Initiation Of Sediment Transport Research Articles

The Physics of Sediment Transport Initiation, Cessation, and Entrainment Across Aeolian and Fluvial Environments

AbstractPredicting the morphodynamics of sedimentary landscapes due to fluvial and aeolian flows requires answering the following questions: Is the flow strong enough to initiate sediment transport, is the flow strong enough to sustain sediment transport once initiated, and how much sediment is transported by the flow in the saturated state (i.e., what is the transport capacity)? In the geomorphological and related literature, the widespread consensus has been that the initiation, cessation, and capacity of fluvial transport, and the initiation of aeolian transport, are controlled by fluid entrainment of bed sediment caused by flow forces overcoming local resisting forces, whereas aeolian transport cessation and capacity are controlled by impact entrainment caused by the impacts of transported particles with the bed. Here the physics of sediment transport initiation, cessation, and capacity is reviewed with emphasis on recent consensus‐challenging developments in sediment transport experiments, two‐phase flow modeling, and the incorporation of granular physics' concepts. Highlighted are the similarities between dense granular flows and sediment transport, such as a superslow granular motion known as creeping (which occurs for arbitrarily weak driving flows) and system‐spanning force networks that resist bed sediment entrainment; the roles of the magnitude and duration of turbulent fluctuation events in fluid entrainment; the traditionally overlooked role of particle‐bed impacts in triggering entrainment events in fluvial transport; and the common physical underpinning of transport thresholds across aeolian and fluvial environments. This sheds a new light on the well‐known Shields diagram, where measurements of fluid entrainment thresholds could actually correspond to entrainment‐independent cessation thresholds.

The Critical Role of the Boundary Layer Thickness for the Initiation of Aeolian Sediment Transport

Here, we propose a conceptual framework of Aeolian sediment transport initiation that includes the role of turbulence. Upon increasing the wind shear stress τ above a threshold value τ t ′ , particles resting at the bed surface begin to rock in their pockets because the largest turbulent fluctuations of the instantaneous wind velocity above its mean value u ¯ induce fluid torques that exceed resisting torques. Upon a slight further increase of τ , rocking turns into a rolling regime (i.e., rolling threshold τ t ≃ τ t ′ ) provided that the ratio between the integral time scale T i ∝ δ / u ¯ (where δ is the boundary layer thickness) and the time T e ∝ d / [ ( 1 − 1 / s ) g ] required for entrainment (where d is the particle diameter and s the particle–air–density ratio) is sufficiently large. Rolling then evolves into mean-wind-sustained saltation transport provided that the mean wind is able to compensate energy losses from particle-bed rebounds. However, when T i / T e is too small, the threshold ratio scales as τ t / τ t ′ ∝ T e / T i ∝ s d 2 / δ 2 , consistent with experiments. Because δ / d controls T i / T e and the relative amplitude of turbulent wind velocity fluctuations, we qualitatively predict that Aeolian sediment transport in natural atmospheres can be initiated under weaker (potentially much weaker) winds than in wind tunnels, consistent with indirect observational evidence on Earth and Mars.

On the numerical modelling of initiation of sediment transport using Smoothed Particle Hydrodynamics

AbstractMobilization of solid particles at the interface between a porous and a free flow domain is a relevant subject in many fields of mechanical, civil and environmental engineering. One example is the initiation of sediment transport as it appears in river beds. To approximate this initiation state, various theoretical models exist. Common approaches use two‐domain formulations as in [1] or one‐domain formulations as in [6]. The named approaches were compared with Direct Numerical Simulations (DNS) using Smoothed Particle Hydrodynamics (SPH) in [3]. The results of these simulations showed that the theoretical models often underestimate the occurring velocities at the interface and therefore critical velocities to initialize the motion of single grains can be lower than predicted by theoretical approaches. In our numerical simulations, we study creeping flow in a free flow domain coupled to flow in a porous media applying various porous structures. To investigate velocities and shear stresses at the interface more intensively we then compare our numerical results to data from experiments that were performed on equivalent microstructures. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Does Downward Seepage Initiate Lateral Channel Shift?

The bed shear stress increased from its critical value when downward seepage was applied to the threshold channel, which resulted in initiation of sediment transport. Experiments have revealed shifting of the channels in lateral directions due to the erosion and deposition of bed material.

