Equilibrium sediment transport, grade and discharge for suspended-load-dominated flows on Earth, Mars and Titan

© 2020 The Authors Recent surface observations have emphasised the importance of bedload transport by rivers and streams on Mars and Titan. Previous “hydraulic” analysis, however, has also shown that transport as suspended load should be possible, if not more prevalent due to lower critical shear stresses, under reduced gravity conditions compared with Earth; where the dominant mode of sediment flux by rivers is as suspended load. A new suspended-load equilibrium condition (zero net erosion and deposition) constraint is advanced, using a flux-balance model applicable to transport (capacity) limited alluvial channels. The theory unifies Gilbert's relation for slope and flow capacity model with the Shields criterion for incipient motion. Shields-stress analysis shows that sediment transport thresholds on Earth, Mars and Titan are dynamically similar. Thus, despite large differences in dimensional shear stresses, variations in the critical slopes and flow depths required for threshold and equilibrium conditions are comparatively modest. Under reduced gravity conditions, there exists a slightly higher potential for sediment transport as suspended load; except for systems involving high particle-fluid density ratios, as may apply to the transport of organic particles on Titan. Critically we show that graded suspended-load channels should develop gentler slopes under reduced gravity, and not steeper slopes as previously inferred based on generalized hydraulic geometry relationships of terrestrial river and submarine channels. We further argue that the use of submarine channels as analogues for channels formed under reduced gravity is not physically justified. Finally, compared with bedload channels, suspended-load channels should also have markedly higher discharges, with implications for mechanistic-based discharge predictions and precipitation models

The coupled dynamics of internal waves and hairpin vortices in stratified plane Poiseuille flow

A simulation of stably stratified plane Poiseuille flow at a moderate Reynolds number (Formula Presented) and Richardson number (Formula Presented) is presented. For the first time, the dynamics in the channel core are shown to be described as a series of internal waves that approximately obey a linear wave dispersion relationship. For a given streamwise wavenumber Formula Presented there are two internal wave solutions, a dominant low frequency mode and a weaker-amplitude high-frequency mode, respectively corresponding to ‘backward’ and ‘forward’ propagating internal waves relative to the mean flow. Analysis of linearised equations shows that the dominant low-frequency mode appears to arise due to a particularly sensitive response of the mean flow profiles to incoherent forcing. Instantaneous visualisations reveal that hairpin vortices dominate the outer region of the channel flow, neighbouring the buoyancy dominated channel core. These hairpins are fundamentally different from those observed in canonical unstratified boundary layer flows, as they arise via quasi-linear local processes far from the wall, governed by background shear. Outer region ejection events are common and can be induced by high amplitude waves. Ejected hairpins are transported into the channel core, in turn ‘ringing’ the prevailing strong buoyancy gradient and thus generating high-amplitude internal waves, high dissipation and wave breaking, induced by spanwise vortex stretching and baroclinic vorticity generation. Such spontaneous and sustained generation of quasi-linear internal waves by wall-bounded sheared turbulence may provide novel idealised solutions for, and insight into, large-scale turbulent mixing in a wide range of environmental and industrial flows

Circulation of hydraulically ponded turbidity currents and the filling of continental slope minibasins

Natural depressions on continental margins termed minibasins trap turbidity currents, a class of sediment-laden seafloor density driven flow. These currents are the primary downslope vectors for clastic sediment, particulate organic carbon, and microplastics. Here, we establish a method that facilitates long-distance self-suspension of dilute sediment-laden flows, enabling study of turbidity currents with appropriately scaled natural topography. We show that flow dynamics in three-dimensional minibasins are dominated by circulation cell structures. While fluid rotation is mainly along a horizontal plane, inwards spiraling flow results in strong upwelling jets that reduce the ability of minibasins to trap particulate organic carbon, microplastics, and fine-grained clastic sediment. Circulation cells are the prime mechanism for distributing particulates in minibasins and set the geometry of deposits, which are often intricate and below the resolution of geophysical surveys. Fluid and sediment are delivered to circulation cells by turbidity currents that runup the distal wall of minibasins. The magnitude of runup increases with the discharge rate of currents entering minibasins, which influences the amount of sediment that is either trapped in minibasins or spills to downslope environs and determines the height that deposits onlap against minibasin walls

