Bed Composition Research Articles

Development and validation of a morphological model for multiple sediment classes

The complexity of sediment dynamics in aquatic systems can be better understood by applying numerical models. The development of a comprehensive morphological model is presented in this paper. The model aims to predict the sediment transport and bed evolution in natural systems composed of different sediment types. The morphological model was implemented in MOHID, a modelling system that solves the three-dimensional hydrodynamics and advection-diffusion transport of suspended sediments. Multiple sediment classes were taken into account (non-cohesive and cohesive) considering the effects of sediment mixtures and bed consolidation on resistance to erosion. To represent bottom stratigraphy, the bottom column can be divided into several layers. The key points of the simulated processes are discussed in this paper. Model results are assessed in six test cases through comparison with analytic solutions or experimental data. The outcomes demonstrate the model׳s capacity to simulate the transport dynamics of non-cohesive and cohesive sediments. The speed up of morphological changes by an acceleration factor permitted modelling bed evolution for long time periods. Moreover, a test case for the Tagus Estuary demonstrated the model׳s capacity for generating realistic sediment distribution based on the local hydrodynamic conditions. Limitations in the availability of bed composition data can be overcome by considering a warm-up run to provide realistic initial conditions for further predictions of morphological developments.

Morphological reactions of schematic alluvial rivers: Long simulations with a 0-D model

The paper presents a 0-D model of an alluvial watercourse schematized in two connected reaches, evolving at the long time-scale and under the hypothesis of Local Uniform Flow. Each reach is defined by its geometry (constant length and width, time-changing slope) and grain-size composition of the bed, while the sediment transport is computed using a sediment rating curve. The slope evolution is provided by a 0-D mass balance and the evolution of the bed composition is computed by a 0-D Hirano equation. A system of differential equations, solved with a predictor-corrector scheme, is derived and applied to the schematic watercourse to simulate the morphological response to changing initial conditions, and the evolution towards long-term equilibrium conditions. Differently from a single-reach 0-D schematization with uniform grain-size, besides the simplifications adopted, the model proposed here simulates the behaviour of alluvial rivers in a physically-based way, showing a grain-size fining in the downstream direction accompanied by milder slopes, and a tendency to develop concave longitudinal profiles.

Process-based morphodynamic modeling of the Yangtze Estuary at a decadal timescale: Controls on estuarine evolution and future trends

Understanding the decadal morphodynamic evolution of estuaries and deltas and their controls is of vital importance regarding management for estuarine function and sustainable development. This work addresses this issue by applying a process-based model system (Delft3D) to hindcast and then forecast the morphodynamic evolution of the Yangtze Estuary at a decadal timescale. Forced by the river and tides, the model considers sand-mud mixture and the variations of river water discharge and sediment discharge. The morphodynamic model is validated against three periods, i.e., an accretion period (1958–1978), an erosion period (1986–1997) and a recent accretion period with human activities (2002−2010). Model results show good performance with respect to spatial erosion and deposition patterns, sediment volume changes, and hypsometry curves. The model reveals quite different behaviors for mud transport between the dry and wet seasons, which is subject to the prescription of river boundary conditions and bed composition. We define six scenarios to project evolution to the year 2030 under decreased river inputs and increased relative sea level. The simulations reveal that overwhelming amount of erosion will likely occur in the inner and mouth bar area of the estuary. Particularly, the mouth zone will shift from net deposition before 2010 to net erosion by 2030, mainly because of decreasing sediment supply. Changes in water discharge have minor effects on the projected trend. Net erosion will be considerable when the sediment supply is extremely low (100Mtyr−1) due to the abundance of erodible modern sediment in the Yangtze Estuary. Erosion within the mouth bar area may be unexpected, including the deepening of the tidal inlet at East Chongming mudflat and the formation of a flood channel on the seaward side of Jiuduansha Shoal. Overall, the model results provide valuable information for sustainable delta management under changing conditions for both the Yangtze system and other similar estuaries and deltas with diminishing sediment supplies.