Biotic interactions influence sediment erodibility on wave-exposed sandflats

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 523:15-30 (2015) - DOI: https://doi.org/10.3354/meps11164 Biotic interactions influence sediment erodibility on wave-exposed sandflats Rachel J. Harris1,*, Conrad A. Pilditch1, Judi E. Hewitt2, Andrew M. Lohrer2, Carl Van Colen4, Michael Townsend2, Simon F. Thrush3 1School of Science, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand 2National Institute of Water and Atmospheric Research Ltd. (NIWA), PO Box 11115, Hamilton 3240, New Zealand 3Institute of Marine Science, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand 4Research Group of Marine Biology, Ghent University, Krijgslaan 281/S8, 9000 Ghent, Belgium *Corresponding author: harris.rachelj@gmail.com ABSTRACT: Biological activities in marine soft-sediments can modify the sedimentary environment through processes that change erosion rates. In low-energy environments, bioturbating macrofauna destabilizes sediments while microbes bind sediments and stabilize them. The degree to which these counter-acting processes influence sediment movement in a physically dynamic environment has not been well quantified. In a field experiment, we established 56 (1 m2) plots on an exposed intertidal sandflat. We used shade cloth and manipulated grazing pressure exerted by the deposit-feeding bivalve Macomona liliana (0-200 ind. m-2) to alter the microphytobenthic community. Three months post-manipulation, initiation of sediment transport (Ʈc) and change in sediment erosion rate with increasing bed shear stress (me) were measured. Mean grain size, density of the spionid polychaete Aonides trifida, density of adult M. liliana, and bulk carbohydrate concentration could account for 54% of the variation in Ʈc (0.3-1.1 N m-2 s-1). Mean grain size was the only significant predictor (p ≤ 0.01) of me explaining 22% of the variability (6-20 g N-1 s-1). Ʈc was negatively correlated with density of several abundant shallow-dwelling bioturbators (indicating sediment destabilization), but we did not observe the expected increase in Ʈc with microbial biomass. Furthermore, there was a positive correlation between adult M. liliana and Ʈc as well as evidence for several positive feedbacks between abundant shallow-dwelling macrofauna and microbial biomass. These study results demonstrate that despite frequent reworking by tidal currents and waves, bioturbating macrofauna are important to initiating sediment transport regardless of their effects on microbial biomass. KEY WORDS: Deposit-feeding · Microphytobenthos · Benthic macrofauna · Sediment stability · Infauna · Positive feedbacks · Macomona liliana · Habitat modifiers Full text in pdf format PreviousNextCite this article as: Harris RJ, Pilditch CA, Hewitt JE, Lohrer AM, Van Colen C, Townsend M, Thrush SF (2015) Biotic interactions influence sediment erodibility on wave-exposed sandflats. Mar Ecol Prog Ser 523:15-30. https://doi.org/10.3354/meps11164 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 523. Online publication date: March 16, 2015 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2015 Inter-Research.

Laboratory Measurements of Coarse Sediment Bedload Transport Velocity Using a Prototype Wideband Coherent Doppler Profiler (MFDop)

AbstractA prototype wideband coherent Doppler profiler (MFDop) was tested for measuring bedload velocity of different gravel and coarse-sand-sized fractions (d = 1–32 mm) in the laboratory. The sediment was spread out on a smooth-surface tray, and motion was initiated by tilting the tray at angles of α = 20°–39° from the horizontal. Particle velocities downslope (u), cross slope (υ), and vertical to the tray (w) were determined for different MFDop parameter settings, such as monostatic/bistatic configuration, acoustic beam angle, and pulse length. Video observations of bed particle velocity were made for comparison to the acoustic measurements. Velocities estimated using the MFDop equal to, on average, 71%–74% of the velocities determined using the video observations. Standard deviations ranged from 21% to 35%, including observed irregular motion. Three stages of sediment motion were observed: (i) single particles moving with u ≤ 5 cm s−1, (ii) varying motion of particles and particle groups with predominantly 5 cm s−1 ≤ u ≤ 20 cm s−1, and (iii) fast sheetlike motion with u ≥ 20 cm s−1. The cross-slope velocity υ and the vertical velocity w were significantly smaller than u, hinting at slipping as the major particle motion rather than rolling or saltation. Comparisons between MFDop and video-determined velocities showed good agreement. Standard deviations for the MFDop velocity estimates ranged from 22% to 35%. The trials with different gravelly sediments and coarse sand revealed a significant influence of grain size, as well as grain shape impacting the initiation of sediment transport and transport velocities.

