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Wave-induced sheet flow leads to intense sediment transport. Fine sand and medium/coarse sand exhibit opposite directions of net sediment transport under velocity-skewed oscillatory sheet flows. A newly developed two-phase mixture model is employed to simulate the sediment transport under these conditions. The model accounts for particle stress, two-phase momentum exchange, and turbulence modulation. The effects of grain size on flow characteristics and sediment transport are primarily investigated. The model effectively reproduces the spatial and temporal distributions of two-phase velocities and sediment concentration as well as the periodic distribution of erosion depth. Comparisons between configurations with medium and fine sand demonstrate that the grain size impacts sediment transport in two main ways. First, the grain size influences the periodic variations in erosion depth and the quantity of suspended sediment. A decrease in the grain size increases the phase residual and phase lag, enhancing offshore sediment transport. Second, suspended sediments modulate the flow dynamics within the oscillatory boundary layer. Through the mobile bed effect and density stratification, the grain size affects two-phase velocities, turbulence, and net sediment transport. [ABSTRACT FROM AUTHOR]

In the Geum River, a representative altered estuary, in-situ mooring and erosion experiments were conducted to reveal the effects of human-controlled discharge on benthic sediment disturbances. The strong discharge through the estuarine dam increased the current velocities within the benthic boundary layer (BBL) up to 79% and 153% during spring and neap tides, respectively. During the discharge period, the suspended sediment concentration in BBL was five times higher than that during the non-discharge period. During the flood phase, a critical shear stress for erosion (τ ce) was in the range of 0.1278 to 0.1391 Pa. Immediately after the ebb phase with strong discharge, τ ce increased to 0.1848 Pa, and the erosion rate (E) decreased from 33.73 to 14.81 mg m−2 s−1. The repetition of human-controlled discharge removed the erodible sediments within BBL, exposing the underlying consolidated sediment bed with low E and high τ ce. The results suggest that an altered estuary is vulnerable to benthic sediment erosion under combined natural and anthropogenic forcings. [Display omitted] • Human-controlled discharge disturbed benthic sediment transport and velocity structures. • Altered estuary is vulnerable to sediment erosion under anthropogenic forcings. • Discharge removed sediments, exposing consolidated sediment with low erodibility. • Erosion experiments present direct evidence on benthic sediment resuspension. [ABSTRACT FROM AUTHOR]

The long-distance transport of Yellow River sediments has formed an omega-shaped ('Ω') distal muddy deposit in the Yellow Sea. Numerous studies have investigated the sediment dynamics and sedimentary evolution of the deposit, but several key questions remain unanswered due to insufficient observations. To reveal the mechanisms of sediment dynamics and quantify sediment transport and deposition processes, this study collected field datasets in the depocenter of the muddy deposit off the Shandong Peninsula. Bottom mounted tripods were deployed from August 18 to 27, 2017, and from February 23 to March 2, 2018, during which two strong wind events with speeds of over 10.8 m/s occurred. The results showed that near-bed suspended sediment concentration responded directly to varying hydrodynamic conditions, and significant differences in suspended sediment concentration between two wind events could be ascribed to a number of factors (e.g., tidal current, wave periods, wind stress, turbulence). The harmonic analysis demonstrated that suspended sediment variations during the spring tide were directed mainly by resuspension, while resuspension and advection contributed equally at neap tide. The variations in background suspended sediment concentration can be ascribed to the residual current. Furthermore, the sediment transport pattern is similar to the pattern of water circulation. Generally, the along-shore suspended sediment flux is about one order of magnitude higher than the cross-shore flux. In summer, the suspended sediment was transported mainly downcoast. In winter, sediment was transported downcoast during wavy periods and upcoast during calm periods. Advection dominated sediment flux in most cases, except during wavy periods when local resuspension increased significantly, and the contribution of resuspension term to along-shore flux was up to 87%. Meanwhile, cross-shore flux also increased significantly under strong winds, which was simultaneously modulated by downcoast and offshore winds. After statistical analysis, we found that offshore winds controlled the cross-shore sediment transport in winter. Using parameterized calculation and statistical analysis, the average accumulation rate of the study site was estimated to be 0.6 cm/yr. During calm periods, lower background sediment concentration in summer led to lower deposition rates than in winter. In addition, the frequent strong wind events in winter cause extremely high erosion rates. This study provides new perspectives on quantitative variations in suspended sediment transport and regional erosion/deposition from synoptic to seasonal variations in the distal mud deposit. • In-situ observations at the bottom boundary layer were conducted in summer and winter on the muddy deposit off the Shandong Peninsula. • The advection dominates sediment flux in most cases, except for during wind gusts when local resuspension increases significantly. • Combining parameterized method and observational data, the accumulation rate is estimated. [ABSTRACT FROM AUTHOR]

Abstract: The geotectonic framework and the evolutionary history of the Japanese Islands need revision in accordance with the various geophysical/geological evidence gathered by new methodologies in the recent years including seismic tomography, vibroseis/ground-breaking seismic experiments, and detrital zircon chronology. These investigations have addressed various themes such as: 1) seismic profile of the crust and mantle beneath the Japanese Islands, 2) high-precision ages of the protoliths of high-P/T metamorphic rocks, and 3) provenance of terrigenous clastics. The results have led to a number of important findings including: 1) detection of a large mass of slab around the mantle boundary layer suggesting the long-term oceanic subduction beneath Japan, 2) confirmation of the subhorizontal piled-nappe structure for the entire crust of Japan, 3) finding a new high-P/T metamorphosed accretionary complex unit that represents the youngest blueschist in Japan, 4) finding of the oldest (Early Cambrian) arc batholith and cover sediments, and 5) the identification of plural arc batholiths which have already been erased from the surface. Based on a synthesis of these new data, this article presents a re-evaluation of the conventional geotectonic subdivision of the subduction-related orogen in Japan, re-definition of the elements and their mutual boundaries, and re-consideration of the geotectonic evolution of the Japanese Islands. In particular, the historical change in provenance suggests that proto-Japan has experienced large-scale tectonic erosion in multiple stages, and the corresponding large amounts of continental crust materials were subducted. For understanding the orogenic growth of Japan during the last ca. 500million years, the significance of tectonic erosion coupled with continental contraction, as well as the oceanward accretionary growth, requires further attention. [Copyright &y& Elsevier]

