Suspended Mud Research Articles

Direct processing of cellular ceramics from a single red mud precursor

The feasibility of recycling red mud waste by its direct transformation into highly-porous cellular ceramics was successfully demonstrated. Ceramic materials with designed cellular porosity were processed by emulsification of red mud suspensions with liquid paraffin. Taguchi method was used to study the effects provided by varying the red mud load in the suspension, gelatine content and emulsion stirring time on the micro structural features of the cellular ceramics. Additional experiments analysed the effects of the organic to suspension ratio and firing temperature. Emulsification of paraffin followed by gelatine consolidation, drying, elimination of the droplets of the discontinuous organic phase and firing, allowed one to design cellular ceramic pieces with open porosity up to 75%, consisting in interconnected cells with adjustable cell size and low resistance to percolation. These results allow one to consider prospective applications of red mud-based cellular ceramics with designed microstructures as highly-porous membranes for the capture of pollutants.

16S rRNA analysis of electrogenic bacterial communities in microbial fuel cells developed from temperate soils

Bergen Community College (BCC) soils were used to generate mud suspensions to construct microbial fuel cells (MFCs) with the anode buried with the mud, while the cathode rested on top. MFCs incubated at 37°C showed more electrical output and electrogenic bacteria than those grown at 25°C. The most productive MFC generated a maximum of 80 microwatts with 1.67 × 109 electrogenic microorganisms. 16S rRNA clone libraries were developed and analyzed using next generation sequencing. The average soil bacterial community was comprised of 17 different bacterial phyla. The most abundant bacterial phyla in soils were Actinobacteria (33.76%), Proteobacteria (25.60%), Chloroflexi (9.70%), Acidobacteria (8.98%), and Planctomycetes (6.10%). The 16S rRNA genes of the biofilm grown on the anode of the most efficient MFCs were amplified and cloned to determine the composition and diversity of electrogenic bacteria. The average bacterial community was comprised of 21 bacterial phyla. The most abundant bacterial phyla in MFCs were Firmicutes (18.98%), Proteobacteria (18.79%), Actinobacteria (13.63%), Chloroflexi (11.43%), and Planctomycetes (7.91%). All MFCs showing electricity generation had an increase in Firmicutes sequences at all taxonomical levels. The MFC lacking large numbers of Firmicutes did not show electrogenic bacteria nor any electrical production. Bacteria belonging to the phylum Firmicutes provided an important contribution to the development of diverse, stable, and sustainable electrogenic bacterial communities.

Design of a parallel plate shearing device for visualization of concentrated suspensions

A modified version of the commercially available RheOptiCAD® was developed to visualize the microscopic structural changes occurring in concentrated suspensions, such as the break-up of flocs in clay suspensions, under shearing action. This is made possible by replacing the inverted microscope used in the traditional RheOptiCAD® set-up by an upright modular microscope equipped with a CMOS camera and epi-illumination. Our device retains the following features of the previous version of RheOptiCAD®: [i] uniaxial translational motion of two parallel plates, [ii] three modes of shear straining, [iii] controlled thermal environment, and [iv] vacuum joining of microscopy glass slides. The validation of the new design was done using a model system of un-flocculated and flocculated kaolin suspensions and concentrated natural mud suspension. The results showed that the constructed device is a promising tool for studying, from fundamental and industrial perspectives, the microstructural behaviour of complex suspended systems under controlled thermal and mechanical conditions.