An experimental method to verify the failure of coastal structures by wave induced liquefaction of clayey soils

Although the dynamics involved in the liquefaction process are understood reasonably well, experimental work to reproduce the sinking of structures due to liquefaction, which is representative and repeatable, has so far not been recorded. In this work, three sets of experiments were performed in an attempt to fill the gaps in the knowledge by modelling a small scale reproduction of the failure. Firstly, an analysis of the role of the proportions of the initial fine sediment and water content is presented; secondly, a group of tests involving a vertical breakwater were performed, and thirdly, an experiment was carried out to reproduce the failure of a submerged structure on a clayey bed in the presence of waves. From these experiments, we were able to set thresholds for bed composition, below which soil liquefaction is likely to occur. It was determined that the potential to liquefy increases with the initial water content and that soils of 40% or more clay content may liquefy. This methodology has proven to be repeatable, allowing the reproduction of the sinking of coastal structures due to liquefaction of the underlying soil.

Experimental and numerical investigation of liquid–solid binary fluidized beds: Radioactive particle tracking technique and dense discrete phase model simulations

Liquid–solid binary fluidized beds are widely used in many industries. However, the flow behavior of such beds is not well understood due to the lack of accurate experimental and numerical data. In the current study, the behavior of monodisperse and binary liquid–solid fluidized beds of the same density but different sizes is investigated using radioactive particle tracking (RPT) technique and a dense discrete phase model (DDPM). Experiments and simulations are performed in monodisperse fluidized beds containing two different sizes of glass beads (0.6 and 1mm) and a binary fluidized bed of the same particles for various bed compositions. The results show that both RPT and DDPM can predict the mixing and segregation pattern in liquid–solid binary fluidized beds. The mean velocity predictions of DDPM are in good agreement with the experimental findings for both monodisperse and binary fluidized beds. However, the axial root mean square velocity predictions are only reasonable for bigger particles. Particle–particle interactions are found to be critical for predicting the flow behavior of solids in liquid–solid binary fluidized beds.

Stochasticism in noise generated by an array of marine hydrokinetic devices

Marine hydrokinetic (MHK) devices generate electricity from the motion of tidal and ocean currents and ocean waves and provide another source of renewable energy. Additionally, MHK devices are also a new source of anthropogenic noise in the marine ecosystem and must meet regulatory guidelines that mandate a maximum amount of noise that may be generated. In the absence of measured levels from in-situ deployments, a model for predicting the propagation of sound from an array of MHK sources in a real environment needs to be established. A 3D finite-difference, time-domain solution to the governing velocity-pressure equations is used, which permits a finite number of complex sources and spatially varying sound speeds, bathymetry, and bed composition. However, deterministic solutions to these types of problems cannot capture uncertainties in the source profiles that may result from operational changes. This work presents the broadband sound pressure levels from an array of MHK sources as the amplitude and frequency from each source are allowed to vary. This Monte Carlo approach demonstrates that the idealized, deterministic solution, vastly underestimates the compounding uncertainty on the final sound field and that these small variations in the source profile cannot be ignored.

Observations and modelling of nearshore sediment sorting processes along a barred beach profile

The understanding of cross-shore sediment sorting is of primary importance for the design of sand nourishments and for assessing the suitability of the seabed to different ecological species. In this paper, sediment sorting processes were investigated by using a combination of physical and detailed numerical modelling. Data from large-scale wave flume experiments were used to validate a 2DV cross-shore Delft3D model. The model solves coupled short-wave averaged equations for flow, sediment transport, bed composition and bed level change. The infra-gravity wave motions were explicitly resolved. In order to investigate sorting processes, eight sediment fractions were used as well as a layered bed stratigraphy. The effects of different wave conditions (high energetic and more moderate energetic waves) on the morphodynamic profile development and sorting processes were investigated. The Delft3D model reproduced the profile development and bar position very accurately. Additionally, model predictions of sediment sorting across the profile fitted very well with the available observations. The numerical model simulations showed the importance of including short-wave grouping and infragravity wave effects in order to reproduce the cross-shore profile development, especially the breaker bar dynamics and sediment sorting processes. Infragravity waves contribute to larger sediment entrainment and more offshore bar development. Besides leading to a better prediction of the bed profile, infragravity waves also lead to a better prediction of the bed composition. Model results are in agreement with experimental data, showing its capabilities in functioning as a tool to predict sorting processes.

