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Sediment Transport and Morphodynamics Marcelo H

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Sediment Transport and Morphodynamics Marcelo H CHAPTER 2 Sediment Transport and Morphodynamics Marcelo H. García ASCE Manual 54 , Sedimentation Engineering, prepared suspended load, fl uid turbulence comes into play, carrying under the leadership of Professor Vito A. Vanoni, has pro- the particles well up into the water column. In both cases, the vided guidance to theoreticians and practitioners’ world driving force for sediment transport is the action of gravity wide on the primary topic of sediment problems involved on the fl uid phase; this force is transmitted to the particles in the development, use, and conservation of water and via drag. Whether the mode of transport is saltation or sus- land resources. First published in 1975, Manual 54 gives pension, the volume concentration of solids anywhere in the an understanding of the nature and scope of sedimentation water column tends to be rather dilute in rivers. As a result, it problems, of the methods for their investigation, and of prac- is generally possible to treat the two phases separately. tical approaches to their solution. It is essentially a textbook In the geophysical domain, the fi eld is much broader on sedimentation engineering, as its title accurately refl ects. than rivers alone. The same phenomena of bed load and sus- Manual 5 4 was the fi rst and most comprehensive text of its pended load transport occur in a variety of other geophysical kind and has been circulated throughout the world for the contexts. Sediment transport is accomplished in the near- past 30 years as the most complete reference on sedimenta- shore of lakes and oceans by wave action. Turbidity currents tion engineering in the world. It has recently been published act to carry suspended sediment into lakes, reservoirs, and again as the Classic Edition (Vanoni 2006). In the spirit the deep sea. Landslides, debris fl ows and mud fl ows pro- of its predecessor, this chapter of Manual of Practice 110 , vide mass transport mechanisms for the delivery of sediment Sedimentation Engineering, aims at presenting the state of from highlands to lowlands. the art concerning the hydraulics of sediment transport in fl u- The solid phase can vary greatly in size, ranging from vial systems based on the knowledge gained in the last three clay particles to silt, sand, gravel, cobbles, and boulders. decades. A concerted effort is made to relate the mechanics Rock types can include quartz, feldspar, limestone, granite, of sediment transport in rivers and by turbidity currents to basalt, and other less common types such as magnetite. The the morphodynamics of lake and reservoir sedimentation, fl uid phase can, in principle, be almost anything that con- including the formation of fl uvial deltas. stitutes a fl uid. In the geophysical sense, however, the two fl uids of major importance are water and air. The phenomenon of sediment transport can sometimes 2.1 SEDIMENT TRANSPORT MECHANICS be disguised as rather esoteric phenomena. When water is AND RELATED PHENOMENA supercooled, large quantities of particulate frazil ice can form. As the water moves under a frozen ice cover, one has The fi eld of sediment transport might just as well be called the phenomenon of sediment transport in rivers stood on its “transport of granular particles by fl uids.” As such, it embod- head. The frazil ice particles fl oat rather than sink, and thus ies a type of two-phase fl ow, in which one phase is fl uid tend to accumulate on the bottom side of the ice cover rather and the other phase is solid. The prototype for the fi eld is than on the riverbed. Turbulence tends to suspend the par- the river. Here, the fl uid phase is river water, and the solid ticles downward rather than upward. phase is sediment grains, e.g., quartz sand. The most com- In the case of a powder snow avalanche, the fl uid phase mon modes of sediment transport in rivers are those of bed is air and the solid phase consists of snow particles. The load and suspended load. In bed load, particles roll, slide, or dominant mode of transport is suspension. These fl ows are saltate over each other, never rising too far above the bed. In close analogues of turbidity currents, insofar as the driving 21 6690-003-2pass-002-r03.indd90-003-2pass-002-r03.indd 2211 11/21/2008/21/2008 55:42:33:42:33 PPMM 22 sediment transport and morphodynamics force for the fl ow is the action of gravity on the solid phase antidunes, and alternate bars. The fi rst three of these can have rather than the fl uid phase. That is, if all the particles drop a profound effect on the resistance to fl ow offered by the out of suspension, the fl ow ceases. In the case of sediment riverbed. Thus, they act to control river depth. Riverbanks transport in rivers, it is accurate to say