This study examines the local, intratidal effects of suspended sediment concentrations (SSCs) on the hydrodynamics and vertical mixing in the Ems Estuary, located on the border between Germany and The Netherlands, during summer and winter seasons when the estuary turbidity maximum (ETM) is located upstream and adjacent to the study site, respectively. Measurements of density, SSCs, turbulent kinetic energy dissipation, and current velocity were collected and analyzed over a semi-diurnal tidal cycle in August of 2018 and January of 2019 as part of the collaborative Ems-Dollard Measurement (EDoM) campaign. During August, the estuary turbidity maximum was located 25 km upstream from the measurement site and local SSCs were low. Results revealed that under these conditions, suspended sediment minimally impacted vertical mixing by stabilizing density near-bottom during flood tide, while typical salinity-induced tidal straining patterns dominated. During January, the ETM was located only 5 km upstream of the measurement site leading to higher local sediment concentrations. Salinity-induced straining of the density occurred on early flood tide, creating stratification that suppressed vertical mixing. The suppression was enhanced by the contribution of vertical gradients in SSC to density, as signified by the gradient Richardson number. Suppression of vertical mixing by sediment-enhanced stratification was most significant within the hour following maximum flood currents when elevated velocity shear occurred. The variability observed between the local dynamics during August and January were attributed to greater sediment concentrations due to the ETM proximity in January. The intratidal asymmetry of vertical mixing observed under higher SSCs likely has implications for sediment transport.

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For the management of estuaries and the preservation of tidal flats it is crucial to understand the tidal flat shape and development. Previous work focused predominantly on the quasi-equilibrium shape of tidal flats along open coasts with a dominant cross-shore flow and wave exposure. This paper evaluates the shape of fringing tidal flats in engineered estuaries, where longshore velocities generally dominate. Using a long-term (20 years) topographic data set of an anthropogenically modified estuary in the Netherlands (the Western Scheldt estuary), we relate key profile shape parameters and changes over time to natural and anthropogenic processes. In an engineered estuary, the tidal flat shape depends on the estuary geometry, hydrodynamic forcings and human interventions. In contrast to open coast tidal flats, the presence of the channel and dominant longshore flow determines the available cross-shore length (accommodation space) of the tidal flat and the shape of the tidal flat. This accommodation space defines the maximum tidal flat height and opportunity for marsh development. We propose the use of the Index of Development, indicating to what extend tidal flats have space to develop. This index is not only influenced by longshore and cross-shore flow, but also (or even more) by hydraulic structures, dike realignments and channel migration. Especially the latter two strongly influence the accommodation space and thereby the maximum tidal flat height and the opportunity for marsh development. For large stretches of the Western Scheldt, the accommodation space is too small, and the majority of the tidal flats do not vertically extent to mean high water. The success of tidal flat and marsh restoration projects depends on the accommodation space.@en

Storm surge barriers and closure dams influence estuarine morphology. Minimizing consequential ecological impacts requires a thorough understanding of the morphological adaptation mechanisms and associated time scales. Both are unraveled using three decades of morphological measurements on the adaptation of the Eastern Scheldt estuary (The Netherlands) to a storm surge barrier and closure dams. Both the storm surge barrier (through a decrease in cross-sectional area) and closure dams (inducing a reduction in surface area of the estuary) contributed to a reduction in tidal prism. As a smaller tidal prism implies a smaller equilibrium volume of the channels, the channels demand sediment to adjust. Consequently, by providing sediment to the channels, the intertidal flats erode. Erosion rates decreased while the sediment demand of the channels attenuated. This attenuation in sediment demand resulted mainly from tidal prism gains, caused by intertidal flat erosion and sea level rise. Erosion rates of the intertidal flats decreased further while they flattened to adapt to the reduced tidal velocities. Furthermore, storms caused erosion events, after which the long-term adaptation pace of intertidal flats suddenly reduced. Despite decreasing erosion, sea level rise enhances the drowning of intertidal flats in sediment-scarce estuarine systems, thereby pressuring these estuarine ecosystems and raising the need for mitigation measures.

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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.@en

The world’s coasts and deltas are progressively threatened by climate change and human activities. The degree at which coastlines can adapt to these changes strongly depends on the sediment availability. The availability of muddy sediments is however poorly known. This study aims at developing a mud budget for the world’s largest system of uninterrupted tidal flats: the Wadden Sea. The resulting mud budget is nearly closed: ~ 12 million ton/year enters the system on its western end, ~ 1.5 million ton/year is added by local rivers, while ~ 12 million ton annually deposits or is extracted by anthropogenic activities. A mud deficit already exists in the downdrift areas, which will only become more pronounced with increased sea level rise rates. Mud is thus a finite resource similar to sand, and should be treated as such in sediment management strategies. Resolving future challenges will therefore require a cross-border perspective on sediment management.@en

