scholarly journals Residual flow, bedforms and sediment transport in a tidal channel modelled with variable bed roughness

Geomorphology ◽  
2017 ◽  
Vol 295 ◽  
pp. 855-872 ◽  
Author(s):  
A.G. Davies ◽  
P.E. Robins
2018 ◽  
Author(s):  
Tian Zhao ◽  
Qian Yu ◽  
Yunwei Wang ◽  
Shu Gao

Abstract. Being a widespread source-to-sink sedimentary environment, the fine-grained dispersal system (FGDS) features remarkably high sediment flux, interacting closely with local morphology and ecosystem. Such exceptional transport is believed to be associated with changes in bedform geometry, which further demands theoretical interpretation. Using van Rijn (2007a) bed roughness predictor, we set up a simple numerical model to calculate sediment transport, classify sediment transport behaviors into dune and (mega-)ripple dominant regimes, and discuss the causes of the sediment transport regime shift linked with bedform categories. Both regimes show internally consistent transport behaviors, and the latter, associated with FGDSs, exhibits considerably higher sediment transport rate than the previous. Between lies the coexistence zone, the sediment transport regime shift accompanied by degeneration of dune roughness, which can considerably reinforce sediment transport and is further highlighted under greater water depth. This study can be applied to modeling of sediment transport and morphodynamics.


2018 ◽  
Author(s):  
Fritz Schlunegger ◽  
Philippos Garefalakis

Abstract. Clast imbrications are presumably the most conspicuous sedimentary structures in coarse-grained clastic deposits. In this paper, we test whether the formation of such a fabric is related to changes from lower to upper flow regime conditions in streams. To this extent, we calculate the Froude number at the incipient motion of coarse-grained bedload for various values of relative bed roughness and stream gradient. We then compare the results with data from modern streams and stratigraphic records. The calculations show that upper flow regime conditions most likely establish where average stream gradients exceed c. 0.5 ± 0.1°, and where relative bed roughness values are larger than ∼ 0.06 ± 0.01. Similarly, data from modern streams reveal that imbricated clasts are found where channels are steeper than c. 0.5 ± 0.2°, and where relative bed roughness values exceed ∼ 0.07. Likewise, imbricated conglomerates are encountered in late Oligocene foreland basin sequences where paleo-slopes were greater than 0.4°. We use these relationships to propose that clast imbrications occur where channel gradients exceed a threshold, which appears large enough for upper flow regime conditions to establish. We finally relate the formation of an imbricated arrangement of clasts to a mechanism where material transport occurs through rolling, or pivoting. This process requires a large shear force and thus a large flow velocity upon transport, which is likely to be associated with shifts from the lower to the upper flow regime. Our results thus suggest that clast imbrications are suitable recorders of upper flow regime conditions upon sediment transport.


1978 ◽  
Vol 1 (16) ◽  
pp. 118
Author(s):  
Larry G. Ward

Processes and sediment transport were investigated in a salt marsh drainage system at Kiawah Island, South Carolina. A general survey of the tidal current was done in the major tidal channel (Bass Creek) for a 10 tidal cycle period in August, 1977. Detailed determinations of current velocity, discharge, and suspended load were conducted during 15 tidal cycles in March, 1977 and again during 8 tidal cycles in July- August, 1977. For each of these periods, mass budget for the total suspended load were computed. The tidal currents have a pronounced time velocity asymmetry with the maximum current velocity occurring nearer high slack water and the peak ebb velocity being 20 - 30% stronger than the flood. Suspended load transport is significantly affected by the time velocity asymmetry. Peak current occurring nearer high slack water causes a net displacement of suspended material in an ebb or seaward direction under normal conditions. This process is enhanced by the stronger ebb currents. Mass budgets reflect the ebb dominance of the system showing a net export of combustible (organic) material during the March sampling period and a net export of both noncombustible (inorganic) and combustible material during the July-August period. Also important to suspended load transport in marsh systems are stressed meteorological conditions. High winds or heavy rains increase suspended load concentration and can cause significant import or export of fine-grained material.


Author(s):  
Zaiyang Zhou ◽  
Jianzhong Ge ◽  
D.S. van Maren ◽  
Zheng Bing Wang ◽  
Yu Kuai ◽  
...  

2014 ◽  
Vol 5 (1) ◽  
Author(s):  
Ravindra Jayaratne ◽  
Sarik Salim

AbstractIn order to understand sediment transport process based on knowledge of their soil properties and hydrodynamic behaviour a series of 2D laboratory controlled small-scale experiments were conducted using the Ahlborn sediment mobile bed tank (4.0×0.6×0.2 m). Experiments were conducted in smooth and rough bed conditions with purposely-built Soil Protrusion Apparatus (SPA) to measure the basic parameters on which erosion depends. Sediment deposition patterns in equilibrium stage associated with different bed roughness and particle size distributionswere fundamentally investigated. Extended physical modelling of crescent zones also included analysing their grain size distribution. Dimensional analysis and multiple linear regression methodswere employed to derive a simple empirical relationship for erosion rate (ER) in terms of the shear stress (τs), average grain diameter (d50) and soil protrusion (z) for smooth and rough sediment bed conditions. These analyses also suggest ways to refine empirical models, examining transport rates to explore the limits of erosion and deposition influences in shallow flow conditions.


1972 ◽  
Vol 1 (13) ◽  
pp. 46 ◽  
Author(s):  
H.A. Einstein

This paper summarizes the results of a continuing study at the hydraulic laboratory at the University of California at Berkeley on this subject, which is supported by the Coastal Engineering Research Center (CERC) and which has resulted over the years in the Theses of Huon ti (1954), M. Manohar (1955), G. Kalkanis (1957, 1963), M. M. Abou-Seida (1965), M. M. Das (1968) and is at this time being continued by T. C. Mac Donald. All these researchers have greatly contributed to the success of this work while the author was mostly responsible for the continuity of the study. The aim of the study was to see if it is possible to establish for the description and prediction of sediment transport by waves a general system of approach similar to that which the author published in 1950 under the title '.'The Bed-Load Function for Sediment Transport in Open Channel Flows". ( It was hoped at the time that many of the basic steps of such a description may at least be similar to those used for uni-directional flow. It became apparent that such similarities of approach were quite feasible; but another difficulty became apparent from the beginning. While in the uni-directional flow many details of the flow, such as velocity distributions, boundary layers and turbulence had been, studied and described previously, such knowledge was almost entirely lacking for wave motion. The first part of the study consisted entirely of hydraulic measurements and of their analysis. In order to determine the necessary scope of such hydraulic studies, the analogy with sediment transport in uni-directional flow was used. Some of the principles governing uni-directional flow transport are: 1. Sediment motion can be divided into bed-load motion or surface creep and suspension. 2. While moving as bed load, the particle weight is to a large part transmitted directly to the nonmoving bed, not to the flow. 3. The rate of bed-load motion is defined by the equilibrium exchange of sediment between the bed-load and the nonmoving bed. 4. This equilibrium gives a direct relationship between the sediment rate and the flow conditions near the bed, including the turbulence. 5. The flow condition near the bed can be predicted for a uni-directional boundary layer as a function of the bed shear and the bed roughness, only.


2014 ◽  
Vol 509 ◽  
pp. 113-126
Author(s):  
RG Macdonald ◽  
DG Bowers ◽  
D McKee ◽  
GW Graham ◽  
WAM Nimmo-Smith

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