Enhanced Macroplastic Transport and Interception in Rivers: The Role of Detached Groynes and Vegetation Patches

Enhanced Macroplastic Transport and Interception in Rivers: The Role of Detached Groynes and Vegetation Patches | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Enhanced Macroplastic Transport and Interception in Rivers: The Role of Detached Groynes and Vegetation Patches Łukasz Przyborowski, Aleksandra Śmietanka, Paweł Gilewski, Gordon Gilja, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7921573/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 5 You are reading this latest preprint version Abstract Small-scale hydrodynamics of plastic debris transport in freshwater environments are insufficiently understood, but it is known that groynes and vegetation play a role in debris accumulation. From the perspective of water management, pollution transport estimates and efficiency of cleaning efforts, this knowledge gap should be addressed. This research paper investigates the impact of groyne setup on macroplastic transport in rivers through observation of the uniform, floating macroplastic particles in laboratory experiments with stationary flow conditions. The effects of two factors were compared by tracking the plastic particles' movement patterns: vegetation presence and groyne configuration. Vegetation was in the form of a circular patch of wooden stems imitating common reed in the middle of the groyne area. The groynes were either in a regular setup, i.e., attached to the bank, or they were detached, allowing the flow along the bank through the groyne field. During the test with a regular setup, typical recirculation of the flow causes plastic litter to accumulate and stay in the upstream corner of the groyne field. The plant patch presence resulted in the temporal accumulation of the plastic pieces between plant stems. When the gap in the groynes was present, all the plastic pieces floating inside the groyne field made their exit from the area, and this behaviour repeated with the vegetation presence. Those results indicate that detached groynes disrupt the flow pattern, which reduces plastic retention time and enhances transport through a groyne field. Those observable changes in macroplastic transport may help in designing best practices of river cleaning activities or more ecologically friendly river structures. macroplastic eco-hydrology particle tracking pollution transport groynes physical model Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Highlights Detaching groynes reduces plastic retention. Vegetation may temporarily trap plastic litter. Gaps in detached groynes and vegetation disrupt typical groyne fields' flow patterns. Trapped plastic litter is most likely to be found in the upstream half of the groyne field. 1 Introduction Over the past few years, there has been growing interest in the problem of plastic pollution in rivers (Blair et al. 2017; Valero et al. 2022) because this is the primary means of transporting plastic to the oceans (Meijer et al. 2021). Moreover, larger pieces of plastic that become stuck along the way undergo a degradation process, transforming into smaller fractions (White and Turnbull 1994), which ultimately harm the environment and human health (Blettler and Mitchel 2021; Nelms et al. 2018). That said, most studies about plastics in rivers focus on monitoring and discerning types of plastics found in or along the rivers (van Emmerik et al. 2019; Gasperi et al. 2014). The transport of macroplastics in rivers is a process strongly dependent on hydrological conditions. Most studies focus on major floods, during which significant amounts of pollutants accumulated on banks and floodplains are remobilised. However, for most of the time, rivers operate under low and medium flow conditions. Under such conditions, hydraulic structures like groynes play a key role in shaping local hydrodynamics. By constricting the flow, groynes create recirculation zones with reduced velocity, which favour sediment deposition but can also act as traps for suspended pollutants (Mohammed 2017; Wu and Qin 2020). Understanding the mechanisms of plastic transport and accumulation on a small scale, specifically under non-flood conditions, is crucial for a complete picture of the fate of plastic in river systems, a point highlighted by numerous studies indicating the need for process-based research taking into account hydrological conditions (Horton et al. 2017; van Emmerik et al. 2023; van Emmerik 2024; Schreyers et al. 2025). What makes predicting plastic paths on a smaller scale difficult is that a few characteristics of the plastic particles must be considered – size, shape, submergence and density. For example, the movement of non-spherical floating macroplastic litter, i.e., fragments bigger than 5 mm, will not be comparable to Lagrangian tracers, which is expected for smaller particles with no inertia (Calvert et al. 2021). In particular, the horizontal movement of floating particles is also affected by forces like surface tension (Valero et al. 2022) and wind (Al-Zawaidah et al. 2021). Another study also showed that floating PET bottles travel farther than submerged ones (Chardon et al. 2025). Plastic litter transport can be influenced by obstructions such as river structures, logjams and vegetation (Lechthaler et al. 2020; van Emmerik and Schwarz 2020). The latter, like widespread species like M. Spicatum and P. Crispus , were tested as efficient in trapping plastic pieces (Gallitelli et al., 2023). Riparian vegetation is reported to accumulate plastic litter (Gallitelli et al., 2025), which is also present in groyne fields, altering the flow patterns (Sukhodolov et al., 2017), and the riparian zones diminish the transport rate of macroplastics (Valyrakis et al., 2024). Recent studies confirm that groyne fields can function as temporary reservoirs for macroplastic, becoming local pollution "hotspots" (Liedermann et al. 2020; Liedermann et al. 2022; Grosfeld et al. 2024; Murphy 2024; Liro et al. 2025) or be specifically designed to redirect litter floating through the channel (Tominaga et al. 2020). Field studies have shown that accumulation on the banks within groyne fields is a dynamic process, strongly linked to water level changes, whereas the role of wind in remobilising settled particles is limited, especially when they are wet and covered with sediment. These observations indicate that the dead zones within groyne fields are key retention areas for plastic. On the other hand, the typical groynes' effect on the flow can be altered by detaching the groynes from the banks (Tritthart et al. 2014), but there are no dedicated field studies on how such a setup affects plastic transport. In conclusion, the hydrodynamic mechanisms governing the entry, circulation, and eventual trapping or escape of particles in either of the groyne’s configurations remain insufficiently understood. This paper aims to fill this gap by analysing macroplastic transport under controlled laboratory conditions that simulate non-flood flow conditions in a groyne field. In summary, the problem of plastic pollution in rivers is an issue that needs further exploration. Given the number of individual processes involved in plastic transport, experimental work on particular cases is needed to support the theory and build a wider perspective. This is especially valid for river management, where future impacts of river engineering and present needs must be considered. Cleaning rivers from plastic pollution is a persistent issue, while river training may require revisiting to apply new solutions that come from ecohydraulics. To explore the impact of detached groynes and vegetation on plastic transport, insights from four experimental scenarios, where plastic particles float through a laboratory channel, are delivered. The described results encompass the measured flow field and the statistics of the tracked plastic particles. In the discussion, the observed behaviour of plastic litter between the individual scenarios is compared. 2 Materials and Methods The experiments were conducted in the Hydraulics Laboratory of the Faculty of Civil Engineering at the University of Zagreb, which is a central research station of the R3PEAT project (e.g., Gilja et al. 2023). 2.1 Laboratory setup The flume has dimensions: 0.8 m wide, 3 m effective length of the bottom with no slope covered with fine sand and 1.5 m high. The flume's bed is made of sand on the concrete lining, while the flume's side walls are glass. Flow in the channel is controlled by a pair of frequency-regulated pumps and a flap gate at the outlet. The middle 3 meters of the channel length were used as a working section. For all experiment scenarios, a series of groynes made of cement-bonded gravel was installed in that section along one side of the channel at right angles to the flow direction, equally spaced (Fig. 1 ). To better simulate real groynes and ripraps, the surface of the groynes was kept rough. The groynes had trapezoidal side sections and the rectangular top of which was 0.1 x 0.05 m. The space between the top and the channel wall was 0.06 m long. It was filled with loose gravel secured with a plastic insert (Fig. 2 ). When testing scenarios with detached groynes, the insert in the gap was removed along with half of the gravel, creating an unobstructed flow path between the groyne and flume’s wall. In each experiment scenario, groynes' tops emerged from the water, which simulates a non-flood flow condition in a river. Four experiment scenarios were executed (Fig. 2 ): A - Regular groynes - attached to the flume’s wall; B - Detached groynes - with the gap between the groyne and the flume's wall; C - Regular groynes with the vegetation patch added in between the last pair; D - Detached groynes with the vegetation patch added in between the last pair. 2.2 Vegetation and plastic particles Vegetation presence was simulated by a circular patch of wooden, rigid stems imitating reeds, which was inserted between the last pair of groynes (Fig. 2 ). The stems had a diameter of d = 0.006 m, the patch density described as a = n*d, where n is the number of stems per patch area, amounted to a = 26.6 m − 1 . Vegetation stems were always protruding above the water level. The spaces between the stems were deliberately left big enough to allow plastic particles to get inside the patch, simulating a smaller fraction of litter that can potentially get stuck inside. The macroplastic particles used for the study were cut out of white polypropylene cups, which, with a density of 0.91 g/cm 3 , float on the water surface. The particles created in this way were flat squares with side dimensions of 6 to 10 mm. The particles were thrown onto the water surface at the tip of the preceding groyne pair (Fig. 1 b), several or a dozen pieces at a time, a total of 1200 in each scenario. This method was chosen to prevent particles from aggregating too much, i.e., combining into larger groups and to ensure more even distribution in the flow. Particles were fished out of the channel after each test from a net securing the channel outlet. 