Efficiency and uncertainties in micro- and mesoscale habitat modelling in large rivers

Habitat modelling has become an increasingly important tool in river sciences to evaluate impacts on running waters and to predict the effects of river restoration in the context of the European Water Framework Directive which aims to reach a good ecological status by 2015. In a scaling framework like the River Scaling Concept, micro- and mesoscale habitat modelling hierarchically integrate point-/local-scale abiotic processes like grain sorting, initiation of sediment transport, bedform development and braiding with biological processes like spawning, daily and seasonal movements, feeding and shifting to refuge habitats. The paper, along with application results, shows that micro- and mesoscale habitat models are complementary especially for large river systems. Overall possibilities, restrictions and future development are discussed. Both rely on dynamic abiotic modelling as a basis as well as on biological data. In future, especially for large rivers, habitat modelling should be addressed on both scales (micro-/meso-) with the main focus on site-specific efficiencies (e.g. minimum efforts) and uncertainties (e.g. transferring suitability indices).

Review of River Mechanics by Pierre Y. Julien

River mechanics is a wide-ranging engineering field. It brings together the complexities and knowledge of climate, watershed, hydrology and hydraulics, geology and geomorphology, and in many cases the interaction of rivers with lakes and estuaries. The field requires a deep understanding of hydrodynamic forces governing the motion of water and sediment in complex river systems. This textbook is designed primarily for teaching, and it is configured based on material for a graduate course on river mechanics taught at Colorado State University. The book also provides scientists and engineers with broad-based reference material. The book contains a good mixture of both theoretical concepts and practical engineering technology, and it illustrates aspects of river engineering using various exercises, general problems, data sets, computer problems, examples, and case studies. The book is subdivided into 12 chapters as follows: ‘‘Introduction to River Mechanics,’’ ‘‘River Basins,’’ ‘‘Steady Flow in Rivers,’’ ‘‘Unsteady Flow in Rivers,’’ ‘‘River Equilibrium,’’ ‘‘River Dynamics,’’ ‘‘River Stabilization,’’ ‘‘River Engineering,’’ ‘‘Physical River Models,’’ ‘‘Mathematical River Models,’’ and ‘‘Waves and Tides in River Estuaries.’’ Chapters vary in length and depth of treatment of their respective topics. Bibliography and Index sections are provided at the end of the book. As designed, the book will be certainly a useful reference for both students and practicing engineers. It is well written and covers a wide range of topics. The derivations are rigorous, explanations given are in a clear and good English, and illustrations are adequate. The text is accompanied by a multitude of problems and numerical examples, and in many cases the ways of solving the problems are discussed. This approach undoubtedly elevates the value of this book beyond its intended use of mainly by students. Although the title of the book is River Mechanics, it is only the first half of the book that deals with river mechanics per se. The second half is devoted almost exclusively to the topics of river engineering, and in the first half, many relevant and prominent topics of river mechanics are presented only in a superficial manner. These include the initiation of sediment transport ~IST!, bedload rate ~BLR!, and suspended-load rate ~SLR!. Other topics are simply omitted as discussed in the next paragraph. For example, IST, BLR, and SLR are dealt with in only two and half pages ~viz., pp. 112–114!. A very large number of expressions and diagrams have been produced on IST, BLR, and SLR for various cases of flow and sediment types by generations of researchers, and further investigation of these topics are still going on. Yet only four diagrams are presented on pp.112–114 and captions of three ending with ‘‘after Julien.’’ This reviewer does not consider this to be a major shortcoming of the book, given that the author is a prominent researcher and world-class scientist in need of no such self-advertisement. It is hoped that some would not interpret this omission of other researchers’ works by the author as a selfadvertisement vehicle.