Land reclamations influence the morphodynamic evolution of estuaries and tidal basins, because an altered planform changes tidal dynamics and associated residual sediment transport. The morphodynamic response time to land reclamation is long, impacting the system for decades to centuries. Other human interventions (e.g., deepening of fairways or port construction) will add more morphodynamic adaptation timescales. Our understanding of the cumulative effects of anthropogenic interference with estuaries is limited because observations usually do not cover the complete morphological adaptation period. We aim to assess the impact of land reclamation works and other human interventions on an estuarine system by means of digital reconstructions of historical morphologies of the Ems Estuary over the past 500 years. Our analysis demonstrates that the intertidal-subtidal area ratio altered due to land reclamation works and that the ratio partly restored after land reclamation ended. The land reclamation works have led to the degeneration of an ebb and flood channel system, transitioning the estuary from a multichannel to a single channel system. We infer that the 20th-century intensification of channel dredging and re-alignment works accelerated rather than caused this development. The centennial-scale observations show that the Ems estuary evolution corresponds to a land reclamation response following tidal asymmetry-based stability theory as it moves toward a new equilibrium configuration with modified tidal flats and channels. Considering the long history of land reclamation in the Ems Estuary, it provides an analogy for expected developments in comparable tidal systems where land reclamations were recently carried out.

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Existing tidal input reduction approaches applied in accelerated morphodynamic simulations aim to capture the dominant tidal forces in a single or double representative tidal cycle, often referred to as a “morphological tide.” These strongly simplified tidal signals fail to represent the tidal extremes and hence poorly allow to represent hydrodynamics in the intertidal areas. Here, a generic method is developed to construct a synthetic spring-neap tidal cycle that (a) represents the original signal; (b) is exactly periodic; and (c) is derived directly from tidal time series or harmonic constituents. The starting point is a fortnightly modulation of the semidiurnal tide to represent spring-neap variations, while conserving periodicity. Diurnal tides and higher harmonics of the semidiurnal tide are included to represent the asymmetry of the tide. The amplitudes and phases of the synthetic signal are then fitted to histograms of water levels and water level gradients derived from the original sea surface elevation time series. A depth-averaged model of the Ems estuary (The Netherlands) demonstrates the effects of alternative tidal input reduction techniques. Adopting the new approach, the along-estuary variation in tidal wave shape is well-represented, leading to an improved representation of extreme tidal conditions. Especially the more realistic representation of intertidal dynamics improves the overall hydrodynamics and residual sand transport patterns, approaching nonschematized tidal dynamics.

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Intertidal areas play a crucial role in controlling tidal hydrodynamics and morphodynamics in estuaries and tidal inlets. As a consequence, widespread land reclamation of the intertidal zone has led to alterations in tidal dynamics and the associated morphodynamics of estuaries worldwide. Comparatively little research has focussed on the impact of width changes on tidal hydrodynamics, and results that do exist are highly ambiguous (Talke and Jay, 2020). Tie channels in between parallel inlets complicate the tidal motion, as multichannel systems exhibit a hydrodynamic response that differs significantly from the response observed in single channel systems. In this study, we combine idealized process-based modelling and historical data analysis to investigate the effects of intertidal land reclamation on the tidal dynamics of single- and multichannel systems. Specifically, we focus on the Scheldt and Pasur-Shibsa estuaries, which represent a single- and multichannel system, respectively.@en

Muddy coasts provide ecological habitats, supply food and form a natural coastal defence. Relative sea level rise, changing wave energy and human interventions will increase the pressure on muddy coastal zones. For sustainable coastal management it is key to obtain information on the geomorphology of and historical changes along muddy areas. So far, little is known about the distribution and behaviour of muddy coasts at a global scale. In this study we present a global scale assessment of the occurrence of muddy coasts and rates of coastline change therein. We combine publicly available satellite imagery and coastal geospatial datasets, to train an automated classification method to identify muddy coasts. We find that 14% of the world’s ice-free coastline is muddy, of which 60% is located in the tropics. Furthermore, the majority of the world’s muddy coasts are eroding at rates exceeding 1 m/yr over the last three decades.

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The morphology of tide-dominated systems is progressively influenced by human activities and climate change. Quantitative approaches aiming at understanding or forecasting the effects of interventions and climate change are often aggregated, thereby simplifying or schematizing the investigated area. In this work, we advance on the knowledge of sediment transport processes shaping tidal systems and on methodologies translating schematized model output into physically realistic variables. In terms of improved physics, we systematically evaluate the influence of sand-mud interaction processes. Most tidal systems are shaped by a mixture of sand and mud. Morphological models typically compute transport of sand and mud independently, despite studies clearly demonstrating that their physical behavior is mutually dependent. We investigate the effects of two interaction mechanisms (erosion interaction and roughness interaction, applied with varying mud erodibility) with a schematized process-based morphodynamic model. We convert model output into metrics that describe the meso-scale configuration of the modeled systems, allowing a quantitative comparison of scenarios. Modeled patterns and intertidal flat shape, size and composition widely vary with mud erodibility settings, but equally depend on the evaluated sand-mud interaction mechanisms (with erosion interaction having a larger effect than roughness interaction). Sand-mud interaction thus needs to be accounted for from a physical point of view, but also to improve predictions of tidal basin evolution models, particularly the (bimodally distributed) sediment composition of intertidal flats.

