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Experiments were performed in the W. M. Keck Laboratory, California Institute of Technology, using a lucite channel 5 meters long, 50 cm. deep, and 15.4 cm. wide. Turbidity-current "surges" were produced by releasing suspensions of plastic beads from a lock at one end of the channel. The bottom of the flume was horizontal and before release of the suspension the water level was the same within the lock and the main channel; the experimental results may therefore be compared with the results obtained by Keulegan (1958) for saline surges. Experiments were performed at two depths (20 and 30 cm.), and two suspension concentrations (about 25% and 45% concentration by volume) were used. The sediment consisted of perfectly spherical plastic beads with a density of 1.52 and an av rage size of 0.18 mm. The results show that the shape of the head, the initial velocity, and the relation between the velocity of the head and its thickness and density are very similar to those reported for saline surges. The velocity of the head is given by the equation
where g^prime = g^Dgre/e, d2 is the thickness of the head, and C is a coefficient with a value of 0.75. In another series of experiments, with saline density currents, it was found that C varies little with increase in slope of the bottom, up to a slope of 0.04.
Two types of graded bed were formed, depending on the concentration of the suspension used. The "normal" type of graded bed, formed by surges with low sediment concentration, shows continuous grading within the bed at nearly all percentiles. The "coarse-tail" type of graded bed, formed by high-concentration surges, shows little grading through most of the bed, except for the coarsest 2-5 per cent of the size distribution. The differences between the two types of grading can be related to the depositional mechanics, which are revealed by slow-motion movies. In low-concentration flows, the bed is deposited layer by layer. In high-concentration flows, most of the bed is deposited first as an expanded or "quick" bed which continues to shear and to be disturbed by waves which form at the i terface between the bed and the flow above. As the bed comes to rest, the waves disappear and the upper surface becomes perfectly flat.
A 300-foot color movie of the experiments will be shown.
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