Miwa Yokokawa

Sand Particle Movement on Migrating Combined-Flow Ripples

ABSTRACT We studied sand particle movement on migrating combined-flow ripples in an experimental flume. The water-surface waves propagated against the flow in the flume. The resultant combined-flow ripples have characteristic rounded profiles. Motion pictures reveal that vortices, generated only close to the lee sides of ripples, are responsible for the rounded profile. These vortices excavate rounded troughs and lift sand particles. The lee sides are nourished by the sand, contributing a convex-upward profile. The flow that separates at the crests is dragged into the vortices at the troughs, so that there is no clear reattachment point with focused erosion, as is the case for current ripples. Sand that fulls to the bed from a floating cloud over the stoss side nourishes the convex upward roun ed stoss side. The relative strength of the oscillatory flows controls the positions at which the vortices lift sand particles to deposit, and determine whether the ripple profiles are rounded or sharp. A lower relative oscillatory flow velocity yields smaller vortices, which cause rounded ripple profiles. Conversely, a higher relative oscillatory flow velocity results in larger vortices and sharper crests. The movement of sand particles is unlike that on ripples beneath either purely oscillatory flows or unidirectional flows.

Grain fabric of experimental gravity flow deposits

Abstract Grain fabric of deposits accumulated from a high-density surge-type gravity (turbidity) current in an experimental flume was measured. Vertical sequential change (0.2-mm interval) in imbrication shows that a bed can be divided into lower, middle, upper and uppermost parts. The lower part is characterized by both up-current and down-current imbrication with a wide range of angles. Dominant up-current imbrication and rare down-current imbrication characterize the middle part. The imbrication angle of this part tends to be smaller than in the lower part. The upper part is represented by intervals with up-current imbrication. Nearly flat imbrication is dominant in the uppermost intervals. Statistically significant preferred orientation was observed from the lower, middle and upper parts, and it deviates up to 13° in both clockwise and anticlockwise directions from the current direction. The lower and middle parts of the bed may correspond to the Bouma A-division judging from the wider range of imbrication angles and the presence of down-current imbrication which have been reported from natural turbidite beds. The upper part, which is characterized by up-current imbrication, is interpreted to be the Bouma B-division. The uppermost interval may coincide with the D-division. The episodically appearing down-current imbrication in the lower and middle parts can probably be attributed to oscillation of an interface between a denser basal layer and superjacent low-density layer of the turbidity current. Measurement of grain orientations in this and previous studies implies that at least 30° of deviation from the flow axis should be considered for paleoflow analyses based on grain fabrics.

Grain-size distributions of sediment carried by single transportation modes in an experimental microdelta system

Abstract Grain-size distributions of three sands in single transport-mode experiments after transport downstream from the brink of an experimental microdelta, are neither gaussian nor log-hyperbolic. Instead, the sands transported by avalanching and by suspension downstream of the microdelta derive many of their grain-size attributes from those of the sand fed into the flume at its upstream end. Relationships between the grain-size distributions of individual transport modes and the texture of the ‘original sand’ are best demonstrated by normalizing the data to the grain size of the ‘original sand’. The experimental data suggest that the suspension process does not prefer a peculiar size among the grains which can be suspended; i.e., grains present in the suspended load are represented in the same proportion as in the ‘original sand’. Avalanching and saltating sands, however, show modified size distributions, even over short distances.