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<br />" <br /> <br />r <br /> <br />- <br /> <br />" <br /> <br />- <br /> <br />111. KYDROLOGIC AND KYORAULIC ~~ALYSIS <br />~lood Chaf~cteristics <br /> <br />Floods in the study area are u8ually caused by general ralnstonus and/or <br />cloudburst storms during the period May through ~pternber. Floods resulting <br />from prolonged heavy rainfall over the watershed are characterired by high <br />pea~ flows of moderate volume and duration. ~e cloudburst type storm is a <br />high-intensity. short-duration rainstorm which produces a flood of high peak <br />flow, short duration, and a small volume of runoff. Peak runoff will usually <br />occur within a few hours after a sIngle storm. For a general one-hour storm <br />over the entire ~sin, Milliken will experience a double peak. ~e initial <br />peak is the concentration of runoff from raInfall over the town itself and <br />occurs within the first hour. The second peak occurs as concentrated runoff <br />from the upper basin reaches Milliken. Time to the second peak is a function <br />of travel distance. On the Eastern and ~eatern Flow Patha, the aecond peak <br />occura just Over an hour after the etart of the atorm. The Lola Park <br />Tributary drains the upper half of the Millken basin and time to aecond peak <br />in town is about three hours after the beginning Of the storm. <br /> <br /> <br />Figure 12. <br /> <br />Ponding west of fire atation, south <br />of Colorado Highway 60, June 25, 1982. <br /> <br /> <br />~'i9urc 13, Flood waters illlpoWlOOd <br />by th~ Highway 60 em- <br />bankmentapreade<lst",..rd <br />dCMn5roadSt:reetfrorr. <br />th"fir"statior., <br />June 25, 1982. <br /> <br />Hydrologic AnaLysis <br />Hydrologic analyaia W1l," conducted to dete"",ine runoff hydro9raphs from a <br />one-hour ator,. at tn.. S-, 10-,25-,50-,100-, ,v,d SOD-year recurrence inter- <br />"<l1a for the Milliken watersh..d. The ~Storm Wat<l" MIlfi$9<'"",nt Model" (SW"-'I) <br />(reference 4) as developed by the Environmental Protection Agency (~PAl and <br />later Il'lOdifi"d by the Mis..ouri River Division of th.. o.s. Army Corps of <br />Engineers was used to estimate the peak flows and runoff volumes. ~ storm of <br />a qiven return period was ass~ed to ~nerate a flood hydrograph and peak of <br />the s",." returnperio<l. Thepara,.etera "sed in the anaiyds were consistent <br />with previous etudles in the area, such aa "Master Dralnageway Planning for <br />Fo~~il Cr~~~, Fort CnlI1~~, Color~~o~ (Fo~~il Crp~k) "nd ~M~~t~r nr"in~~w~y <br />planning tor Cooper Slou9h and Box"lder Cre"k, Larimer County, Colorado" <br />(SoxelderCr"e1<.l (reference 5 and 6) and r6flect"d future development con- <br />dLtions in the stu~y area. <br />The entlre dr<linage ~..in of the Town of Milliken is shown in Fi~re 14. <br />The bdsln was subdlvided into 44 subcatchments and 43 channel units. Figure <br /> <br />H shows the eubcatch....nt ..nd chann..l loc..tions. Snhcatch....nt "..lin- <br />eations .~re based on the assumption that all irrigation ditches are flowing <br />