Insights into the Short-Term Tidal Variability of Multibeam Backscatter from Field Experiments on Different Seafloor Types

Three experiments were conducted in the Belgian part of the North Sea to investigate short-term variation in seafloor backscatter strength (BS) obtained with multibeam echosounders (MBES). Measurements were acquired on predominantly gravelly (offshore) and sandy and muddy (nearshore) areas. Kongsberg EM3002 and EM2040 dual MBES were used to carry out repeated 300-kHz backscatter measurements over tidal cycles (~13 h). Measurements were analysed in complement to an array of ground-truth variables on sediment and current nature and dynamics. Seafloor and water-column sampling was used, as well as benthic landers equipped with different oceanographic sensors. Both angular response (AR) and mosaicked BS were derived. Results point at the high stability of the seafloor BS in the gravelly area (<0.5 dB variability at 45° incidence) and significant variability in the sandy and muddy areas with envelopes of variability >2 dB and 4 dB at 45° respectively. The high-frequency backscatter sensitivity and short-term variability are interpreted and discussed in the light of the available ground-truth data for the three experiments. The envelopes of variability differed considerably between areas and were driven either by external sources (not related to the seafloor sediment), or by intrinsic seafloor properties (typically for dynamic nearshore areas) or by a combination of both. More specifically, within the gravelly areas with a clear water mass, seafloor BS measurements where unambiguous and related directly to the water-sediment interface. Within the sandy nearshore area, the BS was shown to be strongly affected by roughness polarization processes, particularly due to along- and cross-shore current dynamics, which were responsible for the geometric reorganization of the morpho-sedimentary features. In the muddy nearshore area, the BS fluctuation was jointly driven by high-concentrated mud suspension dynamics, together with surficial substrate changes, as well as by water turbidity, increasing the transmission losses. Altogether, this shows that end-users and surveyors need to consider the complexity of the environment since its dynamics may have severe repercussions on the interpretation of BS maps and change-detection applications. Furthermore, the experimental observations revealed the sensitivity of high-frequency BS values to an array of specific configurations of the natural water-sediment interface which are of interest for monitoring applications elsewhere. This encourages the routine acquisition of different and concurrent environmental data together with MBES survey data. In view of promising advances in MBES absolute calibration allowing more straightforward data comparison, further investigations of the drivers of BS variability and sensitivity are required.

FORMATION, TRANSPORT AND BREAKDOWN OF NEAR-BED HIGH-CONCENTRATED MUD SUSPENSION WITHIN THE NORTH PASSAGE OF THE CHANGJIANG ESTUARY

The turbidity maximum region of the Changjiang Estuary would potentially produce near-bed high-concentrated mud suspension (HCMS), which significantly contributes to the sediment transport and morphology within estuary channel. However, the near-bed high-concentrated mud suspension is technologically difficult to be observed and measured in traditional survey methodology. HCMS can frequently occur around an estuary as a result of sediment deposition, mass or bulk erosion, and convergence under physical mechanisms (Bruens et al., 2002; Winterwerp, 2002, 2011).

Formation of Concentrated Benthic Suspension in a Time‐Dependent Salt Wedge Estuary

AbstractThe concentrated benthic suspension (CBS) of mud, as a major contributor of sediment transport in the turbidity maximum of the estuary, is of great challenge to be correctly monitored through field measurements, and its formation mechanism is not well understood. A tripod system equipped with multiple instruments was deployed to measure the near‐bed hydrodynamics and sediments in the North Passage of the Changjiang Estuary, with the aim at determining the formation mechanisms of CBS. The measurements detected a significant dominance of high sediment concentration in the near‐bed 1‐m layer: ~20 g/L at the southern site and ~47 g/L at the northern site. Strong CBS occurred under weak tidal mixing condition and was directly relevant to the sediment‐induced suppression of turbulent kinetic energy and the enhanced water stratification due to saltwater intrusion and sediment suspension. During the weak‐mixing neap period, the typical thickness of CBS was about 0.2–0.3 m, with a life time of ~2.83 hr (suspended‐sediment concentration > 15.0 g/L). Enhanced water stratification reduced vertical mixing and confined the sediment entrainment from the near‐bed layer to the upper column. This enhancement was due to the suppression of turbulent kinetic energy as a result of the sediment accumulation in the near‐bottom column during the slack waterand also due to the appearance of a two‐layer salinity structure in the vertical as a result of saltwater intrusion near the bottom. These physical processes worked as a positive feedback loop during the formation of CBS and can be simulated with a process‐oriented, one‐dimensional vertical CBS model.

