Sediment Transport and Deposition

Flowing water, wind, and glaciers move sediment from where it is produced by rock weathering to deposition sites in river basins, lakes, and oceans. Much of the world’s landscape is shaped directly or indirectly by the movement of sediment. Interpretation of sedimentary deposits, modern and ancient, rests on the understanding of sediment transport and deposition.

Movement of Sediment

Weathering of bedrock exposed on the continents produces solid particles of mineral or rock, ranging in size from the finest clay to large boulders. In most places, sediment moves slowly downslope toward stream channels, largely by gravity's direct or indirect effects. Sediment transport by fluids is highly dependent upon the density and viscosity of the fluid. Fine sediments may be transported by moving air from any location. Once in stream channels, sediment particles of different sizes are moved with varying degrees of efficiency by flowing water.

A flow of water or air exerts a force on a solid particle resting on a loose bed of similar particles. This force, which arises both from the friction of the flowing fluid and from the existence of relatively high fluid pressure on the upstream side of the particle and relatively low pressure on the downstream side, tends to move the particle in the direction of the flow. This force commonly has an upward component, termed lift, and a downstream component, termed drag. When the fluid force on the bed particle, which is counteracted by the particle's weight, is sufficient to lift the particle from its underlying points of support or to rotate the particle downstream around its points of support, the particle begins to move downstream. This condition is called incipient movement, and the overall force per unit area, or stress, that the flow exerts on the bed under those conditions is called the critical or threshold bed shear stress. Another way of looking at incipient movement is defined in terms of competence: the point at which fluid shear stress is just able to initiate the movement of particles of a given size in a sediment bed.

Water and Air Transport

Once a sediment particle is set in motion by a flow of water, it is likely to move by some combination of sliding, rolling, or hopping close to the bed. The material in this kind of motion is called bed load. If turbulent eddies in the flow have upward speeds greater than the downward settling speed of the particles relative to the fluid in their immediate vicinity, some of the moving particles are swept up into the flow to travel long distances downstream before returning to the bed. The material in this kind of motion is called the suspended load. As the strength of the flow increases, a larger percentage of the load travels in suspension, but bed load is always present near the bed, even in strong flows. Suspended particles are not floating in the fluid stream. Rather, they are suspended in the fluid due to the upward force exerted by turbulence currents and are continuously settling downward relative to the surrounding fluid. Ultimately, they are redeposited on the bed after traveling distances ranging from less than 1 meter (for coarser particles) to hundreds or even thousands of kilometers (for the finest particles).

The sediment particles transported by water range in size from the finest clay sizes (of the order of 1 micrometer in size) through silts (a few tens of micrometers) to sands (of the order of 1 millimeter) and gravels (coarser than a few millimeters). Clays and silts are carried mostly in suspension and are deposited only where current velocities are very small. Sands are transported both as bed load and in suspension, depending on the strength of the flow, and gravels are transported mainly as bed load.

In air, the most prominent mode of sediment particle movement is saltation, in which the particles are briefly lifted off the bed to move in a regular arching trajectory upward at a fairly large angle to the bed and then downward at a relatively small angle to the bed. Saltating sand grains under the influence of strong winds commonly rise no more than 1 or 2 meters above the sand surface while traveling as much as several meters downwind. Sands and even small gravel particles are transported in saltation; the wind puts finer particles directly into suspension.

In the oceans, sediment is moved not only by unidirectional currents but also by oscillatory flow resulting from the passage of wind-generated waves at the sea surface. Moreover, unidirectional and oscillatory flows can be superposed to produce combined flows, in which the water at the bed has an oscillatory motion but undergoes net movement in some direction. The concepts of threshold, bed load, and suspension apply to oscillatory, combined, and unidirectional flows.

