12.1: Introduction to Bed Configurations
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A striking characteristic of the transport of granular sediment over a bed of the same material by a turbulent flow of fluid is that in a wide range of conditions the bed is molded into topographic features, called bed forms, on a scale that is orders of magnitude larger than the grains. Little ripples at one’s feet at the seashore, or on a dry river bed, or in the desert, and gigantic dunes in the desert and (even more common, but not apparent to the casual observer) in large rivers and the shallow ocean—all of these are examples of bed forms. Generations of scientists and engineers have marveled at the rich and confusing variety of these features.
First I will introduce some terminology. The overall bed geometry that exists at a given time in response to the flow (the bed configuration) is composed of individual topographic elements (bed forms). The aggregate or ensemble of similar bed configurations that can be produced by a given mean flow over a given sediment bed is the bed state: The bed configuration differs in detail from time to time, and the bed state can be considered to be the average over the infinity of configurations that are possible under those conditions. The term bed phase can be used for recognizably or qualitatively different kinds of bed configurations which are produced over some range of flow and sediment conditions and which are closely related in geometry and dynamics. Finally, the term bedform (one word) is widely used, indiscriminately, for all four of the foregoing aspects of the bed geometry.
Bed configurations that are common in natural flow environments can be generated by purely unidirectional flows, combined flows, and purely oscillatory flows. I pointed out in Chapter 6 that even purely oscillatory flows in natural flow environments can be more complex than those with only one oscillatory flow component, and that a wide range of oscillatory flows can be superimposed on a unidirectional current. (This would be a good point at which to go back and review the nature of oscillatory and combined flows.)
You can well imagine, then, how wide a range of bed configurations we should expect to have to deal with if we endeavor to make an inclusive survey of bed configurations. The enormous range of flows that can generate bed configurations, together with the complex dynamics of the response of the bed, makes for highly varied geometry of the resulting bed configurations. In one sense, though, this is fortunate for sedimentologists, because it provides a great variety of different sedimentary structures which can be used in attempting to make paleoflow interpretations!
Both engineers and geologists have been making laboratory experiments on bed forms for over one hundred years, as well as watching their movement in natural flow environments. Understanding of unidirectional-flow bed configurations is fairly good by now, although by no means perfect. Work on oscillatory-flow bed configurations is less well advanced, I think owing to the difficulty of arranging oscillatory flows with long oscillation periods in the laboratory. Finally, combined-flow bed configurations have still not been much studied.
If the flow changes with time, the bed configuration adjusts in response. In natural flows, equilibrium between the bed and the flow is the exception rather than the rule: usually the bed configuration lags behind the change in the flow. Such disequilibrium is a major element of complexity that makes relationships among bed phases much more difficult to decipher, but its effects are very important in natural flow environments.
In the natural environment most bed forms are seen in sands, but they are produced in silts and gravels as well. Of greatest interest to geologists, oceanographers, and hydraulic engineers are bed forms produced by flows of air or water over mineral sediments with equant grain shape, but a far wider range, important in many engineering applications, can be produced by flows of other fluids with other densities and viscosities over sediments less dense or more dense than the common mineral sediments.
Many natural scientists believe (and I am among them) that there must be enormous numbers of Earth-like planets throughout the universe. The field of extraterrestrial planets is a rapidly growing field nowadays, and it would not surprise me to learn, in the not too distant future, that such Earth-like planets are being discovered. In studying a physical phenomenon like bed configurations, there is an element of danger in restricting our consideration only to the few points in the spectrum of density ratios with which we have at hand: sand in water on Earth; sand in air on Earth; sand in the Martian atmosphere of the Venusian atmosphere; see Figure 8.1.5, in Chapter 8). In a sense, there is nothing special about those particular points in the spectrum!
Apart from their intrinsic scientific interest, bed forms are important in both geology and engineering. Large subaqueous bed forms many meters high can be obstacles to navigation, and their movement can be a threat to submarine structures. Engineers are interested in bed configurations partly because they play an important role in determining the sediment transport rate, but perhaps mainly because of their importance in determining the resistance which a channel presents to a flow. For example, predicting the depth of flow in a channel built with a given slope and designed to carry a given water discharge necessitates knowing the bed configuration to be expected. Sedimentologists have given attention to bed configurations mostly because of their role in the development of stratification in sedimentary deposits; bed forms are one of the most useful tools available for interpreting ancient sedimentary environments.
The status of observations on bed configurations is good, but there is much room for further improvement. It is easy to observe bed configurations in steady unidirectional and simple bidirectional oscillatory flows in laboratory channels and tanks. But there is still a great need for further laboratory work, in part because the usually small width-to-depth ratios of tanks and flumes tend to inhibit full development of the three-dimensional aspects of the bed geometry, but also, and more importantly, because not much work has been done with multidirectional oscillatory flows, and, especially, combined flows. And even the largest of laboratory experiments are restricted to flow depths at the lower end of the range of natural flow depths. In nature, on the other hand, observations on bed configurations are limited by practical and technical difficulties, and the flows that produce them are usually more complicated.