The Neretva River rises southeast of the Zelengora Mountain in Bosnia and Herzegovina at an elevation of 1,095 meters above sea level and flows approximately 220 kilometers to the Adriatic, of which only the final 22 kilometers lie within Croatia. Its topographic catchment spans roughly 10,975 square kilometers, mostly across the Dinaric Mountain massif, with only 692 square kilometers in Croatian territory. The delta itself developed over an alluvial area of about 190 square kilometers, formed during the last ice age when rising sea levels flooded karst areas along the river. Gravel and sand carried from upstream deposited in the valley, gradually raising the ground level. The delta existed in a largely natural state until the end of the 19th century, when the first regulatory works began, initially focused on maintaining the waterway from the sea to Metković for commercial transport. During the 20th century, numerous embankments, dams, and other hydrotechnical facilities were constructed and wetland reclamation transformed much of the former wetland into agricultural land, significantly altering the natural character of the area.

A defining feature of the Neretva system is its hydropower infrastructure. In the second half of the 20th century, a series of hydroelectric power plants with reservoirs were built in the upstream basin, including Jablanica in 1954 with a dam height of 85 meters and a reservoir volume of 318 million cubic meters, Rama in 1969, Grabovica and Salakovac in 1981, and Mostar in 1985. At the beginning of the 21st century, additional plants such as Peć Mlini on the Tihaljina tributary and Mostarsko Blato were commissioned, while the construction of the Ulog plant has begun and Konjic is planned. The Trebišnjica River basin, interconnected with the Neretva through deep and underexplored underground karst, carries its own substantial hydropower developments, including the large Bileća reservoir with a volume of 1,277 million cubic meters. These structures exert a dominant influence on the frequency, extent, and duration of floods in the downstream delta, and they also reduce sediment delivery to the valley, deepening the riverbed and worsening problems of water and soil salinity.

The hydrological regime of the basin reflects a transition between moderate continental and maritime influences. Average annual precipitation ranges from 1,000 millimeters on the coast to 1,900 millimeters in the mountainous parts of the catchment, with maxima exceeding 2,900 millimeters in elevated areas. The rainiest quarter of the year is November through January, which is also the period when high waters in watercourses are most frequent; the highest recorded monthly precipitation in this quarter at Metković was 400 millimeters in November 2021. Most precipitation in the Dinaric karst quickly sinks underground, accumulating in the subterranean system before reemerging through numerous springs along the edges of the valley, feeding the Neretva, Mala Neretva, and Norin watercourses. This interaction of surface and underground water produces stretched runoff hydrographs and means that floods in the delta are caused primarily by inflow of large waters from the upstream basin, with smaller contributions from karst springs and the sea, and the least impact from precipitation falling directly on the delta area.

Historical measurements at Metković, the furthest upstream hydrological station in Croatia, located approximately 21 kilometers from the river mouth, illustrate how the hydropower construction changed the flood regime. The highest water levels, exceeding 430 centimeters, were recorded in the 1950s before the construction of the Jablanica and Rama plants. After 1961, the highest recorded water level was 414 centimeters in December 2010, an event that flooded a large part of the valley. Because tidal influence prevents unambiguous determination of flows at Croatian stations, the study relied on statistical analysis of maximum annual water levels. Five theoretical distribution functions were tested, and the log-normal function was selected for Metković and Prud, while log-Pearson was chosen for the sea boundary. Calculated high waters for the 1,000-year return period reached 548 centimeters at Metković, 163 centimeters at the sea station, and 182 centimeters at Prud on the Norin River.

For the hydraulic simulation, the researchers built one-dimensional and coupled one-dimensional/two-dimensional hydrodynamic models using the MIKE 11 and MIKE FLOOD program packages from DHI, covering the Neretva River and its banks from the Croatian state border to the mouth. Calibration and verification were carried out separately for four areas due to the complexity of the system, using registered water levels from boundary stations and treating central stations as control points. Manning roughness coefficients ranged from 0.033 for the riverbed to 0.050 for inundation areas. The difference between modelled and measured water levels was mostly within an interval of plus or minus 5 centimeters, with larger deviations occurring only rarely during rapid water level changes. The authors attribute residual inaccuracies to insufficiently precise terrain geometry and to marginal karst springs whose variable inflows were not defined as boundary conditions but were indirectly incorporated through calibration against measured levels.

The simulations classified flood extent into three statistical probability levels. A high-probability flood, corresponding to a 25-year return period, would cover 51 square kilometers; a medium-probability 100-year event would inundate 62 square kilometers; and a low-probability 1000-year flood would spread across 110 square kilometers. The most endangered zone is the Vid–Norin area, where the city of Metković has expanded significantly and many residential, economic, and infrastructure facilities are at risk. The mouth of the Norin is a particularly sensitive location, allowing Neretva flood waters to enter that area. The Kuti area, a swampy zone intersected by numerous canals and influenced by karst springs fed from the underground hinterland, also has a high probability of flooding, though the risk there is moderated by low population density. The Opuzen-Ušće area, protected by numerous embankments following the course of the Neretva and Mala Neretva, carries the lowest risk.

Damage calculations considered seven categories, including populated areas, economic areas, agricultural areas, permanent plantations, green areas, water management facilities, and potential human victims. For the 1,000-year flood, total damage amounted to 250 million Euros, while a 100-year flood would cause 93 million Euros in damage. Populated areas would account for more than 90 percent of the total, with the greatest territorial losses expected in the wider area of Metković on both banks. The estimated delta damage represents approximately 1 percent of the total potential flood damage for the entire Republic of Croatia and about 6 percent of the damage for the Adriatic basin area. Flood risk, defined as the product of probability and adverse consequences, was expressed as an average annual potential number of victims of 0.67 per year and an average annual damage of 11.63 million Euros under conditions of 2015, before the construction of the Jerkovac embankment in Metković.

