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II I I I 24 this study a probable future development of South Boulder was assumed in order to arrive at realistic future hydrologic conditions. The per cent of impervious land surface area assigned to the various basins is as follows: 20 per cent for the foothills basins; 3D per cent for low density residentIal basins; and 40 per cent for urbanized basins. DESIGN POINTS I I I I I I There were thirty one hydrologic design points selected for the project. Three were on Gregory Creek, two on King Gulch, four On bluebell Canyon Creek, fiveon Skunk Creek, eight on Bear Canyon Creek, eight on Viele Channel (including two design points in the Anderson Ditch basin) and one design point on David's Draw. The points were selected for their hydrologic and/or hydraulic importance. For example, design points were selected at str"am confluences, major highway crossings, etc. A iist of the design points and the correspond- ing lOO-year design flows are listed in Table IV-2 and shown in Figure IV-2. DETERMINATION OF DESIGN FLOWS The Colorado Urban Hydrograph Procedure was used in determining the magni- tudes of the 100-year flood at the thirty three design points. This proced- ure, which was developed for the Denver Regional Council of Governments, is based on synthetic unit hydrograph theory. I I I The unit hydrograph principle was originally developed by Sherman in 1932. The synthetic unit hydrograph, which is used for analysis when there is no rainfall-runoff data for the basin under study, as is the case in Boulder, was developed by Snyder in 1938. A unit hydrograph is defined as the hydrograph of one Inch of direct runoff from the tributary area resulting from a unit storm. A unit storm is a rain- fall of such duration that the period of surface runoff is not appreciably less for any rain of shorter duration. The unit hydrograph thus represents the integrated effects of factors such as tributary area, shape, street pattern, channel capacities, and stream and land slopes. I I To apply the unit hydrograph, the effective precipitation depth for the "unit storm" periods are multiplied by the ordinates of the unit hydrograph and added to obtain a design storm runoff. I I I I The basic premise of the unit hydrograph is that individual hydrographs result- ing from the successive increments of rainfall excess that occur throughout a storm period will be proportional in discharge throughout their length, and that when properly arranged with respect to time, the ordinates of the individual unitgraphs can be added to give ordinates representing the total storm dis- charge is obtained by summing the ordinates of the individual hydrographs. The derivation and application of the unit hydrograph are based on the fol- lowing assumptions: 1. The rainfall intensity is constant durIng the storm that produces the unit hydrograph.