Flow-Sediment Interaction and Formation Mechanism of Sediment Longitudinal Streaky Structures in Rough Channel Flows
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Hyperconcentrated flow
The saltating mode of sediment transport is the dominant mechanism of bedload transport in a fluvial stream, especially in mountainous rivers. In most simulation studies, the actual shapes of sediment particles are simplified considering the spherical shape of particles, which can affect the accuracy of the estimation of bedload transport rate. In this study, a model of sediment particle saltation by streamflow is presented, simulating the wall-shear turbulent flow over a sediment bed by large-eddy simulation and calculating the forces acting on the saltating particles by Newton's second law of motion. The model was calibrated by using the experimental data from previous studies on particle saltation. To study the impact of particle shapes, the computational results of saltation lengths, heights, and velocities of different-shaped particles were analyzed. Then, three key parameters were used to estimate bedload transport rates: bedload concentration, saltation height, and particle velocity. By summarizing the above results, this work presents a formula of bedload transport rate that improves upon a previous well-celebrated formula from the literature. The proposed formula introduces shape factors of natural sediment particles.
Hyperconcentrated flow
Bedform
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Hyperconcentrated flow
Bedform
Suspended load
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The landforms and deposits associated with AD 1996 debris-flows at three sites in the low-alpine zone, Jotunheimen, southern Norway, are described and analyzed. Parallel levées, composed of diamicton, occur on the valley-side slopes but distinct frontal lobes are absent: instead, low-angle fans, up to about 50 m wide and ca. 500 m long have overridden vegetation in the footslope zone and in the valley bottom. Five facies are recognized in the fans: (1) cobble-rich diamicton, up to 50 cm thick; (2) pebble-rich diamicton, typically up to 30 cm thick; (3) pebbly sand lenses, up to about 15 cm thick; (4) massive silty sand or sandy silt (intermediate-type deposits) of thickness 5–15 cm; and (5) laminated fine sands and silts (typically a few cm thick). These succeed one another in a vertical and lateral sense. The landform-sediment assemblage and proximal-distal trends are explained by a four-stage model of an integrated debris-flow event in which (1) slope failure and sediment disaggregation are followed sequentially by (2) debris flow sensu stricto (cohesive debris flow), (3) wet mudflow or hyperconcentrated flow, and (4) water flow. Debris flow sensu stricto accounts for an estimated 48 to 51%, wet mudflow/hyperconcentrated flow 21 to 26%, and water flow 24 to 31%, by volume of material mobilized during each debris-flow event at two of the sites. Results highlight the potential complexity of debris-flow events and the importance of the associated relatively fine-grained intermediate-type deposits with little or no structure, which are attributed here to wet mudflow and/or hyperconcentrated flow. Water-lain deposits, also integral to the debris-flow event, tend to be thinner and finer, better sorted, and distinctly laminated.
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Cobble
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Hyperconcentrated flow
Ephemeral key
Bedform
River morphology
Sedimentation
Flash flood
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Sedimentary budget
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Stony debris flow transits to sediment sheet flow when the river bed gradient becomes gentle. The sediment sheet flow consists of a water flow layer and a sediment moving layer. Fine sediments are expected to behave as a part of the fluid rather than a solid phase in the sediment moving layer. Further, it can be thought that a part of fine sediment can be suspended in the water flow layer. However, it was not possible to physically express whether the fine sediment behaves as a solid phase or a fluid phase in the numerical simulation model. Here we physically modeled fine sediment behavior in sediment sheet flow. We confirmed the applicability of the new model to describe the longitudinal deposited sediment gradient in flume experiments.
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The volcanic, lacustrine fan-delta deposits in the Pleistocene Lower and Middle Formations of the Yachiho Group, central Japan commonly include debris-flow and hypercon-centrated-flow deposits in the upper fan-delta-plain facies. The hyperconcentrated-flow deposits are recognized by its characteristics and depositional organization with debris-flow and streamflow deposits. Hyperconcentrated-flow deposits usually coexist with cohesionless debris-flow deposits. This indicates that the generation and depositional processes of hyperconcentrated flow are closely related to cohesionless debris flow in the volcanic fan-delta setting. A depositional organization comprising cohesionless debris-flow deposits, hyperconcentrated-flow deposits, and normal streamflow deposits, in ascending order, is commonly observed. This organization indicates the longitudinal segmentation of the composite flows. Cohesionless debris flow preceded dilute hypercon-centrated flow followed by a streamflow that segregated from the initial debris flow.
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An extension of the bedload model of Kaczmarek & Ostrowski (1996), taking account of suspended sediment, is proposed for the calculation of sediment transport features, such as the transport rate and thickness. The paper is focused on the transition region (named as a contact load layer) between the outer region (suspension layer) and the bedload layer within the proposed three-layer sediment transport model.
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