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The structure and propagation of lock-release bottom gravity currents in a linearly stratified ambient with the presence of a submerged canopy are investigated for the first time using large-eddy simulations. The canopy density (i.e. the solid volume fraction), the strength of ambient stratification and the canopy height are varied to study their respective effects on the gravity current. Both denser canopies and stronger ambient stratification tend to switch the horizontal boundary along which the current propagates from the channel bed towards the canopy top (i.e. the through-to-over flow transition). It is found that the dilution of the current density is enhanced by denser canopies but is weakened by stronger ambient stratification. The non-monotonic relationship between front velocity and canopy density proposed by Zhou et al. (J. Fluid Mech., vol. 831, 2017, pp. 394–417) in homogeneous environments is also observed in stratified environments. However, as the ambient stratification is strengthened, the present study shows a shift of the turning point (beyond which increasing canopy density leads to faster current propagation) towards sparser canopies, accompanied by a more pronounced recovery of the front velocity. This is the combined action of three stratification-induced mechanisms: the promotion of through-to-over flow transition (less canopy drag), the upward displacement of current nose in a stably stratified water column (more buoyancy gain) and the weakening of current dilution (less buoyancy loss). Under stronger ambient stratification, the propagation of gravity currents shows a lower sensitivity to the retarding effect of the submerged canopy.

In this paper, the authors review the current state of the science on the dynamics of gravity currents generated by positively and negatively buoyant jet discharges from submerged round outfalls (i.e., a point source) in inland and coastal waters. Specifically, this article focuses on describing gravity currents occurring at both the bottom boundary and the free surface of the receiving fluid. The manmade discharge operations generating both types of gravity currents and their significance to sustainability of the surrounding hydro-environment are first described. The authors then summarize the flow regimes characteristics of these discharges before becoming gravity currents and how those flow regimes influence the dynamics of the gravity currents. The gravity current dynamics in the calm receiving waters are then analyzed. This analysis is followed by an analysis of the influence of the hydrodynamic forces (e.g., currents, turbulence, waves) on the dynamics of gravity currents. Finally, the authors review quantitative modeling approaches for different forms of gravity current, and identify the current knowledge gaps and research needs.

Particle-laden gravity currents propagating in stratified environments, such as turbidity currents induced by floods in estuaries or triggered by landslides in oceans, are important and complicated geophysical processes that require multidisciplinary studies. This paper numerically investigates the dynamic features of lock-release particle-laden gravity currents in linear stratification on a flat bed, with the main focus on the front velocity, entrainment ratio, and energy budget. The direct numerical simulations reveal that the suppressive effect of the ambient stratification on the turbulence may cause a particle-laden current to quickly lose momentum so that the near-constant front velocity of the particle-laden current cannot be maintained if no more particles are resuspended. After the acceleration stage, the entrainment ratio of a particle-laden gravity current barely changes with ambient stratification due to a combined effect from suppressed turbulent structures and deposition of particles. The energy-conversion process is accelerated by particle settling and is suppressed by ambient stratification. Specifically, because of the suppressive effect of a stronger stratification on the turbulence, a larger part of the energy is dissipated by microscopic Stokes flow around particles, while a smaller part of the energy is dissipated by the macroscopic convective motion of the fluids.

We report experimental, theoretical and numerical results on the effects of horizontal heterogeneities on the propagation of viscous gravity currents. We use two geometries to highlight these effects: (a) a horizontal channel (or crack) whose gap thickness varies as a power-law function of the streamwise coordinate; (b) a heterogeneous porous medium whose permeability and porosity have power-law variations. We demonstrate that two types of self-similar behaviours emerge as a result of horizontal heterogeneity: (a) a first-kind self-similar solution is found using dimensional analysis (scaling) for viscous gravity currents that propagate away from the origin (a point of zero permeability); (b) a second-kind self-similar solution is found using a phase-plane analysis for viscous gravity currents that propagate toward the origin. These theoretical predictions, obtained using the ideas of self-similar intermediate asymptotics, are compared with experimental results and numerical solutions of the governing partial differential equation developed under the lubrication approximation. All three results are found to be in good agreement.

