Abstract

Fine-grained tailings discharged from a taconite-ore processing operation near the shore of Lake Superior produced turbidity currents that transported the sediment from a small delta into deep water at Silver Bay, Minnesota. Deposition over nearly 20 years produced a sublacustrine fan with two prominent channels. During 1972 and 1973, a current meter anchored 5 m above the lake floor adjacent to one of the channels recorded episodic turbidity-current flow events lasting as long as two weeks. To understand flow parameters for turbidity currents better, a short-term experiment within a channel on Reserve Fan in 1975 measured those variables not previously directly observed for channelized turbidity currents: flow thickness, flow density, and concurrent velocity. The observed flow thickness, approximately 16 m, is nearly four times the channel depth. Calculations using the average flow speeds (8 to 12 cm/sec) and the dilute concentration of the flow as measured during the experiment yield a value for the drag coefficient that is in remarkable agreement with estimated values commonly used for deriving speeds of turbidity currents using dimensions of submarine channels and properties of the sediments.

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