2.3 Water velocity measurements The water velocity field for each scenario was measured using vertically directed acoustic Doppler velocimeters (manufacturer: Nortek, equipment name: Vectrino Profiler, ADV) on a grid of 44 points covering the space between the last pair of groynes and a strip of the main flow area. The velocities were measured at mid-water height, i.e., 0.07 m, for 3 min at 50 Hz record frequency. The final velocity time series were acquired by applying commonly used filtering and smoothing procedures (described, e.g., in Przyborowski et al. 2019). To better explain flow conditions in the channel, the following numbers were calculated for the channel width from the groyne tip to the opposite flume’s wall. Reynolds number according to the equation: Re = U avg R H / 𝜗 , (1) where U avg is bulk streamwise velocity, R H is hydraulic radius, and 𝜗 is kinematic viscosity. Froude number: 𝐹𝑟 = 𝑈 𝑎𝑣𝑔 / (𝑔𝐷 𝐻 ) 0.5 (2) where 𝑔 is the gravitational acceleration and 𝐷 𝐻 is a hydraulic diameter. Table 1 Hydraulic characteristics of the main flow in the laboratory channel, i.e., limited by the groyne field Channel cross-section limited by the groyne field Parameter [unit] Value of the parameter Discharge Q [m 3 s − 1 ] 0.02 Water depth h [m] 0.12 Bed width B [m] 0.65 Flow area A = h*B[m 2 ] 0.075 Hydraulic radius Rh = A/(B + 2*h) [m] 0.088235 Hydraulic diameter D H = A/B [m] 0.115385 Bulk velocity U = Q/A [ms-1) 0.2666 Kinematic viscosity [m 2 s − 1 ) for t = 20°C 0.0000010034 Reynolds number 23449 Froude number 0.25 The calculated hydraulic parameters (Table 1 ) indicate that the flow in the channel was in a fully turbulent (Re ≈ 23,449) and subcritical (Fr ≈ 0.25) regime. These conditions are representative of typical, non-flood flow states in regulated lowland rivers where groynes are emerged and actively influence the hydrodynamics. The calculations of Re and Fr numbers were performed for the cross-section limited by the groyne tip and the opposite flume wall, as this is the main flow zone whose interaction with the groyne field was the core of the study. The chosen discharge and the resulting hydraulic indices are comparable to conditions used in other laboratory experiments on groyne hydrodynamics (Yeo et al. 2025; Sanju and Nezu, 2017), which allows the results to be placed in a broader scientific context. 2.4 Particle tracking analysis of macroplastic particles' behaviour The space between the last pair of groynes, where imitation vegetation was placed, was used to analyse the behaviour of plastic particles (Fig. 1 ). The movements of plastic particles flowing through this part of the channel were recorded with a GoPro camera. Records were made at a frequency of 30 frames per second, in 4K UHD resolution and with the function of digital elimination of the "fisheye" effect turned on. The camera was installed above the channel at a right angle to the water surface, so that the entire area between the selected groynes was visible. Adjusting the camera height and image resolution ensured that each plastic particle occupied at least a dozen pixels in the image. The Particle Tracking Velocimetry (PTV) method was used to create initial maps of plastic particle trajectories floating on the water from camera recordings. To perform the PTV method, existing MATLAB code was utilised (Capart et al. 2002; Aleixo et al. 2010). Initially generated trajectories were heavily serrated due to problems with particle detection and tracking. To obtain whole paths for each individual particle and subsequently count their exit/residence time within the investigated area, manual completion of the maps was required using vector graphics software Inkscape. MATLAB was also used to create maps of the probability of a particle's presence. The maps visualise the percentage of frames in which macroplastic particles were visible in each of the videos in each pixel. Each map was created by summarising the colour intensity of each pixel from all clipped frames (pictures centred on the groyne field, with mostly macroplastic visible, by subtracting certain colour frequencies in RGB), then dividing it by the number of frames and adding artificial colour to better visualise the percentage. 3 Results 3.1 Water velocity field The flow field in the investigated area was obtained by calculating time-averaged longitudinal and transverse velocities recorded by ADV. The first observation is that typically for groyne compartments, a part of the bulk flow is redirected at the downstream groyne and velocities are substantially diminished inside the groyne area. A recirculation vortex appears in the middle of the groyne area when the downstream groyne deflects the flow back. In scenarios without the vegetation patch present, the vortex spans over the majority of the area between groynes, but it is weaker when groynes are detached. The reason for the weaker recirculation pattern is its disruption by a bleed flow, coming through the gaps between groynes and the channel’s wall. The bleed flow enhances formation of a smaller, counter-rotational vortex that appears just behind the upstream groyne, allowing the flow to exit the stagnant area. This smaller vortex is also present when groynes are attached (as observed in other studies, e.g., Sanju and Nezu 2017; Yeo et al. 2005), but it is less pronounced. The disruption of the main recirculation in scenarios b and d also causes the main flow to enter the groyne cavity further downstream. The vegetation patch caused a similar effect by deflecting and dividing the flow in the groyne compartment, with the small flow recirculation area downstream of the patch, visible in both scenarios (B and D). 3.2 Plastic litter behaviour The used PTV technique was employed to detect particles and visualise their paths in each video created for the four studied scenarios. Each video anaylsis started when the first batch of particles was released and ended when all the particles escaped the investigated area or, in the case of regular groynes when last of the particles were stuck, i.e., did not moved in a more than 5 minutes from the dead zone near the flume fall and upstream groyne (example visible in Fig. 4 - scenario A and C, where few of the paths finish in the left, lower part of the picture). 3.2.1 Average particle path according to initial position In the case of the regular groynes, particles from the main flow entered the groyne field near the downstream groyne and then moved around the area, following the streamlines of the recirculation vortex. The exit points, where particles return to the main flow, were near both the downstream and upstream groyne. The presence of the vegetation patch disturbs those paths, as particles began to move in smaller circles in front of or downstream of the patch (Fig. 4 , scenario A and scenario C), which is consistent with the flow velocity field results (Fig. 3 , scenario A and scenario C). Moving that way, almost half of the particles got trapped inside the vegetation patch from the downstream side, to be flushed out the other side of the patch. Another observation is a situation where the particle, instead of flowing into the vegetation patch, bounces off towards the main flow, exiting that way the investigated area, which is another difference from scenario A. In scenario C with the vegetation patch, more particles entered the groyne field. In both scenarios A and C, less than half of the particles remained in the investigated area (Table 2 ), stopping the movement in the dead zone near the flume wall close to the upstream groyne. The scenarios with detached groynes are characterised by two possible entry and two exit points for the particles, thanks to the presence of the gap (Fig. 4 , scenario B and scenario D). Most of the particles during the experiments entered through the gap and then got swept into the middle of the groyne compartment, moving forwards and backwards in the area closer to the wall and the upstream groyne (Fig. 4 , scenario B). Some particles travelled only along the flume wall to the exit gap (respectively 17% and 22% for scenarios b and d, based on statistics in Table 2 ). What is more, some of the particles entering through the upstream gap escaped to the main flow (in both scenarios B and D). The paths’ visualisation suggests it happened due to turbulent moves in the upstream half of the groyne compartment/around the vegetation patch. Fewer such movements were registered in the downstream half of the groyne field, where flow separates in front of the downstream groyne and moves particles toward the downstream gap. Only one particle did not leave the investigated area. The presence of the vegetation patch caused similar path distortions as in scenario C - particles moved around in two vortices in front and downstream of the patch. However, a bigger percentage of particles entered the cavities between the stems in scenario D, and two particles got trapped in the vegetation beyond the experiment’s duration. Table 2 Results describing statistics of plastics entry and escape points from the investigated groyne field for each scenario. Scenario A Scenario B Scenario C Scenario D Number of particles flowing into the groyne field (directed from main flow) (directed through the gap) 21 (21) - 291 (50) (241) 33 (33) - 185 (40) (145) Number of particles not escaping the groyne field 9 1 10 2 Number of particles escaping the groyne field (to the main flow) (through the gap) 12 (12) - 290 (107) (183) 23 (23) - 183 (80) (103) Number of particles getting inside the vegetation patch - - 13 20 Number of particles travelling only along the flume’s wall - 42 - 32 3.2.2 Average residence time of particles recirculation in the groyne field Results show a difference between the time duration of particle presence in the investigated area between scenarios with regular and detached groynes (Table 3 ). With the additional exit point, particles moved out of the groyne compartment much faster compared to regular groynes, where particles travel many circles around the area before exiting. In particular, without the presence of the vegetation, particles in the detached scenario B tended to move diagonally from the gap to exit into the main flow without delays, spending an average of 8 seconds in the investigated groyne field. The overall difference in particle time spent between regular and detached groynes is even higher, considering that the recorded average time duration also accounts for particles sticking to the flume’s wall, which travelled very slowly (Table 3 , difference between 38.5 and 35 seconds for scenario B), grouping into a long strip of floating pieces. Interestingly, the addition of vegetation in regular groynes reduced the time needed to exit the groyne field by half compared to a base scenario A. However, there was no change in contrast to the detached groynes case in scenario B. On average, particles spend almost a minute (55 seconds) flowing through the stems of the vegetation patch, but only half of it (25 seconds) in scenario D. Table 3 Results describing the time duration plastics spend in the investigated groyne field for each scenario (excluding particles that did not escape the investigated area during the experiment). Scenario A Scenario B Scenario C Scenario D Avg. time duration of particle spend in the groyne compartment [s] 216 38.5 100 36.1 Avg. time duration of particle escaping to the main flow [s] 216 8 100 24.5 Avg. time duration of particle escaping through the gap [s] (without particles sticking only to the flume’s wall) - 35 - 43.5 Avg. time duration of particle spend in the vegetation patch [s] (not counting particles that get stuck inside) - - 55 25 3.2.3 Probability of a particle's presence inside the groyne field The probability of particle’s presence map shows where macroplastics spend the most time inside the groyne field or where they travel with the lowest velocity. In scenarios A and C, a strip near the bottom marks the dead zone area near the flume wall, where the particles get trapped, exhibiting to and fro movements but not moving away. Scenario A shows a higher probability in the upstream half of the groyne field (Fig. 5 ). The presence of the vegetation patch in scenario C changes this picture - the higher probability is visible around the wall-facing stems of the vegetation patch (Fig. 5 ). Due to a higher number of particles that entered the groyne field in scenarios B and D, there is a higher overall probability of macroplastic presence visible (Fig. 5 ). The stripe at the bottom defines the region where particles were sticking to the flume’s wall. However, in contrast to scenarios without the gaps, the strip is moved towards the downstream gap, indicating that after entering the investigated area from the upstream gap, particles were being swept towards the middle of the area. The upstream half of the investigated groyne field is where the probability of particles being present is the highest, especially just behind the downstream groyne. With the addition of the vegetation patch, the probability distribution changes in favour of areas upstream, downstream and inside the vegetation patch, but overall is similar. 