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<br />Iv-4 <br /> <br />I nfi ltration <br /> <br />Part of the rainfall that occurs infiltrates into the ground and <br />does not become part of the immediate storm runoff. As the study area <br />develops, the overall Infiltration rate will change because large areas <br />will become Impervious, due to building and pavng, and other areas will <br />change from pasture to lawn grass. In some cases, the infi 1 tration wi II <br />Increase, and In other cases, it will decrease. <br /> <br />The infll tration rates used In this study are based on both the ex. <br />isting and on the probable future land cover. ~) obtain these rates, <br />numerous field infiltration tests were performed in the study area. <br />Tests were performed on Jawn grass, pasture land, and bare soil. Based <br />on the tests, the average infiltration rate for existing conditions was <br />found to be one inch per hour for pervious areas in the basin. For de- <br />sign purposes under future development, the infiltration rate used was <br />reduced to 0.75 Inches per hour. These design Infiltration rates are <br />for the lOa-year storm, where it is assumed that the ground surface is <br />saturated before the storm occurs. For storms that have no antecedent <br />rainfall, and where the ground is assumed not to be saturated, the ini- <br />tial infiltration would be higher and a ten per Gent 1055 on all Im- <br />pervious areas is assumed. See Table No. IV-5 for infi 1 tration test re- <br />sults. <br /> <br />Determination of Desi~n Flows <br /> <br />The Colorado Urban Hydrograph Procedure wa,. used in determing the <br />magnitudes of the 5, 25, and IDD-year floods at design points. This <br />procedure, whIch was developed for the Denver Region"l Council of Govern- <br />ments, is based on the synthetic unit hydrograph theory. See Table No. <br />iV-2 for basin characteristics used in the program. <br /> <br />The unit hydrograph coefficients C~ and C that were used in this <br />study were adapted from slmi Jar coeffiCIents w~ich were developed by <br />the United States Army Corps of Engineers for the recent Cherry Creek <br />flood study. When these coefficIents are used, a hydrograph is de- <br />veloped that accounts for reasonable stream char.nel storage. <br /> <br />There are over seventy five miles of ditches "nd canals in the basin. <br />It has been assumed that their net effect on trans-basin diversions dur- <br />ing storms Is not significant. The effect of these ditches will be to <br />lengthen the time of concentration and lower the, peak runoffs, in gen- <br />eral. <br /> <br />Table No. IV-3 shows the print-out of a typical computer run for a <br />hydrograph 1 istlng output as well as Input. Tab.le No. Iv-4 1 ists the <br />runoff under future development to be expected from the 5, 25, and <br />