Noritaka Endo

Collision Dynamics of Two Barchan Dunes Simulated Using a Simple Model

The collision processes of two crescentic dunes called barchans are systematically studied using a simple computer simulation model. The simulated processes, coalescence, ejection and reorganization, qualitatively correspond to those observed in a water tank experiment. Moreover we found the realized types of collision depend both on the mass ratio and on the lateral distance between barchans under initial conditions. A simple set of differential equations to describe the collision of one-dimensional (1D) dunes is introduced.

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.

Emergence of a Barchan Belt in a Unidirectional Flow: Experiment and Numerical Simulation

We observed the time evolution of dune fields in a water tank experiment and simulated it by using a simple model without taking complex fluid dynamics into account. The initial sand bed changed its form to transverse ripples, that is, dunes with straight crest lines perpendicular to the flow direction. Then crescentic dunes, called barchans, evolved from transverse ripples.We observed the time evolution of dune fields in a water tank experiment and simulated it by using a simple model without taking complex fluid dynamics into account. The initial sand bed changed its form to transverse ripples, that is, dunes with straight crest lines perpendicular to the flow direction. Then crescentic dunes, called barchans, evolved from transverse ripples.

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.

Response of Channels to Tectonic Activities: Flume Experiments for Bedrock Rivers Affected by Localized Uplift

Response of river channels to localized uplift was investigated using flume experiments where the base level was kept constant by a weir at the river mouth. We found three kinds of channel pattern responses; (1) channels that passed through the localized uplift area (generation of a water gap), (2) channels that avoided the uplift area, and (3) formation of a new river with the origin inside the uplift area. It is difficult to predict which type of response will happen in advance because localized uplift causes complicated changes in the channel network pattern, which results in variation of stream power due to interactions between several channels through confluence, avulsion, etc. In terms of the longitudinal profile of the channels, two types of responses were observed: (1) keeping a straight profile in the case of rivers flowing from much higher areas than the uplift area and (2) having a concave-up profile in the case of a newly formed channel running from the uplift area. A straight type of profile was also typical in the case of no uplift, so the concave-up profile is considered to be a “dynamic” equilibrium state that is realized between the descent due to fluvial erosion and the ascent due to uplift.

Simple stochastic cellular automaton model for starved beds and implications about formation of sand topographic features in terms of sand flux

A simple stochastic cellular automaton model is proposed for simulating bedload transport, especially for cases with a low transport rate and where available sediments are very sparse on substrates in a subaqueous system. Numerical simulations show that the bed type changes from sheet flow through sand patches to ripples as the amount of sand increases; this is consistent with observations in flume experiments and in the field. Without changes in external conditions, the sand flux calculated for a given amount of sand decreases over time as bedforms develop from a flat bed. This appears to be inconsistent with the general understanding that sand flux remains unchanged under the constant-fluid condition, but it is consistent with the previous experimental data. For areas of low sand abundance, the sand flux versus sand amount (flux–density relation) in the simulation shows a single peak with an abrupt decrease, followed by a long tail; this is very similar to the flux–density relation seen in automobile traffic flow. This pattern (the relation between segments of the curve and the corresponding bed states) suggests that sand sheets, sand patches, and sand ripples correspond respectively to the free-flow phase, congested phase, and jam phase of traffic flows. This implies that sand topographic features on starved beds are determined by the degree of interference between sand particles. Although the present study deals with simple cases only, this can provide a simplified but effective modeling of the more complicated sediment transport processes controlled by interference due to contact between grains, such as the pulsatory migration of grain-size bimodal mixtures with repetition of clustering and scattering.

Measurements Concerning Generation of Rhomboid Rills in Flume Experiments: Froude Number and Sand-Topographic Feature

Rhomboid rills, small sand topographies often observed in the swash zone of beach slopes, are apparently similar but different from rhomboid ripples that have been relatively well researched. Rhomboid rills are composed of thin grooves, while rhomboid ripples are of the tilelike geometry. A recent study showed a difference in the dependence of rhombus angle on the slope gradient between rhomboid rills and rhomboid ripples. In this study, we measured Froude numbers during the generation of rhomboid rills and found that the trend of rhombus angles versus Froude numbers was similar to that in the case of rhomboid ripples. We discuss similarities and dissimilarities between rhomboid rills and rhomboid ripples, showing an open problem.