Tidal rivers are defined as the tide-influenced, salinity-free inland reaches of estuaries. Understanding the occurrence of peak water levels (PWLs) is critical for flood risk management, yet the timing and magnitude of PWLs in tidal rivers have been little studied. We address this gap by investigating PWLs during two catastrophic floods (1954 and 2020) in the tidal Yangtze River, which reveals that PWLs are higher during perigean spring tides under intermediate discharges following peak flow. Three factors modulate the PWLs in tidal rivers: river-enhanced tidal damping, flow-elevation hysteresis that raises the falling flow limb of the hydrograph, and low-frequency subharmonics. The latter, arising from nonlinear river-tide interactions, are distinct in tidal rivers and culminate under intermediate river flow as a result of the balance between tidal energy dissipation and the spectral energy transfer to subharmonics. They elevate PWLs by up to 0.3 m during perigean spring tides in the Yangtze case and amplify compound flood risk. Channel degradation has reduced tidal damping compared with circumstances in the 1950s, resulting in larger tidal ranges and record-breaking PWLs during the 2020 flood despite a smaller peak discharge. We identify tidal rivers spanning over 3,380 km in worldwide estuaries and deltas, where prevalent subharmonics like MSf are up to 0.65 m in amplitude. Many of those experience even larger tidal amplification due to channel deepening, implying escalating flooding risk under sea-level rise and human impact. These findings underscore a need to include nonlinear river-tide interactions in flood risk assessment for river-coast transition zones.

Microplastics pose numerous threats to aquatic environments, yet understanding their transport mechanisms remains limited. Drawing from natural sediment research provides valuable insights to address this knowledge gap. One key dimensionless number used to describe sediment transport is the transport stage, referring to the ratio between the flow shear velocity and the particle settling velocity. However, variations in physical properties, such as shape and density, raise concerns about the applicability of existing sediment transport theories to microplastics. To address this challenge, we employed a physical modeling approach, examining 24 different nonbuoyant microplastic particles in a turbulent open channel flow. Utilizing 3D particle tracking, a total of 720 trajectories were recorded and analyzed. Microplastic particles exhibited transport modes akin to natural sediments, including rolling/sliding, saltation, and suspension. The transport stage strongly correlated with these modes, as well as with the mean forward velocity and mean position in the water column. Notably, particle shape emerged as a critical factor influencing transport dynamics. Due to their lower settling velocity, fibers tended to stay closer to the water surface with lower forward velocities compared to spheres. Based on the laboratory results, a new phase diagram for microplastics is introduced analogous to an existing diagram for sediments.

The morphological configuration of estuaries and tidal basins influences future development because the channel-flat pattern and geometry control tidal dynamics and, as a result, residual sediment transport patterns. Large-scale human alteration of estuarine plan-form and channel dimensions, as a result of land reclamation, influences long-term evolution, because the existing balance of sediment import versus export is disrupted. The morphodynamic response to land reclamation is, however, slow, impacting the system for decades to centuries. Consequently, there are usually multiple human interventions cumulatively impacting the system. Our understanding of the cumulative effects of land reclamation and other anthropogenic interference is limited because observations usually do not span the complete morphological adaptation time. The Ems estuary (bordering The Netherlands and Germany) provides an unique site to study the effects of the cumulative impact of land reclamations and 20th-century human interference. Extensive storm surge-formed basins have been gradually reclaimed over a period of 500 years in this well-documented estuary, and dredging works dominated in the past century. Our objective is to quantify the effects of land reclamations and channel dredging on the historic evolution of the Ems estuary from century-scale observations combined with numerical morphodynamic modelling.We compiled a digitized bathymetric dataset, spanning nearly the full reclamation period, from historical maps, nautical charts, and recent sounding observations. The dataset was used to reconstruct the morphological evolution of the estuary over the past 500 years. The centennial-scale morphodynamic trends show that the system responded to land reclamation by subtidal infilling and evolved from a multichannel system separated by shoals to a single channel system flanked by fringing flats. The long-term geometric changes show that the main system-scale morphodynamic adaptation is controlled by the effects of land reclamation. The present-day evolution is additionally influenced by the effects of 20th-century dredging works.A process-based morphodynamic model (Delft3D-FM), forced with a synthetic spring-neap tidal cycle, was used to investigate the Ems estuary channel evolution in response to historical land reclamations. Simulation results showcase the transformation from an initially flat-bed bathymetry to a system with multiple channels and tidal flats when historic storm surge basins provide extensive intertidal areas. Simulations in which these former storm surge basins are reclaimed result in a single-channel system, confirming the influence of land reclamations on the observed evolution. The results of this study emphasize that, contrary to what is generally assumed, pre-dredging estuarine morphologies are often far from pristine. Ongoing research focuses on quantifying the interplay between natural and human-driven factors in century-scale channel evolution.