Rheology of a colloidal suspension of carbon nanotube particles in a water-based drilling fluid

High-pressure and high-temperature rheology of a nanomud suspension is studied. The nanomud is prepared by dispersing a water-based drilling mud suspension with multi-walled carbon nanotubes (MWCNT). Effect of nanoparticle concentration, shear rate, pressure, and temperature are investigated. The pressure and temperature are independently varied from ambient conditions to 170 MPa and 180 °C respectively. The results presented show that the addition of CNT increases the viscosity of the drilling mud. A shear thinning non-Newtonian behavior is observed for the basemud and the nanomud samples at all pressures and temperatures. The basemud showed an increase in viscosity with an increase in pressure. However, with CNT particle addition this trend is observed to have reversed. For the basemud and the nanomud at lower concentrations, the viscosity values are observed to have initially increased with the increase in temperature and to have reduced beyond 100 °C. For a higher particle loading, a consistent decrease in viscosity with temperature is observed. The behavior of higher concentration nanomud samples with respect to pressure and temperature are seen to follow a similar trend to that of water. It is reasoned that an adequate addition of CNT nanoparticles helps retain the mud particles in a more homogenous and stable state, where they are held within the CNT chains. However, with further addition of CNT nanoparticles, the mud particles become agglomerated with the long chains of CNT, eventually separating water from the drilling fluid mixture.

A Shear‐Limited Flocculation Model for Dynamically Predicting Average Floc Size

AbstractThe accuracy of sediment transport models depends on the selection of an appropriate sediment settling velocity. In general, settling velocity is primarily dependent on the size and density of the particles in the water column. Determining this value for mud suspensions can be difficult because the small cohesive particles can aggregate to form flocs whose sizes and density are a function of hydrodynamic and physiochemical conditions of the suspension. Here we present a new model for predicting floc size as a function of hydrodynamic conditions and inherited floc sizes. The new approach is a simple modification to the existing Winterwerp (1998, https://doi.org/10.1080/00221689809498621) floc size model. The modification is significant in that it yields predictions that are more inline with observations and theory regarding the upper limit on floc size. The modification we propose is to make the ratio of the applied stress on a floc, over the strength of the floc, a function of the floc size relative to the Kolmogorov microscale. The outcome of the modification is that flocs are not allowed to surpass the Kolmogorov microscale in size and that calibrated aggregation and breakup coefficients obtained at one suspended sediment concentration can be used to predict floc size under other concentrations without recalibration. In this paper, we present the motivation for the modification, the functionality of the modification, and we make a comparison of the updated model with laboratory and field data. Overall, the modification shows promise as a tool for improved prediction of cohesive mud transport.

