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<br />FUTURE FLOODS <br /> <br />Table 4 <br />SLH-IARYfFPOTENTlALFLOCXJPEAKDISCHAR3ES <br /> <br />In order to study tho flood potential on St. Vrain Creek, <br />two floods, the Intermedi~te Regional and Standard Project Floods <br />were doveloped. The Intermediate Regional Flood ;s one with an aver- <br />agefrequencyofoccurrenceot about once in 100 years and Isincreas- <br />inglybelngaccepted by the public as a mlnl'TIlH" limit for "pplicdlon <br />of local regulations. The Standard Project Flood isofgrwtermagnl- <br />tude and represents a reasonable uppor limit ot expected flooding. <br /> <br />Stream and Location <br /> <br />Intermediate Standard <br />Regional Project <br />C<ood Peak Flood Peak <br />Discharge Di5Charge <br />c.f.s. c.t.s. <br />4,900 12,400 <br />6,450 16,000 <br />10,200 21,800 <br />10,100 28,300 <br /> <br />Intermediate Regional Flood <br />ThO dIscharge-frequency relattonshlps in the St. Vrain Creok <br />basin were based on a statistical analysis of the stream gaging <br />recordsot St. YrainCreekat Lyons and Platteville, Colorado. Flood <br />hyljrO>Jrephsweradevelopedforroutingthrough8uttonRockDi.lmand <br />for routing downstream from Lyons to determIne the discharges through- <br />out the length of the stream. The IntermoolateRegional Floo<lpeak <br />discnarges for eac~ si'ream are shown In table 4. <br /> <br />North St. VrainCreek <br />(Entire Study Reach) <br />South St. Vraln Creek <br />(Entire Study Reach) <br />St,VralnCreek <br />at L'jQnsJ.! <br />St.VralnCrook <br />atdownstreiYrllimltotstudy <br /> <br />II <br />At the Lyons gaga. <br /> <br />Froquency <br /> <br />Ha<:ardsofLargeFloods <br />The effacts ot the Intermediate Regional and St~ndard <br />Project Floods were analy<:ed to determine potential flood ha<:ards <br />in the study reach flood plain. Theanalyslsus.<ldfieldcondltlons <br />reprnsentedbybrldgaandvalleycrosssectlonssurveyed In 1911. <br />Factors considered Include extent of aroa flooded, flood depths. <br />f I<XJJ Jd""'Y"&, <.>b",lrucl I,-,n", 10 f looJ f low~, ....loci tl",& of fi"",J <br />flows, rate of rise of streClm lavel, and duratIon of flooding. <br />Oamagesduring tloo<llng are effected by both depth i.lnd velocity <br />of flows. Obstructionstoflowscanlncreasetlooddepths. In <br />regard to r...man hazards, gonerally flood waters 2 feet or mora <br />deep and flowing at avelocityot3 faot or more per socond croatos <br />a definite danger at injury or drowning. These condifions wOllld <br />occurdtmanylOCCltionsduringa large flood. <br /> <br />StllndardProjoctFlood <br />Storms tl1at would produce tt\e Standard Project Flood ar<l <br />uncommon, and it is difficult to dssign traquoncies of occurrence <br /> <br />with anyraasonablo d"9r..... 0' accuracy. Ho~ever, 1 loads of this <br />si<:e dO occur as avidenced Oy flooding In the Denver area '0 1965. <br />T" Standard Project "000 peak discharges are also shown '0 table ,. <br /> <br />Floods larger than the Standard Project Flood can occur. <br />Hm,c','or, floods """ller than +he 'nter'f\ediatf' R<>gi"n;',! Flood ;,r" "'Or", <br />frequent "nd ..Ithough thoy il1'Jndate sma I lor are..s they alsO cause <br />damdge, hardship, and Inconveniences. The hIstoric peak discharges <br />;ntllbla3 illustratethesmallormllgnitudeotthernorefreqJent <br />floods. <br /> <br />18 <br /> <br />" <br />