This work investigates how a vertical cylindrical obstacle affects the dynamics and mixing of lock-release gravity currents. Gravity currents are produced in the laboratory and density fields are measured using an image analysis. Two Reynolds numbers, three submergence ratios and three flow blockage ratios have been tested. Results show that upstream of the cylinder, gravity currents propagation remains unaffected. Downstream of the obstacle, the front velocity decreases compared to the undisturbed case, with a deceleration becoming larger as the obstacle diameter increases. The deceleration does not trigger the transition from the slumping to the self-similar phase. Greater obstacle height causes an increased current thickness upstream of the cylinder, while larger diameters amplify both the thickness and the reflection of the dense current. The entrainment parameter remains unchanged across all tested configurations. Energy budget analysis reveals an enhancement of mixing in the presence of obstacles.

We analyze the structure of lock-release gravity currents as they approach and flow around a vertical obstacle. Two laboratory setups are used: an undisturbed horizontal channel and the same channel partially obstructed by a circular cylinder. Instantaneous two-dimensional velocity fields were acquired with Particle Image Velocimetry. The structure of the flow is discussed in terms of its instantaneous velocity components and by identifying its characteristic vortical coherent structures employing the Rortex/Liutex vector. The current is divided in three layers, characterized by different vortical structures. The lowermost layer features the hairpin-like vortical structures of smooth boundary layers while the shear layer features the growth of inflectional instabilities. Once the head has passed, the body of the current is upwelled, disrupting the shear layer and creating a region of intense mixing. A complex vortex system is formed with rotation compatible with returning the upwelled flow back to the current. The interaction of this region with the separated ambient flow requires the formation of a horizontal vortex that compatibilizes both flows. In the ambient fluid, the observed rotational structures may be the cores of Von K & aacute;rm & aacute;n vortices whose axis (vertical when shed) were rotated by the upwelling current.

Every day, millions of tons of plastic debris are poured into rivers from industrial and civil waste or due to social carelessness and transported to the ocean. Here they decompose into small fragments, compromising the health and growth of fauna and flora that ingest or absorb them. In recent years the idea of using vegetation to trap and extract plastic waste has developed to limit this phenomenon. The aim of this work is to experimentally quantify the ability of aquatic vegetation in trap plastic and understand whether different biotic factors, hydraulic conditions or debris type influence it. Three of the most abundant macrophytes in European and Asian rivers are tested in this study, Myriophyllum spicatum, Potamogeton crispus and Phragmites australis. Natural samples of vegetation, taken along the Tiber, Ninfa-Sisto and Aniene rivers, are positioned into a recirculating flume, where the flow rate and the water depth can be varied. Once stationary flow conditions are reached, a known quantity of polystyrene fragments of different sizes (macroplastics, mesoplastics and microplastics) is added in the upstream part of the channel. The ratio between the fragments retained in the green barrier and the total added during the experiment defines the species' capacity to retain plastics. A change in seasonality, simulated by changing the water depth and the number of stolons inserted into the flume, is tested and its effects on the trapping efficiency is analysed. Three plant’s densities and two water depths are tested for each species. All three plant species show to effectively retain large and medium-sized plastic debris. Only the Myriophyllum spicatum, whose needle-like leaves form a denser network than the other two species, is also found to be efficient in retaining microplastics. The density of the area occupied by vegetation affects the number of trapped fragments, which increases for all species as the number of inserted stolons increases. The change in water depth has no significant impact on the results obtained. In conclusion, the three macrophyte species analyzed in this work can be used to create a barrier to the transport of plastics from rivers to oceans. A more complex structure of the vegetation allows the trapping of microplastics. A larger density of the area occupied by vegetation induces larger trapping efficiency, while hydraulic conditions appear to have no significant influence for the values tested in this study.