Fault zones have the potential to act as leakage pathways through low permeability structural seals in geological reservoirs. Faults may facilitate migration of groundwater contaminants and stored anthropogenic carbon dioxide (CO$_2$), where the waste fluids would otherwise remain securely trapped. We present an analytical model that describes the dynamics of leakage through a fault zone cutting multiple aquifers and seals. Current analytical models for a buoyant plume in a semi-infinite porous media are combined with models for a leaking gravity current and a new model motivated by experimental observation, to account for increased pressure gradients within the fault due to an increase in Darcy velocity directly above the fault. In contrast to previous analytical fault models, we verify our results using a series of analogous porous medium tank experiments, with good matching of observed leakage rates and fluid distribution. We demonstrate the utility of the model for the assessment of CO$_2$storage security, by application to a naturally occurring CO$_2$reservoir, showing the dependence of the leakage rates and fluid distribution on the fault/aquifer permeability contrast. The framework developed within this study can be used for quick assessment of fluid leakage through fault zones, given a set of input parameters relating to properties of the fault, aquifer and fluids, and can be incorporated into basin-scale models to improve computational efficiency. The results show the utility of using analytical methods and reduced-order modelling in complex geological systems, as well as the value of laboratory porous medium experiments to verify results.

Lateral gravity currents can play a critical role in the exchange of materials between terrestrial and marine ecosystems. The three-dimensional flow structure and mixing process of the gravity current, discharged from a lateral rectangular lock into an ambient fluid, are investigated by solving Unsteady Reynolds-averaged Navier-Stokes equations with the RNG k-ε turbulence model. The accuracy and consistency of the developed model are checked using the experimental data of the lock-exchange in the straight channel, lateral flow in the open channel and mass exchange in the cavity. The agreement between measured and simulated flow and concentration fields is reasonable. The lateral gravity current without the main channel discharge spreads radially out and is arrested by the other bank of the main channel. Before reaching the other bank, the lateral gravity current evolves into the acceleration, slumping, and inertial phases. The gravity current remains in the slumping phase at a straight channel without the tributary. The mixing layer is more diluted at the lateral gravity current. As the main channel discharge increases, the symmetry plume is broken, and the dense fluid propagation is limited toward the confluence upstream. The time-evolution of discharged spatially averaged density decreases due to increasing the main channel discharge. The mixing process assessment indicated that the entrainmant of the lateral gravity current without the main channel discharge is more intense compared with the cases having discharge. The decreasing lateral Froude number associated with increasing main channel discharge leads to a decrease in the entrainment.

In this study, repeated lock-exchange experiments under well-controlled conditions were carried out to evaluate the uncertainty of the macro-propagation and entrainment process and the statistical variation/correlation of the current parameters. The results show that the lobe and cleft structures grow in amplitude while short in wavelength as current propagates, which enlarges the uncertainty of the gravity current propagation. A larger density difference inhibits the split process of the lobe and cleft structures and reduces the fluctuation degree of the current front edge. The macroscopic propagation parameters of the gravity current for the repeated experiment all meet the normal distribution, confirmed by the Shapiro–Wilk test. The mapping relationship between the dimensionless current front velocity and the front height forms a “circle-shaped” mode, while the corresponding relationship between the dimensionless current front velocity and the front height performs wedge-shaped. The evolution trend of the variation coefficient of the mixing layer area is that the two quasi-stationary periods are connected by a sharp decline. The variability of the former quasi-stationary period is stronger than the latter one, and the variation strength of the two quasi-stationary periods is both positively correlated with the initial density difference. The uncertainty at the early stage of the entrainment process is dominated due to the evolution of the mixing layer. However, the lobe and cleft structure provide another uncertainty source of the entrainment coefficient at the later stage. In addition, the uncertainty of the current propagation brought by image resolution is far less than that formed by the evolution of the gravity current itself.

The propagation and hydrodynamic processes of lock-exchange gravity current through vegetation regions were investigated experimentally. Experimental results show that the presence of vegetation can prominently advance the transition position of the gravity current from the slumping phase to the self-similar phase. The process of two-heads propagation can be divided into three stages: the coordinated advance stage (vegetation height controls), the overtaking stage (entrainment process and vegetation friction dominate), and the merge stage (vegetation density controls). After an adjustment period, the bulk slope angle βb of the triangular gravity current converges to a constant terminal value. A modified empirical equation is fitted to better meet the stable convergence of the terminal bulk slope angle βb. The mixing layer formed at the transverse boundary between the vegetation and no-vegetation part can promote fluid entrainment. For the submerged vegetation, the gravity current flows over a new “wall boundary”, i.e. the top of the vegetation region, and causes the negative vorticity. The changes in the vorticity field indicate the presence of vegetation can significantly affect the internal flow-field structure of gravity currents.Article HighlightsThe process of two-heads propagation for gravity current flowing within submerged vegetation can be divided into three stages: the coordinated advance stage (vegetation height controls), the overtaking stage (entrainment process and vegetation friction dominate), and the merge stage (vegetation density controls).A modified empirical equation is fitted to better meet the stable convergence of the terminal slope angle βb for lock-exchange gravity current within vegetation.The vegetation exerts resistance on the gravity current and suppresses the K-H instability at the interface, which weakens the entrainment process, but the mixing layer formed at the transverse boundary of the vegetation region promotes fluid entrainment.