Climate change simulations modified both boundary conditions, assuming sea level rise at the downstream edge and increased inflow at Metković. The greatest predicted mean sea level increase was 87 centimeters, or 61 percent, for the end of the 21st century at the 1,000-year return period, accompanied by a 23 percent increase in discharge. Under these conditions, the average annual number of potential victims would rise to 0.88 and total potential average damage to 16.5 million Euros, increases of 31 and 41 percent respectively compared to the 2015 baseline. Based on a multi-criteria analysis, the researchers proposed optimal protective measures, including construction of an embankment on the right bank near Metković, a concrete defensive wall with a mobile barrier in the town, embankments on Mislina and near Krvavac, rehabilitation of riverbanks in critical sections, and a self-regulating gate at the junction of the Norin and Neretva rivers. Implementation would reduce the average annual potential number of victims by 70 percent, to 0.2 per year, and average annual damage by 83 percent, to approximately 2 million Euros, with a benefit-cost ratio of 18.04.

The study acknowledges limitations, including reliance on hydrological series ending in 2013 and the use of digital terrain models in certain areas instead of geodetic measurements, which introduces topographic inaccuracy. Extending the input series and defining hydrographs for marginal karst springs are identified as priorities for future work. Nevertheless, the researchers argue that the methodology is universal and can be applied to similar estuary and delta systems worldwide. Beyond flood defense, the quantified hydrological parameters such as water depth, flow rate, flood duration, and salinity can support ecological studies, which is particularly important given recommendations to preserve natural wetlands in the Mediterranean region. Because the Neretva basin spans two countries with separate protocols and professional services, transboundary cooperation in water management remains essential for reducing flood risks and improving conditions across the entire delta.

Subject of Research: Fluvial flood risk assessment and damage modelling in the Neretva River Delta, Croatia

Article Title: Delta flood risk analysis: case study from the Neretva River (Croatia)

Article References: Ljubenkov, I., Kvesić, D., & Erceg, J. (2024). Delta flood risk analysis: case study from the Neretva River (Croatia). Estuarine Management and Technologies, 1, 69-93. https://doi.org/10.3897/emt.1.137829

Image Credits: AI Generated

DOI: 10.3897/emt.1.137829

Keywords: Neretva River Delta, flood risk, hydrodynamic modelling, MIKE FLOOD, return period, climate change, sea level rise, Mediterranean deltas, karst hydrology, flood protection, damage assessment, Croatia

The treatment of horizontal diffusion in the particle model deserves particular attention, because it illustrates a subtlety that is often overlooked in applied transport modelling. In a random-walk formulation, particles are displaced by both the resolved advective velocity and a stochastic term representing unresolved turbulent mixing. When the diffusion coefficient varies in space, as it does here, a naive implementation in which the stochastic displacement is drawn independently at each step systematically violates the well-mixed condition, the requirement that a uniformly distributed set of particles remains uniformly distributed over time. The consequence is a spurious drift of particles toward regions of low diffusivity, an artifact that could be mistaken for a physical accumulation zone. The correction introduced by Visser, which the authors adopt, adds an explicit gradient term to the particle displacement equation and evaluates the diffusivity at an intermediate position reached after applying half of the gradient drift. This two-stage procedure substantially reduces the bias, and its inclusion in the model design signals a careful attention to numerical fidelity that strengthens confidence in the resulting exposure maps and dispersion statistics.

The decision to represent the lagoon as a closed basin is a deliberate simplification with a clear evidential basis. Previous modelling work on the same system had shown that, even under idealized conditions with both inlets fully open, the exchange of water between the lagoon and the Adriatic Sea is remarkably weak, with the external sea level exerting a negligible influence on the internal circulation except in the immediate vicinity of the inlets themselves. By removing the open boundary altogether, the authors isolate the internal transport mechanisms, wind-driven circulation above all, and can attribute differences in plume behavior between scenarios directly to variations in the wind forcing rather than to changes in boundary exchange. This framing also carries a practical implication: if the lagoon behaves hydrodynamically like a nearly closed basin, then pollutants introduced into it are, to first order, retained within it, and the timescales over which they are redistributed internally become the relevant timescales for ecological exposure. The two inlets, whose operation is informal and unscheduled, therefore emerge not as reliable flushing conduits but as marginal contributors whose management status nonetheless shapes local conditions near the connections.

The six weekly wind scenarios were extracted from a forty-year historical record, which gives the scenario ensemble a degree of representativeness that single-case studies often lack. Rather than simulating one arbitrary week, the authors sampled the range of metocean conditions that the lagoon actually experiences, so that the contrast between scenarios reflects genuine climatological variability rather than an artifact of scenario selection. This approach sits between the two extremes commonly found in the literature: idealized forcing experiments, which are clean but may not correspond to real conditions, and long-term coupled simulations, which are realistic but computationally expensive and harder to interpret scenario by scenario. The weekly window itself is a meaningful choice for a shallow lagoon, since it is long enough for the wind-driven circulation to establish a coherent pattern and for particles to travel appreciable distances, yet short enough that the forcing can be treated as approximately stationary within each scenario.

