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RAIN STORM MONTROSE Downtown ~ontro~e was virtually turn~d into (I. rhcrTllC.day at ;;:311 p.m. when a heavy rain and hail storm struck without warning...water WaS runningdow'rr the 5treets,ovcrflowing curbs and rushing down the lower part of Main Street. South Townsend Avenue look"d lik~ the Uncompah- grc Rivor for a time as runoff water rushed down the full length of the street and then rushed .'cst down Main Stroct... Catchbusil\& !It the e.~st sidcoftheinter.ection of ~ain and Uncompahgre were clogged with trash, causing a lake to form across the cntir~ j;trc~t after th~ ~toym. SLASHES AREA HYDROLOGY PeakOischarqes The Unco""pahgre Valley Water User. Association r~rortt'd pr~cjpi tilt ion a, 1.Ot>inchc$. Thisappeared to be the heaviest amount oflIDisture in One day in June for over 46 years... The storm soemed to cOme in fNmthc wostor south- west and head east and eircl<, around to the north. The Loutsenhi:er Canal which runs in back of Flat Top had broken, and water carried about a foot of silt ovor one 12 acro plot of sugar beots and ruined the wooleficld. flood di sCh~rges for the major drain~geways in this report were obt~ined fro", a report completed in April, 1979 by Hydro-TriJd, Ltd., (Reference 1). , I , , i\s a pJrtof that previous study, runoff hydrographs were developed using the Soil Conservation Service procedure for the sub-basins. The hydrographs were then combined using Muskingum routing to yield discharges for Montrose Arroyo,CedarCreekandDryCedarCreek. Ditch crossin~s in the study basin, with the exception of the South Canill,wereanalyzed byassurning them to be flowing full at times of flood such that tributary runoff would flow i1cross unimpeded. The South Cilndl, because of its lilrge capacity and safe operating record was assumed to collect runoff draining directly into it and convey it out of the basin. 6-21-67 The impilct of snowmelt runoff was evaluated and determined to be significantonlyforthehigherfrequf'flcyevents. The discharges in Table 5 include the effects of rainfall and snowmelt runoff. Url:>anHydrolo!lY As p~ rt of the prev fous study. peil k di sch~rges were determined for each of the local urban ,ub-basins. These discnorges were calculated us ing the rationill fonllula i1nd r~infall dilta kept by the National '~eather Service, (Reference 3) and the Uncompdhgre Valley Water Users Associ~tion, (Reference 4). The peak discharges calculated for the local urban areas were routed and combined by Gingeryi\ssocidtes to detennine the effect of urban dr ain- age on the study strear:1s. The individual urban sub-baSins wereCOl'tlined by lagging thehyorographs of upper basins to account for travel tine to the lower areas, then ddding the hydrographs. Th!' results ore summarized in Table 6. It should be noted that the urban runoff is of short duration illl(lwill have subsided before the upstream peak flows reach the ci ty. -16- - 17 .