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<br />factors Affecting flooding and Its Imp~ct <br />OtlstructionstoflOO<lflows-Brldqecrossingsoftenresult <br />in ch~nnel constrict Ions ~nd strucTural otlstruCTicns to f lcod flows. <br />Eleven road bridges and One railroad bridge cross St. Vrain Creek <br />within the study reach. R3isedroddsurt<><::esinthefloodplain <br />impede and constrict overbank flows, causing higher water surface <br />elevations at serne IOC3tions. There are seven diversion structures <br />In the study reach none of which wili significantly affoct flood <br />flows. In the reilchesupstream frCtll Lyons plant growth is thick <br />a long the stre1lf1ls and many f 1000ting brilllches and logs are swept down~ <br />stream during floods sometimes causing jams at bridges and other <br />constrictions. Houses, sheds, and fences In the flood plllin at Lyons <br />also Impede overbank flows. The effects of bridge constrictions is <br />shown on the water SlJrface profiles on plates 6 through3,and In <br />tables 5 through 8, Flood Plain Reference Data, On pages 26 throu~h 30. <br /> <br />Erosion may unoercut anc destroy structures whiCh woulC otnorwlse <br />receivo little damago from inundation. Large quantities of rocks .:lre <br />often deposltied In portions of tho Channel leaving little caoacity <br />for future floods. <br /> <br />flood wJrning ~nd forecastl"~ - AQv~nce warning in the <br />event of a furure large flood would be im_ortant and perhaes a matter <br />of life or do~th. At the pro:ent time wJrn;ng of i~pending floods <br />at Lyons comas only from voluntary telophone COMmUnication from <br />people ilving upstreaM along the two main branches and fron personnel <br />at Button Rock Dam on the ~orth St. Vrain Creek. <br /> <br />flood d~age reduction me.:lSUreS - The channels of St. Vrain <br />Creekandtrll>utllriesinthestudyraachare055ontiully in their <br />natural state. Thorearenofloodcontrolstructure5<1tLyons. <br /> <br />flood fighting ~nd emergency evacu<ltion plans - When <br />~ flood Warning is received at Lyons word Can be spread rapidly to <br />residents of the flood plain by town police and volunteer firemen. <br />Flood fighting at Lyons during o~st floods has been a combined elfort <br />of tho coonty civil delenseoffice, county<lnd local pollceotflcer$, <br />and volunteers. <br /> <br />[}Jrten Rock Dam ;, located six miles west of Lyons on ~~or th ". Vri)in <br />Creek. This damwils oollt '0' LongmontrTlJnlcipal water supply 000 <br />,,,,,,,,,,lly ,,, Ilttlefload stor~ge ~ap.:lclty. Longnont JMI, 0", mile <br /> <br />downstre~m from autton Rock Darn, "nd Ueaver ReservoT r neilr the head- <br />w"ters of Swth St. Vrain Cre"k wore considerod to h~ve little effect <br />on ~eak di sChar1e~ in the ~t"dy re~ch. There have oo..n no citv or <br />county ord in~nces providing f load pi ilin regu lation in the study reilcn. <br />There is no evidence of flood pn>ofing of structures in toe flooJ <br />plain. <br /> <br />Other factors and their impacts - Tho steep stream slopes <br />cre<lteswiftcurrentsduringafloodwhtcoproducosaddoddam<l'JOS. <br />Debris carried by the fast moving water not only threatens bridges <br />tlJt""k)Y tl;:Jtter houses or other structurflSOn the floodpi"in. <br /> <br />" <br /> <br />, <br />