The distinction between the two release configurations, instantaneous and continuous, addresses a question that is central to pollutant risk assessment but frequently glossed over in modelling studies. A single pulse release, such as an accidental discharge or an overflow event, produces a compact cloud of particles whose fate depends on the flow field encountered at the moment of release. A continuous release, such as the steady discharge from a sewage outfall, produces instead a stream of particles that sample the flow at many different phases of the circulation, and the resulting plume integrates over that variability. The finding that dispersion in some scenarios is sensitive to release timing, with particles either remaining close to the source or spreading more uniformly across the lagoon, indicates that the advective pathways within the lagoon are strongly organized by the wind regime. When the circulation is dominated by coherent, persistent flow structures, the moment of release largely determines which pathway a particle follows, and two releases separated by only hours or days can end up in entirely different regions of the basin. When the circulation is more dispersive, this sensitivity fades and the plume behaves more predictably. The authors attribute this sensitivity specifically to advection, which is a useful diagnostic: it means that the variability arises from the deterministic structure of the flow rather than from the stochastic diffusion term, and it suggests that even modest improvements in the timing of discharges, where such control is possible, could meaningfully reduce exposure in the most retention-prone conditions.

The exposure maps highlighting accumulation zones along the lagoon boundary carry direct management relevance. In a closed or nearly closed basin, material that is not flushed out must eventually be redistributed by the internal circulation, and the boundaries, where currents weaken and particles can strand in shallow areas or along the shoreline, act as natural sinks for floating matter. For floating pollutants, including oils, plastics, and buoyant sewage-associated debris, these boundary accumulation zones represent the locations where ecological contact and cleanup costs are likely to be concentrated. Identifying them in advance, and understanding how their location shifts between wind regimes, allows managers to prioritize monitoring stations, anticipate where visual or chemical impacts will appear after an event, and design interventions such as containment booms or targeted collection with some foreknowledge of where material will converge. The fact that the release point was chosen on the basis of previous field sampling that identified it as the most contaminated area of the lagoon further anchors the simulations in observed reality, ensuring that the modelled plumes originate from a location where pollutant inputs are known to occur rather than from a purely hypothetical source.

The ecological and economic context of the Narta Lagoon amplifies the significance of these results. The lagoon supports migratory birds, European eels, and local aquaculture activities that employ a substantial share of the nearby workforce, so water quality is not merely an environmental concern but a livelihood issue. The documented pressures on the system, including waste disposal serving Vlorë and surrounding towns, salt production, leather tanning, and incomplete wastewater treatment, mean that the pollutant loading simulated in this study is not an abstract scenario but a plausible representation of ongoing inputs. Shallow lagoons are especially sensitive in this respect because their limited depth means that floating material remains in close contact with the biologically productive surface layer, and their restricted exchange means that dilution rates are low. A pollutant that would be rapidly dispersed in an open coastal setting can persist in a lagoon of this type for weeks, repeatedly exposing the same benthic and pelagic communities.

The finding that advective flow plays the dominant role in determining plume behavior also has implications for how monitoring and modelling of such systems should be designed. If mixing were controlled primarily by diffusion, then coarse descriptions of the flow field would suffice, and dispersion could be parameterized with simple bulk coefficients. The dominance of advection implies instead that the spatial structure of the wind-driven circulation, including its gyres, shear zones, and stagnation points, is what controls where pollutants go. This raises the bar for the hydrodynamic model: the velocity fields must resolve the relevant circulation features accurately, because errors in the flow structure translate directly into errors in predicted exposure. It also suggests that wind forecasting, rather than tidal prediction, is the more valuable operational input for anticipating pollutant behavior in this lagoon, a somewhat counterintuitive conclusion for a coastal water body connected to the sea.

The observation that the northern inlet is prone to seasonal closure through sedimentation driven by littoral transport, and is manually reopened by fishermen roughly twice per year, adds a human dimension to the physical problem. The intermittency of this connection means that the lagoon’s effective degree of isolation varies through the year in an unmanaged way, and the southern inlet’s manually operated sluice gates introduce a further element of discretionary control. The authors’ emphasis on the importance of properly managing the two inlets follows logically from their results: if internal retention is already high when the lagoon is treated as closed, then informal or infrequent inlet operation can only prolong stagnation, and a more deliberate management regime, with scheduled openings timed to coincide with conditions favorable to exchange, could reduce the residence time of both water and pollutants. The study’s framing of the lagoon as a closed basin thus serves a dual purpose: it is a defensible modelling simplification supported by prior evidence, and it is also a conservative bounding case whose results quantify the worst-case retention that inlet management would seek to alleviate.

More broadly, the study contributes to a growing body of work demonstrating that semi-enclosed coastal systems cannot be managed by analogy with open coastlines. The combination of high-resolution hydrodynamic modelling, Lagrangian particle tracking, and statistically grounded dispersion descriptors offers a transferable template that can be applied to other shallow lagoons facing similar pressures, many of which are distributed along the Mediterranean and Adriatic coasts where urbanization, tourism, and aquaculture converge on ecologically fragile water bodies. The scenario-based design, in which a small number of representative forcing conditions is analyzed in depth, strikes a practical balance between computational cost and physical realism, and the explicit comparison of release configurations provides a dimension of insight that single-release studies cannot offer. As climate variability alters wind regimes and sea level modifies inlet dynamics in the coming decades, the kind of scenario library constructed here could be extended and reweighted to reflect future conditions, extending the usefulness of the modelling framework well beyond the present analysis of a single Albanian lagoon.