Discharge and Deposition

The time rate at which sediment is carried across some planar section, real or imaginary, normal to the flow direction, is called the sediment transport rate or sediment discharge. It is expressed as either mass, weight, or volume of sediment per unit time. The transport rate of the bed load and of the suspended load can be considered either separately or together as the total transport rate. The sediment transport rate, expressed per unit width normal to the flow direction, is a steeply increasing function of the bed shear stress or the flow velocity. The great mathematical complexity of turbulent flow carrying discrete solid particles has hindered the development of theories to predict the sediment transport rate as a function of sediment characteristics and flow conditions. Many formulas or equations, often called sediment discharge formulas, have been developed to predict the sediment transport rate. All have been built around one or another physically plausible mechanism that provides the general mathematical form of the equation. The specific form of the equation is then found by fitting or adjusting coefficients in the equation so that the equation conforms to some set of actual measured data on transport rates. None of these sediment discharge formulas is significantly better than any other, and there can be differences by as much as a factor of ten in predicted transport rates.

The volume concentrations of suspended sediment in most flows of water in rivers or the oceans, as expressed in volume of sediment per unit volume of water-sediment mixture, are usually no more than a few percent. In certain situations, however, water-saturated masses of sediment can begin to flow even on a gentle slope of 1 or 2 degrees by liquefaction, either spontaneously or induced by earthquake shocks. Such flows, called debris flows, may have sediment concentrations of up to 70 percent by volume. Debris flows can be formed either on the land surface or underwater.

A fluid medium is not essential for sediment transport. The force of gravity is often sufficient to induce the movement of significant quantities of sediment, at least for short distances. Soil creep on hillsides is the primary example of this influence. The collapse of sediment structures such as sand dunes and the wholesale movement of sediments of many sizes in a landslide are also well-known events.

Glaciers are locally responsible for the transport of significant volumes of sediment. Glaciers derive most of their sediment load from bedrock erosion or preexisting sediment beneath the glacier. However, valley glaciers can carry on their upper surface large quantities of sediment that fall from the valley walls. Glaciers are far less selective of the sediment sizes they carry than are flows of water or air, which is understandable given the order-of-magnitude difference between the viscosities of liquid water and solid ice.

Transported sediment is ultimately redeposited in some way. Deposition is always associated with one or both of two kinds of changes in the flow. One of these is temporal. There is a net loss of load everywhere from the flow to the bed over time as the flow becomes weaker. An example is the redeposition of sediment picked up by a river flood as the flood subsides. The other change in flow, which is usually more important in building thick sediment deposits, is spatial. The flow becomes weaker in the downstream direction, causing the sediment transport rate to decrease in the downstream direction. The only way the sediment transport rate can decrease downstream is for sediment to go into storage at all points on the bed, thus building up the bed. An example is the expansion and weakening of flow in a river delta, where a river meets a large lake or the ocean.

Study of Sediment

Sediment transport is studied in natural flows, in laboratory tanks, and by computer modeling. Laboratory studies have the advantage that sediment transport conditions can be closely controlled, so the various factors considered important in determining the mode and rate of sediment movement can be varied independently. The disadvantage of laboratory work, aside from the necessarily small physical scales of the flow, is that the phenomenon may be too simplified to simulate natural flow environments well. In the laboratory, sediment transport and deposition are studied mostly in open channels, called flumes, in which a flow of water is passed over a sediment bed. Flumes range from a few meters to more than 100 meters long, from about 10 centimeters to a few meters wide, and from several centimeters to about 1 meter deep. The water is usually recirculated from the downstream end to the upstream end to form a kind of endless river. The sediment may also be recirculated, or it may be fed at the upstream end and caught in a trap at the downstream end. In laboratory flumes, the sediment movement may be observed visually or photographed either through a transparent sidewall or from the water surface if suspended sediment is not abundant.

Measurement of sediment transport rates is notoriously difficult, not only in natural flows but also in the laboratory. In flumes and some specially instrumented rivers, it is possible to pass the flow over or through a section where all the sediment in transport as bed load is extracted for measurement. In general, however, bed load must be measured using traps of various designs that are lowered to the bottom, opened for a certain time interval, and brought back to the surface. The problem is that such samplers tend to distort the flow and, therefore, the sediment movement in their vicinity, and there is usually no good way of estimating and correcting for that effect.