Flow of non-Newtonian fluids in open channels

This report details progress made in a Maths in Industry Study Group Project. The key conclusions of the study group were that correctly choosing an appropriate rheological model, and using appropriate data to compute the parameters of that model, is vital in recovering the correct fluid behaviour; and that exact mathematical solutions that exist for simplistic channel shapes can be used to approximate the flow in more complicated geometries. The problems of accurate numerical computation for highly non-Newtonian channel flow, as well as the complications that arise from turbulence, were identified as important areas of further research. References N. J. Alderman and R. Haldenwang, ``A review of Newtonian and non-Newtonian flow in rectangular open channels.'' Hydrotransport 17: The 17th International Conference on the Hydraulic Transport of Solids , The Southern African Institute of Mining and Metallurgy and the BHR Group, 2007. http://hdl.handle.net/11189/5192 J. Burger, R. Haldenwang and N. Alderman, ``Experimental database for non-Newtonian flow in four channel shapes.'' Journal of Hydraulic Research , 48 (2010): 363–370. doi:10.1080/00221686.2010.481849 P. Coussot, ``Steady, laminar, flow of concentrated mud suspensions in open channel.'' Journal of Hydraulic Research , 32 (1994): 535–559. doi:10.1080/00221686.1994.9728354 D. A. Rojas and R. H. A. Janssen, ``Design of open channels for non-Newtonian fluids'', in Proceedings of the 16th International Seminar on Paste and Thickened Tailings , Eds R. J. Jewell, A. B. Fourie, J. Caldwell and J. Pimenta, Australian Centre for Geomechanics, 2013. S. W. McCue, J. R. King and D. S. Riley, ``Extinction behaviour for two-dimensional inward-solidification problems.'' Proceedings of the Royal Society of London A , 459 (2003): 977–999. doi:10.1098/rspa.2002.1059 C. C. Mei and M. Yuhi, ``Slow flow of a Bingham fluid in a shallow channel of finite width.'' Journal of Fluid Mechanics , 431 (2001): 135–159. doi:10.1017/S0022112000003013 H. Schlichting and K. Gersten, Boundary-Layer Theory . Springer-Verlag, Berlin Heidelberg, 2000. doi:10.1007/978-3-662-52919-5 K. X. Whipple, ``Open-channel flow of Bingham fluids: applications in debris-flow research.'' The Journal of Geology , 105 (1997): 243–262. doi:10.1086/515916 R. Chhabra and J. F. Richardson, Non-Newtonian Flow: Fundamentals and Engineering Applications , Butterworth-Heinemann, 1999. P. Coussot, Mudflow rheology and dynamics , Balkema, 1997. F. Holland and R. Bragg, Fluid Flow for Chemical and Process Engineers , Butterworth-Heinemann, 1995. E. Mitsoulis, Flows of viscoplastic materials: models and computations, Rheology reviews , 2007 (2007), pp. 135–178. http://www.bsr.org.uk/rheology-reviews/RheologyReviews/viscoplastic-materials-Mitsoulis.pdf D. Pritchard, B. R. Duffy, and S. K. Wilson, Shallow flows of generalised Newtonian fluids on an inclined plane, Journal of Engineering Mathematics , 94 (2015), pp. 115–133. doi:10.1007/s10665-014-9725-2 J. N. Reddy and D. K. Gartling, The finite element method in heat transfer and fluid dynamics , CRC press, 2010.

Wave-Created Mud Suspensions: A Theoretical Study

We studied wave-created high-density mud suspensions (fluid mud) using a one-dimensional water column (1DV) model that includes k-ε turbulence closure at a high vertical resolution with a vertical grid spacing of 1 mm. The k-ε turbulence model includes two sediment-related dissipation terms associated with vertical density stratification and viscous drag of flows around sediment particles. To this end, the calibrated model reproduces the key characteristics (maximum concentration and thickness) of fluid mud layers created in laboratory experiments over a large range of wave velocities from 10 to 55 cm/s. The findings demonstrate that the equilibrium near-bed mud concentration, Cb, is solely determined from the balance between erosion and deposition fluxes, whereas the thickness of the fluid mud layer is mainly controlled by sediment-induced density stratification, which dissipates turbulence and hence eliminates turbulent sediment diffusivity at the top of the fluid mud layer, the lutocline. Our model stands in contrast to those that suggest that upward sediment diffusion is close to zero at the interface between the fluid mud layer and the overlying fluid. Instead, our model suggests that the upward diffusive flux of fluid mud flows peak at the lutocline and is compensated for enhanced settling fluxes just above it. Our model findings also support the existence of the gelling-ignition process, which is critical for the development of fluid mud beds in modern sedimentary environments.

