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<br />IV. HYDHOLOG',' <br /> <br />Eydr::Jlogic a,nalyse:,; of the study area \"",re carric,d out: -tel <br />establish the peak discharqe-frequency relationships for <br />1'100,:15 of la-and lOa-year recurnonce int:ervals. The <br />Colorado Urban Hydrograph Procedure (CUHP), as outlined in <br />the Urban St,orm Drainage Criteria r,janual, was used to <br />deter-mine the peak discharqe from each drainage suharea. <br />The ';vesterly Creek drainage basi:n vla,~; redefined into sub-- <br />areas to accommodat:e the propose,j i;rp:rovement:s. Ti'le drainage <br />boundaries map is ",hO\"n in Drawinq ]Co, ;;: in Volume II" <br /> <br />Rain Eall <br /> <br />1'he I~ainfall intensity \-las assum"CJd to be the same in t:he <br />entire drainage basin: The rainEall depths for floods of <br />diff'erent recurrence intervals a1d durati.ons were obt:ained <br />from Rainfall/Runoff Information pub,;.ished by the Denver <br />Hegional Council of Governrnen ts ,~nd Urban Drainage and <br />Floo:1 Control District: in !-lay, 1972.. Table 1 shol"s t:he rai:1- <br />fall depth-duration relation~hips used in this study for <br />recurrence intervals of 2-, 10-, ana lOa-years. It should <br />be noted that the values listed in t~is table for rainfall <br />deptls for the lO-year stonn are only 70\:, of the magn:i.tudEOs <br />lisb~d in the Rainfall/Runoff InformatioL. They \,,,ere <br />modifiec1, as sugge;o,ted by i:he Omaha District Corps of <br />Engi:"leerS, while dE~veloping the :S.Epori: on Drainaqe 1'1aster <br />Plan for Westerly Creek, Phase 1\, becau:,e thE! plotted rainfall <br />data had a significant: neg-iltivE'3ke\'!, <br /> <br />Effective Hainfall <br /> <br />Effective rainfall is the storm W2i.tey tha.t reaches a study <br />point after a rainfall. The difference between rainfall and <br />effective rainfall is due too losses by infil.tration, detention <br />and depression storage. The loss ratE'S listed in the manual <br />were used to calculate the losses. ~'he percentaqe of impervious <br />area in each drainage subarea was based on the zoninq maps from <br />Denver and Aurora for future develop:me:nt: and the Aurora Building <br />and Zoning Regulations. The effective rainfall for each subarea <br />was then determined according to the manual. <br /> <br />Unit.Hydrograph <br /> <br />Under the CURP, hydrographs for the specific areas <br />were developed by applying the effective rainfall data to a unit <br />hydrograph. In this study, a unit ",torm duration of 10 minutes <br />was used in accordance with the manual for an urban area of less <br />than 5 square miles. Unit hydro(Jraphs for drainage subareas were <br />devel.oped following the procedures i:~ .the manual. The drainage <br />basin parameters used to develop unit. hyc1rographs are listed in <br />Table 2. Symbols representating the drainage parameters arE~ <br />described in the Urban Drainage Criteria Manual. <br /> <br />-10- <br />