River Bed Forms

Bedforms, produced by water or air flows in natural environments and artificial channels, are a distinctive aspect of the transport of granular sediment. They strongly affect the magnitude of bottom friction felt by the flow. Because of their great variety as a function of flow conditions, bedforms are valuable in interpreting the depositional conditions of ancient sedimentary deposits.

Characteristics

A striking feature of the transport of loose granular sediment over a bed of the same material by a turbulent flow of fluid such as air or water is that in a wide range of conditions of flow and sediment size, the bed is molded into topographic features, called bedforms, on a scale ranging from hundreds to millions of times larger than the grains themselves. Examples of such bedforms are sand ripples at the seashore or on a dry riverbed, sand dunes in the desert, and (less apparent to the casual observer) large underwater sand dunes in rivers and the shallow ocean. The overall geometry of a sediment bed molded by a flow of fluid is called the bed configuration. Bedforms are individual elements of this configuration. The term bedform includes both the overall geometry and the individual elements of that geometry.

The enormous range of bed-form-producing flows, together with the complex dynamics of the response of the bed, makes for striking variety in the scale and geometry of bedforms. Scales typically span a range of five orders of magnitude, from a few centimeters to more than 1,000 meters in spacing. Bedforms may appear as long ridges or circumscribed mounds, and their crests may be rounded or sharp. Most bedforms are irregular in detail, but elongated bedforms tend to show a more or less strong element of regularity in their overall arrangement, some even being perfectly regular and straight-crested. Elongated bedforms tend to be oriented transversely to the direction of flow, although flow-parallel forms are produced under certain conditions, and forms with no strongly preferred orientation are produced in some flows. Most bedforms are approximately wave-shaped and are often likened to waves, but they are waves only in a geometrical sense, not in a functional sense.

The most common bedforms are in sands (sediments having a mean particle size between about 0.1 millimeter and two millimeters), but bedforms are produced in silts (sediments having a mean particle size between about four micrometers and 0.1 millimeters) and gravels (sediment having a mean particle size greater than two millimeters) as well. Bedforms produced by flows of air or water over mineral sediments in natural flow environments are of greatest interest to geologists. A far wider range of bedforms can be produced by flows of fluids with other densities and viscosities over sediments that are more or less dense than the common mineral sediments, which have densities mostly in the range of 2.5 to 3.0 grams per cubic centimeter.

Types of Fluid Flow

Bedforms are made by unidirectional flows of air or water, as in rivers and tidal currents and under sand-moving winds, and by oscillatory flows, as on the shallow sea floor beneath wind-generated surface waves, which cause the water at the bottom to move back and forth with a period the same as that of the waves and with horizontal excursion distances of a few centimeters to a few meters. Bedforms are also made by what are called combined flows, which are superpositions of unidirectional and oscillatory flows. Such combined-flow bedforms are not as well understood as those made by unidirectional and oscillatory flows, but they are common in the shallow ocean. Bedforms made under wind are called subaerial or aeolian bedforms, and bedforms made underwater are called subaqueous bedforms.

It might seem that the natural mode of sediment transport would be over a planar bed surface. In certain ranges of flow, a planar transport surface is indeed the stable bed configuration, but technically, such a plane bed is a bed configuration with no bedforms. In reality, plane-bed transport is the stable configuration only under a very limited set of conditions, whereas rugged bedforms cover the transport surface over a wide range of conditions in both oscillatory and unidirectional flows. Why such bedforms develop at all on transport surfaces is poorly understood. In certain ranges of flow, the planar transport surface is unstable in the sense that small bed irregularities of the kind that can be built at random by the plane-bed sediment transport become amplified to grow eventually into bedforms rather than being smoothed out again. The physics behind this instability is chaotic and complex and is still not clearly understood. An essential element of this complexity is that there is a strong interaction or feedback between the bed configuration and the flow. The flow molds the bed configuration, but the bed configuration, in turn, affects the nature of the flow.