In both the laboratory and nature, suspended load is usually sampled by extracting samples of the suspensate (water plus sediment) at several levels in the flow by sucking or siphoning through small-diameter horizontal tubes with their openings facing upstream. Care must be taken to match the extraction speed to the local flow speed to minimize overcatching or undercatching. Both in rivers and shallow marine environments, such as beaches, the direction and rates of sediment movement have been estimated by the placement of plugs of sediment tagged with short-lived radioisotope tracers and by taking closely spaced sediment samples in the surrounding area later, after the tagged sediment has been dispersed by transport. This kind of measure has the advantage of integrating the transport over a long time.

Significance

Because most of Earth’s topography is produced by erosion, transport, and deposition of particulate material derived from the weathering of bedrock, consideration of sediment transport is essential in any attempt to account for how the landscapes of Earth develop through time. Even in the driest of deserts, most geological work is accomplished by running water: On the rare occasions of heavy rains, the abundant loose sediment produced by weathering is entrained and transported by floods. Erosion and sediment deposition in rivers and estuaries in the processes of channel shifting lead to great changes in river geometry on time scales ranging from days to decades. These changes often make maintaining navigable channels in rivers and harbors difficult. The useful water-storage life of reservoirs is determined by the sediment transport rate from upstream in relation to the reservoir capacity. Because of saltation, reservoir life is typically limited to a few decades rather than hundreds of years. Rivers downstream of reservoirs commonly experience a substantial lowering of the level of the riverbed as the now sediment-free flow seeks to pick up new sediment, leading to what is called degradation.

In coastal zones, sediment is transported by tidal currents in shifting tidal channels and by nearshore currents of various kinds that flow parallel to open shorelines. Sediment movement along open shorelines is augmented greatly by the effect of waves: The strong oscillatory bottom flows produced by waves tend to suspend the sediment, which then can be carried for some distance even by unidirectional currents too weak to move sediment by themselves. Understanding sediment erosion, transport, and deposition rates is essential for dealing with problems of shoreline changes in the coastal zone.

Earth's sedimentary record is the outcome of sediment transport and deposition in the same ways observed in the twenty-first century. Understanding modern sediment movement and deposition processes is essential in interpreting the ancient depositional environments in which Earth’s sedimentary record was produced. Understanding the controls on the complex geometry of sediment bodies—ultimately a matter of sediment transport and erosion—plays an important role in petroleum exploration because petroleum is commonly found in porous sedimentary rocks formed by particle-by-particle deposition of sediments in river systems and oceans.

In the twenty-first century, as concern for the effects of global climate change remains high, attention must be paid to how these changes will affect sediment transport and deposition. Rising global temperatures and increased precipitation are expected to impact erosion rates, leading to more sediment. Glacial melting and the thawing of permafrost will also increase sediment. Changes to the timing of seasonal flows in the spring, whether transport is done by snow or rain, and the intensity of summer flows are all factors that affect sediment transport. These changes will, in turn, affect sediment deposition as well. Further, sediment transport and deposition changes will vary with conditions and regions. 

Principal Terms

bed load: sediment in motion in continuous or semicontinuous contact with the sediment bed by sliding, rolling, or hopping (saltation)

bed shear stress: the force per unit area exerted by the flowing fluid on the sediment bed, averaged over an area that is large compared to individual bed particles

competence: a concept that expresses the ability of a fluid stream to move particles of a given size

debris flow: a flowing mass consisting of water together with a high concentration of sediment with a wide range of sizes, from fine muds to coarse gravels

saltation: a mode of sediment transport in a moving fluid, in which sediment particles move forward in discrete increments rather than continuously, often as one particle bumps into another and drives it forward

sediment discharge: the rate of transport of sediment past a planar section normal to the flow direction, expressed as volume, mass, or weight per unit time; also called sediment transport rate

sediment discharge formula: a formula or equation designed to predict the sediment discharge that would be observed for a given combination of flow conditions and sediment characteristics

suspended load: sediment in motion above the sediment bed, supported by the vertical motions of turbulent eddies

threshold of movement: the conditions under which a flow is just strong enough to move the sediment particles at the surface of a given sediment bed

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