A river is considered supply limited, or hungry, when its capacity to transport sediment (strongly) exceeds the supply of sediment. Common consequences include erosion of bed material and channel incision, as observed in the heavily engineered river Meuse in the Netherlands, where these processes have contributed to the formation of scour holes. This study establishes a sediment budget for the Dutch part of the river Meuse, aiming to better understand the drivers of channel incision, and to assess the potential of mitigation by removing bank protection. We used multibeam data to track riverbed elevation changes, laser altimetry data describing banks and floodplains, a detailed dredging and nourishment database, recent bed material samples and hydrodynamic modelling results to set up a sediment budget for a 250 km stretch of the river from 2011 to 2019. Our findings reveal that in the Dutch Meuse River, annual sediment extraction volumes frequently exceed natural sediment loads several times. Rather than reduced inputs from tributaries, sediment mining and barrages reducing sediment connectivity control the hunger of the river. Removing bank protection alleviates supply-limited conditions, at least temporarily. The comprehensive sediment budget obtained in this study offers a knowledge base for sustainable river management, highlighting the importance of international collaboration in sediment management efforts.

Introducing RUMBA: Revealing underwater macroplastic pollution using acoustic backscatter Session: ITS3.19/HS12.4: Advances in plastic pollution monitoring across the Geosphere The ever-increasing production of (single use) plastics has led to enormous amounts of pollution, threatening ecosystems, livelihood, safety and human health. Large quantities of the littered plastics are trapped in or transported by rivers. Methods for monitoring plastics in rivers mostly focus on floating or deposited plastics, while recent studies show that a substantial proportion of plastics are transported below the water surface. At this stage, mainly nets and heavy machinery are used, making them labor-intensive, expensive and invasive. They are therefore limited to occasional spot measurements. The RUMBA project aims to detect underwater macroplastic pollution (>5 mm) in rivers using acoustic backscatter. While acoustic sensor shows promise for plastic detection (Boon et al., 2023), a comprehensive understanding of how backscatter varies with item characteristics (size, shape, composition, and orientation) under different environmental conditions is still needed. We will test this during controlled, semi-controlled, and uncontrolled settings in Europe and Asia. In this poster presentation we will discuss the aims of RUMBA:(1) identify and distinguish the most common underwater macroplastics, (2) develop an automated detection method, (3) apply and validate the method in field conditions, and (4) use unique historical datasets to uncover trends in plastic transport in Dutch rivers. We anticipate that the results from RUMBA have the potential to provide continuous and/or cross-sectional estimates of underwater plastic transport in rivers, along with measurements of current and sediment concentration. By providing insights into the impact of past interventions on plastic pollution and enabling accurate identification of sources and sinks of plastic litter, this approach could support more effective mitigation and remediation efforts. ReferencesBoon, A., et al. (2023). Detection of suspended macroplastics using acoustic doppler current profiler (ADCP) echo. Frontiers in Earth Science, 11, 1231595.