This study investigates the dynamics and mixing of gravity currents propagating over an array of cylindrical obstacles using laboratory experiments. The effects of obstacle spacing (l/d) and submergence ratio (d/H0) on flow structure, dynamics, entrainment, and energy distribution are examined. High-resolution density measurements reveal that the submergence ratio plays a critical role in controlling current diversion, while obstacle spacing governs the flow pathway. An increase in d/H0 enhances the interactions between the current and the roughness elements, resulting in marked fluctuations in potential energy and mixing intensity that significantly affect the current evolution. Although bottom roughness generally reduces the front velocity and alters entrainment behavior, the effect of obstacle spacing is less important, particularly for low d/H0. Notably, for large d/H0, the current exhibits a shift in mixing dynamics, deviating from the near-linear growth of background potential energy observed in smoother cases. Furthermore, by applying the Thorpe scale to assess turbulent mixing, the study demonstrates that larger obstacle spacing promotes stronger turbulence, leading to greater vertical displacements and enhanced mixing.

The Agulhas Current is a vigorous western boundary current that flows off the southeastern coast of Africa, linking the Indian Ocean to the Atlantic Ocean as part of a broader inter-ocean current system. The Agulhas Current is characterized by intense heat fluxes between the ocean and atmosphere, as well as complex multiscale ocean dynamics. Dynamically unstable currents in this region generate mesoscale eddies, which propagate into the South Atlantic and interact with the Benguela Upwelling system and the Atlantic Meridional Overturning Circulation. These unique features make the Agulhas region an ideal site for studying air-sea interactions in climate models and examining the relative roles of atmospheric and oceanic weather in driving upper-ocean variability. In this study, we investigate how horizontal resolution in climate models affects their ability to represent thermal air-sea interactions over the Agulhas Current region, by comparing several simulations of state-of-the-art models. We identify ocean- and atmosphere-driven regimes using a covariance analysis of sea surface temperatures and turbulent heat fluxes. Our findings suggest that a minimum ocean model resolution of approximately 25 km is necessary to capture the signature of ocean dynamics, leading to a better alignment with theoretical and observational results. Furthermore, we identify a transition scale between ocean-driven and atmosphere-driven regimes within the 2 degrees-5 degrees range: when this critical length scale is exceeded, the ocean-driven behavior is filtered out and the atmosphere-driven regime, which becomes relevant at larger scales, dominates. While both oceanic and atmospheric resolution play a role, we find that increasing the horizontal resolution in the ocean component yields a comparatively larger improvement in the representation of air-sea flux variability.

We present a pioneering experimental study of stratified, rotating exchange flows interacting with a bottom, mobile sediment bed that simulates large estuaries. Two-dimensional velocity fields are coupled with bed scan that allows to reconstruct the bed morphology. The experiments span a large parameter range, notably laminar to turbulent Ekman layer regimes (33

When a gravity current encounters a barrier, it is reflected as a moving hydraulic jump or bore. These reflected flows, which play a significant role in estuarine mixing and sediment transport, are often simplified in theoretical models as purely advective processes with no mixing and dilution effects. This study explores the dynamics of gravity currents fully blocked by various inclined barriers, focusing on the resulting mixing behavior. Using an image analysis technique based on light attenuation to capture instantaneous density fields, we reveal how the presence of a barrier influences the current even before impact. By applying the Thorpe scale to assess turbulent mixing, we show that a barrier’s geometry significantly affects mixing intensity. Notably, this study finds that barriers can increase the local turbulent mixing compared to horizontal surfaces.