that the fl uid phase themselves can also be considered to be a self-formed mor- drags the solid phase along. In the cases of turbidity currents phological feature, thus specifying the entire container. and powder snow avalanches, the solid phase drags the fl uid The container itself can deform in plan. Alternate bars phase along. cause rivers to erode their banks in a rhythmic pattern, thus Desert sand dunes provide an example for which the fl uid allowing the onset of meandering. Fully developed river phase is air, but the dominant mode of transport is saltation meandering implies an intricate balance between sediment rather than suspension. Because air is so much lighter than erosion and deposition. If a stream is suffi ciently wide, it will water, quartz sand particles saltate in long, high trajectories, braid rather than meander, dividing into several intertwining relatively unaffected by the direct action of turbulent fl uctua- channels. Braided rivers are an important component of the tions. The dunes themselves are created by the effect of the Earth’s surface. The deposits of ancient braided rivers may fl uid phase acting on the solid phase. They, in turn, affect the contain signifi cant reserves of water and hydrocarbon. fl uid phase by changing the resistance. Rivers create morphological structures on much larger In the limiting case of vanishing solids, the fi eld reduces scales as well. These include canyons, alluvial fans, and del- to pure fl uid mechanics. As a result, sediment transport must tas. Turbidity currents act to create similar structures in the be considered to be a subfi eld of fl uid mechanics. In the lim- oceanic environment. In the coastal environment, the beach iting case of vanishing fl uid, the problem reduces to that of profi le itself is created by the interaction of water and sedi- the fl ow of a granular substance in a vacuum. The driving ment. On a larger scale, offshore bars, spits, and capes con- force now typically, but not always, becomes gravity. This stitute rhythmic features created by wave-current-sediment problem, as well, can be treated with the techniques of fl uid interaction. The boulder levees often created by debris fl ows mechanics, as long as one is willing to move far afi eld of provide another example of a morphological structure cre- traditional Newtonian fl uid mechanics. Martian rock ava- ated by a sediment-bearing fl ow. lanches constitute a geophysical realization of grain fl ows This chapter is an introduction to the mechanics of sedi- in a near vacuum, and it is likely that the fl uid phase plays ment transport and river morphodynamics. Rivers evolve only a subsidiary role in many terrestrial rock avalanches. over time in accordance with the interaction between the fl ow Another example of grain fl ow is a slab avalanche of snow. If and sediment-transport fi elds over an erodible bed (which they attain suffi cient speed, slab avalanches tend to devolve changes the bed) and the changing morphology of the bed into more dilute powder snow avalanches in which the fl uid (which changes the fl ow and sediment-transport fi elds). This phase plays a greater role. co-evolution is termed morphodynamics. Sediment transport Among the more interesting intermediate cases are debris by turbidity currents and the mechanics of lake and reser- fl ows, mud fl ows, and hyperconcentrated fl ows. In all of voir sedimentation are also considered in this chapter. The these cases, the solid and fl uid phases are present in similar approach is intended to be as mechanistic and deductive as quantities. A debris fl ow typically carries a heterogeneous possible so that readers will be able to gain a fi rm founda- mixture of grain sizes ranging from boulders to clay. Mud tion in the mechanics of sediment transport. This should be fl ows and hyperconcentrated fl ows are generally restricted benefi cial both for understanding the rest of the material pre- to fi ner grain sizes. In most cases, it proves useful to think of sented in the manual as well as for sedimentation engineer- such fl ows as consisting of a single phase, the mechanics of ing and teaching purposes. which is highly non-Newtonian. The study of the movement of grains under the infl u- 2.1.1 The Sediment Cycle in the Environment ence of fl uid drag and gravity constitutes a fascinating fi eld in its own right. The subject becomes even more interest- The sediment cycle starts with the process of erosion, ing when one considers the link between sediment transport whereby particles or fragments are weathered from rock and morphology. In the laboratory, the phenomenon can material. Action by water, wind, and glaciers as well as be studied in the context of a variety of containers, such as plant and animal activities, contributes to the erosion of the fl umes and wave tanks, specifi ed by the experimentalist.
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