Addressing the adverse impacts of soil erosion through effective soil conservation measures (SDG15) requires a thorough understanding of the erosion processes involved. However, existing erosion prediction models are often compared solely to sediment outflow at catchment outlets without explicitly assessing these various processes. In such cases, the observed and simulated rates at the outlet may align well without effectively showing the spatial sediment redistribution patterns in the catchment. Moreover, data availability is often limited for remote catchments. This study combined the less data-intensive Unit Stream Power Erosion Deposition (USPED) model with gully erosion indices to analyse sediment redistribution and to identify erosion hotspots in a data-scarce catchment in the Ethiopian highland. The model performance was evaluated spatially at three scales, each characterised by distinct erosion processes. The main findings indicate that the model displayed overall agreement across the three scales (R² = 0.63, NSE = 0.40, KGE = 0.47, Pbias = 5 %, RMSE = 1.39 t ha⁻¹ yr⁻¹). The catchment experienced an average soil erosion of 32.3 t ha⁻¹ yr⁻¹ from 2000 to 2023, resulting in a total annual loss of 0.32 million tons. The most erosion-prone areas, which comprise just 18 % of the catchment area, contributed approximately 69 % of the total soil erosion. The observed erosion processes vary by scale, emphasising the need for scale-aware modelling, with distinct erosion processes involved. In conclusion, the USPED model, combined with gully erosion indices, effectively captures the dominant erosion processes at various scales and identifies hotspots for targeted conservation amid land use changes.

Water-level measurements, sometimes spanning centuries, offer a valuable historical perspective. Although contemporary tidal data is collected digitally at high frequencies, historical records often merely consist of basic high and low water levels. Recognizing the value of these low-resolution tidal records, recent 'data rescue efforts' focus on digitizing and preserving them. Current tidal analysis methods, optimized for high-frequency data, fall short in exploiting the potential of high- and low-water observations.Here, we introduce a specialized tidal analysis methodology tailored for high- and low-water observations. Leveraging equilibrium tide information and the unique characteristics of these observations, such as a derivative constraint, we enhance the analysis of historical records. Additionally, we explore interpolation methods for high- and low-water observations, aiming to address the possibilities and limitations associated with these data.Our approach has the potential to offer valuable insights into century-scale water level changes, and to unravel the contributions by tides, river discharge, mean sea level, storm surges and interactions among those governing factors to water level variation. A key ambition we have is to reveal the hydrodynamic consequences of human interventions, which are difficult to distinguish from each other. We hope the new technique will encourage to continue digitization of historic high-low-tidal observations, and allow to demonstrate the role of intertidal areas in modulating water level extremes.

Flocculated particles, formed by the aggregation of clay particles, are common in rivers. These flocs exhibit a different behaviour than primary particles: they can deform and break apart, and they have greater settling velocities than the particles of which they are composed. The latter allows flocs, unlike primary clay particles, to deposit on the river bed in mildly turbulent conditions, potentially leading to interactions with the bed. Particularly in sand-bedded rivers, where bedforms shape the riverbed, there exists a potential interaction between flocs and the riverbed.Physical experiments were carried out in an annular flume, using a flocculant to induce flocculation. Different amounts of flocculant and various shear stress conditions were applied, and the resulting floc characteristics and bedform geometry were measured.Under lower shear conditions, the flocs were larger and transport rates were lower than under high shear conditions. However, under both shear conditions, flocs were transported via saltation and in suspension, and they became integrated within the sediment bed either as individual flocs, clusters, or sheets. Deposition occurred predominantly on the leeward side of the dune, revealing distinct stratigraphy patterns. The presence of flocs had a negligible impact on the actual geometry of the bedforms.This investigation highlights the active role of flocculated clay particles in sediment transport in riverine systems, contrary to the general assumption that clay particles behave passively as wash load. This finding has the potential to affect sediment transport rates of fines and contaminants and could have far-reaching impacts on the interpretation of mud deposits in the sedimentary rock record. For modelling and predicting the sediment dynamics in river systems a comprehensive understanding of the transport mechanisms of clay flocs is essential and should be taken into account.

