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<br />" <br /> <br />16 <br /> <br />HYDROLOGIC AND HYDRAULIC ANALYSIS <br /> <br />Hydrolog" Analysis <br />There arc two different general methOdologies that are commonly used <br />tomHhematlcal1y predict peak flood flows. The first is "parametric"or <br />"deterministic" hydrologic modeling which <lnalyus the .a.ious physical <br />parameters of the basin which i1re then used along with knownclim ati,para- <br />meters topredictfloodhydrogrilphs. The second methodology is called <br />"stochastic" 'malysis which relies ",ainly on the statistics of historic <br />Stream flow data. <br />A parametric analysis of the Roaring Fork Basin to determine flood <br />~gnitudes would be extremely complex and probahly only partially reliable. <br />Basin features that would have to be defined and understood for such a study <br />would Include: geology, ground cover, b~sln orlentdtlon, o;ogr~phlc effects <br />em precip i taUon patterns, ,nowmel t-ra infd! I ",Iat Ions, thermal c~paci ty of <br />snow p'ilck, effects "f "",nmade feature, on the n~rur~l ba'" flow, storm <br />patterns,andmanymore. <br />For purposes of thi, report we have cho,en to define the flood ma~nitude - <br />frequency rclatlon,hips basedonastochasticanalySI,of rhea vdildblestream- <br />flow recoro, for the Roaring FClrk RI vcr and Its trl but~rl <os. The Log Pearson <br />Type I I I ~naly,l. was used for this study, <br />I n 1967 the Federa I Wil~e r Re,Qurce. Counc II reco".nended that d II federa I <br />"~encle' thdt were involved in Cd Iculatlnq flood magni tude ~nd frequenc ies <br />u,;o the Leg l'eM,on Type III exlr""",, f,'"quenq dndlpi. unle.s there wd' good <br />justlficaUontouseanothermethod. Table2prc,ents the results to Log <br />I'earson Type III analy,I, on 9 differelH 'et, of data based upon 53 years of <br />r"c<>rdfcon>ROdrln9FoekBa.insteeamgage5. <br />The pc.,k flows ,hown in Table 2 foe 10, SO, 100, drld SOO-y~ar fr~4uenc ie, <br />were u:.cd to dctermi ne th" fl<>"d peof i Ie, and 100-y~ar flopdp I"I~ ~hown on <br />Plates 3 thru 34. <br />Fo r a deta I I cd de~cr i p t Ion of the hyd ro log I c ana Iys i" refer to the <br />teChrllcaladdend""", <br /> <br />flood Characteristics <br />Floodflow' on the Roarin9 Fork River result from rapid melting of the <br />mountain 'nowpdck during the period from May to early July, Snuwmelt runoff <br />may o"a,lonally be augmented by rain. Thcsnow"\Clt runoff I"haracterized <br />by ,usta i ned per lod, of high f low~ and marked d i urna 1 fillet uat i on. E"",mi na- <br />tion of meteorological and ,llm3tological condition, and precipitation and <br />streamflow records sho,., that summer cioudbursts ar<, not il great flood threat <br />onthe5estream" <br />