Red mud carbonation using carbon dioxide: Effects of carbonate and calcium ions on goethite surface properties and settling

Carbonation using CO2 appears as an attractive solution for disposing of red mud suspensions, an aluminum industry hazardous waste since it also offers an option for CO2 sequestration. Here we report the novel findings that CO32− together with Ca2+ can significantly affect the surface properties and settling of goethite, a major component of red mud. Specifically, their effects on the goethite surface chemistry, colloidal interaction forces and settling in alkaline solutions are investigated. The surface potential becomes more negative by the formation of carbonate inner-sphere complexes on goethite surface. It is consistent with the strong repulsion, decreased particle size and settling velocity with increased carbonate concentrations as measured by atomic force microscopy, particle size analysis, and particle settling. Adding Ca2+ that forms outer-sphere complexes with pre-adsorbed carbonate changes goethite surface charge negligibly. Changing repulsion to the attraction between goethite surfaces by increasing calcium dosage indicates the surface bridging, in accordance with the increased settling velocity. The adverse effect of carbonate on goethite flocculation is probably due to its specific chemisorption and competition with flocculants. By forming outer-sphere complexes together with the flocculant-calcium bridging effect, calcium ions can eliminate the negative influence of carbonate and improve the flocculation of goethite particles. These findings contribute to a better understanding of goethite particle interaction with salt ions and flocculants in controlling the particle behavior in the handling processes, including the red mud carbonation.

Prograding muddy shelves in the Paleogene Wilcox deltas, south Texas Gulf Coast

Core facies analysis integrated with subsurface well-log correlations and seismic horizon mapping document two thick mudstone-dominated deltaic successions in the Wilcox Group along the south Texas Gulf Coast. Meters to tens of meter-thick upper slope to outer shelf mudstones form units that lie at the base of upward-coarsening successions which are interpreted as fluvial or storm/wave dominated shelf deltas. The muddy character of the deposits is observed in cores and inferred from well logs, whereas the larger depositional architecture is interpreted from well log correlations and seismic data.Physical sedimentary structures described from a lower Wilcox core reflect deposition on a mud dominated prodelta controlled by hyperpycnal processes. Normally and inversely graded beds are present indicating sustained flows that waxed and waned. Upward-coarsening facies successions contain current ripples, organic matter, and siderite cemented bands, low trace fossil abundance and low diversity, which suggest deposition in a fluvial prodelta to shelf environment. Centimeter-thick interbeds of normal and inversely graded mudstones, siltstones and very fine-grained sandstones with low intensities of bioturbation, reflect the high physical and chemical stresses of hyperpycnal environments. Heterolithic stratification with lenticular, wavy and flaser bedding indicates some tidal influence.On the other hand cores of the middle Wilcox reflect deposition on a muddy wave-storm dominated shelf. Bioturbation intensities tend to be high, with unstressed, fully marine ichnological suites. Pervasively bioturbated sandy mudstones and muddy sandstones with Cruziana ichnofacies, wavy parallel and low angle lamination, and structureless sandstones with Ophiomorpha record deposition in a wave-influenced prodelta to shelf environment. Hyperpycnal flows associated with river floods might have transported mud across the shelf to the outer shelf where prevailing wave and storm conditions facilitated suspension and transport of mud beyond the shelf edge.Previous studies have shown that Wilcox delta sandstones are largely trapped on the shelf by contemporaneous growth faults. However, this paper illustrates that at times the Wilcox shelf margin prograded through mud accretion.

Two-phase flow modelling of sediment suspension in the Ems/Dollard estuary

Understanding and quantifying mud suspension and sediment transport processes are of great importance for effective exploitation and sustainable management of estuarine environments. Event-based predictive models are widely used to identify the key interactions and mechanisms that govern the dynamics involved and to provide the essential parameterisation for assessing the long-term morphodynamic evolution of the estuaries. This study develops a one-dimensional-vertical (1DV) Reynolds averaged two-phase model for cohesive sediments resuspension driven by tidal flows. To capture the time-dependent flocculation process more accurately, a new drag force closure which relates empirically to settling velocity of mud flocs with suspended sediment concentration (SSC) is incorporated into the two-phase model. The model is then applied to simulate mud suspension in the Ems/Dollard estuary during two periods (June and August 1996) of tidal forcing. Numerical predictions of bed shear stresses and sediment concentrations at different elevations above the bed are compared with measured variations. The results confirm the importance of including flocculation effects in calculating the settling velocity of mud flocs and demonstrates the sensitivity of prediction with the settling velocity in terms of flocs concentration. Although the two-phase modelling approach can in principle better capture the essential interactions between fluid and sediment phases, its practical advantages over the simpler single phase approach cannot be confirmed for the data periods simulated, partly because the overall suspended sediment concentration measured is rather low and the interaction between the two phases is weak and also because the uncertainties in the relationship between the settling velocity and flocs concentration.