In this laboratory study, propagation behaviour, particle deposition patterns, and suspension characteristics of non-cohesive particle-driven gravity currents formed under constant-volume release conditions were investigated. The experimental gravity currents were created in a two-dimensional lock exchange type tank using two different particles (silicon carbide and glass beads) with four different median diameters. Video imaging and image processing techniques were utilized to monitor the current propagation, laser diffraction size analysis and dry weighing techniques were utilized to examine the size and mass characteristics of the deposits and suspensions, and acoustic Doppler velocimetry was utilized for flow velocity measurements for turbulence analysis. Our observations showed that the experimental gravity currents experienced two different propagation phases based upon the particle settling regimes. The first propagation phase was named as the propagation with the turbulence-dominated settling (TDS) and the later propagation phase was named as the propagation with gravity-dominated settling (GDS). It is found that a critical turbulent Reynolds number value (estimated to be O(1)) delineates the settling regimes, hence determines the transition between the propagation phases. With increasing particle settling velocity, the observed propagation phases in our experimental currents showed increasing deviations from the slumping, inertia-buoyancy, and viscous–buoyancy propagation phases that have been reported for homogeneous constant-volume gravity currents with no or negligible settling in the literature. Propagation observations showed that the initial median particle diameters of the currents have negligible effect on the current propagation characteristics during the TDS phase, but become important during the GDS phase. The currents with smaller initial median particle diameters propagated faster and a longer distance in the GDS phase than their counterparts with larger median particle diameters. The deposited particle characteristics indicated that particles of different sizes settle at similar speeds during the TDS phase due to turbulent mixing and the settling speed becomes dependent on the particle size during the GDS phase. As a result, size sorting of the deposited particles became more pronounced during the GDS phase. At the earlier stages of propagation, the vertical profiles of suspended particle concentrations in the current head showed some extent of vertical uniformity due to turbulent mixing around the half height of the current head. On the other hand, at the later stages of propagation, suspended particle concentration profiles exhibited an exponential profile. Deposited and suspended particle characteristics showed that horizontal particle sorting, that is size grading of particles in the flow direction, was more pronounced than the vertical particle sorting, that is size grading of particles at different elevations within the current head.

Identifying the spatial extent of volcanic ash clouds in the atmosphere and forecasting their direction and speed of movement has important implications for the safety of the aviation industry, community preparedness and disaster response at ground level. Nine regional Volcanic Ash Advisory Centres were established worldwide to detect, track and forecast the movement of volcanic ash clouds and provide advice to en route aircraft and other aviation assets potentially exposed to the hazards of volcanic ash. In the absence of timely ground observations, an ability to promptly detect the presence and distribution of volcanic ash generated by an eruption and predict the spatial and temporal dispersion of the resulting volcanic cloud is critical. This process relies greatly on the heavily manual task of monitoring remotely sensed satellite imagery and estimating the eruption source parameters (e.g. mass loading and plume height) needed to run dispersion models. An approach for automating the quick and efficient processing of next generation satellite imagery (big data) as it is generated, for the presence of volcanic clouds, without any constraint on the meteorological conditions, (i.e. obscuration by meteorological cloud) would be an asset to efforts in this space. An automated statistics and physics-based algorithm, the Automated Probabilistic Eruption Surveillance algorithm is presented here for auto-detecting volcanic clouds in satellite imagery and distinguishing them from meteorological cloud in near real time. Coupled with a gravity current model of early cloud growth, which uses the area of the volcanic cloud as the basis for mass measurements, the mass flux of particles into the volcanic cloud is estimated as a function of time, thus quantitatively characterising the evolution of the eruption, and allowing for rapid estimation of source parameters used in volcanic ash transport and dispersion models.