013 0.015 360 0.007 0.006 0.008 0.010 0.013 0.015 SUM 1.60 2.00 2.50 3.00 3.40 3.80 There were three major changes made by the UDFCD to the CUHP model during 1984 and 1985. These included an improved "time to peak" algorithm, an improved ''weighted basin slope" algorithm, and the incorporation of a hybrid rational method/unit hydrograph procedure for drainage basins smaller than 90 acres. CUHP is a model used to determine the runoff from a specific rainfall event for one drainage basin. It is based on the unit hydrograph procedure. A unit hydrograph is defined as the hydrograph of 1 inch of direct runoff from the tributary area resulting from a unit storm. The unit hydrograph thus represents the overall integrated effects of factors such as tributary area, shape, and land slope. The basic premise of the unit hydrograph is that individual hydrographs resulting from the successive increments of rainfall excess that occur throughout a storm period will be proportional in discharge throughout their time duration. The hydrograph of total storm discharge is obtained by summing the ordinates of the individual period hydrographs. The unit hydrograph procedure is consistent with DOE requirements for drainage system analysis and design. CUHP uses relationships that take into account rainfall/runoff data collected in the Denver area. CUHP is a unit hydrograph procedure specifically developed for and calibrated to the Denver area. Information regarding rainfall and basin characteristics is required input to the model. A tabulated runoff hydrograph is the generated output of the model. The specific variables used as input to the CUHP model are basin area, length, and centroid; land slope; percent imperviousness; soil infiltration (initial, final, decay coefficient); surface retention; time of concentration; and design storm rainfall depth and distribution. (1) Derived using procedures described in the UDFCD Manual with point precipitation from NOAA Atlas 2, Volume III-Colorado(Department of Commerce, 1973).