Session E29: Geophysical Fluid Dynamics: Sediment Transport

Numerical simulations and modeling of turbidity currents from a moving source

We present numerical simulations of turbidity currents generated by a moving source as a model of particle clouds released by underwater vehicles such as those used in deep-sea dredging and mining. We use two approaches to model the flow and resulting deposits. First, we adapt the shallow-water equations to include a moving source of both momentum and particles. Second, we modify the classical box model to include a moving source. The most important governing dimensionless parameters are a Froude number, Fr, based on the depth of sediments being resuspended and the dimensionless settling speed of those sediments, u_s. We find that Fr is most determinant in the current and deposit shape, with Fr>2 resulting in elongated shapes and Fr<2 resulting in rounder ones, and that u_s determines the rate at which deposits grow. Overall, both models found similar trends, but the box model, while much less computationally expensive, was less accurate when spatial non-uniformities developed, as happens at long times.   

Study of ejecta properties during plume surface interactions using three-dimensional optical diagnostic technique

Plume surface interaction (PSI), which is the interaction between the rocket plume and planetary surface, leads to crater formation and dust plumes at the landing site and was one of the major problems faced during the Apollo program. With the Artemis program, NASA is aiming to return humans back to the moon, and a comprehensive understanding of the PSI process is vital for ensuring the safety of future missions. While the crater formation process during PSI has been studied over the years, accurate data on the behavior of the particle ejecta from PSI process is scarce, particularly from unobstructed full-domain experiments. In this study, we apply three-dimensional optical diagnostic techniques to study the simultaneous temporal velocity and trajectory of the particle ejecta and crater evolution in a full-domain, atmospheric, bench-scale facility. Preliminary results show that the particles were ejected more vertically with reduced velocity over time while the crater radius growth stagnated, and the crater depth continued increasing. Further investigation and analysis of these phenomena along with the contributing mechanisms will be presented.   

In-situ sedimentological analysis of deep sea mining trials generated sediment plumes

The impact of polymetallic nodule mining on the deep sea environment remains poorly understood. To predict the impact of such operations, better insight into the fluid dynamics of sediment plumes produced by deep sea mining operations. In particular, accurate size and settling velocity distributions  (SVD) of suspended sediments are necessary to better inform plume models of sediment transport evolution. Characterizing deep sea particles in situ is crucial in understanding the variability of deep-sea sediments through different timescales and regions. Currently, there is no instrument available that can characterize in-situ these particles with high enough resolution. The RTSSV has thus been developed to measure in real-time, the SVD of deep sea particles by Sequoia Scientific, Inc., with MIT and Scripps Institution of Oceanography. Field experiments were performed in the Belgian area of the Clarion Clipperton Fracture Zone (CCFZ), where the RTSSV was first deployed to measure particle size distribution of sediment laden plumes generated by a collector. This constitutes, to the best of our knowledge, the first in-situ direct imaging of deep-sea sediments during a deep sea mining test operation at such high resolutions and a step closer to predicting the impact of deep sea mining.   

A simple advection-diffusion-settling model for estimating deep-sea mining sediment plume transport

Large regions of the deep ocean are covered in potato-sized nodules rich in metals and minerals that are commonly mined on land for the production of batteries. An emerging deep-sea mining industry proposes to use large vehicles to collect nodules from the seabed. During nodule mining, a collector vehicle will pick up sediment as it drives along the seabed. Most of this unwanted sediment is expected to be discharged at the rear of the vehicle, close to the seabed, resulting in a so-called seabed plume. The remaining sediment is lifted up with the nodules to a surface operation vessel, where it is separated from the nodules and returned back to the ocean, either in the midwater column or near the seabed. The seabed and discharge sediment plumes lie at the core of the indirect impact of deep-sea mining on the ocean environment, yet their evolution remains a topic of speculations and misconceptions. Here, we employ established principles of fluid dynamics, namely advection, diffusion and settling, to derive simple predictions of the evolution of both seabed and discharge plumes over their entire lifespan.   

Large-scale motion flow physics regulate dust entrainment rates

Wind-driven hopping of sand particles (saltation) starts when the imposed aerodynamic stress exceeds a fluid threshold. This results in dust emission from the sediment bed, which continues until the surface stress falls below an impact threshold. Elongated regions of relatively high and low momentum (HMR, LMR) aligned with the dominant flow direction are observed in atmospheric surface layer turbulence. High momentum regions impose high stress, thereby mobilizing sand grains, but fail to entrain dust owing to the associated negative vertical velocity. On the other hand, low momentum regions do not initiate saltation, but embody the positive vertical velocity required for entrainment. This entrainment paradox indicates the presence of alternate entrainment mechanisms. Turbulent stress production by sweeps, ejections, inner interactions and outer interactions reveal three modes of dust entrainment. LMRs immediately behind HMRs encounter hysteretic saltation – the remnant of saltation initiated beneath an earlier HMR – and entrain dust; dust-rich fluid from near the base of HMRs is pumped to the adjacent LMRs by interfacial streamwise vorticity; and outer interactions impose high surface stress and directly entrain.   