Suspended clays and silts: Are they independent or dependent fractions when it comes to settling in a turbulent suspension?

This research examines whether or not mixes of clay and silt in a turbulent suspension act and settle independent of each other. More specifically, we examine the following three questions: (1) does the presence of silt in suspension alter the size of mud flocs relative to those from a pure clay suspension of equivalent concentration? (2) can silt particles become bound inside clay flocs?; and (3) if silt is bound within flocs, how does this change the settling properties of the clay and silt mixture? These questions are explored through a series of laboratory experiments in which: (a) images of flocs and silt particles within the turbulent suspension are used to measure their size distribution as a function of time; and (b) the settling velocity of the individual aggregates from each of the suspensions are measured in a settling column. The experiments use pure clay, pure silt, and two different mixture ratios of silt and clay. The results show three primary conclusions. First, the presence of silt has no significant impact on floc size. Second, most of the silt in suspension became bound up within the floc aggregates. And third, the bound silt within the flocs increased the floc settling velocity by at least 50%. These results have potential implications for the modeling of suspended mud mixtures in rivers, estuaries, and turbidity currents.

A PRELIMINARY STUDY OF SUBMARINE COLD SEEPS BY SEISMIC OCEANOGRAPHY TECHNIQUES

AbstractSubmarine cold seeps are widely distributed in the continental margin seas around the world. In this study, we apply a conventional multichannel seismic reflection (seismic oceanography) method to image the water column near the seafloor in order to detect cold seeps. In addition to analyzing the fluid escape structures, we also describe and discuss the development positions, seismic reflection characteristics as well as features of the cold seeps. The seismic reflection from the water column is very weak; therefore, the seismic sections above and below the seafloor are processed in two different sequences as follows: (1) geometry definition, direct wave attenuation and amplitude recovery, high‐pass filter, common midpoint sorting, constant velocity (seawater sound velocity) stack, and post‐stack FK filter in some sections; and (2) data quality control, amplitude recovery, 6∼100 Hz bandpass filter, multiple attenuation, deconvolution, velocity analysis, normal move‐out correction, common midpoint stack, post‐stack noise attenuation, 4∼70 Hz bandpass filter, and FX migration. The processed sections are then assembled together along the seafloor after carefully adjusting the color scale. The analysis shows that active cold seeps primarily present plume, broom, and/or irregular shapes that have weak and chaotic seismic reflections in the water column. The seismic reflection amplitude is enhanced at times; this could be attributed to the suspension of mud or particles in the water column. Cold seepage activities are typically associated with fluid escape structures, including mud diapirs, pipes, faults, fractures, gas chimneys, seabed pockmarks, and mud volcanoes. This indicates fluid migration from deep to shallow strata, fluid seepage or escape at the seafloor, as well as the formation of widespread cold seep activities. All the results herein are derived from a comprehensive interpretation of the seismic sections of the water columns and strata; however, further field studies, theoretical simulations, and experiments are required to confirm these conclusions.

