Sediment Mass Transport Research Articles

Water movements, sediment transport and deposition, Western Port, Victoria

Western Port is a tidal embayment, and the characteristics of oscillatory tidal currents, overall net and small-scale circulation patterns, and aspects of the mode, frequency and mass transport of sediment are presented. The results of the studies reported in this volume are both corroborative and complementary, and their integration has resulted in a much better understanding of the bay than could otherwise have been obtained. Consideration of dynamic process, sedimentation patterns and morphology has allowed the distinction of four “systems” within the bay: 1. (1) High-energy channel/bank system (Western Entrance to just beyond the Confluence Zone), characterised by strong tidal currents, varying degrees of surface wave activity, and a sandy substrate. Middle Bank represents the largest individual sand body in Western Port and is associated with intense sediment movement. Sand transported from Middle Bank through the Confluence Zone supplies and maintains the sandy deposits and beaches in the Lower North Arm and East Arm. Seismic evidence coupled with estimates of mass transport of sand suggest that during the Holocene, 10 3 to 10 4 g cm −1 yr −1 have been transported from Middle Bank to Cowes Bank in order to produce the extensive sedimentary complex in that region. 2. (2) Ebb/flood differentiated segments with contrasting intertidal sediments (lower North Arm, Corinella Segment), characterised by clearly differentiated flood- and ebb-dominant zones, sandy and muddy intertidal sediment deposition. 3. (3) Subtidal, partly enclosed mud-deposition basin (Rhyll Segment), characterised as a shallow, subtidal, depositional basin collecting fine sediments. A central clay-size zone is flanked by coarser sediments. Circulation is weak and sediment transport is minimal. 4. (4) Extensive intertidal flat systems with minor channels (Embayment Head), characterised by a web of minor channels, and luxuriant seagrass meadows except in the regions of the Bunyip and Lang Lang Rivers. Local flood- and ebb-dominant transport paths are defined by sandy deposits of reworked or autochthonous Pleistocene sands.

Shallow structure of the continental margin of southwestern Africa

Twelve seismic reflections profiles across the continental margin between Cape Town and the Walvis Ridge abutment are used to describe the shallow structure of the margin and to propose a history of sedimentation. Numerous unconformities occur within the shelf sediments; one of them is widespread and possibly mid-Tertiary in age. Beneath the slope is a thick sequence of deposits younger than the widespread unconformity and therefore thought to be Late Tertiary and Quaternary in age. Below the rise is a Tertiary and Quaternary succession overlying Horizon A. The margin has been built outwards and to a lesser extent upwards as a result of mass transport of sediments, current action and subsidence.

Transport of Ocean Sediments by Debris Flow: ABSTRACT

Debris flow, a gravity-transport mechanism commonly observed on land, may be a significant agent of high-density mass transport of sediment in the oceans. Debris flow is distinguished from other mass-transport agents by the mechanism of support of granular solids in the flow. Support is provided mainly by the strength of the debris, but also by buoyant forces. Strength is derived from the fluid phase of the debris (clay minerals plus water), which acts as a plastico-viscous material. Suspension of granular solids by this mechanism does not depend on flow conditions and occurs if the debris is moving very slowly or even not at all. Movement of a debris flow depends on a critical thickness of the debris as well as the internal-friction angle. The slope angle required for de ris flow typically is less than the 18-37° required for normal grain flows or avalanching. Thus, debris flow may carry large amounts of sediment in suspension while moving sluggishly down a gentle slope. The amount of clay, relative to granular solids, necessary completely to support sand-size material is on the order of 10% or less. Thus, sandy debris-flow deposits may be texturally similar to current-deposited sands. End_of_Article - Last_Page 622------------

Transport of bedload as ripples during an ebb current

A large instrumented tripod was used during a period of relatively strong ebb current in Puget Sound, to measure simultaneously the currents within 1.5 m of the sea bed and the changing ripple configuration of a sandy bottom. Measurement of the heights and migration rates of the ripples on time-lapse stereo photographs made it possible to calculate the amount of sediment moved. From the measured velocity profiles, the shear velocity, U∗, and the fluid power expended on the bed, ρU∗ 3 , were determined. These measurements of mass transport are compared with the values predicted by several existing bedload transport theories. Einstein's (1950) and Yalin's (1963) equations predict values with the appropriate trend of the curve of mass transport vs. U∗, but their values are too large, because of a difference in the boundary shear stress measurement used in the comparison. Bagnold (1963) relates immersed weight of transported sediment to fluid power by a proportionality coefficient, K, which supposedly reaches a constant maximum value for any system. Our measurements indicate that K increased exponentially with the growth of ripples, which in turn were related to the flow conditions. Bagnold's basic formulation has been modified with respect to a variable K, and when compared with flume data, the mass transport of sediment was adequately predicted. It appears that K does not reach a constant value in most marine environments but is related to the sediment characteristics, the shear stress, and the turbulence of the flow.