4 Discussion 4.1 Influence of vegetation patch and groynes detachment The analysis of particle tracks revealed how groyne configuration and the presence of a vegetation patch change the transport dynamics inside the groyne field. In the work of Przyborowski et al. (2024), extending the groyne had an impact on macroplastic litter capture rate, by increasing the width of the mixing region in the groyne tip’s wake. In the experiments shown here, all the groynes' tips are in the same position relative to the main flow; therefore, all the changes in dynamics come from either adding vegetation or the gap between the groynes and the flume’s wall. Detaching the groynes had two effects: it allowed macroplastic to enter the groyne field through the gap due to a bleed flow, and it caused more particles to enter from the main flow (Table 2 ) compared to the regular setup in scenario A. The second observation may be connected to the presence of a counter-rotational vortex near the upstream groyne, which, besides disrupting the main circulation in the groyne field, simultaneously uplifts particles that enter through the upstream gap (visible in both scenarios B and D). With the bleed flow observed in the downstream gap, plastic particles spend much less time in the downstream part of the groyne field, escaping rather towards the gap than towards the main flow. Those two facts indicate that detaching the groynes, with the bleed flow strong enough to disturb the typical recirculation pattern inside the groyne field, causes macroplastic litter to predominantly float around the upstream part of the groyne field. Another difference of this study in comparison to experiments shown in Przyborowski et al. (2024) is that the vegetation there was flexible, and here it is rigid, hence a more distinct impact on the flow field and particle paths. The results of particle tracking (Fig. 4 ) and probability map (Fig. 5 ) clearly show how the rigid stems in the form of a circular patch divided the litter paths into two parts, opposite to the regular, unobstructed groyne field. Particle paths also showed how some of the particles, which were about to enter the groyne field, bounced off the vegetation patch back to the main flow. Simultaneously, vegetation patch presence increased the number of particles flowing into the investigated area in the case of regular groynes (from 21 to 33, Table 2 ). However, it decreased for scenarios with detached groynes (from 50 to 40, Table 2 ). Moreover, macroplastic particles spend less time in the investigated area when vegetation is present and groynes are attached. The explanation of this behaviour lies in the observed particles' paths. Litter, that would normally be trapped in a big recirculation vortex for many encirclements, is pushed from the upstream half of the groyne field back to the main flow when trying to flow around or through the edges of vegetation. Regardless of the observable flow patterns, several particles either stopped moving or moved very slowly through the boundary layer of the flume's wall. However, in a natural channel, bank roughness will play a role, potentially limiting plastic transport through gaps and along the banks even more than the flume wall did during the experiments. Grosfeld et al. (2024) reported that more litter from the beach was picked up in the middle of the groyne field, while more accumulation was visible in the corners. Overall, the plastic particles that moved around the investigated area spent less time in scenarios with detached groynes. Additionally, particles moving through the vegetation patch spend less time there than it took on average to move out of the investigated area, which implies that unless particles get trapped, the only impact of the vegetation patch on the plastic behaviour is by changing the flow directions inside the groyne field. 4.2 Other factors that may influence plastic litter transport Groynes in this experiment were placed perpendicularly to the main flow in all scenarios for consistency reasons; however, there are recommendations to put detached groynes at an angle (Sommer and Aberle 2009). Such a setup would have some impact since it would redirect the movement of the macroplastic litter towards the downstream end of the groyne. Following the design of the groyne itself, a rough amalgamate of concrete and gravel was used in the conducted experiments. In real engineering projects, the use of riprap or mesh for groyne protection would alter the roughness and potentially increase the entrapment of litter on the groyne itself or enlarge the area of a dead zone behind the obstruction. Plastic litter transport occurs predominantly during flood conditions (Liro et al. 2020). Such conditions were not simulated in the flume, since the groynes tend to be submerged during floods, and therefore, they would marginally influence the floating particles. However, other possible events, such as the release of a large number of plastic pieces in one place, can be related to conducted experiments, as a large number of particles were released in each experiment. Hauk et al. (2024) reported that after the release of butter tubs due to a flood, their transport along the river was spatially limited. This kind of litter can be described as rigid plastic. In the observed experiment, the pieces of firm cups used as macroplastic particles clogged along the wall, which led to a decreased transport rate. In other words, not only can hydrology and hydrodynamics play a role in the macroplastic transport, but also the number of such objects spilling into a river. The behaviour of macroplastics in a channel, however, can be different to that observed in the experiments for foldable or “soft” plastics. What is more, a recent study revealed that even non-buoyant plastic pieces can be moved to the surface, contradicting the typical Rouse profile of sediment density (Lofty et al. 2024). Alas, the differences in behaviour of plastic particles due to characteristics like dimensions, rigidity, or density are not suitable for discussion here, as there is a lack of comparable studies. The wind is a factor not accounted for in this laboratory channel, but in a real scenario, it is a factor that may influence the plastic accumulation. For example, van Thi et al. (2024) observed how wind direction and speed correlated with the influx of certain plastic types to a river. What is more, GPS tracking on the Danube river showed that plastic debris tends to flow along the same river bank (Liedermann et al. 2022). In other words, if wind could transport plastic litter from inland into the river, it can float downstream, sticking to the initial bank and eventually being intercepted by groyne fields. However, there are no studies showing whether wind can actually move floating plastic across the river’s width. 4.3 Hydrological perspective and link to transport modelling Macroplastic transport in rivers is a large-scale process that begins within the catchment area. The characteristics of the catchment, including the degree of urbanisation and land use, determine the sources and quantity of plastic that can enter a river. The initiating factors for terrestrial transport are often atmospheric phenomena, such as rainfall generating surface runoff and wind (Mellink et al. 2022). Once in the river channel, the subsequent fate of the particles is controlled by complex hydrodynamic processes. Modern plastic transport models attempt to describe these processes, often using a probabilistic or force-balance approach. For example, the "travel distance" model developed by Newbould et al. (2021) conceptualises transport as a series of "step" (movement) and "rest" periods. The fate of a particle depends on its probability of being trapped, p(T) , in "traps" such as vegetation, bank irregularities, or meanders. Our study provides a detailed, mechanistic insight into the functioning of one such key retention point—the groyne field. The results showing how particles are retained in recirculation zones, how this is affected by vegetation, or how they escape through the gap in a detached groyne can be used to parameterise and calibrate the probability p(T) for this type of structure in large-scale models (Newbould et al. 2021). Another modelling approach, the "Plastic Pathfinder" (Mellink et al. 2022), is based on the principle that transport occurs when driving forces (e.g., wind, surface runoff) overcome resistive forces (e.g., surface friction). Although this model pertains to terrestrial transport, its concept is universal. In a river system, the driving force is the water current (along with turbulence), while resistive forces are generated by "traps," such as the complex and slowed flow within a groyne field. Our experiment illustrates in detail these "resistive" hydrodynamic conditions, which are often simplified in large-scale models. Understanding how detaching a groyne or the presence of vegetation modifies these resistive forces is crucial for building more physically realistic transport models that can better predict the locations of plastic accumulation zones in rivers. 4.4 Ecological impact of the conducted investigation New types of groynes, including detached ones, are now considered as one of the more sustainable methods of river training (Manual on Good Practices in Sustainable Waterway Planning, 2010; Gilja et al. 2019). Researchers have also explored the impact of vegetation presence on hydrodynamics in a groyne field (Sukhodolov et al. 2017). Rigid vegetation imitation used in this investigation was built to imitate a reed colony, and the presence of riparian vegetation is one of the colonisation stages on sandy bars inside groyne fields (Luna 2016). Cesarini and Scalici (2022) reported from a river monitoring campaign that reeds trapped the smallest number of plastic pieces, but at the same time, those were predominantly macroplastic pieces. Gallitelli et al. (2024) showed that in small and medium rivers, riparian vegetation traps mostly plastic litter. The conducted experiments confirm that macroplastic pieces can get trapped within the reeds, even with the particles’ diameter smaller than the gaps between the stems. Exploring the possible impact of detached groynes on plastic transport, the conducted study shows that this new groyne setup can be beneficial in this regard. Although there is a higher chance for macroplastic litter to flow into the groyne field, there is a smaller possibility that it will stay there. Also, following the particle paths patterns, it is possible to discern the most efficient place for a plastic skimmer to catch the biggest number of floating litter, which would be in the upstream part of the groyne field in the vicinity of the vegetation or, in the case of the groyne initialising the groyne fields, it would be where the bleed flow occurs. This investigation focused only on the floating macroplastic litter, but the observed patterns may likewise be expanded to plastics with neutral buoyancy or sinking ones. In the work of Sukhodolov et al. (2002), fine sediment accumulation within groyne fields was spatially connected to the occurrence of vortices. Following the particles' paths and the probability map, it is possible to detect places where bigger and smaller vortices were forming during the experiment. Those places can therefore be attributed to the likelihood of finding microplastic particles, which are likely to distribute in rivers in a similar way to natural sediments (Waldschläger et al. 2022). 