Physical and sedimentary processes on the tidal flat of central Jiangsu Coast, China: Headland induced tidal eddies and benthic fluid mud layers

An 11-tidal-cycle record time series of current, wave, suspended-sediment, and bed-level characteristics was analyzed to identify physical and sedimentary processes on the tidal flat of Jiangsu Coast, China. A tripod observation system was placed near the transition between mid and upper tidal flat south of a newly constructed harbor for hydrodynamic and sediment dynamic measurements from 27 Apr 2013 to 3 May 2013. The observations confirm the stable longshore northward (ebb direction) current s with residual velocities ~59.2° anti-clockwise from the offshore direction. This phenomenon can be attributed to the effects of headland (the Harbor) induced tidal eddies based on comparable frictional length scale and the headland length scale. Benthic fluid mud layers occurred in 2 of 11 tidal cycles, with the conditions of strong waves during the flood phase. The fine sediment was resuspended by the waves and currents from the lower area, transported upward and concentrated at the observation station, resulting in the formation of a fluid mud layer with thickness of ~15cm and SSC of ~8kg/m3 at 10cmasb. Once formed, the fluid mud layer dramatically modified the flow structure, showing a large reduction of current speed from 20cmasb to 10cmasb, when the gradient Richardson number was around the critical value of 0.25, inferring that sufficient turbulence from waves and currents exists to maintain fluid mud suspension. The fluid mud processes appear to occur episodically and may play an important role of sediment dynamics on the tidal flat.

Suspended matter and fluid mud off Alleppey, southwest coast of India

Time series measurements on suspended particulate matter (SPM) were made at one non-mud bank (M1) and two mud bank stations (M2 and M3) off Alleppey, south west coast of India. The mean SPM was low in surface (6.2 mg/l) and mid-depth (3.7 mg/l) waters and higher in bottom-depth waters (24.6 mg/l) of these stations, during both pre-monsoon and monsoon seasons. Near bed suspended sediment results indicated low SPM during July (0.042 g/l) and September (0.018 g/l) at M1, but very high SPM at M2 (9.2 g/l) and M3 (6.2 g/l) during July that decreased (M2: 0.033 g/l; M3: 0.1 g/l) again in September. Observations based on LISST-25X indicated that optical transmission (OT) was high (80–100%) in surface and mid-depth waters but decreased with increasing depth. Near bed waters at M2 and M3 stations showed ∼1 m thick fluid layer with 0% OT and with high particle volume concentrations (150–200 μl/l) during monsoon. Bottom sediments were clayey silts. Sediments of <63 μm fraction were skewed towards finer size at M1 but to coarser size at M2 and M3 stations during monsoon. The water content of the sediment decreased and wet bulk density increased at M2 and M3 stations, while it was the opposite at M1 station during the monsoon. Our results indicated the presence of fluid mud close to the bottom at M2 and M3 stations. As the wave height decreased from ∼3 m in May to 0.76 m in July, the dissipated wave energy probably liquefied, eroded and re-suspended the bottom sediment to form fluid mud. Upwelling currents may have been involved in the up keep and transportation of fluid mud. The suspended mud resettled at the bottom soon after the monsoon event.

Enhancing mud supply from the Lower Missouri River to the Mississippi River Delta USA: Dam bypassing and coastal restoration

Sand transport to the Mississippi River Delta (MRD) remains sufficient to build wetlands in shallow, sheltered coastal bays fed by engineered diversions on the Mississippi River (MR) and its Atchafalaya River (AR) distributary. But suspended mud (silt & clay) flux to the coast has dropped from a mean of 390 Mt y−1 in the early 1950s, to 100 Mt y−1 since 1970. This fine-grained sediment travels deeper into receiving estuarine basins and plays a critical role in sustaining existing marshes. Virtually all of the 300 Mt y−1 of missing mud once flowed from the Missouri River (MOR) Basin before nearly 100 dams were built as part of the Pick-Sloan water development project. About 100 Mt y−1 is now intercepted by main-stem Upper MOR dams closed in 1953. But the remaining 200 Mt y−1 is trapped by impoundments built on tributaries to the Lower MOR in the 1950s and 1960s. Sediment flux during the post-dam high MOR discharge years of 1973, 1993 and 2011 approached pre-dam levels when tributaries to the Lower MOR, including the Platte and Kansas Rivers, contributed to flood flows. West bank tributaries drain a vast, arid part of the Great Plains, while those entering from the east bank traverse the lowlands of the MOR floodplain. Both provinces are dominated by highly erodible loess soils. Staunching the continued decline in MR fine-grained sediment flux has assumed greater importance now that engineered diversions are being built to reconnect the Lowermost MR to the MRD. Tributary dam bypassing in the Lower MOR basin could increase mud supply to the MRD by 100–200 Mt y−1 within 1–2 decades. Such emergency measures to save the MRD are compatible with objectives of the Missouri River Restoration and Platte River Recovery Programs to restore MOR riparian habitat for endangered species. Rapid mobilization to shunt fine-grained sediments past as many as 50 Lower MOR tributary dams in several U.S. states will undoubtedly require as much regional coordination and funding in the 21st century as the monumental effort it took to build the dams in the last century.