The hydrostatic imbalance between two adjacent fluids, driven by density variations associated with temperature, salinity, or sediment concentration gradients, often initiates the formation of gravity currents. These phenomena play a crucial role in various geophysical and engineering applications, influencing atmospheric, terrestrial, and subaqueous environments. In recent years, there has been growing research interest in understanding the interaction of gravity currents with obstacles on the seafloor. These obstacles can be artificial structures like pipelines and gas pipelines situated in the oceanic environment. Therefore, it is essential to investigate the dynamics of gravity currents over complex topography and analyze their characteristics and behavior as the initial conditions vary. This study experimentally examines the evolution of bottom-propagating gravity currents in the presence of an array of submerged cylindrical obstacles. The laboratory experiments were conducted within a Perspex tank with dimensions of 3 m in length, 0.3 m in height, and 0.2 m in width, using the lock-release technique by filling the left and right volumes of the tank to the same water depth. The density difference was reproduced through a salinity gradient. Submerged roughness was introduced by arranging a series of rigid plastic cylinders at a specified location, covering the entire width of the channel. Two different diameters, 2 cm and 2.5 cm, were analyzed, and the initial current depths were varied. A total of 24 full-depth lock-exchange experiments were performed. We employ an innovative image analysis technique based on light reflection to evaluate the instantaneous density fields. To apply the light attenuation technique and visualize the dense fluid, a controlled quantity of dye was introduced into the saline water. A calibration method was used to establish the correlation between light intensity and dye concentration for each pixel in the captured images. The conducted study clearly illustrates that an adequate height of obstacles results in a substantial portion of denser fluid being impeded by the foremost obstacle in an array. Additionally, transitioning from densified to less-densified array geometries induces distinct changes in flow morphologies. Upon concluding the analysis of this study, it is evident that all the experiments are affected by the presence of substantial bottom roughness.

Gravity currents are flows driven by density gradients between two or more contacting fluids and play a key role in nature and industrial environments via global ocean circulations, climate variability and the distribution of airborne pollutants. In the present work, we study, experimentally, the changes induced by an emergent vertical PVC cylinder on the mean and turbulent flow fields of an unsteady bottom-generated lock release gravity current. Tests were carried out, with and without the cylinder, in refractive index matching conditions and instantaneous velocities were acquired with a Particle Image Velocimetry system. The mean velocity field, Reynolds stresses and terms of turbulent kinetic energy (TKE) budget for the currents head were presented and discussed. The results show that the adverse pressure gradient generated by the cylinder induces a uniform deceleration of the current head. Hence, there are no appreciable differences on the spatial distribution of the mean velocities in the current head, compared to the undisturbed current. On the other hand, the changes on the turbulent flow field are remarkable. The total diffusion of TKE decays in the inner part of the head while becoming stronger at the interface between the two fluids, as the current approaches the cylinder. This is associated to an increase of the diffusion term due to pressure fluctuations, that acts against diffusion due to velocity fluctuations and contributes to disrupt the transport of TKE from the interface between the fluids and the inner part of the current. As a result, in the presence of an obstacle, Reynolds stresses are suppressed in the inner part of the current head and enhanced at the interface.

Lock-exchange gravity currents experiments are performed to characterize the influence of bottom roughness on the dense flow through Particle Image Velocimetry, PIV. The surface properties over which gravity currents propagate in natural environments play a key role in the dynamics and structure of dense current. Roughness was introduced by an array of obstacles that may slow down the gravity current and significantly modify its capacity to entrain sediments compared to a current propagating over a flat bottom. The instantaneous maps of the flow velocity show how the bottom roughness impacts the flow dynamics; as an example recirculation zones develop among the elements. These structures, are constant over time with a longitudinal development and height depending on the height of the elements; the greater the height of the elements and the greater the size of the recirculation areas.

We investigate the interaction between lock release gravity currents and a vertical emergent cylinder. Two-dimensional instantaneous velocity fields were measured in vertical planes with Particle Image Velocimetry and mean velocity and Reynolds stresses are analyzed. The presence of the obstacle produces a deceleration in the longitudinal direction associated to the adverse pressure gradient. It was found that the cylinder does not change appreciably the structure of the mean flow. Turbulence is more expressively affected. Reynolds stresses are suppressed in the inner part of the head as the current approaches the cylinder. Third order moments are affected in a way that suggests that turbulence produced at the shear layer is not effectively fed back into the current. This is compatible with pressure diffusion counter-acting the effect of turbulent diffusion. This effect of the adverse pressure gradient on the current head has not been reported before.

