{A numerical scheme is developed to simulate the transport of natural gravel. Starting with computerized tomographic (CT) scans of natural grains, our method approximates the shapes of these grains by “gluing” spheres of different sizes together with overlaps. The conglomerated spheres move using a Discrete Element Method (DEM) which is coupled with a Lattice Boltzmann Method (LBM) fluid solver, forming the first complete workflow from particle shape measurement to high resolution simulations with hundreds of distinct shapes. The simulations are quantitatively benchmarked by flume experiments. The numerical tool is used to further validate a recently proposed modified sediment transport relation, which takes particle shape effects into account, including the competition between hydrodynamic drag and material friction. Unlike a physical experiment, our simulations allow us to vary the hydrodynamic drag coefficient of the natural gravel independently of the material friction. Our studies support the modified sediment transport relation. The simulations also provide insights on the particle-level kinematics, such as particle orientations, in the bedload transport process. Particles below the bed surface prefer to orient with their shortest axes perpendicular to the bed surface, but the tendency goes down as the packing fraction decreases far from the bed surface. The particles rotate freely in the dilute particle flow regime. }

Titan is unique among icy moons for its active surface processes and extensive erosional features. The presence of coarse sediment suggests that mechanical weathering breaks down Titan’s surface material, but the exact processes of mechanical weathering are unknown. We tested the idea that topographic features perturb ambient crustal stresses enough to generate or enhance fractures. We used a boundary element model to predict the likely stress state within hypothetical Titan landforms, including river valleys and isolated ridges, and to model the locations and types of resulting fractures. Our results suggest that topographic stress perturbations are indeed sufficient to generate fractures and drive mechanical weathering, with little dependence on the density of the material making up Titan’s crust and landforms and no dependence on its elastic moduli. For material density of 800 to 1200 kg/m3, opening-mode failure is predicted to occur within hypothetical Titan landforms with relief exceeding 20 m at ambient horizontal stresses up to 1 MPa of compression, which encompasses typical predicted tidal stresses ranging between 10 kPa of compression and 10 kPa of tension. Under the same conditions, shear fracture is predicted to occur if cohesion of the material is less than 100 kPa or if hydrocarbon fluids reduce local effective normal stresses. We therefore suggest that Titan’s crust may be highly fractured and permeable, and that the predicted fractures could create pathways for sediment generation and subsurface transport of fluids.

Near the threshold of grain motion, sediment transport is “on-off” intermittent, characterized by large but rare bursts separated by long periods of low transport. Without models that can predict the presence of intermittency, measurements of average sediment flux can be in error by up to an order of magnitude. Despite its known presence and impact, it is not clear whether on-off intermittency arises from the grain activity (the number of moving grains) or grain velocities, which together determine the sediment flux. We use laboratory flume experiments to show that the on-off intermittency has its origins in the velocity distributions of grains that move by rolling along the bed, whereas grain activity is not on-off intermittent. Improved predictions of sediment flux require that the types of intermittency we identify be incorporated into stochastic models of sediment flux. Their recognition opens the door to physically based uncertainty estimates of time-averaged sediment flux.

Sediment transport in rivers near the threshold of grain motion is characterized by rare but large transport events. This intermittency makes it difficult to relate average sediment flux to average flow conditions, or to define an unambiguous threshold for grain entrainment. Although intermittent sediment transport can be observed and characterized, its origins are unclear. In this study we investigate bedload sediment transport near the threshold of grain motion in an experimental flume to examine the origins of intermittency. We apply image-processing techniques to high-speed video of grains in a narrow flume, which allows us to track individual particles and measure statistics of particle motion. Bedload sediment transport near the threshold of grain motion is very low, allowing us to approximate the time evolution of the sediment flux via a polynomial expansion, including a linear growth rate and a nonlinear term which saturates the growth. We introduce a noisy coefficient to the linear growth rate term (“multiplicative noise”), rather than adding the noise to the equation, to model the inherent fluctuations in the system. We demonstrate that multiplicative noise near the threshold of grain motion can account for the observed intermittency. We use analytical results from bifurcation theory in the presence of multiplicative noise to analyze our experimental results, quantifying the noise responsible for the intermittency and calculating the critical shear stress for grain entrainment in a novel way that is consistent with the physics of grain motion at low transport stages.

Large alluvial rivers transport water and sediment across continents and shape lowland landscapes. Repeated glacial cycles have dominated Earth's recent climate, but it is unclear whether these rivers are sensitive to such rapid changes. The Amazon River system, the largest and highest-discharge in the world, features extensive young terraces that demonstrate geologically rapid change temporally correlated with changes in runoff from Quaternary climate cycles. To test the plausibility of a causal relationship, we use a simple model to estimate from empirical measurements how quickly a river profile responds to changes in discharge or sediment supply. Applying this model to data from 30 gauging stations along alluvial rivers throughout the Brazilian Amazon, we find that many rivers of the Amazon basin can respond faster than glacially induced changes in runoff or sediment flux. The Amazon basin is unusually responsive compared to other large river systems due to its high discharge and sediment flux, narrow floodplains, and low slopes. As a result, we predict that the Amazon basin has been highly dynamic during Quaternary glacial cycles, with cyclical aggradation and incision of lowland rivers driving repeated habitat and environmental change throughout the region. This dynamic landscape may have contributed to the exceptional biodiversity of the region and patterns of ancient human settlement.