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<br />factors Alfecting Flooding and Its Imp~ct <br />OtlstructionstoflOO<lflows-Bridqecrosslngsoftenresult <br />in ch~nnel constrict ions ~nd struCTural otlstructlcns to flood flows. <br />Eleven road bridges and One railroad bridge cross St. Vrain Creek <br />withlnthe study reach. R3isedrOddsurt3Cesillthefloodplain <br />impede and constrict overbank flows, causing higher "ater surface <br />elevations at seme IOC3tlons. There are seven diversion structures <br />In the study reach nOlle of whlch wIll significantly aftoct flood <br />flows. In the reilchesupstream frCtllLyonsplentgrowtM is thick <br />a long the stremns and many floating branches and logs are swept down~ <br />stream during floods sometlmes causing jams at brldges and other <br />constrlctlons. Houses, sheds, and fences in the flood plllin at Lyons <br />also impede overbank flows. The effects of bridge constrlctlons is <br />sho"non the water 5lJrface proflles on plates 6 through 3. and In <br />tables 5 through 8, Flood Plain Reference Data, On pages 26 through 30. <br /> <br />Erosion may unOercut anc destroy structures "hlch woulC otnorwise <br />receive little damago from Inundation. Large quantities of rocks "re <br />otten deposltled in portions of the Channel leavi"g little caoaclty <br />tor futuro floods. <br /> <br />flood warning ~nd forucasfln9 - Aduance "arning in the <br />ev"nt of a furure large flood would be important and perhaps a matter <br />of lile or death. At the present time wdrning of I~pending floods <br />"t Lyons comes only from voluntery telophone COMmUnication trom <br />people living upstrea~ "long the two main branches and Iron personnel <br />at Button Rock Dam On the ~orth St. Vrain Creek. <br /> <br />Flood d~age reduction measures - Tho channols of St. Vraln <br />Creekandtril>utllrleslnthestudyroachare055ontiillly in their <br />natural state. There are no flood control structures at Lyons. <br /> <br />Flood fighting and emergency evacuation plans - Whon <br />" flood wdrnlng is received at Lyons "ord Can be spread rapidly to <br />residents of the flood plain by town police end volunteer tiremen. <br />Flood lighting at Lyons during oast floods has been" combined ellort <br />of thO coonty civil del"nseofflce, count yond local policeotflcers, <br />and volunteers. <br /> <br />[}Jrten Rock Dam ;, located six ml1eswostof Lyons on i~or th ". Vrain <br />Creek. This dam was t>ullt '0' LOllgmontrTlJnlcipal water supply 000 <br />"urr1Ully ,,, Ilttlefload storage capacIty. Longnont JMI, 0", mile <br /> <br />downstre<lm trom 2<.Jtton Rock Dam, and Ueaver Reservoir near the head- <br />w"ters of Swth St. Vrain Cre"k were considerod to have little efluct <br />on peak discharges in the study nmch. There haWI oo..n no city or <br />cm<ntyordinancos providing flood plilinregulation in the study rcacn. <br />There is no evidence of flood pn>ellng of structuros in the flooJ <br />plain. <br /> <br />Other factors and their impacts - Tho ste"p stream slopes <br />creates"ifteurrentsduringafIOO<lwhtchproducesaddoddama'Jes. <br />Debris carrlod by the fast moving water not only threatens bridges <br />tlJt""k)Y tl;:Jtter houses or other strucrureson the/1oodpl<lin. <br /> <br />" <br /> <br />, <br />