5 Summary Transport and accumulation patterns of macroplastic particles inside the groyne field were investigated in four scenarios. Experiments were conducted in a laboratory flume with the use of the same flow conditions and the same uniform plastic particles. Two scenarios included an artificial vegetation patch in the middle of the groyne field, and in two scenarios, the groynes were detached. Results, including both the particles’ paths and the water velocity field in the investigated groyne field, were used to understand the litter behaviour. The overall pattern that emerges from flume observations is that detaching the groynes enhances transport of macroplastic particles in the groyne field. At the same time, a rigid vegetation patch alters the flow pattern and leads to the interception of macroplastic particles. What was observed for regular setup of groynes, that plastic litter rarely enter the area and then follow the big recirculation pattern of the flow, with the accumulation zone near the wall behind the upstream groyne (Fig. 6 ). The addition of the vegetation does two things: it divides the flow inside the groyne field into smaller, separate vortices; it also either traps the plastic particles inside or push them faster back to the main flow (Fig. 6 ). In other words, with the vegetation patch presence, 50% more plastics pieces entered the groyne field, but on average spend in the investigated area 50% less time. Detaching the groynes increased the number of plastics entering the area from the main flow but also allowed a much larger number of particles to enter through the gap (Fig. 6 ). All the plastics eventually left the area with twice the number of particles that escaped to the main flow than got in that way. Vegetation patch in this configuration caused again the same changes as in the first setup, which is visible in probability maps: plastics tended to float most often in a smaller vortex near the upstream groyne, and some particles got caught within the vegetation stems. The results of this investigation show a new perspective on the phenomenon of macroplastic transport in the case of a trained river. Although such laboratory experiments do not cover various factors related to known and unknown plastic behaviours in the natural environment, as explained in the discussion, the observed patterns are relevant to ecohydrology. Detached groynes can act as a more sustainable way to train a river, and the experiments showed that they can store floating plastic litter only temporarily instead of trapping it in the dead zone. The vegetation presence is profitable in this aspect, as it creates a natural barrier and most of the litter pieces small enough to enter the vegetation are gradually washed away. The findings of this study also carry practical implications for river management. Demonstrating that groyne fields, especially in their traditional (attached) configuration, act as effective traps for macroplastic indicates that they should be treated as potential pollution accumulation hotspots. Consequently, waterway managers should consider these areas as priority locations for monitoring activities and clean-up campaigns. Furthermore, the observed accumulation patterns (mainly in the upstream part of the groyne field) can help in the optimal placement of debris-capturing devices. Finally, the fact that detached groynes (with a gap at the bank) reduce plastic retention provides valuable insights for the design of future, more ecologically sustainable hydraulic structures. Declarations Acknowledgments We thank Rui Aleixo for sharing code for particle tracking and Robert Filszar for the help with laboratory experiments. Funding This work is supported by the National Science Centre, Poland (Grant number: 2021/05/X/ST10/00443) and in part by the Croatian Science Foundation under the project R3PEAT (Grant number: UIP-2019-04-4046). We thank Aleksandra Śmietanka for her assistance in analysing video records. Author Contributions Łukasz Przyborowski, Gordon Gilja. and Manousos Valyrakis contributed to the conception and design of this study and secured the funding. Łukasz Przyborowski conducted experiments. Łukasz Przyborowski, Aleksandra Śmietanka and Paweł Gilewski performed data analysis. All authors contributed to writing the manuscript. All authors have read and agreed on the final version of the manuscript. Ethical Approval This is not applicable Consent to participate All the authors agreed to participate in coauthorship. 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Eng. 9:29-38. https://doi.org/10.1007/BF02829094 Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Major Revision 04 Dec, 2025 Reviewers agreed at journal 10 Nov, 2025 Reviewers invited by journal 10 Nov, 2025 Editor assigned by journal 28 Oct, 2025 First submitted to journal 23 Oct, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7921573","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":542752613,"identity":"a6f1b309-8fd9-423f-a4a2-7f86afab3c7a","order_by":0,"name":"Łukasz 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1","display":"","copyAsset":false,"role":"figure","size":1273624,"visible":true,"origin":"","legend":"\u003cp\u003eConducted experiment’s setup in the laboratory flume: a) - down-looking scheme of the investigated groyne field and main flow; b) - actual photo of the flume before the experiment, drawn borders coloured to match the zones depicted in the scheme (blue - main flow, yellow - groyne field, red - particle tracking area), macroplastic litter injection point marked at the preceding groyne.\u003c/p\u003e","description":"","filename":"Fig1.png","url":"https://assets-eu.researchsquare.com/files/rs-7921573/v1/c0f29a55c3fba698219f0f4d.png"},{"id":96315273,"identity":"56731028-39c6-4b90-b725-3f984cffa04c","added_by":"auto","created_at":"2025-11-19 17:24:29","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":4422894,"visible":true,"origin":"","legend":"\u003cp\u003eFour experiment scenarios: A - regular groynes attached to the flume’s wall; B - detached groynes with the gap between the groyne and flume’s wall; C - regular groynes with the vegetation patch added in between the last pair; D - detached groynes with the vegetation patch added in between the last pair.\u003c/p\u003e","description":"","filename":"Fig2.png","url":"https://assets-eu.researchsquare.com/files/rs-7921573/v1/4a5e64f8d387f6ba9d24a908.png"},{"id":96315274,"identity":"c5e2be76-5974-404f-b2e9-66d65c52cb02","added_by":"auto","created_at":"2025-11-19 17:24:29","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":723775,"visible":true,"origin":"","legend":"\u003cp\u003eVelocity field from ADV measurements for scenarios A, B, C and D.\u003c/p\u003e","description":"","filename":"Fig3.png","url":"https://assets-eu.researchsquare.com/files/rs-7921573/v1/512c46462c9c55aa75a0ae9c.png"},{"id":96315278,"identity":"f528f460-39ad-4747-b4a9-d045c304cd4c","added_by":"auto","created_at":"2025-11-19 17:24:29","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":7525462,"visible":true,"origin":"","legend":"\u003cp\u003eVisualised paths of particles in the groyne field - example results covering 8 to 12 minutes of analysed videos. The majority of particles passing only in the main flow are not drawn.\u003c/p\u003e","description":"","filename":"Fig4.png","url":"https://assets-eu.researchsquare.com/files/rs-7921573/v1/88e6ff4c9968c3f209108bf3.png"},{"id":96315276,"identity":"79e6bb61-81a4-4baf-9459-9f8d489e7aab","added_by":"auto","created_at":"2025-11-19 17:24:29","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1751741,"visible":true,"origin":"","legend":"\u003cp\u003eProbability of particle’s presence inside the groyne compartment.\u003c/p\u003e","description":"","filename":"Fig5.png","url":"https://assets-eu.researchsquare.com/files/rs-7921573/v1/062395c7edd1541504ee89aa.png"},{"id":96315277,"identity":"ef1f88d0-5e17-4f04-96d1-a7e887c698d6","added_by":"auto","created_at":"2025-11-19 17:24:29","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":696670,"visible":true,"origin":"","legend":"\u003cp\u003eSummary of the observed plastic litter behaviour in different tested configurations of the groyne field.\u003c/p\u003e","description":"","filename":"Fig6.png","url":"https://assets-eu.researchsquare.com/files/rs-7921573/v1/cbec90bd22215817c71e8a9b.png"},{"id":96452960,"identity":"461399ac-db9a-4b24-b66e-7046612feba3","added_by":"auto","created_at":"2025-11-21 09:56:01","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":16021087,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7921573/v1/658ef794-79ef-46fc-9b9d-5e05e6bc3027.pdf"}],"financialInterests":"","formattedTitle":"Enhanced Macroplastic Transport and Interception in Rivers: The Role of Detached Groynes and Vegetation Patches","fulltext":[{"header":"Highlights ","content":"\u003cp\u003eDetaching groynes reduces plastic retention. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eVegetation may temporarily trap plastic litter. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eGaps in detached groynes and vegetation disrupt typical groyne fields\u0026apos; flow patterns.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTrapped plastic litter is most likely to be found in the upstream half of the groyne field.\u003c/p\u003e"},{"header":"1 Introduction","content":"\u003cp\u003eOver the past few years, there has been growing interest in the problem of plastic pollution in rivers (Blair et al. 2017; Valero et al. 2022) because this is the primary means of transporting plastic to the oceans (Meijer et al. 2021). Moreover, larger pieces of plastic that become stuck along the way undergo a degradation process, transforming into smaller fractions (White and Turnbull 1994), which ultimately harm the environment and human health (Blettler and Mitchel 2021; Nelms et al. 2018). That said, most studies about plastics in rivers focus on monitoring and discerning types of plastics found in or along the rivers (van Emmerik et al. 2019; Gasperi et al. 2014).\u003c/p\u003e\u003cp\u003eThe transport of macroplastics in rivers is a process strongly dependent on hydrological conditions. Most studies focus on major floods, during which significant amounts of pollutants accumulated on banks and floodplains are remobilised. However, for most of the time, rivers operate under low and medium flow conditions. Under such conditions, hydraulic structures like groynes play a key role in shaping local hydrodynamics. By constricting the flow, groynes create recirculation zones with reduced velocity, which favour sediment deposition but can also act as traps for suspended pollutants (Mohammed 2017; Wu and Qin 2020). Understanding the mechanisms of plastic transport and accumulation on a small scale, specifically under non-flood conditions, is crucial for a complete picture of the fate of plastic in river systems, a point highlighted by numerous studies indicating the need for process-based research taking into account hydrological conditions (Horton et al. 2017; van Emmerik et al. 2023; van Emmerik 2024; Schreyers et al. 2025).\u003c/p\u003e\u003cp\u003eWhat makes predicting plastic paths on a smaller scale difficult is that a few characteristics of the plastic particles must be considered \u0026ndash; size, shape, submergence and density. For example, the movement of non-spherical floating macroplastic litter, i.e., fragments bigger than 5 mm, will not be comparable to Lagrangian tracers, which is expected for smaller particles with no inertia (Calvert et al. 2021). In particular, the horizontal movement of floating particles is also affected by forces like surface tension (Valero et al. 2022) and wind (Al-Zawaidah et al. 2021). Another study also showed that floating PET bottles travel farther than submerged ones (Chardon et al. 2025).\u003c/p\u003e\u003cp\u003ePlastic litter transport can be influenced by obstructions such as river structures, logjams and vegetation (Lechthaler et al. 2020; van Emmerik and Schwarz 2020). The latter, like widespread species like \u003cem\u003eM. Spicatum\u003c/em\u003e and \u003cem\u003eP. Crispus\u003c/em\u003e, were tested as efficient in trapping plastic pieces (Gallitelli et al., 2023). Riparian vegetation is reported to accumulate plastic litter (Gallitelli et al., 2025), which is also present in groyne fields, altering the flow patterns (Sukhodolov et al., 2017), and the riparian zones diminish the transport rate of macroplastics (Valyrakis et al., 2024).