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<br />I <br /> <br />I <br /> <br />15 <br /> <br />I <br /> <br />I <br /> <br />tests were performed in the Boulder area. AI I of these tests were per- <br />formed on lawn grass. Based upon the tests and from various soils maps <br />of the area, North Boulder was divided into two infiltration zones. <br />The northern zone, which includes Fourmile Canyon Creek and Wonderland <br />Creek basins, was estimated to have an infiltration rate of just over <br />one inch per hour. The southern zone, which includes Twomile Canyon <br />Creek, Elmer's Twomile Creek and Goose Creek basins, was estimated to <br />have an infiltration of less than one-fourth inch per hour. <br /> <br />I <br /> <br />I <br /> <br />These infiltration rates are for the 100-year design storm where it is <br />assumed that the ground surface is saturated before the storm occurs, <br />For storms that have no antecedent rainfall and where the ground is <br />assumed not to be saturated, the initial infiltration would be much <br />higher. <br /> <br />I <br /> <br />Percent Impervious Land <br /> <br />I <br /> <br />I <br /> <br />Land that is totally impervious will produce nearly a 100 percent <br />runoff. Pervious land will absorb and retain a significant amount of <br />rainFal I. In this study a probable future development of North Boulder <br />was assumed in order to arrive at real istic future hydrologic conditions. <br /> <br />I <br /> <br />The percent of impervious land surface area assigned to the various <br />basins is as fol lows: 20 percent for the foothills basins; 30 per- <br />cent for the lower ends of Fourmile Canyon and Goose Creeks; 30 per- <br />cent for upper Wonderland; and 40 percent for all other basins. <br /> <br />I <br /> <br />DES I GN POINTS <br /> <br />I <br /> <br />There were thirty hydrologic design points selected for the project. <br />Seven were on Fourmi Ie Canyon Creek, five on Wonderland, five on <br />Elmer's Twomi Ie Creek, four on Twomile Canyon, and nine on Goose <br />Creek. The points were selected for their hydrologic and/or hydraulic <br />importance. For example, design points were selected at stream con- <br />fluences, major highway crossings, mouths of canyons, etc. A list <br />of the design points and the corresponding 100-year design flows are <br />listed in Table IV-2 and shown in Figure IV-2. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />DETERMINATION OF DESIGN FLOWS <br /> <br />I <br /> <br />The Colorado Urban Hydrograph Procedure was used in determining the <br />magnitudes of the 100-year flood at the thirty design points. This <br />procedure, which was developed for the Denver Regional Council of <br />Governments, is based on synthetic unit hydrograph theory. <br /> <br />I <br /> <br />The unit hydrograph principle was originally developed by Sherman in <br />1932. The synthetic unit hydrograph, which is used for analysis when <br />there is no rainfall-runoff data for the basin under study, as is the <br />case in Boulder, was developed by Snyder in 1938. <br /> <br />I <br /> <br />I <br />