|
<br />..,
<br />
<br />-.
<br />
<br />J-
<br />
<br />1
<br />I
<br />, klcrmincd by the collision conditions
<br />I :'etwccn the grains. Equation 8 suggests
<br />. hat P varies as D2 for a given velocity
<br />'J ;l(Jil.:nt and that when grains of a mixed
<br />,i/l' :In: sheared together, as in the now
<br />-,ludied here, the larger grains should drift
<br />\I\'ward, away from the bed and toward
<br />'h<.' lnllL' of least shear stress. Here, at the
<br />~ll:l of the 110w, the larger boulders expe-
<br />I i,'llll' the largest f()TWard velocity rdative
<br />\ll 111L' hed and should he expected 10
<br />IJli~'I;IIC forward toward the front uf the
<br />!'\,l\\', rhis is c.xaetly what was observed in
<br />the ~rain,now deposit. Thus, Hagnold's
<br />l~ll'\:llanisl11 provides a viable explanation
<br />I\lt' ILl' observed longiwdinal sorting.
<br />B;l~'110ld'.. Ihcllry ab,) pro\'idL'~ a llH:ans
<br />ror c;l!ud;lling the vertical vclo(ity gradient
<br />within <l grain flow. This is Jone by
<br />t'qu<lling the gravitationally induced shear
<br />~trt'S.., on the now to its inlernal shear re-
<br />,i~!;llh'e, it being assumed that these two
<br />'l!l'~"l'~ are equal 011 a given slopt..: ,ll
<br />(l':1'(;111t vdol:ity. \\'ht..:ll lhi~ is done, the
<br />\du~'ity gradient is calculated
<br />
<br />h -' J~_i_n~l~_I~1 -I __~o ~ .::_p )~I yl~'!, \1.))
<br />\ (,\ I !I'
<br />
<br />\1 ~II-'r..: ;\ is the linear ~r.lill con..:entration,
<br />,!\'~-~I\,:d by Hagnold as Dls where s is the
<br />11.L''::', free distance between grains of
<br />,il.l:1:L'tcr n. Equation <) incorporates some
<br />1',ljt~~:.::-i,;al codfidents determined experi-
<br />!lLL':,:,dly by Bagnold (1954). por the grain
<br />: "y,t ..1 tidied here, it is assumed that {3
<br />L',;'. .,:' the measured value of 60, Pc! equals
<br />1,~("1 ~g/ml (the demity of the matrix),
<br />,) cquah 2,700 kg/ml, R equals 0.62, ^
<br />npl,d~ 17, and y equals 1.0 Ill. Using these
<br />~"1, ,;:ltlt \'<.llues, the vdo(ity graJit'l1t is
<br />l\Junu to vary .IS liD.
<br />1]1'.: vCrlic<tl grain ~i7.e distribution and
<br />tc,u\1ing vc!ol:ity distribution within the
<br />11.,'..\llg now art" unknown, but ,Ill al"lproxi-
<br />'ll;l\illll of lht: mean rOr\\'ard vclorily or
<br />: Ill' I1nw GW be made with some simpli-
<br />fying 3:.sumptions. Assuming 3 linear dis-
<br />tribution of grain size between the bed and
<br />
<br />I,,; (1\ (lC;Y
<br />
<br />surface of the now and a depth of 1.0 m,
<br />an average velocity of 1.5 m/s is calcu-
<br />lated. for the same conditions but on
<br />a slope of 100, the velocity is 1.901/5.
<br />These velocities arl' substantially less than
<br />lhose of the Iloml w.we (Table I).
<br />Equation 9 also suggests that the grain
<br />110\\' could have stoflped quite suddenly as'
<br />the Oowing mass collapsed, the mean free
<br />distance between the grains appro<lched
<br />7cro, and the boulders and cobbles inter.
<br />h)ckcJ, When this occurs, the ratio Dls
<br />(/1 in equation 9) approaches infinity, and
<br />DU Idy approaches zero, This condusioll
<br />is also supported by the fact that longitu-
<br />dinal and vertical size sorting is retained
<br />in Ihe depo...il. It wa.\; induced dyn~\rTlkally
<br />and pn:servcd even after the movement
<br />stopped.
<br />
<br />DISCUSSION
<br />
<br />The exceptionally large discharge and
<br />rainfall intl.'llsity reponed in this parl.'f
<br />are high estimates, because the nood wave
<br />consisted of water and a large but un-
<br />known volume of solid material. Gerson
<br />(1977) measured up to 350/1) by volume
<br />SllSP<"lHkd load in Iloodwaters from small
<br />desert watersheds. If a similar solids con-
<br />centration is assumed for the Avery Peak
<br />llood, the wata discharge would be only
<br />650'/0 of the total calculated discharge.
