The channelized flow of turbidity currents with application to Monterey Deep-Sea Fan Channel

Abstract

Whenever a turbidity current is confined to a channel its upper surface has a cross-channel slope, owing to the combined effects of the Coriolis and centrifugal forces. This cross-channel surface slope for channel full flows may cause a difference in the heights of the levees that have developed on opposite sides of the channel. An equation is developed that balances the Coriolis and centrifugal forces against the pressure force that results from the surface slope. This equation can be used to calculate curves of average velocity versus density, the two variables in the equation, and the two principal unknowns of turbidity current flow. The autosuspension mechanism of turbidity current flow provides a lower limit on the range of permissible velocities and, hence, by way of the equation, on the range of densities. Grain-to-grain friction becomes prohibitively large for densities greater than about 1.18 g/cm3 so that an upper limit is placed on the range of permissible densities and velocities. The slope equation is applied to fathograms of twelve crossings taken along a 100-km stretch of the channel that extends from Monterey submarine canyon off the coast of central California and crosses Monterey fan. This portion of channel consists of a large meander curving to the right followed by a broad curvature of the channel to the left. The cross-channel surface slope of the flow, as reflected in the difference in the heights of the levees, behaves as expected in response to the combined Coriolis and centrifugal forces. Solutions of the slope equation for the twelve crossings, in view of the permissible densities, give velocities ranging between 600 and 2000 cm/sec. This agrees in magnitude with the velocities obtained from ordered sequences of submarine cable breaks. Calculations of curves of velocity versus density with a modification of the Bagnold [1962] equation, based on the concept of autosuspension and utilizing different channel parameters, agree favorably with the results obtained with the cross-channel slope equation. Using velocities deduced from the levee height differences, it is found that a reasonable range of Pleistocene slump volumes in Monterey Canyon is capable of generating channel-full turbidity currents of adequate length and time of passage. The simultaneous solution of the cross-channel slope equation and the Bagnold equation, assuming a wide range of sediment discharge rates, indicates that there has not been a substantial amount of erosion of the channel since the levee height differences were developed.