Sediment sorting at the Sand Motor at storm and annual time scales

Bed sediment composition, with a focus on the median grain size D50, was investigated at a large-scale nourishment (The ‘Sand Motor’) at the Dutch coast (∼21.5 millionm3 sand). Considerable alongshore heterogeneity of the bed composition (D50) was observed as the Sand Motor evolved over time with (1) coarsening of the exposed part of the Sand Motor (+90 to +150μm) and (2) a depositional area with relatively fine material (50μm finer) just North and South of the Sand Motor. The alongshore heterogeneity of the measured D50 values was most evident outside the surfzone (i.e. seaward of MSL −4m). Coarsening of the bed after construction of the Sand Motor was attributed to hydrodynamic sorting processes, because the alongshore heterogeneity of the D50 showed a similar spatial pattern as the mean bed shear stresses. The observed alongshore heterogeneity of the D50 and correlation of D50 with modelled mean bed shear stresses suggest that preferential erosion of the finer sand fractions has taken place. The selective transport of finer sand fractions results in a coarser top layer of the bed at the Sand Motor. The preferential transport is most dominant during mild and moderate conditions when hydrodynamic forcing conditions are close to the critical bed shear stresses for transport. The measurements also show the impact of a storm, which consists of a ∼40μm finer D50 of the offshore bed composition in front of the Sand Motor (i.e. where a considerably coarser bed was in place). Additionally, storms may generate a (temporary) zone with fine bed material at the toe of the deposition profile. This means that the coarsening of the bed is reduced by storms as a result of the mobilization of both coarse and fine sediment and mixing of the bed with the relatively finer substrate.

Assessment of a numerical model to reproduce event-scale erosion and deposition distributions in a braided river.

Numerical morphological modeling of braided rivers, using a physics‐based approach, is increasingly used as a technique to explore controls on river pattern and, from an applied perspective, to simulate the impact of channel modifications. This paper assesses a depth‐averaged nonuniform sediment model (Delft3D) to predict the morphodynamics of a 2.5 km long reach of the braided Rees River, New Zealand, during a single high‐flow event. Evaluation of model performance primarily focused upon using high‐resolution Digital Elevation Models (DEMs) of Difference, derived from a fusion of terrestrial laser scanning and optical empirical bathymetric mapping, to compare observed and predicted patterns of erosion and deposition and reach‐scale sediment budgets. For the calibrated model, this was supplemented with planform metrics (e.g., braiding intensity). Extensive sensitivity analysis of model functions and parameters was executed, including consideration of numerical scheme for bed load component calculations, hydraulics, bed composition, bed load transport and bed slope effects, bank erosion, and frequency of calculations. Total predicted volumes of erosion and deposition corresponded well to those observed. The difference between predicted and observed volumes of erosion was less than the factor of two that characterizes the accuracy of the Gaeuman et al. bed load transport formula. Grain size distributions were best represented using two φ intervals. For unsteady flows, results were sensitive to the morphological time scale factor. The approach of comparing observed and predicted morphological sediment budgets shows the value of using natural experiment data sets for model testing. Sensitivity results are transferable to guide Delft3D applications to other rivers.

Wall effects on particle separation in air jigs

This study investigates the influence of wall effects on particle separation in an air jigging device. Tests were conducted with a ternary mixture of concrete, brick and gypsum particles in the size range from 12 to 20mm for three different jigging times. It is shown that an unusual, remarkable segregation occurred in the direction perpendicular to the lateral walls, giving rise to a horizontal segregation pattern. In all cases, heavier particles concentrated more in the center of the particle bed while lighter particles concentrated along the side walls. Composition measurements of the stratified bed revealed that wall effects can significantly influence the composition of jig products. Linear predictions of jigging evolution suggest that stratification progress linearly after the first initial stages. Also, time evolution and analysis of local compacities indicate that stratification starts from the bottom to the upper layers of particle bed. Based on the experimental results, a mechanism for the pattern formation was proposed, which was enhanced by a geometry model of bed rearrangement. Prediction results for bed composition correlated well with experimental data and endorsed the role of lateral walls on the segregation pattern formed. The results obtained also show practical significance for the use of air jigging in the sorting of recycled aggregates.