\u003c/p\u003e\u003cp\u003eRecent studies confirm that groyne fields can function as temporary reservoirs for macroplastic, becoming local pollution \"hotspots\" (Liedermann et al. 2020; Liedermann et al. 2022; Grosfeld et al. 2024; Murphy 2024; Liro et al. 2025) or be specifically designed to redirect litter floating through the channel (Tominaga et al. 2020). Field studies have shown that accumulation on the banks within groyne fields is a dynamic process, strongly linked to water level changes, whereas the role of wind in remobilising settled particles is limited, especially when they are wet and covered with sediment. These observations indicate that the dead zones within groyne fields are key retention areas for plastic. On the other hand, the typical groynes' effect on the flow can be altered by detaching the groynes from the banks (Tritthart et al. 2014), but there are no dedicated field studies on how such a setup affects plastic transport. In conclusion, the hydrodynamic mechanisms governing the entry, circulation, and eventual trapping or escape of particles in either of the groyne\u0026rsquo;s configurations remain insufficiently understood. This paper aims to fill this gap by analysing macroplastic transport under controlled laboratory conditions that simulate non-flood flow conditions in a groyne field.\u003c/p\u003e\u003cp\u003eIn summary, the problem of plastic pollution in rivers is an issue that needs further exploration. Given the number of individual processes involved in plastic transport, experimental work on particular cases is needed to support the theory and build a wider perspective. This is especially valid for river management, where future impacts of river engineering and present needs must be considered. Cleaning rivers from plastic pollution is a persistent issue, while river training may require revisiting to apply new solutions that come from ecohydraulics. To explore the impact of detached groynes and vegetation on plastic transport, insights from four experimental scenarios, where plastic particles float through a laboratory channel, are delivered. The described results encompass the measured flow field and the statistics of the tracked plastic particles. In the discussion, the observed behaviour of plastic litter between the individual scenarios is compared.\u003c/p\u003e"},{"header":"2 Materials and Methods","content":"\u003cp\u003eThe experiments were conducted in the Hydraulics Laboratory of the Faculty of Civil Engineering at the University of Zagreb, which is a central research station of the R3PEAT project (e.g., Gilja et al. 2023).\u003c/p\u003e\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Laboratory setup\u003c/h2\u003e\u003cp\u003eThe flume has dimensions: 0.8 m wide, 3 m effective length of the bottom with no slope covered with fine sand and 1.5 m high. The flume's bed is made of sand on the concrete lining, while the flume's side walls are glass. Flow in the channel is controlled by a pair of frequency-regulated pumps and a flap gate at the outlet. The middle 3 meters of the channel length were used as a working section. For all experiment scenarios, a series of groynes made of cement-bonded gravel was installed in that section along one side of the channel at right angles to the flow direction, equally spaced (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). To better simulate real groynes and ripraps, the surface of the groynes was kept rough. The groynes had trapezoidal side sections and the rectangular top of which was 0.1 x 0.05 m. The space between the top and the channel wall was 0.06 m long. It was filled with loose gravel secured with a plastic insert (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). When testing scenarios with detached groynes, the insert in the gap was removed along with half of the gravel, creating an unobstructed flow path between the groyne and flume\u0026rsquo;s wall. In each experiment scenario, groynes' tops emerged from the water, which simulates a non-flood flow condition in a river. Four experiment scenarios were executed (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e):\u003c/p\u003e\u003cp\u003eA - Regular groynes - attached to the flume\u0026rsquo;s wall;\u003c/p\u003e\u003cp\u003eB - Detached groynes - with the gap between the groyne and the flume's wall;\u003c/p\u003e\u003cp\u003eC - Regular groynes with the vegetation patch added in between the last pair;\u003c/p\u003e\u003cp\u003eD - Detached groynes with the vegetation patch added in between the last pair.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Vegetation and plastic particles\u003c/h2\u003e\u003cp\u003eVegetation presence was simulated by a circular patch of wooden, rigid stems imitating reeds, which was inserted between the last pair of groynes (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The stems had a diameter of d\u0026thinsp;=\u0026thinsp;0.006 m, the patch density described as a\u0026thinsp;=\u0026thinsp;n*d, where n is the number of stems per patch area, amounted to a\u0026thinsp;=\u0026thinsp;26.6 m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Vegetation stems were always protruding above the water level. The spaces between the stems were deliberately left big enough to allow plastic particles to get inside the patch, simulating a smaller fraction of litter that can potentially get stuck inside.\u003c/p\u003e\u003cp\u003eThe macroplastic particles used for the study were cut out of white polypropylene cups, which, with a density of 0.91 g/cm\u003csup\u003e3\u003c/sup\u003e, float on the water surface. The particles created in this way were flat squares with side dimensions of 6 to 10 mm. The particles were thrown onto the water surface at the tip of the preceding groyne pair (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb), several or a dozen pieces at a time, a total of 1200 in each scenario. This method was chosen to prevent particles from aggregating too much, i.e., combining into larger groups and to ensure more even distribution in the flow. Particles were fished out of the channel after each test from a net securing the channel outlet.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Water velocity measurements\u003c/h2\u003e\u003cp\u003eThe water velocity field for each scenario was measured using vertically directed acoustic Doppler velocimeters (manufacturer: Nortek, equipment name: Vectrino Profiler, ADV) on a grid of 44 points covering the space between the last pair of groynes and a strip of the main flow area. The velocities were measured at mid-water height, i.e., 0.07 m, for 3 min at 50 Hz record frequency. The final velocity time series were acquired by applying commonly used filtering and smoothing procedures (described, e.g., in Przyborowski et al. 2019).\u003c/p\u003e\u003cp\u003eTo better explain flow conditions in the channel, the following numbers were calculated for the channel width from the groyne tip to the opposite flume\u0026rsquo;s wall. Reynolds number according to the equation:\u003c/p\u003e\u003cp\u003e\u003cem\u003eRe\u0026thinsp;=\u0026thinsp;U\u003c/em\u003e\u003csub\u003e\u003cem\u003eavg\u003c/em\u003e\u003c/sub\u003e \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003eH\u003c/em\u003e\u003c/sub\u003e \u003cem\u003e/ \u0026#120599;\u003c/em\u003e, (1)\u003c/p\u003e\u003cp\u003ewhere \u003cem\u003eU\u003c/em\u003e\u003csub\u003e\u003cem\u003eavg\u003c/em\u003e\u003c/sub\u003e is bulk streamwise velocity, \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003eH\u003c/em\u003e\u003c/sub\u003e is hydraulic radius, and \u003cem\u003e\u0026#120599;\u003c/em\u003e is kinematic viscosity. Froude number:\u003c/p\u003e\u003cp\u003e\u0026#119865;\u0026#119903; = \u0026#119880;\u003csub\u003e\u0026#119886;\u0026#119907;\u0026#119892;\u003c/sub\u003e / (\u0026#119892;\u0026#119863;\u003csub\u003e\u0026#119867;\u003c/sub\u003e)\u003csup\u003e0.5\u003c/sup\u003e (2)\u003c/p\u003e\u003cp\u003ewhere \u0026#119892; is the gravitational acceleration and \u0026#119863;\u003csub\u003e\u0026#119867;\u003c/sub\u003e is a hydraulic diameter.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eHydraulic characteristics of the main flow in the laboratory channel, i.e., limited by the groyne field\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"2\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e\u003cp\u003eChannel cross-section limited by the groyne field\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eParameter [unit]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eValue of the parameter\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eDischarge Q [m\u003csup\u003e3\u003c/sup\u003es\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.02\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eWater depth h [m]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.12\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eBed width B [m]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.65\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFlow area A\u0026thinsp;=\u0026thinsp;h*B[m\u003csup\u003e2\u003c/sup\u003e]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.075\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eHydraulic radius Rh\u0026thinsp;=\u0026thinsp;A/(B\u0026thinsp;+\u0026thinsp;2*h) [m]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.088235\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eHydraulic diameter D\u003csub\u003eH\u003c/sub\u003e = A/B [m]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.115385\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eBulk velocity U\u0026thinsp;=\u0026thinsp;Q/A [ms-1)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.2666\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eKinematic viscosity [m\u003csup\u003e2\u003c/sup\u003es\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) for t\u0026thinsp;=\u0026thinsp;20\u0026deg;C\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.0000010034\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eReynolds number\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e23449\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFroude number\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.25\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe calculated hydraulic parameters (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) indicate that the flow in the channel was in a fully turbulent (Re\u0026thinsp;\u0026asymp;\u0026thinsp;23,449) and subcritical (Fr\u0026thinsp;\u0026asymp;\u0026thinsp;0.25) regime. These conditions are representative of typical, non-flood flow states in regulated lowland rivers where groynes are emerged and actively influence the hydrodynamics. The calculations of Re and Fr numbers were performed for the cross-section limited by the groyne tip and the opposite flume wall, as this is the main flow zone whose interaction with the groyne field was the core of the study. The chosen discharge and the resulting hydraulic indices are comparable to conditions used in other laboratory experiments on groyne hydrodynamics (Yeo et al. 2025; Sanju and Nezu, 2017), which allows the results to be placed in a broader scientific context.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4 Particle tracking analysis of macroplastic particles' behaviour\u003c/h2\u003e\u003cp\u003eThe space between the last pair of groynes, where imitation vegetation was placed, was used to analyse the behaviour of plastic particles (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The movements of plastic particles flowing through this part of the channel were recorded with a GoPro camera. Records were made at a frequency of 30 frames per second, in 4K UHD resolution and with the function of digital elimination of the \"fisheye\" effect turned on. The camera was installed above the channel at a right angle to the water surface, so that the entire area between the selected groynes was visible. Adjusting the camera height and image resolution ensured that each plastic particle occupied at least a dozen pixels in the image.