Subject of Research: Lagrangian particle-tracking simulation of pollutant plume dispersion in the Narta Lagoon under variable metocean conditions

Article Title: Assessing lagoon plume dispersion under variable metocean conditions

Article References: Scovenna, M., & Besio, G. (2026). Assessing lagoon plume dispersion under variable metocean conditions. Frontiers in Environmental Engineering, 5, Article 1882143. https://doi.org/10.3389/fenve.2026.1882143

Image Credits: AI Generated

DOI: 10.3389/fenve.2026.1882143

Keywords: coastal lagoon, Lagrangian particle tracking, plume dispersion, metocean conditions, Narta Lagoon, water quality, hydrodynamic modelling, advection, pollutant transport, Delft3D, exposure maps, Albania

SURF was developed by Mathew J. Owens and Luke A. Barnard of the University of Reading and described in the journal Solar Physics. The framework packages together two modelling options. The first is HUXt, a well-established reduced-physics model that treats the solar wind as a one-dimensional advection problem and has already found use in both forecasting and a diverse range of scientific applications, from planetary studies to comet-tail analysis. The second, and the centrepiece of the new work, is a newly developed one-dimensional compressible hydrodynamic solver called hydro, which adds physically consistent compression effects while retaining the computational efficiency needed for ensemble forecasting, uncertainty quantification and large parametric studies.

The distinction matters because the solar wind is a compressible fluid. Fast streams emitted from coronal holes eventually catch up with slower wind ahead of them, piling plasma into compressed stream interaction regions bounded by shock-like fronts. HUXt, which lacks physics-based compressibility, agrees with full three-dimensional magnetohydrodynamic models to within about five percent for the same boundary conditions, but its largest deviations appear precisely at those compression fronts. SURF-hydro addresses this weakness by solving the one-dimensional Euler equations of mass, momentum and energy conservation in spherical geometry, using finite-volume methods with Riemann solvers and second-order spatial reconstruction.

The numerical machinery is sophisticated but conceptually standard in computational fluid dynamics. The model discretises the radial domain into spherical shell cells and computes fluxes across cell interfaces using a Harten-Lax-van Leer-Contact solver, which handles the discontinuities that arise at shocks. Two reconstruction schemes are available: a first-order piecewise constant method for maximum speed, and a default second-order piecewise linear method with a monotonized central limiter that keeps sharp features like shocks intact without introducing spurious oscillations. Because the flow is radial in spherical geometry, the model includes a geometric source term in the momentum equation, reflecting the fact that the surface area of a sphere grows as the square of distance from the Sun.

Validation against an analytical benchmark is impressive. The team compared SURF-hydro with an exact solution for steady-state, pressure-driven expansion of a uniform spherical wind, treating the solar wind as isentropic flow through a nozzle whose cross-sectional area grows with distance squared. The second-order solution reproduced the analytical solar wind speed with an error of just 0.04 percent, with density and temperature errors below one percent, and conserved mass to within one percent across the whole domain. Speed matters too: a five-day simulation for a single longitude takes about 0.1 seconds on a standard desktop processor, roughly ten thousand times cheaper than a full three-dimensional magnetohydrodynamic run.

Perhaps the most consequential contribution is a new way of setting the model’s inner boundary conditions. Solar wind models typically start at 0.1 astronomical units, about a fifth of Mercury’s orbital distance, where the flow is already super-magnetosonic. Coronal models supply speed and magnetic field at that boundary, but density and temperature must be inferred from the speed, usually by assuming some form of equilibrium such as constant mass, momentum or kinetic energy flux. Owens and Barnard instead mined thirty years of near-Earth OMNI observations, removed all periods contaminated by coronal mass ejections, and derived empirical relations between solar wind speed, density and temperature at 1 astronomical unit. They then back-mapped those relations to 0.1 astronomical units using the analytical nozzle solution, producing a non-equilibrium look-up table that can be interpolated for any inner-boundary speed.

The payoff shows up in hindcast tests. For a representative 27-day interval of recurrent solar wind in 2019, a SURF-hydro hindcast driven by back-mapped in situ observations reproduced the observed base-level proton density of around five particles per cubic centimetre, along with the sharp density spikes of tens of particles per cubic centimetre at stream interaction regions, and temperatures ranging from about 50,000 kelvin in slow wind to 500,000 kelvin in compressed regions. By comparison, archived operational WSA-Enlil forecasts for the same period showed almost no density variation and temperatures systematically an order of magnitude too low. Crucially, the authors show this is not a flaw in Enlil’s physics but in its boundary conditions: when the same WSA coronal maps drove SURF-hydro with the new non-equilibrium relations, the variability in speed, density and temperature all improved markedly.

Extending the comparison across four years of observations reinforced the point. WSA-Enlil systematically under-dispersed solar wind speeds and produced far too little density variability, while its temperatures remained far too low even accounting for the reduced speed range, implicating the equilibrium assumption at the inner boundary. WSA-SURF-hydro, using the empirical relations, matched the observed ranges and trends much more closely, with the main discrepancy being somewhat elevated densities and temperatures at intermediate speeds, likely because the WSA coronal model produces too many fast streams. The authors suggest that operational systems could be significantly improved simply by adopting similar empirical density and temperature relations, an approach transferable to other solar wind models beyond SURF.

The framework also shines a light on an under-explored source of forecast uncertainty: the assumed properties of coronal mass ejections themselves. Operational systems insert CME perturbations at 0.1 astronomical units that are over-dense, typically four times the ambient density, partly to compensate for the neglected internal magnetic pressure of the cone-model representation. Yet observations at 1 astronomical unit show that interplanetary coronal mass ejections are actually cooler and more tenuous than the surrounding wind, partly from adiabatic expansion in transit and partly because significant expansion and cooling has already occurred close to the Sun. A super-posed epoch analysis of 45 fast magnetic clouds confirmed this picture, with the ejecta body characterised by declining speed and lower density and temperature than the ambient solar wind.