Spectrally based mapping of riverbed composition

Remote sensing methods provide an efficient means of characterizing fluvial systems. This study evaluated the potential to map riverbed composition based on in situ and/or remote measurements of reflectance. Field spectra and substrate photos from the Snake River, Wyoming, USA, were used to identify different sediment facies and degrees of algal development and to quantify their optical characteristics. We hypothesized that accounting for the effects of depth and water column attenuation to isolate the reflectance of the streambed would enhance distinctions among bottom types and facilitate substrate classification. A bottom reflectance retrieval algorithm adapted from coastal research yielded realistic spectra for the 450 to 700nm range; but bottom reflectance-based substrate classifications, generated using a random forest technique, were no more accurate than classifications derived from above-water field spectra. Additional hypothesis testing indicated that a combination of reflectance magnitude (brightness) and indices of spectral shape provided the most accurate riverbed classifications. Convolving field spectra to the response functions of a multispectral satellite and a hyperspectral imaging system did not reduce classification accuracies, implying that high spectral resolution was not essential. Supervised classifications of algal density produced from hyperspectral data and an inferred bottom reflectance image were not highly accurate, but unsupervised classification of the bottom reflectance image revealed distinct spectrally based clusters, suggesting that such an image could provide additional river information. We attribute the failure of bottom reflectance retrieval to yield more reliable substrate maps to a latent correlation between depth and bottom type. Accounting for the effects of depth might have eliminated a key distinction among substrates and thus reduced discriminatory power. Although further, more systematic study across a broader range of fluvial environments is needed to substantiate our initial results, this case study suggests that bed composition in shallow, clear-flowing rivers potentially could be mapped remotely.

RELATIONSHIP BETWEEN POWDER COMPOSITION AND SINGLE SCREW EXTRUSION FOR MANUFACTURING RELIABLE RANDOM CERAMIC PACKED BED ACCORDING STATISTICAL APPROACH

The present investigation provides a detailed relationship between the powder composition and reliability of random ceramic beds. This evaluation is important due to standing in the liquid-gas contactors as well as predicting lifetime. It is still unclear whether the normal distribution is the most suitable function for estimation of failure. By developing the application of ceramic beds in the chemical plants, a special attention has been paid in screening strength distributions. To achieve this goal, an experimental–theoretical study was presented on compressive strength distribution. The powder compositions were prepared according to the statistical response surface methodology and then were formed by a single screw extrusion as Raschig rings. The compressive strength of specimens was measured to evaluate the strength data sets by normal and Weibull distributions. The results were analyzed by the Akaike information criterion and the Anderson–Darling test. The accuracy of distributions in prediction fracture was discussed.

Introduction: an Atlas of Submarine Glacial Landforms

Glacial landforms and sediments exposed sub-aerially have been the subject of description, analysis and interpretation for more than a century (e.g. De Laski 1864; De Geer 1889). Indeed, such features provided important initial observations informing Louis Agassiz's ideas that ice was a key instrument in sculpting the landscape and that glaciers and ice sheets had extended to mid-latitudes during the past, implying that Earth's climate must have changed considerably through time (Agassiz 1840). It is only in the last few decades that attention has begun to focus on the marine evidence for the past growth and decay of ice sheets that is recorded in submarine landforms and sediments preserved on high-latitude continental margins. This interest has been driven, in part, by the recognition that sediments deposited below wave-base are often well preserved in the Quaternary geological record, and may be less subject to erosion and reworking than their terrestrial counterparts. In addition, new marine-geophysical technologies have enabled increasingly high-resolution imaging and penetration of the high-latitude seafloor, most notably using multibeam swath-bathymetric and three-dimensional (3D) seismic-reflection methods, and modern ice-strengthened and ice-breaking research vessels have allowed the effective deployment of these increasingly sophisticated instruments in the often ice-infested waters of the Arctic and Antarctic seas. The geological record from high-latitude continental margins is now recognized to provide key information on former ice-sheet extent, the direction and nature of past ice flow and dynamics, and a well-preserved window on the detailed form and composition of former ice-sheet beds (e.g. Ottesen et al. 2005; Anderson et al. 2014; Jakobsson et al. 2014). The geometry and distribution of submarine glacial landforms on the seafloor, and the underlying glacial-sedimentary stratigraphic record with which they are associated, is the topic of this volume. The aims and purpose of the Atlas are: (1) to …

Macroalgal composition and community structure of the largest rhodolith beds in the world

