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<br />HYDRAULIC ENGINEERING '94
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<br />needed to lest this may nol occur in natural mountain rivers. Generally the
<br />sleeper the ('.hannel slope, the lower is the relative submergence at which
<br />bankfull and flood conditions are attained. At a slope of 1 %. for example,
<br />values of dID for inbank nows may typically nol exceed aboul 10. In Fig. I
<br />the group I r:lationship appears to be retur~ing to the ~mil()g~rithmic law
<br />but this is likely to be the result of increasmg ~ank resistance as the now
<br />begins to spill out of bank, rather than a bed re!i;lstance effect.
<br />
<br />fn conclusion this study shows that the familiar semilogarithmic
<br />resistance relationship significantly overestimates the resistance at high flows.
<br />Also, the at-a.-site variation in flow resistance is found to have some
<br />connection with the factors which affect the development of the S-shaped
<br />velocity profile. However, with present data it is not. possi~le to ~lIap~ t~C'
<br />curves of Fig. I to onc general relationship. Unravelhng thiS rela!lOnshlp Will
<br />require considerably more data on at.a.site variations in flow reslstanC'c fot a
<br />range of channel slopes and bed material size distributions.
<br />
<br />References
<br />
<br />Aguirre..Pe, J., and Fuentes, R. (1990). "Resistance to now in steep rough
<br />Slreams." J. Hydrall/. En!('!l., ASCE, 116( 11). 1374-1381.
<br />Balhursl, J.e. (1978). "Flow resislance of large-scale roughness." J. HydrUltl.
<br />Viv. ASCE, 104(HYI2). 1587-1603.
<br />Balh~rst, J.e. (1985). IlFlow resistance estimation in mountain rivers."
<br />J. Hydrall/. Eng'!l., ASCE, 111(4),625-643.
<br />.Balhursl, J.e. (1988). "Velocily profile in high-gradien!, bould~r-bed channel~."
<br />/'roc. Ifill. Au, Hydraul. Res. International Conference on Huvlalllydruuilc.~
<br />'88, Budapesl, Hungary, pp. 29-34. "
<br />Balhursl, J.C. (1992). "Flow resislance Ihrough the channel nelwork. In
<br />Channel Network Hydrology, K. Beven and M.J. Kirkby (eds), John Wiley
<br />and Sons. Chichesler, UK, pp. 69-98. .
<br />Flammer, G.H., Tullis, J.P.. and Mason, E.S. (1910). "Free surlace. velOCIty
<br />gradienl now p..1 hemisphere." J. Hydrall/. Viv., ASCE, 96(HY7), 1485-
<br />1502. .
<br />Hey, R.D. (1979). "Flow resistance in gravel-bed rivers." J. Ilydrau/. f)n.'.,
<br />ASCE, 105(HY4), 365-379.
<br />Jarrell, R.D. (1984). "Hydraulics of high- gradi en I slreams." J. Hydrall/. Eng'!l..
<br />ASeE, 110(11). 1519-1539. " . .
<br />Marchand, J.P., Jarrell. R.D., and Jones, L.L. (1984). Velocity proltle. ""ater-
<br />surface slope, and bed-material size for selected streams m Colorado.
<br />Open-File Rep. 84-733. U.S. Geol. Surv., Lakewood.Colo. _ .
<br />1110rne, C.R., and Zevenbergen, L.W. (1985). "Eslimating mean velOCIty III
<br />mounlain rivers." J. Hydmul. En!('!l.. ASeE. 111(4).612-624.
<br />Wiberg, P.L. and Smilh. J.D. (1991). "Velocily dislribulion and bed roughness
<br />in high-gradienl slreams." War. Resour. Re.,., 27(5), 825-838.
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<br />FLOW-BED INTERACTION AND ENERGY WSS IN l'ORR.:NTS
<br />
<br />luis Ayala (II
<br />Alt:iandro LOpez (I'
<br />
<br />Abstract
<br />
<br />This paper summari~ the resu.lts obtained in field and laboratory investigarions
<br />concernmg the hyd~ultc and ~hment transport behaviour of torrents representative
<br />of Los Andes CordIllera dramage systems of Central Chile. Generally speaking.
<br />t~ Slreams have large strepnesses, coarse and well graded sediments, small
<br />rel~llve submergences and Froude Numbers attaining or exceeding unity, particularly
<br />dUllng. n~ events. In order to achieve reliable energy loss or slage-discharge
<br />predictions II ~rs as ~ecessary 10 introduce in 'hese relations a pavement layer
<br />rou~hness,. whlc.h ~n ~ mdependenUy predicted or estimated using the original bed
<br />sC(hment Size distribution. However~ flume and "'ree"" river data do not seem to
<br />agree completly, probably due 10 differenl geomelry effects of bolh Iypes of system.
<br />
<br />Energy loss evaluation
<br />In defining a flow resistance equation several authors have recommended the use of
<br />a sedimen! diame~r represenla~i~e of the bed roughness. Usually (his is defined as
<br />a I~r~er Size fraction of the onglnal bed sediment sieve analysis curvc,ie.:Du,O\ll'
<br />ThiS IS because the pavement D,so is somewhat similar to a bed 0 or a D for
<br />normal pavement forming flow conditions. However, this may no~ hold lru~ for
<br />other conditions such as those encountered near or below the threshold of sediment
<br />motion. It is not infrequent to have pavement coats formed for large discharge flows
<br />~ that present roughness conditions may not be representative of the ones associaled
<br />with lower flows.
<br />
<br />Q"it~ ,a large n~mber of equations have been proposed, based on Keulegan
<br />logaruhmlc relahon (Ayala, 199/; Ayala. 1993). They may be wriUen under
<br />
<br />tI) Prof., Civ.lJng: Dept., Univ. of Chile. Casilla 228/3, Santiago, Chile
<br />(2) Assoc. pror, CIV. Eng. Dept., Univ or Chile. Casilla 228/3, Sanliago, Chile
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