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The study of sediment transport has been handicapped greatly because most of the pertinent processes are too small and too fast to be observed, controlled, and measured. This is the reverse of the problem which confronts the student of tectonics: the processes which he wishes to clarify are too large and too slow to be observed, controlled, and measured. He attempts to solve this problem by scaling down the phenomena involved, building scaled models in which the model ratio of length is commonly on the order of 10-4 to 10-8, and other model ratios are set accordingly. The sedimentologist who would like a closer look at sediment transport can apply the same general methodology.
Scaled models have been employed, in the general field of hydrodynamics, for many years. The usual practice, however, is to select a reach of a river, or a stretch of beach, or a complete estuary, or perhaps even a complete ocean, and scale this study area down. In order to examine, in detail, sediment transport, the geologist must be prepared to design, build, and operate devices in which the model ratio of length is about 102: that is, he must scale up.
Consider a grain 0.4 mm in diameter. With a ratio of length = 102, one can design a grain which should behave as the tiny prototype does. The model grain will have a diameter of 4 cm. The shape and surface markings ideally should duplicate the original, and the density can be adjusted if necessary. The remaining criterion which must be satisfied is the Reynolds number, which involves fluid viscosity.
Once a few trials have been made, the experimenter can alter his variables so that more favorable velocities and viscosities are used. This alteration can be pushed until a change in behavior sets a limit; thereafter, all runs must be made on the correct side of the limiting value, but without necessarily matching the precise requirements of the model ratios. This permits considerable freedom in design and operation of the models.
Models of this kind show that turbulent regime holds, for settling grains, for Reynolds numbers of about 10, and more. The triple vortex trial which developes behind (above) the grain imparts both a spin and a spiral motion to the grain. Similar models can be operated and evaluated for grain pick-up.
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