Annotation of the results obtained in 2022
Three articles from the Q1 list were published within the framework of this project in 2022: • Sedakov et al., 2022. Large chocked lagoon as a barrier for river – sea flux of dissolved pollutants: case study of the Azov Sea and the Black Sea. Marine Pollution Bulletin (impact factor 7.001) • Osadchiev et al., 2022. Lateral border of a small river plume: Salinity structure, instabilities and mass transport. Remote Sensing (impact factor 5.349) • Korotenko et al., 2022. Mesoscale eddies in the Black Sea and their impact on river plumes: numerical approach and satellite observations. Remote Sensing (impact factor 5.349) Also, a monograph by A.A. Osadchiev "River plumes" was published, which summarizes the results of the project since 2018. An analysis of field measurements and aerial observations made it possible to identify two types of frontal instability that form at the boundary of small river plumes. The first type of instability arises in the estuarine inertial zone of a river plume in the presence of a large velocity shear at the boundary between the plume and the sea (>20–30 cm/s). These instabilities manifest themselves as relatively small eddy structures at the plume boundary (~3–7 m) with asymmetric vorticity and are Kelvin-Helmholtz instabilities caused by the velocity shear gradient. They form when the Richardson number is less than 0.25. The second type of instability is generated in the outer part of plumes and is characterized by a large spread in the size of eddy structures (~5–50 m). These instabilities have symmetrical vorticity and are Rayleigh-Taylor instabilities caused by the pressure gradient at the plume-sea interface. Rayleigh-Taylor instabilities are formed when the Richardson number is more than 1, the linear size of the vortex structures is proportional to the Atwood number. Both instabilities induce water exchange across the plume-sea boundaries, which changes the salinity structure of the plume boundaries and enhances the lateral mixing of plumes from small rivers and marine waters. On the basis of numerical modeling, the synoptic and seasonal variability of plumes of small rivers in the northeastern part of the Black Sea was studied. Modeling of river plumes in low and high discharge years showed that the average annual sizes of river plumes in coastal areas depend more on the duration of floods than on wind forcing. The seasonal variability of plume areas depends mainly on the volume of river runoff, wind, and the shape of the banks at the mouths of the rivers. Wind variability causes significant changes in plume structure on synoptic, diurnal, and hourly time scales. The results of the first long-term (within one year) monitoring of floating debris on the Don (2016-2017), Sochi (2020-2021) and Matsesta (2021-2022) rivers flowing into the northeastern part of the Black Sea are analyzed, and general estimates of the removal are obtained floating debris from the rivers to this region. Observations have established that the average flows of river debris on the Don, Sochi and Matsesta rivers are 6-101, 1-107, 1-24 items/hour, respectively. A joint analysis of river discharge data and monitoring of floating debris made it possible to establish the relationship of these characteristics for the rivers under consideration. Based on the dependencies obtained, the total annual flow of floating debris for the Don, Sochi and Matsesta rivers is estimated at 1-2•10^5, 2-4•10^4, 1-3•10^4 items/year, respectively. Assuming that the dependences for the Don, Sochi and Matsesta rivers are representative for all large, medium and small rivers of the region under consideration, the total annual flow of river debris from all rivers into the northeastern part of the Black Sea was calculated, which is 5•10^5 items/year. Numerical experiments were carried out to study the interaction of an isolated Caucasian anticyclonic eddy with Lagrangian particles, simulating the process of river plume involvement in the eddy as it moves along the coast. Unlike cyclonic eddies, which carry trapped waters to the coastal zone, anticyclonic eddies accumulate river waters and suspended and dissolved substances of river origin. The successive phases of the distribution of particles marking the waters of river plumes revealed important details of their movements in the presence of an eddy: (1) at the stage of development, a young anticyclonic eddy draws water from river plumes and keeps them inside the core, (2) the main particle leakage occurs at a mature stage, despite the still remaining coherence of the vortex, (3) as a result of further damping of the vortex, most of the particles leave it, but some of them are still captured by the vortex. By the end of the experiment, 40% of the particles were carried out of the coastal zone into the open part of the Black Sea, which indicates that the effect of self-cleaning of the coastal zone by anticyclonic eddies can be significant. The cross-shelf transport of plume waters from the coastal zone to the open sea increases as consecutive anticyclonic eddies pass. Numerical modeling was used to study the process of spreading and mixing of the Sea of Azov waters and the pollution they carry to the Black Sea. Numerical experiments have shown that dissolved pollutants of river origin entering the Sea of Azov from the Don River accumulated in the western part of the sea in the first few years, after which, after 5-10 years, they evenly spread over the entire area of the sea. In the Black Sea, elevated concentrations were observed along the continental slope and on the northern and northwestern shelves, while the areas of divergence in the central part of the sea have the least pollution. Uniform distribution of pollutants in the Black Sea was observed 10-15 years after they began to flow from the Don to the Sea of Azov. The presence of a strong halocline in the Black Sea prevents the penetration of dissolved pollutants below a depth of 150 m, keeping them localized in the upper layer of the sea, while the maximum concentrations of pollutants in the Black Sea are two orders of magnitude lower than in the Sea of Azov. Numerical modeling clearly shows that the Sea of Azov plays the role of an effective barrier for the transfer of dissolved pollutants from the river to the sea, which is not observed in open estuaries. In particular, the Sea of Azov delays the inflow of 50% of river pollution into the Black Sea for 4 years, which is the average residence time of pollutants in the estuary. This result shows the extent to which the Sea of Azov slows down the continuous influx of background pollution from the Don River into the Black Sea. 95% of discharged river pollution enters the Black Sea only after a time lag of 15 years, which can be estimated as the size of the time interval necessary for the self-purification of the Sea of Azov. On the other hand, 5% of the discharge of river pollution reaches the Black Sea after only 9 months. This relatively short time lag demonstrates how quickly a pollutant release emergency signal in the Don River can reach the Black Sea.

