|
<br />"'"
<br />
<br />-.
<br />
<br />assuming a cubic boulder, and solving
<br />for the velocity,
<br />
<br />1."",[~J~~.t;111;3)1,,:()~;3)((}-
<br />/1,(;,
<br />
<br />p,lj"
<br />. (7)
<br />
<br />Assuming the same valut's used above, the
<br />vdo(ity lH:Ci..'S~ary for ovcrtufIling is
<br />3.2 mh;.
<br />This discussion of boulder transport
<br />clllph<lsizes thaI ob~l'rvcd boulder mov('-
<br />ment can be explained by the Oood vclod-
<br />lk, c;llcLJl;llnl ....lrlier. Rt't),;lldk~s nf
<br />wilelhef bOllldt.'r~ hq;:IJl Illoving by sliding
<br />or overturning, SilCS up to 1.4 III cuuld be'
<br />moved by the lluid.dynalllic S[rt;SSl'S of
<br />water in the .,hannd between 3,080 and
<br />J, 170 m elevation.
<br />
<br />MASS TRANSPORT BY GRAIN FLOWS
<br />
<br />Oeposits below approximately 3,050 III
<br />suggest that the nood water became more
<br />heavily laden with fine sediment, which
<br />plohably l.'OI!lbilll'd \\i[h cobble:-- and
<br />houlders near thl' be-u. This sedilllcnt IO;ld
<br />lIh)Vl'd as a ...kll.'ie inC:llial flow of cubbks
<br />and boulders, which will be referred to as
<br />a "grain /1ow," !"oJJlming Bagnold (1954).
<br />"Ill; depo~it!> from gr;jill flow" werc di.~-
<br />~l"(tl"d at Illany hxations in the (hannel;
<br />this fa(iJitatcd illSPl'Clioll.
<br />The grain flows movl:u around trees
<br />and other vegetation in the nood path .qnd
<br />left deposits 0.5 to 1.0 III deep against trees,
<br />at corners of ehanneb, and at changes in
<br />!;radi(.'nl. The deposils indk~He th<lt lhc"
<br />larger boulders Wr;'re transported near thc
<br />tops of the nows, presumably due to dis-
<br />per.~ive stresses (Bagnold, 1954). SOIllC of
<br />the ll~}\\'s hold an illICfsljtiallllalli,'\ of fillc
<br />lll,lll'l'ial, whercas olllcrs, il\dlldill~ SOIllC
<br />thaI !1o.....cd laLcrJlJy beyond the flood
<br />~'h;llllll'l, were ...'olllposnl almost l'l1tirdy
<br />of boulder-, cobblc-, and gravel-sized
<br />Illa[erial with llO finc-gl'aincJ matrix.
<br />Ikc;Jusl: thc dcpmil.) in llll: ch;lIl1lcl werc
<br />t'[lltku, it is assulllcd tll;lt lhcir motioll
<br />n':l.\l.J prior to tIn: end of [he flood, po:,-
<br />\ibly at a r~lirly high stage. Although mallY
<br />llf lhe grain !1owunovco below the water
<br />\urfacc, ont: now ~llrgc continlled !o au-
<br />\ .IlKt' llnh) tht' alluvial LilI afl!.:'r [Ill: llond
<br />le(c"ded (Fig. 2). Thi:-; p;lrticlllar flow ob-
<br />\ i"t1~ly did lIlll n:quil c a :-oupl'l'illlposnl
<br />tlood to proviJc mobility through fluid
<br />boumLH)' shear stresS and implies that lhe
<br />flain Jlows in Ihe dl:WJld may ;dso have
<br />Illoved independelltly of the Oood wave.
<br />
<br />~ {j
<br />
<br />
<br />'..
<br />
<br />...
<br />
<br />h;pue J. (;rajn.l1owdl'posi( (poin[ g. Fig. 2). I'hO(llgr;lph t:lkt'n hl\\;Hd C:ISl, looking up dl'JlOsit.
<br />Trullk.~ of lrl'~'.~ in kft foreground have lll'l'fl ahr:ldL'u hy flow :.Jllll arc 10 to 15 em in diameter.