\u003c/p\u003e\u003cp\u003eThe Particle Tracking Velocimetry (PTV) method was used to create initial maps of plastic particle trajectories floating on the water from camera recordings. To perform the PTV method, existing MATLAB code was utilised (Capart et al. 2002; Aleixo et al. 2010). Initially generated trajectories were heavily serrated due to problems with particle detection and tracking. To obtain whole paths for each individual particle and subsequently count their exit/residence time within the investigated area, manual completion of the maps was required using vector graphics software Inkscape.\u003c/p\u003e\u003cp\u003eMATLAB was also used to create maps of the probability of a particle's presence. The maps visualise the percentage of frames in which macroplastic particles were visible in each of the videos in each pixel. Each map was created by summarising the colour intensity of each pixel from all clipped frames (pictures centred on the groyne field, with mostly macroplastic visible, by subtracting certain colour frequencies in RGB), then dividing it by the number of frames and adding artificial colour to better visualise the percentage.\u003c/p\u003e\u003c/div\u003e"},{"header":"3 Results","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Water velocity field\u003c/h2\u003e\u003cp\u003eThe flow field in the investigated area was obtained by calculating time-averaged longitudinal and transverse velocities recorded by ADV. The first observation is that typically for groyne compartments, a part of the bulk flow is redirected at the downstream groyne and velocities are substantially diminished inside the groyne area. A recirculation vortex appears in the middle of the groyne area when the downstream groyne deflects the flow back. In scenarios without the vegetation patch present, the vortex spans over the majority of the area between groynes, but it is weaker when groynes are detached. The reason for the weaker recirculation pattern is its disruption by a bleed flow, coming through the gaps between groynes and the channel\u0026rsquo;s wall. The bleed flow enhances formation of a smaller, counter-rotational vortex that appears just behind the upstream groyne, allowing the flow to exit the stagnant area. This smaller vortex is also present when groynes are attached (as observed in other studies, e.g., Sanju and Nezu 2017; Yeo et al. 2005), but it is less pronounced. The disruption of the main recirculation in scenarios b and d also causes the main flow to enter the groyne cavity further downstream. The vegetation patch caused a similar effect by deflecting and dividing the flow in the groyne compartment, with the small flow recirculation area downstream of the patch, visible in both scenarios (B and D).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Plastic litter behaviour\u003c/h2\u003e\u003cp\u003eThe used PTV technique was employed to detect particles and visualise their paths in each video created for the four studied scenarios. Each video anaylsis started when the first batch of particles was released and ended when all the particles escaped the investigated area or, in the case of regular groynes when last of the particles were stuck, i.e., did not moved in a more than 5 minutes from the dead zone near the flume fall and upstream groyne (example visible in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e - scenario A and C, where few of the paths finish in the left, lower part of the picture).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cdiv id=\"Sec10\" class=\"Section3\"\u003e\u003ch2\u003e3.2.1 Average particle path according to initial position\u003c/h2\u003e\u003cp\u003eIn the case of the regular groynes, particles from the main flow entered the groyne field near the downstream groyne and then moved around the area, following the streamlines of the recirculation vortex. The exit points, where particles return to the main flow, were near both the downstream and upstream groyne. The presence of the vegetation patch disturbs those paths, as particles began to move in smaller circles in front of or downstream of the patch (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, scenario A and scenario C), which is consistent with the flow velocity field results (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, scenario A and scenario C). Moving that way, almost half of the particles got trapped inside the vegetation patch from the downstream side, to be flushed out the other side of the patch. Another observation is a situation where the particle, instead of flowing into the vegetation patch, bounces off towards the main flow, exiting that way the investigated area, which is another difference from scenario A. In scenario C with the vegetation patch, more particles entered the groyne field. In both scenarios A and C, less than half of the particles remained in the investigated area (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), stopping the movement in the dead zone near the flume wall close to the upstream groyne.\u003c/p\u003e\u003cp\u003eThe scenarios with detached groynes are characterised by two possible entry and two exit points for the particles, thanks to the presence of the gap (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, scenario B and scenario D). Most of the particles during the experiments entered through the gap and then got swept into the middle of the groyne compartment, moving forwards and backwards in the area closer to the wall and the upstream groyne (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, scenario B). Some particles travelled only along the flume wall to the exit gap (respectively 17% and 22% for scenarios b and d, based on statistics in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). What is more, some of the particles entering through the upstream gap escaped to the main flow (in both scenarios B and D). The paths\u0026rsquo; visualisation suggests it happened due to turbulent moves in the upstream half of the groyne compartment/around the vegetation patch. Fewer such movements were registered in the downstream half of the groyne field, where flow separates in front of the downstream groyne and moves particles toward the downstream gap. Only one particle did not leave the investigated area. The presence of the vegetation patch caused similar path distortions as in scenario C - particles moved around in two vortices in front and downstream of the patch. However, a bigger percentage of particles entered the cavities between the stems in scenario D, and two particles got trapped in the vegetation beyond the experiment\u0026rsquo;s duration.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eResults describing statistics of plastics entry and escape points from the investigated groyne field for each scenario.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eScenario A\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eScenario B\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eScenario C\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eScenario D\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNumber of particles flowing into the groyne field\u003c/p\u003e\u003cp\u003e(directed from main flow)\u003c/p\u003e\u003cp\u003e(directed through the gap)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e21\u003c/p\u003e\u003cp\u003e(21)\u003c/p\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e291\u003c/p\u003e\u003cp\u003e(50)\u003c/p\u003e\u003cp\u003e(241)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e33\u003c/p\u003e\u003cp\u003e(33)\u003c/p\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e185\u003c/p\u003e\u003cp\u003e(40)\u003c/p\u003e\u003cp\u003e(145)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNumber of particles not escaping the groyne field\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNumber of particles escaping the groyne field\u003c/p\u003e\u003cp\u003e(to the main flow)\u003c/p\u003e\u003cp\u003e(through the gap)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e12\u003c/p\u003e\u003cp\u003e(12)\u003c/p\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e290\u003c/p\u003e\u003cp\u003e(107)\u003c/p\u003e\u003cp\u003e(183)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e23\u003c/p\u003e\u003cp\u003e(23)\u003c/p\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e183\u003c/p\u003e\u003cp\u003e(80)\u003c/p\u003e\u003cp\u003e(103)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNumber of particles getting inside the vegetation patch\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e13\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e20\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNumber of particles travelling only along the flume\u0026rsquo;s wall\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e42\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e32\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section3\"\u003e\u003ch2\u003e3.2.2 Average residence time of particles recirculation in the groyne field\u003c/h2\u003e\u003cp\u003eResults show a difference between the time duration of particle presence in the investigated area between scenarios with regular and detached groynes (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). With the additional exit point, particles moved out of the groyne compartment much faster compared to regular groynes, where particles travel many circles around the area before exiting. In particular, without the presence of the vegetation, particles in the detached scenario B tended to move diagonally from the gap to exit into the main flow without delays, spending an average of 8 seconds in the investigated groyne field. The overall difference in particle time spent between regular and detached groynes is even higher, considering that the recorded average time duration also accounts for particles sticking to the flume\u0026rsquo;s wall, which travelled very slowly (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, difference between 38.5 and 35 seconds for scenario B), grouping into a long strip of floating pieces. Interestingly, the addition of vegetation in regular groynes reduced the time needed to exit the groyne field by half compared to a base scenario A. However, there was no change in contrast to the detached groynes case in scenario B. On average, particles spend almost a minute (55 seconds) flowing through the stems of the vegetation patch, but only half of it (25 seconds) in scenario D.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eResults describing the time duration plastics spend in the investigated groyne field for each scenario (excluding particles that did not escape the investigated area during the experiment).\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eScenario A\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eScenario B\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eScenario C\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eScenario D\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAvg. time duration of particle spend in the groyne compartment [s]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e216\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e38.