Understanding gravity currents developing on complex topography, which involve turbulence and mixing processes on a wide range of spatial and temporal scales, is of importance for estimating near ground fluxes in oceanic and atmospheric circulation. We present experimental results, based on high resolution velocity and density measurements, of constant upstream buoyancy supply gravity currents flowing from a horizontal boundary onto a tangent hyperbolic shaped slope. The mean flow, turbulence characteristics, and mixing properties, the latter expressed in terms of mixing lengths and eddy coefficients, are determined, highlighting their dependency on topography. These mean flow and mixing characteristics are compared with the field measurements in katabatic winds by Charrondière et al. [“Mean flow structure of katabatic winds and turbulent mixing properties,” J. Fluid Mech. 941, A11 (2022)], which are gravity flows that develop over sloping terrain due to radiative cooling at the surface. The results obtained show that the mean katabatic flow structure is substantially different from that of the upstream buoyancy supply gravity current. However interestingly, dimensionless mixing lengths and eddy coefficients compare well despite the difference in the mean flow structure and a two order of magnitude difference in the Reynolds number.

Plastics accumulate in the environment affecting biota and ecosystems. Although rivers are vectors of land-based plastics to the sea, macroplastics and microplastics in rivers are recently studied. Most studies focused on floating plastic transport to the sea through rivers considering only abiotic hydromorphological factors. In this view, among biotic factors, vegetation has recently been found to entrap plastics. Indeed, the role of vegetation is pivotal in affecting riverine plastic transport. While marine vegetation blocking plastics has been studied, research in freshwater ecosystems is neglected. Since hydrological factors have a pivotal role in riverine plastic transport and few is known on plant entrapment, the interaction between hydrological variables and plastic entrapment by vegetation has not yet been investigated. Given that the composition, transport, and fate of "submerged" plastics in the water column are neglected, we aimed at investigating the behaviour of plants in entrapping plastics within a specific laboratory flume tank. Specifically, we assessed whether (i) aquatic plants block different plastic sizes within the water column and (ii) different factors (e.g. water level, density of plants) affect plastic entrapment. Our results showed that, according to plant density, the higher the plant density the higher the entrapment of plastics by plants - independently of plastic size. Considering the water level, macro-, meso-, and microplastics were trapped similarly. Moreover, Potamogeton crispus blocked fewer microplastics compared with Myriophyllum spicatum. Our results might have impact as plants acted as temporary plastic trappers and can be used as tools for mitigating plastic pollution. Future research might investigate if this laboratory approach can be applied in field for recollecting plastics and consequently mitigating the problem. In conclusion, good management of plants in watercourses, canals, and rivers should be ideal for enhancing river functionality and ecosystem services for human well-being (i.e. the plastic entrapment service by plants).

<p><strong></strong>: Gravity currents are flows generated by density differences within two contacting fluids. In this work the interaction between lock-release gravity currents propagating over a horizontal rectangular channel and an emergent cylinder is analyzed through velocity measurements obtained through PIV. Two-dimensional instantaneous velocity fields are measured in a plane perpendicular to the bottom along the center axis of the channel upstream of the obstacle. The experiments were also conducted without the cylinder for comparison purposes and ten repetitions were carried out for each configuration. The analyses focus on the effects that the presence of an adverse pressure gradient has on both the mean velocity field and the turbulence of the leading part of the current, the head, before the impact. The mean velocity field is not affected by the presence of the obstacle and since no differences were found in the spatial distribution of the mean velocity components, the necessary cylinder-induced deceleration occurs uniformly. Turbulence is studied through the components of the Reynolds stress tensor and their fluxes within the head. In the configuration with the cylinder, there are no fluxes of Reynolds stresses in the inner part of the section. Consequently, the Reynolds stress intensity decreases inside the head compared to the configuration without the obstacle. In conclusion, the presence of an adverse pressure gradient stops the mechanism of Reynolds stress distribution from the main source of production, i.e. the front region, to the inner region of the flow. This leads to a decrease in Reynolds stresses in the inner part of the head and an increase in the frontal region.</p> <p><strong>Acknowledgements</strong>: This work was partially supported by Foundation for Science and Technology's through funding UIDB/04625/2020 (CERIS research unit).</p> <p><strong>Keywords: </strong>Gravity currents, lock release, Particle Image Velocimetry, adverse pressure gradient, Reynolds stress.</p>