Soil erosion has on– and off-site detrimental effects, including decreased soil quality and sediment buildup in reservoirs. Predicting and monitoring soil erosion is challenging due to the spatio-temporal variation of its triggering factors. Therefore, developing and successfully implementing appropriate intervention measures requires a thorough understanding of its redistribution at the catchment scale. However, many previous soil erosion prediction models have been calibrated/validated based on sediment yield at catchment outlets. This approach does not provide any insight into the sources and sinks of erosion and deposition within the catchments. Furthermore, this approach has limited applicability in regions with no (limited) measured data. Therefore, exploring spatial patterns of erosion and deposition using the recent advances in remote sensing and GIS technologies is advisable. This research integrates the semi-distributed Unit Stream Erosion Deposition (USPED) model, and gully erosion threshold indices, described by stream power index (SPI) and topographic wetness index (TWI), to evaluate the sediment redistribution dynamics of a sub-humid catchment located in Omo-basin in southwestern Ethiopia. The catchment (~77 km2) has a rugged topography with an average slope of 35.8 %. It consists of four primary types of land use and cover (LUC): rangelands (20%), forest areas (19%), built-up areas (7%) and cultivated lands (54%). The (preliminary) results revealed that the gentle and mild slopes contribute more (53%) to the overall annual catchment soil loss (42.5 t.ha-1) from the hillslope. This is because the sediment deposited in the downstream sinks remobilizes, shifting an erosion-limited to a transport-limited system. Moreover, the total contribution of rangelands and forest areas is comparable to that of cultivated lands. Therefore, by focusing our management efforts on these areas, instead of the steeper slopes, we can make a greater impact on the overall sustainability of the catchment.

Microplastics pose numerous threats to aquatic environments, yet general understanding of the mechanisms governing their transport remains limited. Drawing upon research on natural sediment provides a valuable resource to address this knowledge gap. One key dimensionless number used to describe sediment transport is the transport stage, referring to the ratio between the fluids shear velocity and the particle settling velocity. However, differences in physical properties (e.g., shape, density) raise concerns regarding the applicability of existing sediment transport theories to microplastics. To address this challenge, we employed a physical modelling approach to examine the trajectories of 24 negatively buoyant microplastic particles varying in size, shape and density. Utilizing a 3D particle tracking setup, we captured the movement of the particles in turbulent open channel flow. A total of 720 trajectories where recorded and analysed. The results revealed a strong correlation between the transport stage and the mean forward velocity of the particles, as well as their mean position in the water column. Notably, particle shape emerged as a critical factor influencing particle transport dynamics. Fibres showed a tendency to be transported closer to the water surface while experiencing slower mean forward velocities compared to spheres. The microplastic particles exhibited rolling/sliding, saltation and suspension as transport modes, comparable to natural sediment. The results show a strong correlation between the transport stage and the percentage of time in which microplastics experienced a certain mode of transport. Based on the laboratory results, a new phase diagram for microplastics is introduced, analogous to an existing diagram for sediments.