Abstract: Sandy sediments in shallow coastal waters of the Baltic Sea are often characterised by large numbers of biogenic structures which are produced by macrozoobenthos species. A series of experiments was devised to quantify how the interaction of such structures with the near-bed flow regime affects the sediment flux. Most experiments were done with simplified replicates of structures generated by typical species commonly found in the Mecklenburg Bight, starting with solitary structures and regularly-spaced arrays in a range of characteristic population densities, followed by a complex benthic macrofauna community, both artificial and alive. A laboratory flume channel, equipped with an acoustic Doppler flow sensor and a topography scanning laser, was used for high-resolution measurements (2 mm horizontal step size and 0.3 mm vertical resolution) of sand erosion (220 µm median grain size, at 20 cm s−1) and fine particle deposition (8 µm grain size, at 5 cm s−1). Sediment transport threshold values were measured for each layout. As a rule-of-thumb, both the erosion fluxes and the deposition of suspended matter increased considerably at low population densities (below 2%, expressed as percent of the sediment surface covered, i.e. roughness density RD). Above densities of 4%, erosion almost stopped inside the test arrays, and deposition remained well below the level of unpopulated areas. An attempt to extrapolate these findings to field conditions (using field current velocity data from 2001) showed that the net flux switched from erosion to deposition for densities above 5%. These parameters can now be integrated into a numerical sediment transport model coupling waves, currents, sediment dynamics and biological processes, which is currently under construction at the Baltic Sea Research Institute (IOW), Rostock, Germany. [Copyright &y& Elsevier]

The upward seepage through the bed sediment downstream of an apron of a sluice gate structure is a common occurrence due to afflux of the flow level between the upstream and downstream reaches of a sluice gate. The result of an experimental investigation on the characteristics of the scour hole and the flow-field downstream of an apron due to submerged jets under the influence of upward seepage through the bed sediment is presented. Experiments were run for the conditions of submerged jets, having submergence factors from 0.99 to 1.72 and jet Froude numbers from 3.15 to 4.87, over beds of sediments (median sizes=0.8, 1.86, and 3 mm) downstream of an apron under upward seepage velocities. The characteristic lengths of the scour hole determined from the scour profiles are: the maximum equilibrium scour depth, the horizontal distance of the location of maximum scour depth from the edge of the apron, the horizontal extent of the scour hole from the edge of the apron, the dune height, and the horizontal distance of the dune crest from the edge of the apron, all of which were found to increase with an increase in the seepage velocity. Using experimental results, the time variation of the scour depth is scaled by an exponential law, where the nondimensional time scale decreases linearly with an increase in the ratio of the seepage velocity to the issuing jet velocity. The flow field in the submerged jets over both the apron and within the scour hole was detected using an acoustic Doppler velocimeter. The vertical distributions of time-averaged velocities, turbulence intensities and Reynolds stress at different streamwise distances, and the horizontal distribution of bed-shear stress are plotted for the conditions of scour holes with and without upward seepage. Vector plots of the flow field show that the rate of decay of the submerged jet decreases with an increase in the seepage velocity. The flow characteristics in the scour holes are analyzed in the context of the influence of upward seepage velocity on the decay of the velocity and turbulence intensities and the growth of the boundary layer. [ABSTRACT FROM AUTHOR]

For moderate Shields number flows, the characteristic minimum sediment transport prediction error is estimated to be of the order of a factor of 2, even in stationary and horizontally homogeneous flows over a flat bottom. For oscillatory flows over ripples the same error is judged to be comparable to a factor of 4.In actual flows when the flow, sediments and bottom shape have to be predicted, moderate estimation errors for the near bottom turbulent stress results in characteristic minimum sediment transport error that is probably at least of the order of a factor of 10.The equations for the large-scale average flow require nonstandard parameterizations of the smaller scale ripple flow and boundary conditions. Only weakly founded closure relations are available for this. The error of the large-scale sediment transport can therefore not be quantified without extensive data comparisons. As extrapolated from the simpler estimates, it appears plausible that it could be significantly larger than a factor of 10.When the sediment transport variability is large, both in reality and in models, many models can probably be adjusted to fit the few data available. This may not be relevant for judging the prediction accuracy. [Copyright &y& Elsevier]

Results of field experiments using the Virginia Institute of Marine Science' Sea Carousel and tripod system reveal a highly dynamic sediment activity at the Clay Bank site in the York River. At the water-sediment interface, the critical bed shear stress for sediment erosion varied between 0.026 Pa and 0.1 Pa. For the well consolidated sediment below the interface, the critical bed shear stress increased significantly, to more than 0.6 Pa. The seasonal variation of erosion rate for the surficial sediment is significant. For the well consolidated sediment below the surficial sediment, however, the seasonal variation diminished and erosion rate approached a constant. Experimental results from the Carousel deployments imply that the erosion process for quasi-steady tidal flows is always near equilibrium. For this reason, a constant rate erosion model is proposed for the time during tidal acceleration phases. For other times, the erosion rate would be zero. The measured suspended sediment concentration at 10 cm above bed from the VIMS tripod system also Supports this model. [ABSTRACT FROM AUTHOR]