When a gravity current interacts with a barrier the flow is reflected back in the form of a moving hydraulic jump or bore. This latter phenomenon has important implications for estuarine mixing and sediment reworking in the oceanographic context. In order to examine those reflection processes two-dimensional gravity currents interacting with slopes and overhanging barriers are examined by laboratory experiments and theoretical analysis. Relevant parameters such as the initial volume of the dense fluid, the opening of the fixed gate used for the dense fluid release and the angle of the slope or overhang positioned inside the tank, were varied. An image analysis technique based on light attenuation was applied to evaluate the instantaneous density fields. The theoretical predictions, based on shallow-water theory, give reasonable agreement compared with the laboratory experiments about the spatial position of the dense current and the definition of the point where the reflection process starts. Moreover, the analysis of the density fields highlights the presence of areas within the dense current characterized by a greater dilution due to the reflection of the current. Nevertheless, the bulk entrainment parameter is only slightly affected by the presence of a steep slope or overhang.

<p>Sea surface temperature (SST) has been thought to be linked with air-sea surface heat fluxes (SHF). General knowledge is that the high frequency variating atmosphere properties modify oceanic quantities due to their slower response. However, recent studies show how in regions where SST gradients and heat losses are stronger – in the Western Boundary Currents region (WBC) – variabilities in SST and SHF are due to internal ocean processes and water dynamic effects. Theoretical models suggest that the correlation between SST and SHF and between SST tendency (namely the time derivative) and THF can be used to retrieve the sources of variations of these two quantities distinguishing to influences due to ocean or atmosphere dynamics (ocean or atmosphere driven regimes). In this study, We use observational data and numerical model outputs with different resolutions to distinguish different regimes of variability and to investigate spatial resolution effects over the Agulhas Current region and the Eastern South Atlantic. In these regions waters flowing southward from the Indian Ocean along the eastern coasts of Africa interact with bathymetry and cold waters of the Antarctic Circumpolar Current (ACC) and the SubTropical Front and generate turbulence and eddies that propagates into the South Atlantic carrying warm and salty waters (Agulhas Leakage). Hence this methodology is particularly effective due to the mesoscale length scale of the physical phenomena that occur here. Observations are retrieved from OAFlux dataset and J-OFURO3. Model data come from the Coupled Model Intercomparison Project (CMIP6). The increase of ocean resolution leads to a better representation of the cross-covariance patterns and cross-correlation forms, indicating an improvement of the eddy-permitting from the eddy-parametrized models’ capability. Covariance maps have been calculated to highlight qualitative patterns for the lead-lag symmetry. We concluded that, while high resolution model data have similar covariance patterns and correlation values to the observations, their low-resolution counterpart, in two cases, fails to reconstruct the signal caused by the ocean dynamics. The stronger impact on the capability of reproduce this interaction phenomenon belongs to the ocean part of the coupled model: the higher, the better is the symmetric properties of the correlation functions (symmetry index) and the greater the transition scale is, implying the needs of a wider filtering window to cancel out the ocean driven regime signal.</p>

Where temperature gradients and heat losses are higher, variabilities in sea surface temperature and surface heat fluxes are induced by internal ocean processes. Theoretical models suggest that the correlation between temperature and heat fluxes can be used to distinguish sources of variability: ocean or atmosphere driven regimes. Here we used numerical model outputs with different resolutions to distinguish different regimes of variability and to investigate spatial resolution effects. We focused over the Agulhas Current region and the Eastern South Atlantic. There, waters flowing southward from the Indian Ocean along the eastern coasts of Africa generate turbulence. The mesoscale properties of the physical phenomena that occur in this area can be the key to find an ocean driven regime signal. Observations are used as comparison. The increase of resolution gives a better representation of the cross-covariance patterns and cross-correlation forms, indicating an improvement on the eddy-resolving capability. Their low-resolution counterpart fails to reconstruct the signal due to the ocean dynamics.

