EXPERIMENTAL INVESTIGATION OF EQUILIBRIUM OSCILLATORY AND COMBINED FLOW BED FORMS

The response of cohesionless sediment to oscillatory and especially combined flow conditions is presently poorly understood. As a result, a number of aspects of the shallow marine sedimentary record remain unresolved. In particular, the origin of hummocky cross-stratification (HCS) and the nature of combined flow bed forms and stratification are two of the most fundamental issues. Recent experimental, theoretical, rock record, and modern environments studies have greatly advanced our understanding of various aspects of oscillatory and combined flow sedimentation. Nevertheless, these studies poignantly illustrate the need for a more detailed knowledge of the various equilibrium oscillatory and combined flow bed forms and configurations generated under a wide range of natural flow conditions.

This experimental study investigates equilibrium bed forms and bed configurations produced under oscillatory and combined flow conditions. Numerous runs were conducted in a 15 m long, 1.25 m wide, 0.65 m deep wave tunnel while adjusting the variables governing the flow, sediment, and fluid. Conditions explored are: oscillatory velocities (Uo) ranging between 20-120 cm/s, unidirectional velocities (Uu) ranging between 0-20 cm/s, oscillation periods of 9.4 sec and 7 sec, and grain sizes 0.11 mm and 0.17 mm. Waves and current are colinear.

Preliminary results are consistent with existing phase diagrams (Southard et al., 1990; Arnott and Southard, 1990). With increasing Uo bed phases are: rolling grain ripples, orbital vortex ripples, and gently undulating quasi-planar bed. For a given Uo and increasing Uu, the bed generally becomes progressively more asymmetrical and three dimensional, but eventually is planed out. Asymmetric hummocky bedding surfaces develops at moderate (55 cm/s) to high (100 cm/s) oscillatory speeds. At high Uo, low amplitude, long wavelength 2.5D hummocks are the equilibrium bed forms when Uu greater or equal to 15 cm/s is added. With moderate Uo and Uu between 0-10cm/s, higher amplitude, shorter wavelength 2.5D hummocks coexist with vortex ripples and combined flow "dunes". Conceivably these hummocks might produce anisotropic HCS if the bed was aggrading.