Because SURF-hydro can sample parameter space rapidly, the team ran sensitivity tests varying the initial density and temperature of a model CME launched into a structured ambient wind. Even without magnetic forces, the model reproduced the key observed features of CME evolution at 1 astronomical unit, including a hot, dense sheath ahead of the ejecta, an expanding body cooler and less dense than its surroundings, and durations of roughly 24 hours consistent with observations. The sensitivity results were striking: for one particular structured solar wind, varying the CME’s initial density and temperature at 0.1 astronomical units changed the transit time and arrival speed at Earth by 15 to 20 percent. Hotter, denser CMEs arrived sooner, faster and with stronger shocks. Since these parameters are observationally unconstrained and currently ignored in ensemble forecasting, the authors argue they merit systematic perturbation in future operational ensembles. With the SURF code freely available on GitHub and installable from PyPI and conda-forge, the framework offers researchers and forecasters alike an efficient bridge between idealised models and full three-dimensional simulations, and a practical tool for interrogating the assumptions that quietly shape every space-weather forecast.

Subject of Research: Compressible hydrodynamic modelling of the solar wind and coronal mass ejection propagation for space-weather research and forecasting

Article Title: Space-Weather Utilities for Research and Forecasting (SURF): A Tool for Investigating Hydrodynamic Aspects of Solar Wind and Coronal Mass Ejection Expansion and Evolution

Article References: Owens, M. J., & Barnard, L. A. (2026). Space-Weather Utilities for Research and Forecasting (SURF): A Tool for Investigating Hydrodynamic Aspects of Solar Wind and Coronal Mass Ejection Expansion and Evolution. Solar Physics, 301(9), Article 147. https://doi.org/10.1007/s11207-026-02738-7

Image Credits: AI Generated

DOI: 10.1007/s11207-026-02738-7

Keywords: space weather, solar wind, coronal mass ejections, SURF, HUXt, hydrodynamic modelling, OMNI observations, WSA-Enlil, forecasting, heliospheric physics, boundary conditions, ensemble forecasting

The logic behind the integrated approach is straightforward but demanding. In-situ measurements remain the gold standard for identifying what is actually floating in the water, but point samples cannot capture the highly complex spatial and temporal pathways that carry litter from urban and agricultural sources through rivers, along coasts and eventually into sinks such as sediments, vegetation or the open sea. Remote sensing, in turn, offers wide coverage but cannot chemically confirm what it sees. Numerical models can forecast transport, but only if they are fed realistic inputs about how much plastic enters the water, when, and with what physical properties. The Po Delta project was designed to make each tool compensate for the weaknesses of the others.

At the source end, the team installed a low-cost Raspberry Pi camera system on bridges along the Po River, capturing one image every two seconds, with a GoPro Hero5 alongside for quality comparison. From 18,848 images, 3,526 were labelled, containing 2,914 pieces of litter ranging from multicoloured fragments and wood to lids, sheets and Styrofoam. A Faster Region-based Convolutional Neural Network, pre-trained on the COCO dataset and fine-tuned with both a proxy dataset from earlier river-monitoring work and the new Po River images, was then tasked with detecting floating items automatically. Performance was evaluated using mean Average Precision at an Intersection-over-Union threshold of 0.5, averaged across ten cross-validation folds.

The results revealed both the promise and the fragility of automated river monitoring. The COCO pre-trained configuration reached a mean mAP50 of 0.60, outperforming a model trained from scratch at 0.56, and combining all available training data beat using the Po River dataset alone at 0.57. More striking were the diagnostic tests. On imagery containing floating organic matter such as leaves and twigs, detection performance collapsed to an mAP50 of 0.25, compared with 0.83 on scenes free of natural debris. Yet when the same high-organic scene was captured with a 300 dpi Nikon DSLR instead of the 96 dpi Raspberry Pi, performance rose to 0.46—suggesting that much of the apparent confusion with organic matter stems from limited ground sampling distance rather than the neural network itself. For system designers, the lesson is that improving camera resolution or mounting positions may pay off more than tweaking algorithms.

At the coastal sink end, a DJI Matrice 600 drone equipped with a 20-megapixel Zenmuse X5 camera and a MicaSense RedEdge-3 multispectral sensor flew over Po Delta beaches in October 2021, generating georeferenced orthomosaics that were analysed with the LITTERDRONE software originally developed as a European Commission Blue-Labs activity. The software detects and classifies beached objects larger than 2.5 centimetres based on colour, shape and background, with an expert validating outputs in a human-in-the-loop step. North Pila beach emerged as the hotspot, with 118 items detected including 41 plastic fragments of 2.5 to 50 centimetres, 28 drink bottles and 12 plastic caps. Validation against physical sampling showed how sensitive the technique is to flight altitude: at 25 metres over Barricata beach, only 1 of 49 plastic items found on the ground was detected in the imagery, whereas at 15 metres over North Pila the software recovered 115 of 182 items. Driftwood whose colour closely matched the sand also masked litter on some beaches.

Satellite observations extended the picture offshore. A task-requested WorldView-2 image of the delta, acquired on 24 March 2021 at 1.6-metre resolution, was processed with two complementary approaches: an object identification method fused with edge detection on the panchromatic band, and a spectral band anomaly algorithm that subtracts an averaged water background from Rayleigh-corrected reflectance and computes a near-infrared-minus-green proxy. Both flagged suspected fishing boats, windrows—linear aggregations of floating material—and fishing buoys, with the automated maps consistent with visual inspection of true-colour composites. In parallel, operational Sentinel-2 and Sentinel-3 data were used to map total suspended matter as a tracer of river plumes, the likely hotspots where plastics aggregate, and a customised web interface delivered near-real-time daily views to guide the fieldwork.

