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<br />Iv-4 <br /> <br />I nfI ltration <br /> <br />Part of the rainfall that occurs infiltrat<!s 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 />wiil 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 wIll <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 infIitration rates are <br />for the 100-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 loss 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 Desiqn Flows <br /> <br />The Colorado Urban Hydrograph Procedure wa,. used In determIng the <br />magnitudes of the 5, 25, and 10o-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 simi Jar coeffICIents w~ich WE,re developed by <br />the United States Army Corps of Engineers for the recent Cherry Creek <br />flood study. When these coefflclents 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 and 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. 1'1-3 shows the print-out of a typical computer run for a <br />hydrograph 1 istlng output as well as input. Tab'!e No. Iv-4 1 ists the <br />runoff under future development to be expected from the 5, 25, and <br />