Soil erosion is a process accelerated by natural and anthropogenic disturbances over time and space, leading to land degradation and causing geomorphological change. It is difficult to investigate the spatial and temporal distribution of soil erosion and sedimentation in data-scare areas, in that case, the use of simplified methods to analyze soil erosion and sediment connectivity variations over time and space can help. Sediment connectivity denotes the transfer of sediment from source to sink areas through channel systems of landscape compartments within a watershed. In this study, we aimed to investigate sediment yield (SY) variation over time and space and understand the link between hillslope soil erosion and sediment connectivity to identify hotspot areas in the Rogativa catchment (∼53 km2) in Southeast Spain. The (specific) sediment yield (S)SY was estimated by combining the Revised Universal Soil Loss Equation (RUSLE) model with the sediment delivery ratio (SDR). The SDR was calculated based on the Index of Connectivity (IC). In the channels, 100% delivery was assumed. In the Rogativa catchment, 58 check dams were constructed in 1976/77. Their trapping efficiency, obtained from field observations of sediment retained behind the checkdams in 2003, was included in the SDR estimation of the checkdams. SY was estimated from accumulated hillslope soil erosion in the local stream network while accounting for sedimentation through the SDR. Soil erosion, IC, SDR, and (S)SY were quantified and compared for the years 1956, 1977, 2001, and 2016, for which different land use maps were available. SY model results for the year 2001 were compared with observed SY (in 2003) behind the check dams. Only for about half of the checkdams, model results were comparable. This is investigated further and could be explained by complex sediment dynamics within the channels and between checkdams (i.e. one check dam retaining part of the sediment, the next downstream checkdam as well, etc) – these dynamics are not included in the RUSLE-SDR model. The RUSLE-generated soil erosion and sediment connectivity signatures (IC, SDR, and (S) SY) showed higher values in the channels and croplands than in hillslopes and decreased over time due to significant changes in land use and construction of check dams in the catchment. Moreover, the combined proportion of erosion-connectivity patterns showed about 7% of the area adjacent to some of the streams was found both highly erodible and highly connected, which indicates an adverse erosion-prone part. It is possible to apply this method to understand SY amount and distribution and identify hotspot locations in drainage systems with limited field data in data-scarce semi-arid areas like the Rogativa catchment. However, more field observations to validate the models to identify hotspot locations and investigate river network systems rather than focusing only on hillslopes, which could help to know where to intervene in the catchment.Keywords: Soil erosion-RUSLE, Sediment connectivity, Sediment delivery ratio, Sediment yield, hotspot location

Soil erosion is a natural process that can be accelerated by natural and anthropogenic disturbances and lead to land degradation and geomorphological changes. Analyzing soil erosion and catchment sediment dynamics is a complex process. In such cases, simplified methods can be applied to analyze soil erosion and sediment connectivity variations and to understand sediment flux in a river basin to inform watershed management. In this study, we tested the combined method of the Revised Universal Soil Loss Equation (RUSLE), the Index of Connectivity (IC), and the Sediment Delivery Ratio (SDR) to estimate sediment yield (SY) and investigate the spatiotemporal variation of soil erosion rates and sediment connectivity in the Mediterranean Rogativa catchment (similar to 53 km(2)), Southeast Spain. In this 'RUSLE-IC-SDR' approach, the sediment delivery ratio was estimated from the spatially distributed index of connectivity, calculated using SedInConnect and accounting for the trapping efficiency of 58 check dams in the channels, while assuming 100 % sediment delivery in other parts of the channels. The sediment delivery ratio was calibrated, and sediment yield was verified for the year 2001 using observed sediment yield (in 2003) behind the non-silted check dams. Predicted soil erosion, connectivity (IC, SDR, and SY), and soil erosion-connectivity maps were quantified and compared over time and space, revealing the impacts of rainfall, land use, and check dams. These maps show higher values for areas closer to the channels than on the hillslopes, and higher values on croplands than other land use types, as well as a decrease over time due to land use change and the construction of check dams. The relatively simple 'RUSLE-IC-SDR' approach was found to be effective in identifying the sources and hotspots on the hillslopes of a complex Mediterranean catchment. Future studies should consider the channel erosion processes as the RUSLE-IC-SDR does not take these into account.