<br />Therefore, the rainfall intensity required'
<br />would be reduced to 10.9 to 13.5 em in
<br />10 min. Nevertheless, even these reduced
<br />estimates are an order of magnitude greater
<br />than intensities assumed in 1100d design,
<br />Perhaps the rainfall intensity assumed in
<br />design should be revised upward in
<br />\ome alpine <In..':\s.
<br />Iligh-vdocity lloods with a large
<br />sedimellt load may become so heavily'
<br />laden with coarse-grained sediment that
<br />parts of the hed load become decollpled
<br />from Ihe lloot! wave ~Hld continue to ad.
<br />va[1((..' even :l'i the Iloot.! rc(edes. (jrain
<br />nows advancing at the velocilies calculated
<br />in this paper can be potentially damaging
<br />to buildings located in their paths.
<br />
<br />~
<br />
<br />REFERENCES CITED
<br />
<br />Bagllold, R. A., 1954, Experimcnts on a
<br />gravily.frct.' disp<.'rsiol1l)( large solid sphercs
<br />ill:J Nl'wlolllan nuil! lUhler shc>Jr: I~oyal
<br />Sodl'ly I London I f'roct.'l'dillgs, v. 225,
<br />p.49-bJ.
<br />Baker, V. \t., 1977, Slre;Jm.channd reSponSe
<br />1') n"Hds, with e:cllllph's fronl central Texas:
<br />t;e()lo~ical Sodety of I\illt'fica Bulletin,
<br />v. XK, p. 1051-1071.
<br />IhmgllcrlY, l{. I.., aul! h;II\'llui, J. n., 11J6S,
<br />Hllid uwdlanit's wilh cnginecring appfil.:a-
<br />li01l\: New Yort.., tlll(;T;1\v.llill \Iouk ('0"
<br />574 p.
<br />(;t'TS"ll, l{., 1<)77, Sedilllt'lll trallsport for desert
<br />walersheds in crotlihk materials: Earth
<br />Surbce I'roct:Sscs, \". 2, p. 343-361,
<br />Ildley, E. J., t\)69, Fkld measurelllent (lfthe
<br />initiation of large bed particle motion in
<br />Blue Crct:k n<.';H KI;llllatl1, C<.\lifornia: U.S.
<br />(;eotoRical ::;urvey Profession at Paper
<br />$(,2,(;, III P.
<br />john""'l, '\. M., 1')70, l'llysilal processes in
<br />gcnlll):Y: Sa1\ l.r;llld"o.:o, I'reemall, Cooller
<br />;111\1 (",mpan)', 577 l'
<br />Lins!cy, It. K., Kohler, M. A,. and I'aullllls,
<br />1.1,.11., 1')5~. Ilyuro]ogy for cllgin~crs:
<br />New York, Md;raw.llillllook Cu., 340 p.
<br />So.:olt, K. M., and Gr;lvke, C.CO, 1968, Fluou
<br />sllr~l' ('II lilt: \{uhicoll Hivl'r, C;lliforllia-
<br />1Iydrol(.):y. llydr;l\l]io.:~ a1l\1 h"llhl~'r Ir;llls,
<br />p',rl: II.S. (;c(,I"~i(~11 Surv<:y l'rot't:ssiollal
<br />l'aper 422.M, 40 p.
<br />Soil Con~c.r\'ation S~'rvic..., t971, l'roCt:t1ures for
<br />dcterfllinin,l: I"-'ak flows in Colorauo: Wash.
<br />il\~!ol\, n.c., tl.S. lh-p:Jrlmcnt of Agrio.:uiture
<br />S'1I1 ('O!IWI'V;Jli"ll S~'rvio.:'" (i:raphs, lll;JpS,
<br />alld l;Jh1c~).
<br />SOllk, .1. M., I{,,~t'rs, W. I'" and Shelton, D, C.,
<br />I ()76, (;eol/)~lc h;J/Hds, gCI)lJHHphk fea.
<br />tUfes, and land.usc illl]llic:Jliolls in the aTl~a
<br />of thc 1<)76 IIiI' Thulllpson floou, Larimer
<br />County, Colorado: Colorado Geological
<br />Survl'y FllViroll11H:nlal t;eology I () (m:Jps
<br />alldy~xl).
<br />
<br />ACKNOWLEDGMENTS
<br />
<br />Reviewed by William C. Br<ldley and Nel
<br />('<lint'. Rolf Killl (lNST^AR s~dill1~lltol()~y
<br />laboratory, University or ColurJdo) provided
<br />the an.dysis of the malrix m;lt~rial from the
<br />!!rainf111w.
<br />
<br />I M^~US(,RIPT IU-:CUVED MAY 9, 197MsMANliSCRIPT ACCEPTED SEI)T. is. 1978
<br />
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