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e100\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e36.1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAvg. time duration of particle escaping to the main flow [s]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e216\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e100\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e24.5\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAvg. time duration of particle escaping through the gap [s] (without particles sticking only to the flume\u0026rsquo;s wall)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e35\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e43.5\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAvg. time duration of particle spend in the vegetation patch [s] (not counting particles that get stuck inside)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e55\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e25\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section3\"\u003e\u003ch2\u003e3.2.3 Probability of a particle's presence inside the groyne field\u003c/h2\u003e\u003cp\u003eThe probability of particle\u0026rsquo;s presence map shows where macroplastics spend the most time inside the groyne field or where they travel with the lowest velocity. In scenarios A and C, a strip near the bottom marks the dead zone area near the flume wall, where the particles get trapped, exhibiting to and fro movements but not moving away. Scenario A shows a higher probability in the upstream half of the groyne field (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). The presence of the vegetation patch in scenario C changes this picture - the higher probability is visible around the wall-facing stems of the vegetation patch (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eDue to a higher number of particles that entered the groyne field in scenarios B and D, there is a higher overall probability of macroplastic presence visible (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). The stripe at the bottom defines the region where particles were sticking to the flume\u0026rsquo;s wall. However, in contrast to scenarios without the gaps, the strip is moved towards the downstream gap, indicating that after entering the investigated area from the upstream gap, particles were being swept towards the middle of the area. The upstream half of the investigated groyne field is where the probability of particles being present is the highest, especially just behind the downstream groyne. With the addition of the vegetation patch, the probability distribution changes in favour of areas upstream, downstream and inside the vegetation patch, but overall is similar.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e"},{"header":"4 Discussion","content":"\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\u003ch2\u003e4.1 Influence of vegetation patch and groynes detachment\u003c/h2\u003e\u003cp\u003eThe analysis of particle tracks revealed how groyne configuration and the presence of a vegetation patch change the transport dynamics inside the groyne field. In the work of Przyborowski et al. (2024), extending the groyne had an impact on macroplastic litter capture rate, by increasing the width of the mixing region in the groyne tip\u0026rsquo;s wake. In the experiments shown here, all the groynes' tips are in the same position relative to the main flow; therefore, all the changes in dynamics come from either adding vegetation or the gap between the groynes and the flume\u0026rsquo;s wall. Detaching the groynes had two effects: it allowed macroplastic to enter the groyne field through the gap due to a bleed flow, and it caused more particles to enter from the main flow (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) compared to the regular setup in scenario A. The second observation may be connected to the presence of a counter-rotational vortex near the upstream groyne, which, besides disrupting the main circulation in the groyne field, simultaneously uplifts particles that enter through the upstream gap (visible in both scenarios B and D). With the bleed flow observed in the downstream gap, plastic particles spend much less time in the downstream part of the groyne field, escaping rather towards the gap than towards the main flow. Those two facts indicate that detaching the groynes, with the bleed flow strong enough to disturb the typical recirculation pattern inside the groyne field, causes macroplastic litter to predominantly float around the upstream part of the groyne field.\u003c/p\u003e\u003cp\u003eAnother difference of this study in comparison to experiments shown in Przyborowski et al. (2024) is that the vegetation there was flexible, and here it is rigid, hence a more distinct impact on the flow field and particle paths. The results of particle tracking (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) and probability map (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) clearly show how the rigid stems in the form of a circular patch divided the litter paths into two parts, opposite to the regular, unobstructed groyne field. Particle paths also showed how some of the particles, which were about to enter the groyne field, bounced off the vegetation patch back to the main flow. Simultaneously, vegetation patch presence increased the number of particles flowing into the investigated area in the case of regular groynes (from 21 to 33, Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). However, it decreased for scenarios with detached groynes (from 50 to 40, Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Moreover, macroplastic particles spend less time in the investigated area when vegetation is present and groynes are attached. The explanation of this behaviour lies in the observed particles' paths. Litter, that would normally be trapped in a big recirculation vortex for many encirclements, is pushed from the upstream half of the groyne field back to the main flow when trying to flow around or through the edges of vegetation.\u003c/p\u003e\u003cp\u003eRegardless of the observable flow patterns, several particles either stopped moving or moved very slowly through the boundary layer of the flume's wall. However, in a natural channel, bank roughness will play a role, potentially limiting plastic transport through gaps and along the banks even more than the flume wall did during the experiments. Grosfeld et al. (2024) reported that more litter from the beach was picked up in the middle of the groyne field, while more accumulation was visible in the corners.\u003c/p\u003e\u003cp\u003eOverall, the plastic particles that moved around the investigated area spent less time in scenarios with detached groynes. Additionally, particles moving through the vegetation patch spend less time there than it took on average to move out of the investigated area, which implies that unless particles get trapped, the only impact of the vegetation patch on the plastic behaviour is by changing the flow directions inside the groyne field.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\u003ch2\u003e4.2 Other factors that may influence plastic litter transport\u003c/h2\u003e\u003cp\u003eGroynes in this experiment were placed perpendicularly to the main flow in all scenarios for consistency reasons; however, there are recommendations to put detached groynes at an angle (Sommer and Aberle 2009). Such a setup would have some impact since it would redirect the movement of the macroplastic litter towards the downstream end of the groyne. Following the design of the groyne itself, a rough amalgamate of concrete and gravel was used in the conducted experiments. In real engineering projects, the use of riprap or mesh for groyne protection would alter the roughness and potentially increase the entrapment of litter on the groyne itself or enlarge the area of a dead zone behind the obstruction.\u003c/p\u003e\u003cp\u003ePlastic litter transport occurs predominantly during flood conditions (Liro et al. 2020). Such conditions were not simulated in the flume, since the groynes tend to be submerged during floods, and therefore, they would marginally influence the floating particles. However, other possible events, such as the release of a large number of plastic pieces in one place, can be related to conducted experiments, as a large number of particles were released in each experiment. Hauk et al. (2024) reported that after the release of butter tubs due to a flood, their transport along the river was spatially limited. This kind of litter can be described as rigid plastic. In the observed experiment, the pieces of firm cups used as macroplastic particles clogged along the wall, which led to a decreased transport rate. In other words, not only can hydrology and hydrodynamics play a role in the macroplastic transport, but also the number of such objects spilling into a river.\u003c/p\u003e\u003cp\u003eThe behaviour of macroplastics in a channel, however, can be different to that observed in the experiments for foldable or \u0026ldquo;soft\u0026rdquo; plastics. What is more, a recent study revealed that even non-buoyant plastic pieces can be moved to the surface, contradicting the typical Rouse profile of sediment density (Lofty et al. 2024). Alas, the differences in behaviour of plastic particles due to characteristics like dimensions, rigidity, or density are not suitable for discussion here, as there is a lack of comparable studies.\u003c/p\u003e\u003cp\u003eThe wind is a factor not accounted for in this laboratory channel, but in a real scenario, it is a factor that may influence the plastic accumulation. For example, van Thi et al. (2024) observed how wind direction and speed correlated with the influx of certain plastic types to a river. What is more, GPS tracking on the Danube river showed that plastic debris tends to flow along the same river bank (Liedermann et al. 2022). In other words, if wind could transport plastic litter from inland into the river, it can float downstream, sticking to the initial bank and eventually being intercepted by groyne fields. However, there are no studies showing whether wind can actually move floating plastic across the river\u0026rsquo;s width.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\u003ch2\u003e4.3 Hydrological perspective and link to transport modelling\u003c/h2\u003e\u003cp\u003eMacroplastic transport in rivers is a large-scale process that begins within the catchment area. The characteristics of the catchment, including the degree of urbanisation and land use, determine the sources and quantity of plastic that can enter a river. The initiating factors for terrestrial transport are often atmospheric phenomena, such as rainfall generating surface runoff and wind (Mellink et al. 2022). Once in the river channel, the subsequent fate of the particles is controlled by complex hydrodynamic processes.\u003c/p\u003e\u003cp\u003eModern plastic transport models attempt to describe these processes, often using a probabilistic or force-balance approach. For example, the \"travel distance\" model developed by Newbould et al. (2021) conceptualises transport as a series of \"step\" (movement) and \"rest\" periods. The fate of a particle depends on its probability of being trapped, \u003cem\u003ep(T)\u003c/em\u003e, in \"traps\" such as vegetation, bank irregularities, or meanders. Our study provides a detailed, mechanistic insight into the functioning of one such key retention point\u0026mdash;the groyne field. The results showing how particles are retained in recirculation zones, how this is affected by vegetation, or how they escape through the gap in a detached groyne can be used to parameterise and calibrate the probability \u003cem\u003ep(T)\u003c/em\u003e for this type of structure in large-scale models (Newbould et al. 2021).\u003c/p\u003e\u003cp\u003eAnother modelling approach, the \"Plastic Pathfinder\" (Mellink et al. 2022), is based on the principle that transport occurs when driving forces (e.g., wind, surface runoff) overcome resistive forces (e.g., surface friction). Although this model pertains to terrestrial transport, its concept is universal. In a river system, the driving force is the water current (along with turbulence), while resistive forces are generated by \"traps,\" such as the complex and slowed flow within a groyne field. Our experiment illustrates in detail these \"resistive\" hydrodynamic conditions, which are often simplified in large-scale models. Understanding how detaching a groyne or the presence of vegetation modifies these resistive forces is crucial for building more physically realistic transport models that can better predict the locations of plastic accumulation zones in rivers.