Collapse of sand columns submerged in water: the role of the aspect ratio and the grain size.

Granular flows occur in numerous natural phenomena and industrial applications. However, they often exhibit counter-intuitive behavior and many of their features are still not well understood. In this talk, we report on results from laboratory-scale simulations of an archetypal type of such flows, namely, the collapse of sand columns, submerged in water, under their own weight. Our simulations are based on a two-pressure, two-velocity flow model for granular suspensions. This model incorporates a non-linear representation of the granular rheology and an evolution equation for the volume fraction of the granular phase. The algorithm used to solve the governing equations is based on a predictor-corrector time-integration scheme and incorporates a generalized projection method for the dynamic pressures of the two phases. First, we briefly describe the mathematical model. We then present results from our parametric studies with respect to the aspect ratio and the grain size. More specifically, we elaborate on the effect of these parameters on the time evolution of the column collapse as well as on the shape and dimensions of the remnant pile. The emerging flow patterns and the amount of sand transported away from the column are also discussed herein.   

Not Participating

Wave storms transport and sort coarse gravel along coasts. This fundamental process is important under changing sea-levels and increased storm frequency and intensity. However, limited information on intra-storm clast motion restricts theory development for coastal gravel sorting and coastal management under stormy longshore transport. Here, we use 'smart boulders' equipped with loggers recording underwater, real-time, intra-storm clast motion, and measured longshore displacement of varied-mass marked boulders during storms. We utilize the unique Dead Sea setting where rapidly falling water levels allow isolating boulder transport and sorting during individual storms. Guided by these observations, we developed a new model quantifying the critical wave height for a certain clast mass entrainment. Then, we obtained an expression for the longshore clast displacement under the fluid-induced pressure impulse of a given wave. Finally, we formulate the sorting enforced by wave-height storm distributions, demonstrating how sorting is a direct manifestation of regional, turned-into-local hydroclimatology.   

Investigation of sediment transport in an oscillatory boundary layer using Eulerian-Lagrangian simulations.

Areas where the sea bottom is made of loose sediments are particularly prone to erosion caused by the action of waves, which is exacerbated by rising sea levels. The successful modeling of these effects requires an in depth understanding of the interaction between turbulent flow features and sediment grains. In this talk, we discuss the effects of oscillating pressure gradients caused by surface waves on the formation of bedforms and grain entrainment in a sandy seabed using high-fidelity Eulerian-Lagrangian simulations. In this approach, conservation equations for the fluid phase are solved on a Cartesian grid, while each individual sand grain is tracked and updated in a Lagrangian frame. A particle bed is placed at the bottom of the domain with height 25 particle diameters, and a particle diameter of 500[endif]-->m. Four cases are considered where the period of oscillations is maintained constant at 7s and the magnitude of the pressure gradient is varied to yield the Reynolds numbers 100, 200, 400 and 1790. The first three cases yield an oscillatory boundary layer in the laminar regime, whereas the fourth case falls under the intermittently turbulent regime. Statistics from the case at Reynolds 1790 are shown to agree with prior experimental observations, thus validating the numerical approach. The bedforms and particle transport statistics at the four different Reynolds numbers are compared and related to the varying turbulence intensities in the carrier fluid. Furthermore, to understand the role of the sediment grains in modifying flow features, auxiliary simulations with no particles, i.e., simulations of oscillatory boundary layers on a smooth flat wall, are conducted at the four same Reynolds numbers and compared with the simulations with sandy beds. These comparisons quantify the effect of particle dynamics upon the oscillating flow.   

Wet granular flows over a rough incline: frictional and cohesive rheology

Multi-phase flows encountered in nature or industry (such as landslides, mudflows, powder mixtures, ...) exhibit non trivial rheological properties, that can be understood better thanks to model materials and appropriate rheometers. Here, we use unsaturated wet granular materials: assemblies of frictional spherical particles bonded by a small quantity of a wetting liquid, over a rough inclined plane. This later is relevant for free-surface flows of particulate solids, because of applications and  theoretical concerns about constitutive modelling, as regimes of steady uniform flows lead to effective friction controlled experiments.

Our results show steady uniform flows for a wide range of parameters (the inclination angle and the mass flow-rate).  

A theoretical model, based on the Mohr-Coulomb yield criterion, is extended to inertial flows: τ = τ_c + μ(I) P,  in which τ_c and μ(I) are the cohesion stress and the internal friction coefficient respectively. The cohesion stress induces the emergence of a height threshold for shear and leads to localization of shear in the bottom layer with a non-sheared top layer (plug flow). 

Predictions are in quantitative agreement with experimental measurements only when one considers that dry and wet samples have straightforwardly different internal friction coefficients commonly described by the so-called μ(I)-rheology.

The liquid bridges bounding grains not only induce cohesion, but modify the internal friction of the wet assemblies, by enhancing it.