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10.1: Introduction to the Movement of Sediment by Water Flows

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  • A simple flume experiment on sediment movement by a unidirectional current of water in a flume serves to introduce the material in this chapter. Place a layer of sediment in the flume, level it to have a planar surface, and establish a uniform flow at a certain depth and velocity. Gradually, in steps, increase the strength of the flow beyond the condition for incipient movement. The magnitude of the flow strength relative to what is required for incipient movement of the bed sediment is conventionally called the flow intensity, and is usually taken to be the ratio \(\tau_{\text{o}}/\tau_{\text{o}c}\) (or, what is the same, \(u_{*}/u_{*c}\)), where the subscript \(c\) denotes the threshold (“critical”) condition.

    At first the particles move as bed load, by hopping, rolling, and/or sliding. Particle movement is neither continuous nor uniform over the bed: brief gusts or pulses of movement affect groups of particles locally, and seemingly randomly, on the bed. Particles move a short distance, stop, and then move again. Even when they are moving, they are generally not moving as fast as the fluid near the bed surface.

    As the flow becomes stronger, some of the particles moving near the bed are lifted upward by upward-moving turbulent eddies and travel for more or less long distances downstream as suspended load. The stronger the flow and/or the finer the sediment, the greater is the concentration of suspended sediment, the higher it can travel in the flow, and the longer it moves downstream before returning to the bed. Of course, the particles are not really suspended in the way that a picture is suspended on the wall by a nail; they are continuously settling through the surrounding fluid, and eventually they return to the bed. If the sediment is fine and the flow is strong, however, the particles are likely to travel for the entire length of the flume.

    If you introduce a small quantity of very fine clay-size sediment into the flow, you would find that it too travels in suspension, but the essential difference between this part of the suspended load and the coarser part you observed before is that even if you add large quantities of it to the flow, it would not be represented in the bed. Fine sediment of this kind is called wash load. Extremely fine particles, in the size range of small fractions of a micrometer, can be kept in effectively permanent suspension, because their mass is so small that they can be moved about by the random bombardments of the molecules constituting the fluid itself. These random motions are a manifestation of Brownian motion.

    For flow intensities not much above threshold, it is fairly easy to observe the particle motions in the bed load, provided that you have clear water, good lighting, and sharp eyes (close-up slow-motion vision would be a big help), but as the flow intensity increases, the concentration of particles in motion as bed load increases, and it becomes difficult or impossible to observe the motions of individual particles. Unfortunately, no one yet seems to have devised a good way to see into the dense layer of moving bed-load particles at high flow intensities to study its characteristics. This important aspect of sediment transport remains contentious and inadequately studied.

    To gain an appreciation of a rather different mode of sediment movement, you need to resort to a wind tunnel. It is not difficult to build one: all you need to do is construct a rectangular duct resting on the floor, leading from a flared entrance at the upwind end to a large empty chamber at the downwind end, with an exhaust fan in the side of the chamber to create a wind through the duct. A louver just downwind of the fan lets you adjust the wind velocity. Especially when the ratio of sediment density to fluid density is very large, as with quartz sand in a wind tunnel, sediment particles are entrained impulsively by the flow at middling to steep take-off angles and move downstream in long arching trajectories little affected by the fluid turbulence to make impact with the bed at low angles. This characteristic mode of movement, known as saltation, is especially important in the transport of sand by wind. Its manifestation in transport of particles that are not much denser than the transporting fluid, however, is much less striking or distinctive.