Abstract

A flume study indicates that grainflow on slipfaces accounts for most cross-strata formed in unidirectional, shallow-water flows. The slipfaces studied were on small megaripples and delta-like steps (0.06-0.28 m high). During intermittent avalanching, at relatively low flow velocities, periods between avalanches were marked by grainfall onto the slipface, the intensity of which was greatest near the brink of the slipface and increased with current velocity. Nearly all grainfall deposits, however, were incorporated into subsequent grainflows. Grain flow cross-strata were made up of relatively distinct layers, at least near the base of the slipface. Continuous avalanching at high flow velocity was marked by a steady stream of grains forming more poorly defined cross-strata. Although the fundamental cause of grain flow is the gradual buildup of sediment on the upper slipface to the angle of initial yield, four other processes were recognized as promoting avalanching: 1) migration of superimposed bedforms to the brink, 2) generation of turbulent pulses upstream of the brink, 3) lee-eddy impingement on the lower slipface, and 4) extension of the lee eddy above the brink. The lee eddy proved very significant in slipface processes by redistributing grainfall sediments and both promoting and impeding grainflow. Regression analyses showed that the slipface advance per avalanche, S a , is strongly correlated with the slipface height, H, expressed approximately by S a = 0.060H. In addition, S a is a direct function of the rate of slipface advance, V b . The relationship among S a , H, and V b can be expressed as S a /H = 0.0385[1 - 0.134 (min/cm) V b ] (super -1) . Cross-strata dip angles between 28 degrees and 34 degrees show no systematic relation to H and V b , but dip angles greater than 34 degrees occurred only when both H and V b were small, and dip angles less than 28 degrees occurred only when both H and V b were large.

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