The transport of plastic in rivers is affected by a wide variety of factors, such as river discharge, wind drag, and tides. These dynamic processes include the travelling time or distance, retention time or location, and remobilization rate of plastic items, which can be quantified by using GPS-based trackers. However, these properties are still unknown in some specific locations, including river bifurcations where the changes in river discharge, flow velocity, and river morphology are significant. Here, we demonstrate the behaviour of plastic transport in a river bifurcation area not influenced by tides during flood season in the Red River system of Vietnam. While all trackers retained somewhere in the river after hours or days, we found that after 10.5 kilometers downstream of the bifurcation, 9 out of 10 trackers followed the main channel retained in the same approximately 6-kilometer-long river segment. Meanwhile, 50% of the 6 trackers that left the main channel to enter the tributary also retained in the same 2.5-kilometer-long river segment 9 kilometers downstream of the bifurcation. These findings are linked to the concept of rivers as plastic reservoirs, as none of the trackers that stranded on the riverbanks for several days was remobilized. Furthermore, the retention of trackers in the same area after leaving the bifurcation clearly indicates shared driving factors on plastic transport, which are likely the river discharge, wind direction or velocity, and river morphology. Our results underscore the need for future research on delineating exact accumulation zones of the plastics in riverbanks considering the effects of wind, river discharge, and river morphology.

In fluvial systems worldwide, multiple scales of bedforms coexist. Where most research has focused on the larger, primary dunes, recent studies have indicated the importance of the small, secondary bedforms that are superimposed on the primary ones (Galeazzi et al., 2018, Zomer et al., 2021). The secondary bedforms migrate fast and the bedload sediment transport associated with secondary bedform migration equals that associated with the much larger primary dunes. Depending on the primary lee side slope, secondary bedforms disintegrate or persist at the primary dune lee. Secondary bedforms might have large implications for hydraulic roughness, for local flow dynamics and may interact with the development of primary dunes. Current work focusses on understanding the competition and interaction between primary and secondary bedforms in a lowland river, based on a large, multiyear dataset of bed elevation scans as well as a dedicated field campaign that maps the dynamics of both primary and secondary dunes. A first objective of the study is to understand the competition between primary and secondary bedforms. Previous work has indicated inverse correlations between secondary bedform height and primary dune lee slope or height. The bed elevation scans indicate a spatial variability in secondary and primary bedform properties and locations where either secondary or primary dunes are dominant. This work aims to map and explain the mechanisms that affect the development and (semi-)equilibrium dune size and shape of both scales as well as the dependence on the discharge and bed grain size distribution. A second objective is to shed light on the interaction between migrating secondary and primary dunes. Where secondary bedforms disintegrate at the primary lee, the secondary bedform migration contributes to primary dune migration. Secondary bedforms are also observed to persist over the primary dune lee however. Both scales are then actively migrating. Preliminary results suggest that sediment transport associated with secondary dune migration varies depending on the position of the small dunes on the primary dune. Sediment transported by secondary dunes seems to increase over the primary stoss and decrease on the primary lee. The variability in sediment transport indicates net erosion of the primary dune stoss and net deposition on the primary dune lee, resulting in a downstream migration of the primary dune. References: Galeazzi, C. P., Almeida, R. P., Mazoca, C. E., Best, J. L., Freitas, B. T., Ianniruberto, M., ... & Tamura, L. N. (2018). The significance of superimposed dunes in the Amazon River: Implications for how large rivers are identified in the rock record. Sedimentology, 65(7), 2388-2403. Zomer, J. Y., Naqshband, S., Vermeulen, B., & Hoitink, A. J. F. (2021). Rapidly migrating secondary bedforms can persist on the lee of slowly migrating primary river dunes. Journal of Geophysical Research: Earth Surface, 126(3), e2020JF005918.