Ground truth came from two research campaigns in 2021. From 14 to 27 March, the coastal northern Adriatic from Venice to Ancona was surveyed aboard the research vessel G. Dallaporta across 12 transects and 85 stations, with Secchi disk readings showing water transparency increasing offshore from 1 to 19 metres, 79 CTD casts, and a 330-micrometre Manta net towed at 45 locations. A second campaign in October sampled four coastal sites and five river branches. Back in the laboratory, putative microplastics larger than 300 micrometres were sorted under a stereomicroscope and identified by Attenuated Total Reflectance Fourier Transform Infrared spectroscopy against a 128-record polymer library. Polyethylene, polypropylene and polystyrene dominated, consistent with other regional studies, and most particles were white, transparent or blue fragments—likely weathered pieces of single-use packaging.

The transect data revealed sharp spatial structure. Along transect T2 near the Adige River mouth, nearshore waters were essentially clean, but concentrations spiked to 1.98 pieces per cubic metre before declining seaward to 0.02. Closer to the Po Delta mouth, transect T4 showed far higher values ranging from 0.02 to 54.26 pieces per cubic metre, following a sinusoidal pattern with a maximum halfway along the transect. Polymer diversity tracked abundance, with polyethylene present at more than 50 percent at five of six stations. The offshore maxima in both transects align with the Po River plume forming an accumulation front where freshwater meets seawater, while numerical trajectories indicate that particles near the Adige are transported southwards, leaving nearshore waters there largely plastic-free.

The modelling component used HYDROMOD-3D, a hydrodynamic coastal model running on 100-metre horizontal grids, with tracer modules modified to account for particle size, wind exposure above the water surface, and sinking versus non-sinking behaviour. Simulations driven by 2016 discharge, wind and water-level data tested two characteristic wind regimes of the northern Adriatic: the north-easterly Bora and the south-easterly Scirocco. The contrast was dramatic. Under Bora conditions, 200-millimetre particles travelled in a narrow band straight along the coast, whereas Scirocco winds drove them into a circular gyre generated by a topographically induced counter-current. Larger 500-millimetre objects were more strongly affected by wind drag. The simulations produced probabilistic accumulation zones rather than exact landing points, but those forecasts could be checked by drone surveys of recurrent beaching locations—closing the loop between prediction and observation.

The authors are candid about the limitations. Two field campaigns provide only a narrow spatio-temporal snapshot compared with multiyear surveys of Italian waters, and the modular system was assembled to assess technology readiness rather than rigorously cross-validated across all platforms, which they identify as the essential next step. Still, the proof of concept carries real weight for policy: physiochemical descriptors of the litter feed directly into instruments such as Italy’s Salvamare Law, the Marine Strategy Framework Directive and the EU Single-Use Plastics Directive, and model forecasts of accumulation hotspots could optimise clean-up operations at sea and along shorelines. If the remaining gaps—harmonised protocols, balanced training datasets, and better fusion of high-resolution imagery with operational satellite data—can be closed, the Po Delta experiment suggests that an affordable, multi-tool observing system capable of tracking plastic from source to sink is no longer a distant ambition but an engineering problem within reach.

Subject of Research: Integrated source-to-sink monitoring of plastic litter leakage in the Po Delta using remote sensing, machine learning, field sampling and hydrodynamic modelling

Article Title: A case study on the Po Delta in Italy on advancing the synergy of monitoring strategies for leakage litter from source-to-sink

Article References: Franke, J., Garaba, S. P., Brand, A. K., Duwe, K., Davison, S., Falcieri, F. M., Laforsch, C., Löder, M. G., López-Samaniego, E., Mantas, V., & Pérez-Gómez, J. P. (2026). A case study on the Po Delta in Italy on advancing the synergy of monitoring strategies for leakage litter from source-to-sink. Environmental Challenges, 25, Article 101651. https://doi.org/10.1016/j.envc.2026.101651

Image Credits: AI Generated

DOI: 10.1016/j.envc.2026.101651

Keywords: plastic pollution, Po Delta, marine litter, remote sensing, machine learning, hydrodynamic modelling, microplastics, Adriatic Sea, drone monitoring, satellite imagery, riverine plastic, Environmental Challenges

The method, called Salinity Zones Distribution (SZD) mapping, was created by T. S. Franklin of the Federal University of Bahia, together with P. C. C. Rosman of the Federal University of Rio de Janeiro and R. C. Carvalho of James Cook University. Rather than simply averaging salinity values across space and time, the technique counts how often each salinity class occurs at every point in the estuary over a full calendar year. The class that dominates most frequently is assigned to that location, producing a map of the most permanent ecological salinity conditions rather than a smoothed statistical blur.

To generate the underlying salinity fields, the team used TELEMAC-2D, a depth-averaged hydrodynamic model that solves the vertically averaged Navier-Stokes equations for free-surface flow and salinity transport. The model domain was discretized with a finite-element mesh of 34,699 triangular elements ranging from 16 to 700 meters, built from bathymetric data supplied by the Brazilian Navy and earlier field campaigns. Tidal forcing came from harmonic constituents measured at the Madre de Deus terminal, while salinity at the ocean boundary was set from moored sensor data collected between 2012 and 2014 near the estuary mouth in Todos os Santos Bay.