Evidence of Micro-Rotational Cells in Fluid Flow and Their Possible Implications for Sediment Transport

Considerable attention has been given to the determination of the initiation of sediment transport in both sand and gravel-bed rivers. The underlying purpose of these works has been to establish a predictive relationship between these variables and the rate of sediment transport. However, due to the numerous contributing variables involved, it has been difficult to arrive at the definitive relationship between all variables and rates of sediment transport. This paper investigates the relationship between developing micro-rotational cells and rates of sediment transport in a medium-sized stream containing a sand bed. Analysis of the relationship between flow vectors at different time intervals allows for the quantification of this pressure force which operates either on or off the bed, depending on the direction of rotation. Results from 21 near-bed sediment traps and fluid speed data at those locations show a strong relationship between variability in pressure near the bed and sediment transport rate (r 2 = .516) over a range of transport rates between 0.3 to 83 kg m-1 hr-1. This is an important result because it helps explain variation in transport rates when the relationship between fluid speed and transport rate is poor (r 2 = .138).

A method for the formulation of Reynolds number functions

A general method for the formulation of flow characteristics which are functions of the Reynolds number of the system is presented. It is assumed that the flow characteristics exhibit a strong variation with the Reynolds number when the Reynolds number is "small," and that they become independent of it when the Reynolds number is "large." The method is illustrated by finding mathematical expressions for the experimentally determined "roughness" function curve and for the sediment transport initiation curve (Shields' curve), which are relevant for the analysis of flow and sediment transport in pipes and open channels. The two expressions thus obtained can be used in practice for computational purposes.Key words: Reynolds number functions, mathematical expression, roughness function, Shields' curve.

Measurements of critical shear stress in sewers

Critical shear stress of sediment located in a grit chamber and in a sewer was determined in full-scale by a measuring device constructed in the laboratory of the Institute of Hydrosciences at the Federal Armed Forces University, Munich. The estimation of critical shear stress is actually based on the measurement of the corresponding critical water discharge. Incipient motion was recorded by a video camera and the recorded pictures were treated by an automatic image processing system. The measurement results were compared with data taken from the literature. A good agreement between measurements and literature data was obtained. The automatic image processing contributes to a more objective quantification of the sediment transport initiation.

Coarse bedload transport in a mountain river

Coarse bedload transport dynamics are investigated utilizing hydrodynamic and sediment transport data obtained in an extensively instrumented study reach located in Squaw Creek, Montana, USA. During 1991 and 1992, a number of discrete bedload transport events associated with the daily rise and fall in stream discharge were investigated. Data show that initiation of sediment transport was accompanied by a reduction in bed roughness and by changes in bulk hydraulic parameters. For larger discharges, coarser fractions of the bed material mobilized, and bedload transport rates and average hydraulic parameters stabilized. As discharge reduced, mobile coarse particles became less frequent and deposited fine particles were removed, resulting in an increase in bed roughness. These observations are attributed to the downstream translation of bar sediments during the passage of a hydrograph. Bedload pulses were aperiodic but spatially variable. Flow turbulence and velocity profile data obtained during low flows allowed comparison between average bed shear stress and apparent bed roughness estimates obtained using different approaches. © 1998 John Wiley & Sons, Ltd.

Coarse bedload transport in a mountain river

Coarse bedload transport dynamics are investigated utilizing hydrodynamic and sediment transport data obtained in an extensively instrumented study reach located in Squaw Creek, Montana, USA. During 1991 and 1992, a number of discrete bedload transport events associated with the daily rise and fall in stream discharge were investigated. Data show that initiation of sediment transport was accompanied by a reduction in bed roughness and by changes in bulk hydraulic parameters. For larger discharges, coarser fractions of the bed material mobilized, and bedload transport rates and average hydraulic parameters stabilized. As discharge reduced, mobile coarse particles became less frequent and deposited fine particles were removed, resulting in an increase in bed roughness. These observations are attributed to the downstream translation of bar sediments during the passage of a hydrograph. Bedload pulses were aperiodic but spatially variable. Flow turbulence and velocity profile data obtained during low flows allowed comparison between average bed shear stress and apparent bed roughness estimates obtained using different approaches. © 1998 John Wiley & Sons, Ltd.