Current tidal input reduction approaches applied for accelerated morphological simulations aim at capturing the dominant tidal forces in a single or double representative tidal cycle, often referred to as a "morphological tide''. The existing methods may provide appropriate boundary conditions to simulate representative residual transport fluxes and the resulting morphological changes of the tidal channels. However, heavily simplified tidal signals fail to represent the tidal extremes. They poorly represent intertidal areas, which exert a major impact on the development of tidal asymmetry and the associated residual transport fluxes. Here, we aim to develop a generic method to construct a synthetic representative tidal signal that incorporates tidal extremes. Such a synthetic cycle should adhere to several criteria to make it applicable for long-term (i.e. subdecadal) accelerated simulations. The synthetic signal should: (1) represent the original signal, particularly preserving asymmetries present in the original signal; (2) be exactly periodic, to ensure coherency between consecutive cycles and to control the relative phasing with other types of forcings (e.g. wind, waves, discharge); and (3) remain valid during the long-term simulations in which the bathymetry in the modelling domain changes shape. The starting point for the construction of the synthetic signal is a fortnightly modulation of the semi-diurnal tide to represent spring-neap variations, while conserving periodicity. Diurnal tides and higher harmonics of the semi-diurnal tide are included to represent the asymmetry of the tide. The amplitudes are then scaled to give a best fit to the full tidal signal. Statistical measures of the synthetic signal show that it gives a better representation of the amplitude variation and asymmetries present in the original signal, compared to existing approaches of tidal input reduction. A hydrodynamically validated depth-averaged model of the Ems estuary (The Netherlands) demonstrates the effects of different tidal input reduction techniques on residual sediment transport patterns. Adopting the new approach, the shape of the tidal wave is better represented over the entire length of the estuary, and inundation of shallow parts of the basin is modelled in closer resemblance to the real-world intertidal dynamics.

After major storm surge protection works in the Rhine-Meuse Delta, referred to as the Delta Works (Vellinga et al., 2014), the New Waterway has become the only remaining open channel connecting the estuary to the North Sea. Like in many harbour areas, continuous deepening of this channel for navigation purposes has led to strong stratification and often salt wedge conditions, which likely has a strong impact on the marine sediment import. The sediment balance for various fractions is highly uncertain (Cox et al., 2021). Based on field measurements and sediment transport modelling, we aim to unravel the mechanisms controlling residual sediment fluxes in highly stratified estuarine channels, by focusing on the New Waterway. A measurement campaign was set up consisting of two 13-hour surveys, one during spring tide and one during neap tide. A measurement frame was equipped with a LISST-100x, a Seapoint turbidity meter and a CTD probe. Suspended sediment samples are collected every hour at three depths, next to water temperature, salinity and turbidity. The flow was monitored continuously based on a vessel-mounted ADCP transects across and along the channel . The ADCP-measurements show a clear distinction in flow magnitude and direction between the upper fresh water layer and lower saline layer, confirming the high degree of stratification especially during neap tide. After low water slack, most suspended sediment is found in the lower half of the water column. Suspended sediment concentrations (SSCs) increase during the flood acceleration phase, suggesting local resuspension during this phase of the tide. When reaching high water slack, SSCs decrease with flow velocity. At high water slack, the ADCP-backscatter profiles indicate settling of the suspended sediment on top of the pycnocline. During the ebb phase, SSCs increase again, and the water column is better mixed compared to the flood phase. Preliminary results of the grain size analysis indicate coarsening of the suspended sediment at the end of the flood acceleration and ebb acceleration phases. Ongoing analysis of these data and numerical modelling of SSC will provide more insight in the suspended sediment transport processes under various degrees of stratification. Cox, J. R., Huismans, Y., Knaake, S. M., Leuven, J. R. F. W., Vellinga, N. E., van der Vegt, M., et al. (2021). Anthropogenic effects on the contemporary sediment budget of the lower Rhine-Meuse Delta channel network. Earth's Future, 9, e2020EF001869. https://doi.org/10.1029/2020EF001869 Vellinga, N. E., A. J. F. Hoitink, M. van der Vegt, W. Zhang, en P. Hoekstra. ‘Human Impacts on Tides Overwhelm the Effect of Sea Level Rise on Extreme Water Levels in the Rhine–Meuse Delta’. Coastal Engineering 90 (1 augustus 2014): 40–50. https://doi.org/10.1016/j.coastaleng.2014.04.005.