Validation was extensive and multi-metric. Water levels at stations near the estuary head and mouth achieved Model Prediction Skill values above 0.9, with root-mean-square errors of 0.12 and 0.29 meters respectively, well below the local tidal range. Depth-averaged currents at three mid-channel sections reached Skill values between 0.85 and 0.95. Salinity validation across five stations produced Skill values from 0.53 to 0.91, and the researchers were careful to explain that low Skill at the euhaline mouth station reflects the extremely narrow observed salinity range there rather than poor physical performance. Crucially, all salinity biases remained smaller than the width of any salinity class in the Venice System, the classification scheme adopted for the ecological zoning.

That classification, first consolidated at the 1958 Venice symposium, divides aquatic environments into five zones from limnetic fresh water below 0.5 practical salinity units to marine waters above 30 psu. It remains the most widely used framework for salinity-based ecological zones, and the SZD method can accommodate it or any alternative scheme, including multivariate classifications derived from local biological data. For each month of the simulated year, the researchers computed the dominant salinity class at every mesh node, resolving ties in favor of the class with the narrower salinity range to minimize classification uncertainty.

The team ran two contrasting scenarios for 2010, a year chosen because its wet and dry seasons were typical and because fish survey data were available. In the Natural scenario, freshwater inflow to the estuary equaled the river inflow entering the dam reservoir. In the Regulated scenario, inflow reflected the actual operation of Pedra do Cavalo, which releases water in daily pulses of roughly 45 cubic meters per second for four to eight hours, driven by electricity generation norms and a minimum sanitary discharge of about 10 cubic meters per second. The contrast between these two worlds proved dramatic.

Under regulated flows, polyhaline conditions between 18 and 30 psu dominated Iguape Bay for most of the year, whereas natural flows would have produced euhaline conditions above 30 psu much of the time. During the dry month of February 2010, the limnetic zone extended only 3 kilometers downstream of the dam under natural conditions but stretched 10 kilometers under regulation. Conversely, during the April 2010 flood, when natural discharges exceeded 1,000 cubic meters per second, the upper estuary shifted abruptly to limnetic conditions and the mesohaline zone expanded deep into Iguape Bay. The regulated regime, in short, holds the estuary in a persistently saltier state in the bay while pushing fresher, more variable conditions into the upstream channel.

The ecological consequences follow directly from the maps. Previous fish surveys in the Paraguaçu estuary identified three guilds: estuarine residents that complete their entire life cycle within the estuary, estuarine migrants with larval stages outside it, and marine stragglers that venture in from the sea. Estuarine residents concentrate in salinities of 18 to 26 psu, squarely within the polyhaline class that regulation now sustains in Iguape Bay, suggesting these fish may benefit. But the simulations also show that regulated discharges impose limnetic conditions along the channel upstream of the bay, implying significant stress for the oligohaline and mesohaline biota that historically inhabited those reaches, including larval and juvenile fishes that depend on low-salinity nursery habitats.

Benthic communities tell a parallel story. Local studies of the Paraguaçu system show a clear longitudinal replacement of taxa along the salinity gradient, with bivalves of the families Tellinidae and Veneridae, cirolanid isopods, cyclopoid copepods and nereidid polychaetes dominating low-salinity sectors, while nuculid bivalves, cirratulid polychaetes and amphiurid brittle stars characterize high-salinity, finer-sediment areas. The SZD maps can therefore be read as habitat-suitability predictors: expansion of polyhaline and euhaline zones under regulation should favor the marine-affiliated assemblages, while shrinking the spatial footprint available to freshwater-tolerant taxa. The framework also connects to broader phenomena, since the position of the low-salinity front relates to the estuarine turbidity maximum, where nutrients and phytoplankton concentrate and where algal blooms, including the red tide recorded in the bay in 2007, can originate.

Beyond its immediate findings, the study positions SZD mapping as a versatile management tool. The authors argue it can support environmental flow planning, help resolve conflicts between dam operators and artisanal fishing communities such as those in the Iguape Bay Extractive Reserve, guide aquaculture siting for species like the mangrove oyster whose larvae favor 25 to 30 psu, and even serve as a climate-change indicator by mapping how sea-level rise and altered rainfall shift salinity zones. Because the method works with any salinity classification and any estuary with hydrodynamic model output, the researchers believe it offers a practical, visually intuitive foundation for sustainable catchment management worldwide, from the lagoon of Venice to dam-removal sites on the Elwha River.

Subject of Research: A novel salinity zone mapping method for assessing how dam-regulated freshwater inflow alters ecological salinity zones in estuaries

Article Title: Evaluating salinity alterations in estuaries through ecological mapping

Article References: Franklin, T. S., Rosman, P. C. C., & Carvalho, R. C. (2026). Evaluating salinity alterations in estuaries through ecological mapping. Discover Oceans, 3(1), Article 46. https://doi.org/10.1007/s44289-026-00159-9

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00159-9

Keywords: estuary, salinity, hydrodynamic modelling, Venice System, freshwater regulation, Paraguaçu River, Todos os Santos Bay, ecological zonation, environmental flows, benthic fauna, fish habitat, dam impacts

The research, led by Gunnel Göransson and colleagues at the Swedish Geotechnical Institute, examines the Göta River, the Nors River, the Säve River and the Ångerman River, all of which flow through fine-grained or mixed sediments where landslide susceptibility is high. The five case studies, conducted between 2009 and 2022, were carried out within a national programme for assessing and mapping future landslide hazards along watercourses. Together they form one of the most sustained efforts anywhere in the world to connect climate-driven fluvial erosion with probabilistic geotechnical slope-stability analysis.