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec17\" class=\"Section2\"\u003e\u003ch2\u003e4.4 Ecological impact of the conducted investigation\u003c/h2\u003e\u003cp\u003eNew types of groynes, including detached ones, are now considered as one of the more sustainable methods of river training (Manual on Good Practices in Sustainable Waterway Planning, 2010; Gilja et al. 2019). Researchers have also explored the impact of vegetation presence on hydrodynamics in a groyne field (Sukhodolov et al. 2017). Rigid vegetation imitation used in this investigation was built to imitate a reed colony, and the presence of riparian vegetation is one of the colonisation stages on sandy bars inside groyne fields (Luna 2016). Cesarini and Scalici (2022) reported from a river monitoring campaign that reeds trapped the smallest number of plastic pieces, but at the same time, those were predominantly macroplastic pieces. Gallitelli et al. (2024) showed that in small and medium rivers, riparian vegetation traps mostly plastic litter. The conducted experiments confirm that macroplastic pieces can get trapped within the reeds, even with the particles\u0026rsquo; diameter smaller than the gaps between the stems.\u003c/p\u003e\u003cp\u003eExploring the possible impact of detached groynes on plastic transport, the conducted study shows that this new groyne setup can be beneficial in this regard. Although there is a higher chance for macroplastic litter to flow into the groyne field, there is a smaller possibility that it will stay there. Also, following the particle paths patterns, it is possible to discern the most efficient place for a plastic skimmer to catch the biggest number of floating litter, which would be in the upstream part of the groyne field in the vicinity of the vegetation or, in the case of the groyne initialising the groyne fields, it would be where the bleed flow occurs.\u003c/p\u003e\u003cp\u003eThis investigation focused only on the floating macroplastic litter, but the observed patterns may likewise be expanded to plastics with neutral buoyancy or sinking ones. In the work of Sukhodolov et al. (2002), fine sediment accumulation within groyne fields was spatially connected to the occurrence of vortices. Following the particles' paths and the probability map, it is possible to detect places where bigger and smaller vortices were forming during the experiment. Those places can therefore be attributed to the likelihood of finding microplastic particles, which are likely to distribute in rivers in a similar way to natural sediments (Waldschl\u0026auml;ger et al. 2022).\u003c/p\u003e\u003c/div\u003e"},{"header":"5 Summary","content":"\u003cp\u003eTransport and accumulation patterns of macroplastic particles inside the groyne field were investigated in four scenarios. Experiments were conducted in a laboratory flume with the use of the same flow conditions and the same uniform plastic particles. Two scenarios included an artificial vegetation patch in the middle of the groyne field, and in two scenarios, the groynes were detached. Results, including both the particles\u0026rsquo; paths and the water velocity field in the investigated groyne field, were used to understand the litter behaviour.\u003c/p\u003e\u003cp\u003eThe overall pattern that emerges from flume observations is that detaching the groynes enhances transport of macroplastic particles in the groyne field. At the same time, a rigid vegetation patch alters the flow pattern and leads to the interception of macroplastic particles.\u003c/p\u003e\u003cp\u003eWhat was observed for regular setup of groynes, that plastic litter rarely enter the area and then follow the big recirculation pattern of the flow, with the accumulation zone near the wall behind the upstream groyne (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). The addition of the vegetation does two things: it divides the flow inside the groyne field into smaller, separate vortices; it also either traps the plastic particles inside or push them faster back to the main flow (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). In other words, with the vegetation patch presence, 50% more plastics pieces entered the groyne field, but on average spend in the investigated area 50% less time. Detaching the groynes increased the number of plastics entering the area from the main flow but also allowed a much larger number of particles to enter through the gap (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). All the plastics eventually left the area with twice the number of particles that escaped to the main flow than got in that way. Vegetation patch in this configuration caused again the same changes as in the first setup, which is visible in probability maps: plastics tended to float most often in a smaller vortex near the upstream groyne, and some particles got caught within the vegetation stems.\u003c/p\u003e\u003cp\u003eThe results of this investigation show a new perspective on the phenomenon of macroplastic transport in the case of a trained river. Although such laboratory experiments do not cover various factors related to known and unknown plastic behaviours in the natural environment, as explained in the discussion, the observed patterns are relevant to ecohydrology. Detached groynes can act as a more sustainable way to train a river, and the experiments showed that they can store floating plastic litter only temporarily instead of trapping it in the dead zone. The vegetation presence is profitable in this aspect, as it creates a natural barrier and most of the litter pieces small enough to enter the vegetation are gradually washed away.\u003c/p\u003e\u003cp\u003eThe findings of this study also carry practical implications for river management. Demonstrating that groyne fields, especially in their traditional (attached) configuration, act as effective traps for macroplastic indicates that they should be treated as potential pollution accumulation hotspots. Consequently, waterway managers should consider these areas as priority locations for monitoring activities and clean-up campaigns. Furthermore, the observed accumulation patterns (mainly in the upstream part of the groyne field) can help in the optimal placement of debris-capturing devices. Finally, the fact that detached groynes (with a gap at the bank) reduce plastic retention provides valuable insights for the design of future, more ecologically sustainable hydraulic structures.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe thank Rui Aleixo for sharing code for particle tracking and Robert Filszar for the help with laboratory experiments.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work is supported by the National Science Centre, Poland (Grant number: 2021/05/X/ST10/00443) and in part by the Croatian Science Foundation under the project R3PEAT (Grant number: UIP-2019-04-4046). We thank Aleksandra Śmietanka for her assistance in analysing video records.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eŁukasz Przyborowski, Gordon Gilja. and Manousos Valyrakis contributed to the conception and design of this study and secured the funding. Łukasz Przyborowski conducted experiments. Łukasz Przyborowski, Aleksandra Śmietanka and Paweł Gilewski performed data analysis. All authors contributed to writing the manuscript. All authors have read and agreed on the final version of the manuscript. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical Approval\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis is not applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll the authors agreed to participate in coauthorship. The authors have no competing interests to declare that are relevant to the content of this article.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to publish\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll the coauthors agreed with the content of this article, and they all provided explicit consent for submission. The authors obtained consent from the responsible authorities at the institute/organization where the work was carried out before the work was submitted.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u0026nbsp;\u003c/strong\u003e \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe datasets and materials used and/or analyzed in the current study are available in the repository https://puh.srce.hr/s/tm3dk6fk3FCbmHE \u0026nbsp;\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAleixo R, Soares-Fraz\u0026atilde;o S, Zech Y (2011) Velocity-field measurements in a dam-break flow using a PTV Vorono\u0026iuml; imaging technique. 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Eng. 9:29-38. https://doi.org/10.1007/BF02829094\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"environmental-science-and-pollution-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"espr","sideBox":"Learn more about [Environmental Science and Pollution Research](https://www.springer.com/journal/11356)","snPcode":"11356","submissionUrl":"https://submission.nature.com/new-submission/11356/3","title":"Environmental Science and Pollution Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"macroplastic, eco-hydrology, particle tracking, pollution transport, groynes, physical model","lastPublishedDoi":"10.21203/rs.3.rs-7921573/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7921573/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSmall-scale hydrodynamics of plastic debris transport in freshwater environments are insufficiently understood, but it is known that groynes and vegetation play a role in debris accumulation. From the perspective of water management, pollution transport estimates and efficiency of cleaning efforts, this knowledge gap should be addressed. This research paper investigates the impact of groyne setup on macroplastic transport in rivers through observation of the uniform, floating macroplastic particles in laboratory experiments with stationary flow conditions. The effects of two factors were compared by tracking the plastic particles' movement patterns: vegetation presence and groyne configuration. Vegetation was in the form of a circular patch of wooden stems imitating common reed in the middle of the groyne area. The groynes were either in a regular setup, i.e., attached to the bank, or they were detached, allowing the flow along the bank through the groyne field. During the test with a regular setup, typical recirculation of the flow causes plastic litter to accumulate and stay in the upstream corner of the groyne field. The plant patch presence resulted in the temporal accumulation of the plastic pieces between plant stems. When the gap in the groynes was present, all the plastic pieces floating inside the groyne field made their exit from the area, and this behaviour repeated with the vegetation presence. Those results indicate that detached groynes disrupt the flow pattern, which reduces plastic retention time and enhances transport through a groyne field. Those observable changes in macroplastic transport may help in designing best practices of river cleaning activities or more ecologically friendly river structures.\u003c/p\u003e","manuscriptTitle":"Enhanced Macroplastic Transport and Interception in Rivers: The Role of Detached Groynes and Vegetation Patches","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-19 17:24:25","doi":"10.21203/rs.3.rs-7921573/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major Revision","date":"2025-12-04T06:38:58+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2025-11-10T18:51:32+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-11-10T14:35:44+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-10-28T04:22:42+00:00","index":"","fulltext":""},{"type":"submitted","content":"Environmental Science and Pollution Research","date":"2025-10-24T03:36:14+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"environmental-science-and-pollution-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"espr","sideBox":"Learn more about [Environmental Science and Pollution Research](https://www.springer.com/journal/11356)","snPcode":"11356","submissionUrl":"https://submission.nature.com/new-submission/11356/3","title":"Environmental Science and Pollution Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"818db6cb-348c-45e7-bc54-ac2edf5c51c9","owner":[],"postedDate":"November 19th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-02-07T00:05:48+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-19 17:24:25","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7921573","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7921573","identity":"rs-7921573","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}