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<br />]. i:-:\Irc 2. Drainage basin on west side nr ^Vl'ry Peak, showing area (ahtwc 3,170 -m contour)
<br />ill \\ hid! il\!clI.'il' r:lin(;ll! ul:nlfrt:d. ~bl1nill,g for1l111b was ,lpplit'll at llon\! noss ~l'diollS a througll
<br />f. (;raiJ\ !low Jis('ussed in fl.'XI 3dvrlIH;l'd 10 pIJint g. Base frolll U.S. Geologic:!1 Survey Gothic,
<br />(', .[<>rado, topugraphic qU;Hh3nglc map (196 t).
<br />
<br />a drainage basin of equivalent size, al.
<br />thouf!h little data have been obtained
<br />from such small basins.
<br />The Avcry Peak basin is small, of
<br />rd,lli\'l'ly l.;Ollstanl illfiltriltion potential,
<br />~nd (If ~illlP1c geometry, all of which Sllg-
<br />p.'~t that the nuional formula may be
<br />applied 10 estimate the precipitation in-
<br />Il'milY ncccssary to (JUSt' the nood.
<br />I ill'ky <tnd others (1958) defined the
<br />1.:(,' '!i.11 formula as
<br />
<br />{}== Ci A,
<br />
<br />where C is a runoff coefficient. i is the
<br />1.11nfall intensity, and A is the drainage
<br />\',;\'.:1\ area, In equation 3 the relevant pre-
<br />~'l~'ilalioll intensity (i) is defined over the
<br />~\11h.'<':lllfation lime of the basin. previo\lsly
<br />,~t'!l'rmined to be 10 min. Assuming a run-
<br />III t' I,:ncfficicnt (0.9) and taking results
<br />.I!rndy derived (Q = lOR to 134 ml/s;
<br />
<br />I f I, _f"LUQO Uf~A~I(T~IN(; 1-l~'1(il:jf, r(1I1~'-'~!,-
<br />
<br />f I~, , " "
<br />", ('"I (mi.) (",lnl
<br />J, I ~ 'I I_il I," 12', 2,13
<br />3,1,', I,()/l , , 112 ,,01
<br />J,()!O , 21 , n Illb 2,G2
<br />l,(HI') O,ilL 4_', 110 1,(,&
<br />,',''-'" 1.:'(, " \\ IH I ,~(,
<br />.','!LJ tJ_'JII " , ", , 1\\
<br />
<br />""'.~~": I .,3/1 f'~u"I\ n.l rh
<br />
<br />"'J':~ """!;H, "'iu~1 In 1;'.':,''-. ..nt.r~ '" I, \h~ ~r.,.\_
<br />.."I He,.l,"'.IIOn,
<br />
<br />t,t ()lU(,Y
<br />
<br />A = 430,000 m2), the rational formula
<br />suggests a 10-min rainfall intensity of
<br />bet\veen 16.8 and 20.7 em. This is a high
<br />estimate (discussed below) and approxi~
<br />mates lhe maximum evcr reported (Raker,
<br />1977). Similar intcnsities havc never been I
<br />reported at high elevations.
<br />
<br />BOULDER TRANSPORT
<br />BY FLOOD WATER
<br />
<br />(3)
<br />
<br />^ 200-m reach of the channel between
<br />3,130 and 3,080 rn elevation was studied
<br />to determine the effect of such high dis~
<br />charges on boulder transport. Within this.
<br />reach all boulders appeared to have been
<br />transported by turbulent need waters.
<br />lkrl1l deposits at the channel margins con~
<br />tained none of the interstitial fines charac~
<br />. ,
<br />teristic of mud or debris flows. Below i
<br />I
<br />3,080 m. boulders were transported by !
<br />both Oood water and grain flows (discussed
<br />below), and so the crfcCl of flood (r.ms- I
<br />pOrl could not be easily separated from
<br />that of grain-flow transport.
<br />Above 3.080 m, the laraelit boulder
<br />found In the channel measured 4,2
<br />x 1.5 x 1.2 m, and the average length
<br />of the intermediate diameters, D, of the
<br />SL'Ve-1l large-st boulders was 1.4 m. This
<br />,lveragc figure will be used in calculations
<br />because it represellls a conservative esti-
<br />m:lte of the maxiOlum.size boulder tr<'lIlS.
<br />
<br />ported by the nood. Particle movement
<br />On a stream bed begins when the drag
<br />force, Fd. exerted by the moving fluid
<br />exceeds the friction force, rj. holding
<br />the particle in place at the stream bed.
<br />The dr<lg force on the boulder is
<br />
<br />Fd = V,CdRPrl" ,
<br />
<br />(4)
<br />
<br />where Cd is a coefficient of drag, B is the
<br />area of the boulder projected normal to the
<br />flow, Pfis the fluid unit weight, and Vis
<br />fluid velocity (Daugherty and Franzini,
<br />1965). The force Fforroses Fd and results
<br />from the submerged weight of the boulder
<br />hormal to the bcd times a friction co-
<br />efficient. minus the component of the sub-
<br />merged weight acting down the bed. Thus
<br />
<br />!'j ~(o - PI) K g (~co,~ -- sin~), (5)
<br />
<br />where u is the boulder unit weight, K is
<br />the bOJ.llder volume, g is the gravitational
<br />acceleration, p. is the frictional coefficient
<br />between the static boulder and the stream
<br />bed, and (3 is the gradient of the stream
<br />bcd. When a boulder is submerged in the
<br />flow, equations '4 and S may be equated
<br />and solved in terms of V, the mean veloci-
<br />ty just S1l fficient to indute sliding. For the
<br />simple case of a cubic boulder, similar in
<br />shape to those observed, with K .equal to
<br />f)), the critical mean velocity becomes
<br />
<br />v = [2(<1 - PI) f) ~ (I1CO'O - Sin~)] 'ii, (6)
<br />l Pled
<br />
<br />Within the reach of the sampled boulders
<br />the flow height was approximately equal to
<br />the, height of the largest boulders, ~ ~ 10.,
<br />(1 = 2.700 kg/ml, Pjis estimated to equal
<br />1,200 kg 1m' because of lhe increased bulk
<br />fluid density due to fine material in sus-
<br />pension, Cd = 1.2 (Daugherty and Fran-
<br />zini. 1965), ~ is estimaled as 0.6, corre-
<br />sponding approximately to the static
<br />friction coefficient in granular material,
<br />and D is given the mean value of 1.4 m.
<br />Tne mean flow velocity necessary to initiate
<br />bouhl~r blidtn~, ttum lltiuAtltm 6j is ~:~ ml!l:
<br />Alternatively, the velocity (musing
<br />boulder movement can be calculaled by
<br />equaling the overturning moment resulting
<br />from nuid drag to the resisting moment re-
<br />sulting from the weight of tne boulder on
<br />the bed (Hcllcy, 1969). Equating moments,
<br />
<br />55
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