The influence of bottom roughness on the development of lock-release gravity currents is investigated through laboratory experiments using Particle Image Velocimetry. The bottom roughness is represented by arrays of vertical LEGO bricks with a constant spacing while varying $$\lambda $$ , the relative height of the roughness elements to the gravity current depth. Depending on $$\lambda $$ , the roughness elements may affect the gravity current propagation: for small $$\lambda $$ the current behaves like moving over a smooth bottom, while for larger $$\lambda $$ the propagation speed is reduced and the internal structure of the current, including both the head and the tail, is significantly modified. As $$\lambda $$ increases, stronger recirculation areas between the roughness elements develop interacting with the overlying layer and giving rise to small-scale vortical structures of opposite sign within the whole depth of the current. The additional drag force induced by the bottom roughness adds significant complexity to the flow dynamics and modifies the characteristics of the current.

Sea surface temperature (SST) has been thought to be linked with air-sea surface heat fluxes (SHF). General knowledge is that the high frequency variating atmosphere properties modify oceanic quantities due to their slower response. However, recent studies show how in regions where SST gradients and heat losses are stronger – in the Western Boundary Currents region (WBC) – variabilities in SST and SHF are due to internal ocean processes and water dynamic effects. Theoretical models suggest that the correlation between SST and SHF and between SST tendency (namely the time derivative) and THF can be used to retrieve the sources of variations of these two quantities distinguishing to influences due to ocean or atmosphere dynamics (ocean or atmosphere driven regimes). In this study, We use observational data and numerical model outputs with different resolutions to distinguish different regimes of variability and to investigate spatial resolution effects over the Agulhas Current region and the Eastern South Atlantic. In these regions waters flowing southward from the Indian Ocean along the eastern coasts of Africa interact with bathymetry and cold waters of the Antarctic Circumpolar Current (ACC) and the SubTropical Front and generate turbulence and eddies that propagates into the South Atlantic carrying warm and salty waters (Agulhas Leakage). Hence this methodology is particularly effective due to the mesoscale length scale of the physical phenomena that occur here. Observations are retrieved from OAFlux dataset and J-OFURO3. Model data come from the Coupled Model Intercomparison Project (CMIP6). The increase of ocean resolution leads to a better representation of the cross-covariance patterns and cross-correlation forms, indicating an improvement of the eddy-permitting from the eddy-parametrized models’ capability. Covariance maps have been calculated to highlight qualitative patterns for the lead-lag symmetry. We concluded that, while high resolution model data have similar covariance patterns and correlation values to the observations, their low-resolution counterpart, in two cases, fails to reconstruct the signal caused by the ocean dynamics. The stronger impact on the capability of reproduce this interaction phenomenon belongs to the ocean part of the coupled model: the higher, the better is the symmetric properties of the correlation functions (symmetry index) and the greater the transition scale is, implying the needs of a wider filtering window to cancel out the ocean driven regime signal.

Results are presented for new laboratory-scale numerical simulations of uni- and bi-directional stratified flows generated within an idealized trapezoidal channel topography, as representative of net-barotropic exchange flows generated through sea straits between water masses with different densities. These simulations utilise the Bergen Ocean Model (BOM), a three-dimensional general ocean circulation model, both in non-rotating and rotating frames of reference. The results from the BOM simulations are aimed at simulating the large-scale experimental data obtained in the LEGI Coriolis rotating platform in Grenoble, within which velocity and density fields for exchange flows were measured through particle image velocimetry (PIV) and micro-conductivity density probes, respectively. The BOM simulations are shown to reproduce the main dynamic flow patterns and trends of the large-scale exchange flows through the trapezoidal channel, with the lower layer saline intrusion flux shown to reduce (i.e. due to partial blockage) as the upper freshwater flow is increased. These simulations represent preliminary numerical results and we expect that future simulations will improve overall representation of the interfacial region between the counter-flowing layers, and thus be relevant to many exchange flows generated in wide estuaries, sea straits and deep-ocean channels.