Bedform occurrence and geometry in sand-bedded rivers is traditionally predicted with phase diagrams and empirical equations, in which regional river characteristic are used. Field observations supporting these equations are often made in regions where bedform fields are known to be present and are spatially uniform. However, bedforms occurrence and geometry can vary significantly at the scale of the river width, limiting the applicability of bedform diagrams and questioning the objectivity of field study area selection. To enable the prediction of dune geometry, its spatial variability needs to be better understood. In this study, we aim to relate spatial variations in dune characteristic to grain size characteristics, river geometry, and local flow variation governed by the sub-bedform topography. We hypothesize that curvature-induced bars and pools drive local hydrodynamics, which in turn determine local dune characteristics. To test this hypothesis, bathymetric field data and sediment samples were collected in the fluvial-to-tidal-transition zone of the Fraser river, a sand-bedded lowland river in British Columbia, Canada. A 2D hydrodynamic model was created to explore the impacts of spatial variation in hydraulic conditions. We find that the cross-sectional variability in dune geometry is larger than the longitudinal variability, and that the transition of one type of dune field into another is abrupt rather than gradual. Phase diagrams do not capture these observations accurately. Local hydraulic conditions are more important in determining spatial variability in dune geometry than regional scale changes in river geometry, grain size variation and tidal influence. Dune height has an ambiguous relationship with river depth: the spatial variation in dune height depends on local shear stresses governed by the sub-bedform topography characterized by pools and troughs.

Recent studies suggest that more land-based plastics are accumulated and remobilized in riverine environments than exported to the ocean. Hydrodynamics and other factors like wind drag and navigation can drive plastics to riverbanks, where they can be retained in plants, on floodplains, or in stagnant water bodies. With this study, we provide additional observational evidence that a substantial share of floating plastics may not flow downstream. Every week from May to December 2021, we measured floating plastics at three bridges located in the Hong-Duong bifurcation of the Red River, Vietnam. These locations were chosen to monitor both the input plastics entering the bifurcation and the output plastics exiting the bifurcation in both branches. The upstream location is Nhat Tan, which is located on the Red River in northern Hanoi. The downstream location on the main stream is Long Bien, approximately 8 kilometers south of Nhat Tan on the Red River, while the third location is in Dong Tru, on the distributary Duong River, about 7 kilometers from Nhat Tan. We collected data on the plastic mass balance of various plastic categories in the bifurcation, including PET, PO-soft, PO-hard, multilayers, PS, and PS-E. The results indicated that the plastic mass balance does not close in general; there is more plastic upstream than in the two downstream locations combined. The dry season, from October to December, was more balanced, with a 4% difference. Meanwhile, between May and September, approximately 16% of floating plastics were discovered to be missing. Additionally, the majority of floating plastics remained in the main stream, with only 8% entering the distributary, and the division rate kept constant throughout the study period. However, the balance differed for specific categories. Five categories had missing downstream records compared to upstream, and PO-soft featured a more intricate balance mechanism with alternating changes between missing and abundant records from month to month. In the meantime, PS was seen upstream but was never detected downstream. For PS-E, most of the items found in the upstream were either detected in the distributary, or disappeared in the bifurcation; less than 1% was identified downstream. The variation in transport between PS and PS-E may be caused by deposition, extraction, accumulation, or sub-surface transport. Finally, except for PO-hard, the rate of plastic transport was found to be higher near the river banks than in the thalweg. These findings suggest that the transport mechanism of macroplastics in rivers may be more complex than previously assumed, and prompt further studies. Our data allow modelling plastic transport for different plastic categories, designing suitable monitoring or clean-up methods, and understanding the roles of hydrological components such as discharge and flow velocity in transporting plastics.