Mud-Dominated Basin-Margin Progradation: Processes and Implications

Abstract The accretion of coarse-grained material at the shelf-edge rollover has been emphasized in studies of basin-margin progradation, despite fine-grained sediment (clay and silt) representing a volumetrically more significant component of subaqueous clinothems. The timing and processes of fine-grained sediment transport across the shelf and onto the slope remains an understudied facet of sedimentary-basin stratigraphy. Three exhumed basin-margin-scale clinothems of the Permian Waterford Formation, in the Karoo Basin, South Africa, offer outcrop examples of margin development through the accretion of mud during flooded shelf conditions. The progradation of wave-influenced sandy shelf topset deposits over a thick mudstone succession and beyond a previously established sand-rich shelf-edge rollover suggests that some periods of basin-margin progradation took place exclusively via dilute mud-rich gravity flows. Detailed outcrop and core study of offshore mudstones reveals a high content of organic debris and mica. Individual beds show normal and inverse grading, internal erosion surfaces, and moderate to low bioturbation, reflecting relatively stressed conditions in frequently supplied outer-shelf to upper-slope regions. The estimated low gradient (< 0.7°) of the Karoo Basin margin and prevailing wave and/or storm conditions facilitated prolonged suspension of fluid mud and transport across the shelf and beyond the shelf-edge rollover in sediment gravity flows. This study represents a rare example of mudstone-dominated shelf-edge rollover deposits documented at outcrop and core, and demonstrates how accretion of fine-grained sediment can play a significant role in basin-margin progradation. Conventional depositional models do not adequately account for progradation of basin margins in the absence of sand supply, which implies potential risks in the identification of shelf-edge rollover positions and application of trajectory analysis in strongly progradational margins.

Field studies of estuarine turbidity under different freshwater flow conditions, Kaipara River, New Zealand

We present a first interpretation of three days of measurements made in 2013 from the tidal reaches of the Kaipara River (New Zealand) under both low and high freshwater inputs and a neap tidal cycle. During the first day, we occupied two stations that were approximately 6 km apart in a tidal reach that runs for 25 km from the river mouth to the upstream limit of tidal influence. During the second day, longitudinal surveys were conducted over a distance of 6 km centred on the upstream station. The data reveal a turbidity maximum in the form of a high-concentration ‘plug’ of suspended mud that was advected downstream on the ebbing tide past the upper (HB) measurement station and which exchanged sediment with the seabed by settling at low slack water and by resuspension in the early flooding tide. The data suggest that fine sediment is transported landwards and trapped in the upper part of the tidal reach under these low-flow conditions. On the third day of measurements we repeated the experiments of the first day but later in the year, for a much higher freshwater flow. This interpretation of our data set highlights the potential contribution of a range of processes to the generation of the observed suspended-sediment signals, including resuspension of local bed sediment, advection by the tidal current, settling of suspended sediment over a long timescale compared to the advection timescale, advection of longitudinal gradients in suspended sediment, and suppression of vertical mixing by density stratification of the water column. The level of temporal and spatial detail afforded by these measurements allows a much clearer understanding of the timing and importance of vertical stratification on the transport of suspended particulate matter than is generally possible using fixed-point sensors.