In unidirectional flow, current ripples are formed in fine sands as soon as the current is strong enough to move the particles, and they persist to moderate currents of about 0.5 meters per second. In a vertical cross-section parallel to the flow, these current ripples are triangular in shape, with downstream slopes about equal to the angle of repose of sand underwater (about 30 degrees) and with gentler upstream slopes. In the top or horizontal view, they are oriented mostly transverse to the direction of flow and are irregular in detail. Their spacings range from ten to twenty centimeters, and their heights are a few centimeters. This characteristic size changes little with flow strength or sediment size. With increasing unidirectional-flow speed, ripples give way to dunes, which are geometrically fairly similar to ripples but are of much larger scale, ranging from meters to thousands of meters in spacing and tens of centimeters to tens of meters in height, depending in a complex and poorly understood way on conditions of flow as well as sediment size. With a further increase in flow speed to about one meter per second, dunes are replaced by a plane-bed configuration. The sequence of bed configurations with increasing unidirectional flow speed over coarse sands is different from that over fine sands: Transport is over a plane bed at flow strengths just above the threshold for sediment movement, and then dunes develop with further increase in flow strength. In sediments coarser than about 0.6 millimeters, current ripples are not formed in any range of flow speeds.

Current ripples and dunes move upstream or downstream at a speed far lower than the flow speed. This movement proceeds by erosion of sediment on the upstream side of the bedform and deposition on the downstream side. When an individual ripple or dune can be watched carefully for a time, it is seen to change its size, shape, and speed irregularly, eventually disappearing by being absorbed into a neighboring bedform. Offsetting this loss of bedforms is the production of new ones by a kind of subdivision of one larger form into two smaller ones. Inflows of water with a free upper surface, like rivers and tidal currents, undulatory bedforms called antidunes make their appearance at high flow speeds and shallow flow depths. Antidunes, so named because they tend to move slowly upstream by erosion of sediment on the downstream sides and deposition on the upstream sides, come about by a complex effect of standing surface waves (waves that move upstream about as fast as the flow is moving downstream) on the sediment bed.

In oscillatory flow, regular straight-crested bedforms called oscillation ripples, with sharp crests and broadly rounded troughs, are formed as soon as the sediment begins to be moved. At their smallest, their spacing is a few centimeters, but they grow in size to more than one meter in spacing as the period and amplitude of the oscillatory motion increase. In fine sands, these larger ripples become irregular and mound-shaped, but observations are not yet adequate for a detailed description. When the maximum flow speed during a single oscillation reaches about one meter per second, the bedforms are washed out to a plane-bed mode of transport. Combined flows produce a whole range of ripples intermediate between current ripples and oscillation ripples.

If the flow changes with time, as is the rule rather than the exception in natural flows, the bed configuration adjusts in response. Usually, the bed configuration lags behind the change in the flow, resulting in the bed configuration being more or less out of equilibrium. Dunes formed by reversing flow in a tidal channel are a good example of such disequilibrium. Commonly, the weaker of the two flows (whether ebb or flood) modifies the shape of the dunes but does not reverse the asymmetry. Bedforms on a riverbed during the passage of a flood also tend to lag behind changes in the flow.

Study of Bedforms

Bedforms can be observed and studied in natural flow environments, laboratory tanks, and channels. Each approach has advantages and disadvantages. In nature, observations on bed configurations are limited by various practical and technical difficulties. In laboratory tanks and channels, the bedforms can be studied much more easily, but water depths are generally unnaturally shallow.

A laboratory open channel in which a flow of water is passed over a sediment bed is called a flume. Flumes range from a few meters to more than 100 meters long, from about ten centimeters to a few meters wide, and from several centimeters to about one 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 a flume, it is fairly easy to measure the profile of the bed configuration with a mechanical pointer gauge or with a sonic depth sounder, and the bedforms can be studied visually and photographed through the sidewalls and also from the top when the absence of surface turbulence permits. Time-lapse motion pictures of bed-form movement are also instructive. Oscillatory-flow bedforms can be studied either in long open tanks, in which water waves are passed over a sand bed to produce oscillatory flow at the bed, or in closed horizontal ducts, in which water is pushed back and forth in a regular oscillation between tanks at the ends of the duct.