<br />
<br />Sill\ital .\ubfluvial tran.)port rcalul'c.:.~ afe
<br />rcportcd in Scott and Gra....lec.: (1lJ6H).
<br />This dcpo~it is studied in dctail bel:3usc
<br />ilS fcalures arc unaffected by Oood erosion
<br />and arc well rre~cl'ved. I:igurc 3 shows lhe
<br />depo:,jt, whit-II adV;\Il(ed for approximately
<br />50 III 011 a slope of 50 to 6.50 and buried
<br />a Ihick stand of \\'illow~ and previously
<br />dl'positl'd lload debris. The deposit is
<br />wmposed primarily of gravel- to cobble.
<br />sized lll.:J.terial with occasionJI larger sur-
<br />face boulders. It varies in thickness from
<br />0.6 [0 1.2 III .\!ld contains an interstitial
<br />matrix of material consisting of 59.5\1/0
<br />(by weight) s;.lIld. 27.70/0 silt, and 12.80/0
<br />clay. Tht.' matrix material is cla.~sificd as
<br />a silty sand. The surface of the ueposit is
<br />flat trig. J), suggesting:\ low .~IH':;lr stn.:ngth
<br />ill the moving now in COIl[ra.'it 10 typical
<br />!;lI11ill;\I' dl:bris flows, whidl Illay ddorlll
<br />;IS a Bingham plastic matcrial (Johnson,
<br />1970). The fact lhat thc now cOilSi:,tcu
<br />pI illlarily o[ larger clasts, wilh the matrix
<br />l)ccupyillg only a small part of Ihe tolal
<br />VOIUllli:, sllgg~.<,t~ (ontilluous (.'olli.<,iolls and
<br />1ll1J1l1i:lItullle.xdlangc bc[wct:1l Ihe ranidcs,
<br />again ill contrast to debris~tlow movement
<br />as dc~cribed by Johnson (1970).
<br />I ,;Irger bOlllJl:I'~ Irall~pllrlcd by lhe
<br />flow wefe concclltralcd loward lhe front
<br />of thc depnsit. III ordcr 10 quantify thi:-;
<br />efkl'l, [Ill' inlermedia[c diamclers of lhe
<br />lell large~l bouldcr~ fOLlnd at the ~lIrface
<br />Were llleasured at the tail, midway, and at
<br />lhe front of the now. The means and
<br />
<br />stallLiaru deviations at each sample site
<br />arc given in Table 2. An analysis of vari-
<br />ance of these data shows that the differ.
<br />cnees observed betwcen the samples are
<br />significant at the 50/0 level. Thus, longilll-
<br />din;\l sOfling occurred within the moving
<br />/low. Vertical sorting also occurred as
<br />progrc.'isivcJy larger boulders were found
<br />farther above the bed of the now. This
<br />was not, however, tested quantitatively.
<br />Bagnold (l954) suggested a mechanism
<br />for grain Oow. He pointed out that a flow
<br />of grains can only occur after an upward
<br />dilation of the mass, which increases the
<br />mean free distance between the grains. The
<br />energy necessary for this dilation result.~
<br />from II net momentum flux away from the
<br />hounJary of the now-in this case thc
<br />oed. This is analogous to the net Oux of
<br />1ll0111l'ntulll away frolll a boundary in
<br />turbulenl nuid now but results from
<br />random collisions between the grains.
<br />Ilagnold's exprcssion ror the upward
<br />slrcss or di.<.persivc pressure (P) is
<br />
<br />{':." 0.U42 /)2 (dUfdy)2 (:0:'0:, (~)
<br />
<br />where dU Idy is the vdo(.'jty gradient per-
<br />pl:IH.Jk:ular to thc bt:d, iind u is an angk
<br />
<br />IAllll 1<1)11111111', II/I 1111 1,~,^n;.lllJ'" "IJI(I All
<br />
<br />\''''1')''1'''.'1'','' ^""""I"d""""t,'r '.l"",j,"',\.I1"'IJtiQn
<br />(em)
<br />
<br />1_,)
<br />Mi,ll'o'''t
<br />Front
<br />
<br />t~ 'J
<br />40./
<br />52.6
<br />
<br />'J.l,
<br />10.1
<br />11.3
<br />
<br />IN-JUAHY lSl/9
<br />
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