 

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Annotation of the results obtained in 2021
In 2021, within the framework of the project, a study was carried out of the process of formation and distribution of the Bzyb plume based on field data and aerial remote sensing. Wind forcing is the main factor influencing the dynamics of the river plume. The direction and speed of the wind determine the position, shape and size of the river plume. Over the past decades, a significant amount of research has been devoted to the response of river plumes to wind forcing. At the same time, almost all work was based on numerical modeling, while direct field measurements of this process were practically not carried out due to their complexity. In April 2021, direct measurements of the response of a small river plume to wind action were carried out for the first time. Almost continuous aerial remote sensing of the plume of the Bzyb River was carried out using quadrocopters during daylight hours during three days. Aerial remote sensing was accompanied by synchronous in situ measurements of wind forcing (with a discreteness of 1 minute), as well as the thermohaline structure and flow velocity in the plume. Based on these data, estimates of the response rate of the plume spreading dynamics to changes in wind conditions were obtained. In particular, the velocities of the outer boundary of the plume were reconstructed with an unprecedented high spatial (~ 10 m) and temporal (~ 1 minute) resolution. It was found that the speed of movement of the outer boundary of the plume linearly depends on the wind speed (and is approximately 1/20 of the wind speed) with a very short response time (10-20 minutes). Under conditions of moderate wind impact (<5 m / s), when the wind direction reverses, the plume undergoes a complete restructuring for several hours until the alongshore direction of plume propagation changes by 180 degrees. The change in the direction of propagation affects only the near-field part of the plume, which leads to detachment and mixing of the outer part of the plume. The obtained results are of key importance for understanding and modeling the dynamics of the spread of river plumes. Also, as part of the field work in the spreading area of the Bzyb plume, studies of internal waves, which are generated in many small river plumes located in various regions of the world, were continued. Aerial remote sensing using quadcopters and synchronous field measurements of the thermohaline structure and current velocity in the plume showed that the propagation and dissipation of these waves does not induce horizontal mass transfer inside the plume due to shear instability, which was suggested in a recent study of this process using numerical simulations. It was shown, that the phase velocity of propagation of internal waves attenuates with distance from the source, and numerical estimates of this process were obtained (halving of the velocity at a distance of 1 km from the river mouth). Aerial remote sensing of small river plumes on the northeastern coast of the Black Sea (Mzymta, Kodora, Bzyb) recorded a vortex-like structure of long sections of their sharp outer boundaries, manifested in the alternation of specific lobe and cleft segments 5-30 m long and 2-10 m wide, which intensively mix with the surrounding sea, no vortex-like structure was observed. Such vortex-like fronts start from river mouths and limit the river stream flowing into the sea. Vortex-like fronts were not observed in the outer part of the plume and in the surf zone during periods of active wave breaking due to intense mixing. Quadrocopter video recorded a repetitive circulation process along vortex-like fronts. After the formation of a convex segment at the plume boundary, it begins to increase, expanding towards the sea. As they grow, neighboring vortex-like structures merge, mechanically capturing a small area of salty seawater (0.1–0.5 m2 in area) and carrying them across the plume – sea interface. The merged convex segments dissipate, and the trapped area of seawater is mixed inside the plume, after which the process of the formation of new convex segments in this section of the plume boundary resumes. This repetitive process of continuous formation, expansion, coalescence, and dissipation of convex segments was observed along the entire length of the eddy fronts of the small river plumes under study. The lifetime of an individual segment from its formation to dissipation was 1–2 minutes. The vortex-like structure of the plume sharp boundary appears to be formed due to baroclinic instability between the plume and the surrounding sea. Field measurements carried out on vortex-like fronts showed that a large pressure gradient across the plume boundary is a source of potential energy causing the formation of a vortex-like frontal structure. A slight perturbation of the plume boundary and the formation of a local convex segment leads to an increase in the local length of the boundary and, therefore, to an increase in advection along the normal to the boundary, induced by the pressure gradient. This process causes the convex segment to expand continuously until it merges with the adjacent convex segment. The merger of two segments with the capture of a region of salty seawater and its subsequent mixing with the plume leads to a decrease in the local salinity anomaly and, consequently, to a decrease in the local pressure gradient. This provides negative feedback and prevents further formation of a convex segment in this section of the boundary, increasing the likelihood of disturbance in adjacent sections. Based on the thermohaline measurements, estimates were obtained of the intensity of mixing of the Bzyb plume across the lower boundary with the sea, caused by a velocity shift between the plume and the sea, and the intensity of mixing of the plume across the lateral boundary with the sea, caused by the formation of a lateral pressure gradient at this boundary. It was found that the intensity of mixing across the lateral boundary is only 5 times less than through the lower boundary of the plume, which indicates the important role of this poorly studied process in mixing river and sea waters and the formation of the structure of the river plume. With an increase in the plume, the length of its boundary increases linearly, and its area quadratically. Thus, with a decrease in the plume size, the relative intensity of lateral mixing (compared to vertical mixing at the lower boundary of the plume) increases, and with an increase in size, it decreases. Because of this, lateral mixing seems to play a significant role for small plumes and negligible for large plumes. As a result of the project, two articles were published in 2021: Osadchiev et al., 2021. Response of a small river plume on wind forcing. Frontiers in Marine Science (impact factor 4.435) and Osadchiev et al., 2021. Internal waves as a source of concentric rings within small river plumes. Remote Sensing (impact factor 4.848), both articles from the Q1 list. Also, based on the results of research carried out within the framework of the project, starting in 2018, a monograph by A.A. Osadchieva "River plumes". The monograph is scheduled to be printed in early 2022.

 

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