The Abrolhos Bank, encompassing a wide portion of the Brazilian continental shelf, harbors the world's largest rhodolith bed, which plays an important role in calcium carbonate production in the South Atlantic Ocean. Little is known about the community structure and species composition of this habitat. The aim of this study was to test the hypothesis that latitudinal differences exist in the community structure of rhodolith beds and their associated flora along the Abrolhos Bank. We sampled a total of 33 sites of rhodolith beds located in three regions—northern, central, and southern—of the Abrolhos Bank between depths of 20 and 70 m. Rhodolith density (rhod. m−2) within the beds ranged from 990 ± 347.6 to 57 ± 18.7, with mean diameter ranging from 9.4 ± 3.2 to 3.1 ± 1.4 cm. A total of 146 macroalgae species were identified, including 14 rhodolith-forming species of crustose coralline algae. Abrolhos Bank supports the world’s greatest species richness of rhodolith-forming CCA, with regional distinctiveness. Observed differences in bed structure among regions can be related to differences in shelf width, slope and depth. The studied rhodolith beds constitute a unique habitat supporting a distinctive diversity of associated organisms, and thus require special attention. We highlight the importance of local and regional differences for defining appropriate conservation strategies to protect the rhodolith bed diversity of Abrolhos Bank.

CaCO3 calcination by the simultaneous reduction of CuO in a Ca/Cu chemical looping process

The exothermic reduction of CuO to Cu using a fuel gas as a source of heat to carry out the simultaneous calcination of CaCO3 in a fixed bed is evaluated. The absence of apparent large energy penalties, as in other chemical looping processes, is an indication that there is great potential for achieving a high level of energy efficiency with this Ca/Cu looping concept. A dynamic pseudo-homogeneous model has been developed to describe in detail the transient behavior of this operation in a fixed-bed reactor under adiabatic conditions. A sensitivity analysis of the main operating parameters (i.e. the CuO/CaCO3 molar ratio, starting temperature and fuel gas composition) confirms the theoretical viability of this operation. A balanced CuO/CaCO3 ratio ensures a suitable bed performance allowing the reduction and calcination fronts to advance together, reach moderate maximum temperatures of around 900°C and leave behind totally converted solids. The use of CO as fuel gas significantly reduces the CuO/CaCO3 ratio required for the CaCO3 calcination. A careful adjustment to bed composition must be carried out, since an excess of CuO in the bed will generate more heat than required for the calcination, and consequently hot spots higher than 1000°C will form along the bed. In contrast, an excess of CaCO3 will increase the energy demand and part of the bed will be left uncalcined.

Bed-to-surface heat transfer in conical spouted beds of biomass–sand mixtures

The effects of superficial gas velocity, bed composition and other operating parameters on bed hydrodynamics and bed-to-surface heat transfer were investigated the influence of bed height and gas velocity sawdust, sand and 30–70, 40–60 and 50–50 mixtures of biomass and sand in a conical spouted bed. Experiments were performed at minimum spouting velocity and 10 and 20% above this. At each superficial velocity, experiments were conducted at three heights, 0.1, 0.2 and 0.3m above the inlet. A specially designed heat transfer probe measured the bed-to-surface heat transfer coefficient. The heat transfer coefficient increased from the wall to the spout, and also was observed to increase with increasing percentage of biomass in the blend. The experimental results are compared with published results, showing good agreement with the influence of height of the bed and gas velocity.

Modeling sorting dynamics of cohesive and non-cohesive sediments on intertidal flats under the effect of tides and wind waves

We extend a numerical model to explore the morphodynamics of intertidal flats, with a specific focus on the sorting dynamics of sand and mud. We investigate the effect of tidal currents, wind waves, sediment properties, external sediment supply, flocculation and initial bed composition on the cross-shore profile shape and sediment sorting of intertidal flats. Consistent with existing analytical theories and benchmark simplified numerical solutions, the equilibrium cross-shore profile of intertidal flats simulated by the extended model is convex-up when tidal currents dominate and it progressively becomes concave-up when the strength of wind waves increases. The equilibrium profile is influenced by the external sediment supply which can lead to the seaward advance of intertidal flats. In line with field observations, mud tends to deposit on the upper intertidal flats when wind waves are relatively weak, while sand is mainly distributed on the middle and lower tidal flats. When wind waves are strong, both sand and mud are more easily resuspended and eroded, resulting in a noticeably concave-up profile near the high water mark. The initial bed composition (e.g., percentage of mud and sand fractions) is also found to play an important role: the intertidal flat is more convex-up in a muddier environment. Numerical modeling demonstrates that sediment properties (e.g., critical shear stress for erosion, settling velocity) and flocculation can pronouncedly influence the sediment sorting dynamics by modifying the initiation threshold and the advection distance of entrained sediments. Application of the extended model to a natural study site indicates a qualitative agreement with field observations.