The methodological framework that emerged was iterative, refined case by case, but consistently followed seven steps: compiling previous studies and measurements, conducting hydroacoustic surveys and sediment investigations, running hydrodynamic models to derive erosion parameters, selecting future flow scenarios, modelling erosion, validating results and assessing uncertainty, and finally integrating erosion forecasts into geotechnical stability analyses. Multibeam echosounder surveys mapped bathymetry, side-scan sonar characterized bedforms, backscatter analysis classified sediments, and physical sampling determined how erodible the riverbed materials actually were.

Hydrodynamic modelling sat at the heart of each assessment. The teams used two-dimensional models, including Delft3D, TELEMAC-2D, MIKE 21 C and MIKE 21 FM, to simulate water levels, flow velocities and bed shear stresses under a range of hydrological conditions. Two-dimensional modelling was deliberately chosen as the best balance between computational demand, data requirements and accuracy; three-dimensional simulation, while more detailed, was not considered justified given the uncertainties inherent in forecasting erosion over nearly a century. Erosion estimates rested on the relationship between calculated bed shear stresses, critical shear-stress thresholds and sediment erodibility coefficients, with cohesive sediments handled through the Partheniades formulation within a GIS framework.

The choice of erosion model depended on the geology. In clay-dominated systems such as the Göta, Nors and Säve rivers, the GIS-based analyses were supplemented with the Bank Stability and Toe Erosion Model, known as BSTEM, which explicitly couples hydraulic toe erosion with geotechnical bank-failure mechanisms. The Ångerman River, by contrast, is dominated by frictional sediments such as silt, sand and gravel, so the team adopted a full morphodynamic approach using the MIKE 21 C multi-fraction model, representing sediment transport and channel evolution explicitly. Future flows were derived from downscaled RCP climate projections supplied by the Swedish Meteorological and Hydrological Institute, alongside alternative hydropower regulation strategies.

The erosion projections were then translated into future channel cross-sections that served as direct input to slope-stability calculations performed with Slope/W, following Swedish geotechnical practice. Both present-day and year-2100 conditions were analysed, accounting for projected changes in groundwater, pore-water pressures and erosion-modified slope geometry. To capture uncertainty, the team applied the Point Estimate Method, a computationally efficient alternative to Monte Carlo simulation, deriving failure probabilities from statistical distributions of shear strength, unit weight, pore pressure and geometry. These probabilities were classified into five classes and combined with consequence classes in a GIS-based risk matrix to produce landslide risk maps sensitive to climate change.

Among the most striking findings is the role of hydropower regulation. In heavily regulated rivers, erosion driven by operational flow management can exceed that caused by climate-related changes in discharge, potentially obscuring the climate signal altogether. On the Ångerman River, short-term regulation generated rapid fluctuations in water level and flow velocity that dwarfed projected climate-driven hydrological changes. The study also revealed that the temporal resolution of discharge data matters enormously: simulations based on 14-day averaged flows suggested reduced future erosion, while high-resolution data capturing short-duration flow peaks indicated the opposite, because those peaks contribute disproportionately to bed shear stress but vanish when flows are averaged.

The synthesis also clarified how erosion interacts with inherent susceptibility. In areas already highly prone to landslides, even minor erosion can significantly raise the probability of a catastrophic failure, whereas in stable terrain substantial erosion is needed before risk begins to climb. Sediment composition determines which climate-related driver dominates: in cohesive clay valleys such as those of the Göta, Nors and Säve rivers, fluvial erosion is the dominant climate-related trigger of slope instability, while in the coarser deposits of the Ångerman valley, changes in groundwater and pore-water pressure are likely to matter more.

The authors are candid about uncertainty. Scarcity of sediment-transport measurements, limited repeated bathymetric surveys, positional inaccuracies on steep underwater slopes and the sheer difficulty of simulating long-term fluvial geomorphology all constrain quantitative precision. Their response is a call for recurrent, systematic monitoring, particularly comprehensive bathymetric surveys, and for assessments that are treated as living documents, updated regularly and immediately after any landslide occurs, since such an event would fundamentally reshape river morphology. The projections are best read as semi-quantitative tools for identifying erosion-prone areas and prioritizing detailed investigations rather than as precise predictions.

Perhaps the most valuable export of the study is its transferable workflow. Because it is grounded in fundamental principles of hydrology, hydraulics, sediment transport and erosion, the framework can be adapted beyond Sweden by adjusting the hydrological forcing, whether the driver is monsoon rainfall, tropical cyclones, drought or rapid glacier retreat. The authors emphasize that success depends on interdisciplinary collaboration across geomorphology, hydrology, sediment transport, hydraulics and geotechnics, and on adaptive rather than static management. As extreme precipitation intensifies worldwide, the lesson from the Swedish rivers is clear: the ground beneath riverside communities is being reshaped now, and the only responsible way to plan for it is to model, monitor and adapt continuously.

Subject of Research: Projecting climate-induced river erosion to assess future landslide susceptibility in Swedish river valleys

Article Title: Projecting climate-induced river erosion for assessing future landslide susceptibility: methodological insights and lessons learned from five Swedish cases

Article References: Göransson, G., Odén, K., Bergdahl, K., & Bolin, P. (2026). Projecting climate-induced river erosion for assessing future landslide susceptibility: methodological insights and lessons learned from five Swedish cases. Environmental Earth Sciences, 85(15), Article 396. https://doi.org/10.1007/s12665-026-13128-4

Image Credits: AI Generated

DOI: 10.1007/s12665-026-13128-4

Keywords: river erosion, landslide susceptibility, climate change, slope stability, hydrodynamic modelling, sediment transport, bathymetry, hydropower regulation, geotechnical risk, Sweden, Göta River, morphodynamics