THE INFLUENCE OF RAINFALL ON TRANSPORT OF BEACH SAND BY WIND

This paper deals with the effect of rainfall on the process of wind erosion of beach sands and presents results from both field and wind tunnel experiments. Although sediment transport by splash is of secondary importance on coastal dunes, splash–saltation processes can move sediments in conditions where no motion is predicted by aeolian processes. The effect of raindrop impact on the movement of soil particles by wind was measured on a sand beach plain using an acoustic sediment sampler. In general, an increase of particle movement by wind at the sensor heights was observed during rainfall. Rainfall also affected the wind erosion process during and after rain by changing the cohesive conditions of the surface. The influence of the surface moisture content on the initiation of wind erosion and on the vertical distribution of transported sand particles was studied in a wind tunnel. Moisture significantly increased threshold wind velocities for the initiation of sediment transport and modified vertical sediment profiles.

Transport of recently settled soft-shell clams ( Mya arenaria L.) in laboratory flume flow

A laboratory flume was used to examine the retention of juvenile soft-shell clams (2 wk postsettlement) in sediment when exposed to flow. Clams resisted erosion until the initiation of sediment transport, after which they were rapidly advected from sections of test substrate. Comparisons between living and killed clams indicated that burrowing behavior was instrumental for maintaining position in flow ≤ the critical erosion velocity for sediment movement. The ability of low density, shallow-burrowing juvenile bivalves to avoid transport as bedload or resuspended particles is probably minimal during erosional periods, despite that fact that the position of larger, deeper living individuals may be unaffected. In energetic regimes, infaunal recruitment patterns can be dominated by hydrodynamic forces affecting juveniles.

Sediment in Sewers: Initiation of Transport

The initiation of sediment transport in sewers was investigated in field and laboratory studies. From the field studies it was concluded that some deposits in sewers are permanent due to the insufficient capability of the flow to erode the deposits. From the laboratory studies it was concluded that the upper limit of the critical shear stresses for cohesive sewer sediments may not exceed 5-7 N/m2. Non-cohesive sediments are eroded at lower shear stresses than predicted by Shields' criterion. The shear stresses were calculated using the general equations of continuity and motion and Einstein's separation technique for channels of compound roughness. Experiments showed the validity of this method.

The sediment threshold over tidally induced megaripples

Formulae to predict the initiation of sediment transport over bedforms are derived and field tested. Unlike a Shields-type curve which is valid only for plane beds, both the form drag and skin friction are accounted for. The method relies on successful isolation of the skin friction component of the flow energy losses from the total loss which includes an ineffective form drag component. This is achieved using the Vanoni and Hwang equation for form drag losses while the skin friction is taken from the Yalin sediment threshold curve for plane beds. In conjunction, formulae are derived for the total friction velocity and the speed at 1 m above the bed at the sediment threshold. The theory is tested using thresholds measured in mesotidal estuaries.

The impact of wave loads and pore-water pressure generation on initiation of sediment transport

The build-up of pore-water pressure by waves can lead to sediment liquefaction and subsequent transport by traction currents. This process was investigated by measuring pore-water pressures both in a field experiment and laboratory wave tank tests. Liquefaction was observed in the wave tank tests. The results suggest that sand is less susceptible than silts to wave-induced liquefaction because of the tendency to partially dissipate pore-water pressures. However, previous studies have determined that pore-water pressures must approach liquefaction before current velocities necessary to initiate transport are reduced. Once liquefaction has occurred more sediment can be transported.

A comparison between Shields' threshold criterion and the movement of loosely packed gravel in a tidal channel

ABSTRACTObservations of the threshold of movement of loosely packed gravel in a tidal current are described. For gravel with equivalent ‘spherical’ diameters D in the range 0.2 ≲D≲ 5.0cm the critical friction velocity u*c, corresponding to the initiation of sediment transport, is given by u*c=7.0 D0.2. At large values of D within the quoted range, the value u*c is significantly lower than would be obtained by a Shields experiment (u*c∞D0.5). By comparing our values of u*c with those obtained under well‐controlled laboratory conditions, the discrepancy with Shields is shown to be due to the open spacing between, and exposure of, individual pebbles on the seabed. By comparing our results with those from upland gravel streams and flume experiments, it is suggested that Shields assumed an excessively large water depth to particle size ratio as a constraint within which the critical sediment entrainment number 0c is valid.