Buried and Massive Ground Ice on the West Coast of Baidaratskaya Bay in the Kara Sea

Using data on the structure, conditions of occurrence (bedding), and the isotope composition of massive ice beds on the West coast of Baydaratskaya Bay it was established that the massive ice beds even occurring in the same outcrop may be related to different genetic types. There are two groups of the massive ice: 1) the «upper» thick (> 3 m) massive ice beds composed by buried basal glacier ice; and 2) the «lower» small ice beds (< 3 m), formed both intrasedimentally and as a result of burial of initially surface ice bodies. Sand thickness which included both groups of the massive ice started its formation before the glacial ice burial. As a result of advancing and later degradation of the glacier, probably moving from the Pay-Khoy ridge or from the Polar Ural, its lower (basal) parts were preserved within the permafrost thickness.

Early Middle Jurassic (possibly Aalenian) Tsagan-Ovoo Flora of Central Mongolia

The Tsagan-Ovoo Flora from terrigenous, coal-bearing deposits of the Bakhar Formation, central Mongolia, is documented for the first time. The Tsagan-Ovoo Flora contains 32 megafossil plant taxa belonging to horsetails, ferns, cycadaleans, ginkgoaleans, leptostrobaleans, conifers and gymnosperms of unknown systematic affinity. The floristic composition of the plant-bearing beds allows us to date them as early Middle Jurassic (possibly Aalenian). This flora is typical of the West Siberian Province of the Siberian Region and corroborates our previously obtained conclusion that the boundary between this province and the North Chinese Province of the same region should be positioned further south than its previous placement. The plants of the Tsagan-Ovoo Flora probably formed a mosaic of communities that inhabited swampy lowlands and lake margins. The flora reflects a wet, most probably warm-temperate climate with some seasonality in temperature and/or precipitation. Three new ginkgoalean species of the Tsagan-Ovoo Flora are described: Ginkgo badamgaravii Kostina et Herman, Pseudotorellia gobiense Kostina et Herman, and Pseudotorellia mongolica Kostina et Herman.

Mixed-sediment transport modelling in Scheldt estuary with a physics-based bottom friction law

In this study, the main object is to investigate the performance of a few new physics-based process models by implementation into a numerical model for the simulation of the flow and morphodynamics in the Western Scheldt estuary. In order to deal with the complexity within the research domain, and improve the prediction accuracy, a 2D depth-averaged model has been set up as realistic as possible, i.e. including two-way hydrodynamic-sediment transport coupling, mixed sand–mud sediment transport (bedload transport as well as suspended load in the water column) and a dynamic non-uniform bed composition. A newly developed bottom friction law, based on a generalised mixing-length (GML) theory, is implemented, with which the new bed shear stress closure is constructed as the superposition of the turbulent and the laminar contribution. It allows the simulation of all turbulence conditions (fully developed turbulence, from hydraulic rough to hydraulic smooth, transient and laminar), and the drying and wetting of intertidal flats can now be modelled without specifying an inundation threshold. The benefit is that intertidal morphodynamics can now be modelled with great detail for the first time. Erosion and deposition in these areas can now be estimated with much higher accuracy, as well as their contribution to the overall net fluxes. Furthermore, Krone’s deposition law has been adapted to sand–mud mixtures, and the critical stresses for deposition are computed from suspension capacity theory, instead of being tuned. The model has been calibrated and results show considerable differences in sediment fluxes, compared to a traditional approach and the analysis also reveals that the concentration effects play a very important role. The new bottom friction law with concentration effects can considerably alter the total sediment flux in the estuary not only in terms of magnitude but also in terms of erosion and deposition patterns.