Effect of hydro-climate variation on biofilm dynamics and its impact in intertidal environments

Funding: This research has been supported by the European Research Council under the European Union's Horizon 2020 research and innovation program (grant no. 725955).Shallow tidal environments are very productive ecosystems but are sensitive to environmental changes and sea level rise. Bio-morphodynamic control of these environments is therefore a crucial consideration;however, the effect of small-scale biological activity on large-scale cohesive sediment dynamics like tidal basins and estuaries is still largely unquantified. This study advances our understanding by assessing the influence of biotic and abiotic factors on biologically cohesive sediment transport and morphology. An idealised benthic biofilm model is incorporated in a 1D morphodynamic model of tide-dominated channels. This study investigates the effect of a range of environmental and biological conditions on biofilm growth and their feedback on the morphological evolution of the entire intertidal channel. By carrying out a sensitivity analysis of the biomorphodynamic model, parameters like (i) hydrodynamic disturbances, (ii) seasonality, (iii) biofilm growth rate, (iv) temperature variation and (v) bio-cohesivity of the sediment are systematically changed. Results reveal that key parameters such as growth rate and temperature strongly influence the development of biofilm and are key determinants of equilibrium biofilm configuration and development under a range of disturbance periodicities and intensities. Long-term simulations of intertidal channel development demonstrate that the hydrodynamic disturbances induced by tides play a key role in shaping the morphology of the bed and that the presence of surface biofilm increases the time to reach morphological equilibrium. In locations characterised by low hydrodynamic forces, the biofilm grows and stabilises the bed, inhibiting the transport of coarse sediment (medium and fine sand). These findings suggest biofilm presence in channel beds results in intertidal channels that have significantly different characteristics in terms of morphology and stratigraphy compared abiotic sediments. It is concluded that inclusion of bio-cohesion in morphodynamic models is essential to predict estuary development and mitigate coastal erosion.Publisher PDFPeer reviewe

Lobbying regulations and unintended consequences of interest group influence

This paper studies and analyzes the effects of lobbying and campaign finance regulations on interest group influence in state legislatures. The conventional wisdom supports the belief that lobbying regulations lead to decreased interest group influence. However, this may not be the case; there is some evidence that lobbying regulations may increase interest group influence rather than decrease it. To test these conflicting findings, I utilize the theory of punctuated equilibrium to claim that interest group mobilization is the missing link between these studies. I only end up finding partial support for my theory. Specifically, I find two independent effects. While interest group mobilization does not mediate for lobbying regulations and interest group influence, I do find evidence that interest group mobilization increases interest group influence in the short term. I also find that lobbying regulations increase interest group influence, which is contrary to conventional wisdom. These findings show that more research is needed to fully understand the effects of lobbying regulations on interest group influence in state legislatures. If interest group mobilization is not the cause of rising influence, then there may be some other factors that scholars should consider when studying this in the future

Effect of hydro-climate variation on biofilm dynamics and its impact in intertidal environments

Shallow tidal environments are very productive ecosystems but are sensitive to environmental changes and sea level rise. Bio-morphodynamic control of these environments is therefore a crucial consideration; however, the effect of small-scale biological activity on large-scale cohesive sediment dynamics like tidal basins and estuaries is still largely unquantified. This study advances our understanding by assessing the influence of biotic and abiotic factors on biologically cohesive sediment transport and morphology. An idealised benthic biofilm model is incorporated in a 1D morphodynamic model of tide-dominated channels. This study investigates the effect of a range of environmental and biological conditions on biofilm growth and their feedback on the morphological evolution of the entire intertidal channel. By carrying out a sensitivity analysis of the bio-morphodynamic model, parameters like (i) hydrodynamic disturbances, (ii) seasonality, (iii) biofilm growth rate, (iv) temperature variation and (v) bio-cohesivity of the sediment are systematically changed. Results reveal that key parameters such as growth rate and temperature strongly influence the development of biofilm and are key determinants of equilibrium biofilm configuration and development under a range of disturbance periodicities and intensities. Long-term simulations of intertidal channel development demonstrate that the hydrodynamic disturbances induced by tides play a key role in shaping the morphology of the bed and that the presence of surface biofilm increases the time to reach morphological equilibrium. In locations characterised by low hydrodynamic forces, the biofilm grows and stabilises the bed, inhibiting the transport of coarse sediment (medium and fine sand). These findings suggest biofilm presence in channel beds results in intertidal channels that have significantly different characteristics in terms of morphology and stratigraphy compared abiotic sediments. It is concluded that inclusion of bio-cohesion in morphodynamic models is essential to predict estuary development and mitigate coastal erosion.</p

