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ABSTRACT: Experiments on Combined-Flow Bed Forms and Stratification

John B. Southard

Experiments on equilibrium bed configurations in fine sands under combined flows have been made in a large oscillatory-flow water tunnel. The oscillatory-flow component had oscillation periods of 8-10 sec and maximum oscillatory flow velocities of up to almost 1 m/sec; the unidirectional flow component ranged from zero to about 0.25 m/sec.

The bed configuration varies widely in scale and geometry as a function of flow conditions. At combinations of relatively low unidirectional and oscillatory velocities, ripples become larger (over 1 m in spacing) but without a sharp break in size with increasing velocity. There is a striking increase in the degree of asymmetry of the bed forms with increasing unidirectional velocity; a unidirectional velocity of only 0.05-0.10 m/sec produces strong asymmetry of the ripples. Exploratory work on stratification styles produced by the existence of relatively large ripples at combinations of high oscillatory velocities and nonnegligible unidirectional velocities during slow net aggradation of the bed show a characteristic hummocky-swaly geometry of cross-stratification with noticeable asym etry of dip directions.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990