In some natural flow environments, such as rivers and tidal currents, the geometry of the bedforms can be studied when they are exposed to low water. Observations of the bedforms when they are being molded by the flow, however, are more difficult. If the flow is not too strong, divers can make direct observations. Profiling of the bed geometry along lines or even across wide areas is usually possible using various sonar techniques, whereby the travel times of sound pulses reflected from the bed are converted to water depths. If the water is clear enough, bottom photographs of small areas can be taken. Current velocities can be measured with current meters anchored on the bed. The movement of large bedforms is difficult to measure because bed-form speeds are slow, and usually, no fixed reference points are available.

The status of observations on bed configurations leaves much to be desired. Even in laboratory channels and tanks, where the major outlines are fairly well known, there is much room for further work for two reasons: The narrowness of the flow tends to distort the three-dimensional aspects of the bed geometry, and little work has been done on bedforms in combined flows. There is a need for more observations on the geometry and movement of bedforms in natural flows as a function of flow strength, as well as on the effect of disequilibrium.

Significance

Bedforms are ubiquitous in natural flow environments. They are most apparent to the casual eye in fields of sand dunes in deserts and in certain coastal environments where the wind molds available loose sand into dunes. They are widely present but less obvious in rivers and the shallow ocean. Apart from their intrinsic scientific interest as a widespread natural phenomenon, bedforms are of importance in both engineering and geology for various reasons. Large underwater bedforms many meters high in rivers and marine currents can be obstacles to navigation, and their movement can be a threat to submarine structures. Also, the inexorable movement of desert dunes or coastal dunes can bury roads and buildings. In other engineering applications, technology imitates nature, and various processes to be carried ou,t rely on the flow of a particle-bearing fluid medium, as is the case in fluidized bed processes. A comprehensive understanding of the behavior of the particle load within the fluid is essential to enhancing and ensuring the efficiency of those processes.

The rugged topography of ripple and dune bedforms leads to a pattern of flow over each bedform in which the pressure on the upstream surface is relatively high and the pressure on the downstream surface is relatively low, much like an unstreamlined motor vehicle on the highway or a house in a strong wind. This pressure difference adds greatly to the force the flow exerts on the bed and, conversely, to the force the bed exerts on the moving flow. Hydraulic engineers have spent much effort studying the effect that this resistance force has on the depth a river assumes when it is given a particular rate of flow to carry. A river with a planar bed can pass a given flow rate at a shallower depth and greater velocity than can a river with a bed roughened by large dunes, which exert a large resistance force on the flow and make the velocity smaller and the depth greater.

Geologists have given attention to bedforms partly because of their effect on the geometry of the stratification that develops as a sediment bed is deposited while bedforms are active on the sediment surface. It is often possible to tell the kind of bedform that was present just by examining the stratification in a sedimentary rock like a sandstone. If the flow conditions responsible for making that kind of bedform are already known from laboratory experiments or from observations in modern natural flow environments, the depositional conditions of that sedimentary rock (which may be geologically very ancient) can be interpreted. Such interpretations are one of the tools used in mapping the geometry of subsurface petroleum reservoirs in sedimentary rocks.

Principal Terms

aeolian deposits: material transported by wind

angle of repose: the natural angle formed between the horizontal plane and the side of a free-standing pile of sediment particles under the force of gravity in a fluid medium

antidune: an undulatory upstream-moving bedform produced in free-surface flow of water over a sand bed in a certain range of high flow speeds and shallow flow depths

bed configuration: the overall geometry of a sediment bed molded by sediment transport in a flowing fluid

bedform: an individual geometrical element of a bed configuration

combined flow: a flow of fluid with components of both unidirectional and oscillatory flow superposed on one another to produce a more complex pattern of fluid motion

current ripple: a small bedform, oriented predominantly transverse to the direction of flow, produced at low to moderate flow speeds in unidirectional water flows

dune: a large bedform, oriented predominantly transverse to the direction of flow, produced at moderate to high flow speeds

flume: a laboratory open channel in which water is passed over a sediment bed to study the nature of the sediment movement

oscillation ripple: a small to large bedform, oriented predominantly transverse to the direction of flow, produced at low to moderate flow speeds in oscillatory water flows

oscillatory flow: a flow of fluid with a regular back-and-forth pattern of motion

plane bed: a bed configuration without rugged bedforms produced in both unidirectional and oscillatory flows at high flow speeds

unidirectional flow: a flow of fluid oriented everywhere and at all times in the same direction

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