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<br />OCT-30-S0 WED 14:00 <br /> <br />BRR BLD 53 DFC <br /> <br />FAX NO, 2305034 <br /> <br />p, 03 <br /> <br />Utah. This letter .ummarizee the Interprel8tlon. of theee paleoflood Investigalions. <br /> <br />Summery <br /> <br />IR..ll2 <br /> <br />Methoda <br /> <br />Paleollood discharge' were estill\lltCld using the continuity equation and ,/ope-cDllvllyance method <br />for slream' within the area subject 10 each raln8foon. Flood dQpth for a paleoflood was obtalned <br />from the height of paleoslage indicator. (PSIs) llrlll8rvld in stream channels, which essentially <br />ara old hlgh.walar marks (HWMs). In this /nye.tigation, PSI. COMist of flood bar. havin\l sand <br />through boulller-alzed material, erosion scel'll, tree scars. lllI well as eyewitness estimate. of <br />flood dllpth. Looal rolliden13 indicated that no .ubatantiallloocls nUWB have oc:curred due to <br />raJnlaIlIIoodS~aY. ocCurre<bsinc. 1958 in )( Morgan and 11190 in tho Opal atudy areaa, The <br />I~ of &Oil development on Ikxxl deposits, lIlI10UN 01 decay of woody material, IInd post.flood <br />healing 01 tree. scars were conelGtent with nlaldente' lICCOUnla of the Interpreted age ur the <br />1958 (Utah) or 1990 (Wyoming) storma. Il(lc:Umenlation of actual flood height and PSI height <br />lor racent floods in .!reams in the Rocky Mounlilin region. Paclnc Northwest and Alaska are <br />shown In figure 1 for flood bars (s.. attached abstract) and "\lure 2 for tree xafl!. The data <br />shown in ligure 1 Indlc:ate that on average IIld with Iilllll varlllllon, the lop of flood-depoaited <br />sedimllflll provido Q good estimate of the ac;1lIa1 flood heighL The Ilillll in IIgure 2 Indleste mat me <br />elevation of the lap 01 Iree scaR have more variabDity and averages snllhlly b!9h!t than <br />measured flood depUII. Thus. use of lhelllp of tree acars reflects thll malrlmllm flood stage and <br />peak dischaflle, The key to the reliability 01 paleoflood reconstructions ill utilization of a <br />number 01 PSII for each palllnfkw:j lite. Average Rood velocity for a erosa eection WQG bUR on <br />enalysis of flood velocities frullI rllctmt flood6 of sImilar magnitude and hydraulic conditions. <br />Typically, for stream having grsdient& greatcr Ihen about 5 pa~ent and flood depths Ie 55 UUIn <br />abDut 2 flit average vllloclty used range Imlll 7 to 10 filet per second and for depths greater <br />thllll about 3 feet, average velocity range from 12 10 15 feet per second, For the lower gradient <br />channels (..0.001 to 0.01 MI) for most sitea in the OpQ/ 1Il1a, the average velocity used was <br />8 fils fur U1S main channel snd 3 ITIs for the dense sage on !hI! overbank lI1sas. It is unlibly , <br />that average flood veloclty exeeeded these raJ111e., therefDre, paleoflood estimates probably are <br /> <br />2 <br />