Progressive and biased divergent evolution underpins the origin and diversification of peridinin dinoflagellate plastids

Dinoflagellates are algae of tremendous importance to ecosystems and to public health. The cell biology and genome organization of dinoflagellate species is highly unusual. For example, the plastid genomes of peridinin-containing dinoflagellates encode only a minimal number of genes arranged on small elements termed "minicircles". Previous studies of peridinin plastid genes have found evidence for divergent sequence evolution, including extensive substitutions, novel insertions and deletions, and use of alternative translation initiation codons. Understanding the extent of this divergent evolution has been hampered by the lack of characterized peridinin plastid sequences. We have identified over 300 previously unannotated peridinin plastid mRNAs from published transcriptome projects, vastly increasing the number of sequences available. Using these data, we have produced a well-resolved phylogeny of peridinin plastid lineages, which uncovers several novel relationships within the dinoflagellates. This enables us to define changes to plastid sequences that occurred early in dinoflagellate evolution, and that have contributed to the subsequent diversification of individual dinoflagellate clades. We find that the origin of the peridinin dinoflagellates was specifically accompanied by elevations both in the overall number of substitutions that occurred on plastid sequences, and in the Ka/Ks ratio associated with plastid sequences, consistent with changes in selective pressure. These substitutions, alongside other changes, have accumulated progressively in individual peridinin plastid lineages. Throughout our entire dataset, we identify a persistent bias toward non-synonymous substitutions occurring on sequences encoding photosystem I subunits and stromal regions of peridinin plastid proteins, which may have underpinned the evolution of this unusual organelle.Wellcome Trus

Streamwise turbulence modulation in non-uniform open-channel clay suspension flows

Cohesive sediment particles are ubiquitous in environmental flows. The cohesive properties of clay promote the formation of clay flocs and gels and relatively small suspended clay concentrations can enhance or suppress turbulence in a flow. Furthermore, flows are naturally non-uniform, varying in space and time, yet the dynamics of non-uniform open-channel clay suspension flows is poorly understood. For the first time, the adaptation time and length scales of non-uniform clay suspension flows were quantified using novel experiments with spatially varying but temporally uniform flow. Different levels of turbulence enhancement and attenuation were identified as the flow decelerates or accelerates. Results highlight that decelerating clay suspension flows crucially have a longer adaptation time than accelerating clay suspension flows. This is explained by the longer timescale required for the formation of bonds between cohesive particles in turbulence attenuated flows after deceleration than the rapid breakdown of bonds in turbulent flows after acceleration of clay suspension flows. This hysteresis is more pronounced for higher concentration decelerating flows that pass through a larger variety of clay flow types of turbulence enhancement and attenuation. These different adaptation time scales and associated clay flow type transitions are likely to affect clay flow dynamics in a variety of fluvial and submarine settings

Dynamics and deposition of sediment-bearing multi-pulsed flows and geological implication

Previous studies on dilute, multi-pulsed, subaqueous saline flows have demonstrated that pulses will inevitably advect forwards to merge with the flow front. On the assumption that pulse merging occurs in natural-scale turbidity currents, it was suggested that multi-pulsed turbidites that display vertical cycles of coarsening and fining would transition laterally to single-pulsed, normally graded turbidites beyond the point of pulse merging. In this study, experiments of dilute, single- and multi-pulsed sediment-bearing flows (turbidity currents) are conducted to test the linkages between downstream flow evolution and associated deposit structure. Experimental data confirm that pulse merging occurs in laboratory-scale turbidity currents. However, only a weak correspondence was seen between longitudinal variations in the internal flow dynamics and the vertical structure of deposits; multi-pulsed deposits were documented, but transitioned to single-pulsed deposits before the pulse merging point. This early transition is attributed to rapid sedimentation-related depletion of the coarser-grained suspended fraction in the laboratory setting, whose absence may have prevented the distal development of multi-pulsed deposits; this factor complicates estimation of the transition point in natural-scale turbidite systems