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<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 infdl I ",Iat Ions, thermal c~paci ty of
<br />snow p'ilck, effects "f "",nmade feature, on the n~tur~1 base flow, storm
<br />patterns,andmanymore.
<br />For purposes of thi, report we have chosen to define the flood ma~nitude -
<br />frequency relationships basedonastochasticanalySI,of thea 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 I T 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. Table 2 presents the results to Log
<br />I'earson Type III analysl, on 9 different 'et, of data based upon 53 years of
<br />r"c<>rdfcon>ROdrln9ForkBa.instreamgage5,
<br />The pe.,k flows ,hown in Table 2 for 10, SO, 100, drld SOO-y~ar fr~4uenc ie,
<br />were u,,~d to determi ne th" flo<>d prof i Ie, and 100-y~ar flopdp I"I~ ~hown on
<br />Plates 3 thru 34.
<br />Fa 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 />teChrllcaladdcnd",",
<br />
<br />flOOd Characteristics
<br />Floodflow' on the Roarin9 Fork River result from rapid melting of the
<br />mou~tain 'nowpdck during the period from May to "arly July, Snuwme I t runoff
<br />may o"a,lonally be augmented by rain. Thcsnow"",lt runoff 1"hara'terTzed
<br />by ,usta i ned per lad, of high f low~ and marked d i urna 1 f I u,t uat i on. E"",mi na-
<br />tion of meteorological and ,llm3tologi,al conditions and precipitation and
<br />streamflow records sho,., that summer cioudbursts ar<, not il great flood threat
<br />onthe5estream"
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