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Deltaic intertidal areas disappear worldwide. This impacts delta morphology, because the extent and physiological character of the tidal floodplains control the tidal regime and, as a result, residual sediment transport patterns. Extensive reclamation of former tidal flats, effectively changing the ratio of channel volume to intertidal storage volume (Vs/Vc), drastically changes the functioning of the estuarine system. This might result in morphodynamic feedback loops that reach a tipping point towards an alternative stable regime (Van Maren et al., 2023). Our capacity to predict the consequences of future land reclamation or depoldering methods, a measure frequently suggested to cope with the effects of sea level rise, is limited, because the conceptual framework describing estuarine response to tidal flat reclamation fails to predict such regime transitions.@en

At a global scale, intertidal areas are being reclaimed for agriculture as well as urban expansion, imposing high human pressure on the coastal zone. The Ganges-Brahmaputra Delta (GBD) is an exponent of this development. In this delta, land reclamation accelerated in the 1960's to 1980's, when polders were constructed in areas subject to regular marine flooding. A comprehensive analysis of tidal channel evolution in the southwest GBD reveals how land reclamation leads to tidal amplification, channel shoaling, bank erosion, and interaction between channels in which one tidal river captures the storage area of a neighbouring river. We identify-two positive feedback mechanisms that govern these morphological changes. First, reclaiming intertidal areas results in immediate loss of tidal storage, which leads to amplification and faster propagation of the tides. In systems with abundant sediment supply, the blind tidal channels progressively fill in with sediment, leading to a continued loss of tidal storage and therefore further distorting the tides. Secondly, when intertidal areas of parallel (and inter-connected) river delta distributaries are asynchronously or unevenly reclaimed, one channel distributary may expand its intertidal area at the expense of the other. This is initiated by an increasing propagation speed of the tidal wave in the partially reclaimed distributary, travelling into the non-reclaimed distributary through connecting channels. These connecting channels progressively expand while the pristine channel shoals, and potentially degenerates. Both positive feedback loops are very stable and are responsible for pluvial flooding of polders, large-scale bank erosion, and poorly navigable primary waterways, including the navigation channel accessing Bangladesh's second-largest port. Interventions aiming to solve these problems have to account for the complex positive feedback mechanisms identified in this paper and be nature-based and holistic.

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Many estuaries and tidal basins are strongly influenced by various human interventions (land reclamations, infrastructure development, channel deepening, dredging and disposal of sediments). Such interventions lead to a range of hydrodynamic and morphological responses (a changing channel depth, tidal amplitude and/or suspended sediment concentration). The response time of a system to interventions is determined by the processes driving this change, the size of the system, and the magnitude of the intervention. A quantitative understanding of the response time to an intervention therefore provides important insight into the processes driving the response. In this paper we develop and apply a methodology to estimate the response timescales of human interventions using available morphological and hydraulic data. Fitting an exponential decay function to data with sufficient temporal resolution yields an adaptation timescale (and equilibrium value) of the tidal range and deposited sediment volumes. The method has been applied in the Dutch Wadden Sea, where two large basins were reclaimed and where long-term and detailed bathymetric maps are available. Exponential fitting the morphological data revealed that closure of a very large part of a tidal basin in the Wadden Sea initially led to internal redistribution and import of coarse and fine sediments, and was followed by a phase of extensive redistribution while only fine-grained sediments are imported. Closure of a smaller part of a smaller basin led to shorter response timescales, and these response timescales are also more sensitive to rising mean sea levels or high waters. The method has also been applied to tidal water level observations in the Scheldt and Ems estuaries. Exponential fits to tidal data reveal that adaptation timescales are shortest at the landward limit of dredging. The adaptation time increases in the landward direction because of retrogressive erosion (Scheldt) or lowering of the hydraulic roughness (Ems). The seaward increase in adaptation time is related to the seaward widening of both systems.@en

The sediment load in the Yangtze River downstream of the Three Gorges Dam (TGD) has substantially declined in recent decades. The decrease is more profound below the TGD, e.g., a 97% decrease at Yichang, compared with that at the delta apex, 1200 km downstream, e.g., a 75% decrease, implying along-river sediment recovery. Two large river-connected lakes, i.e., Dongting and Poyang Lakes, may play a role in the re-establishment of the river’s morphodynamic equilibrium, but a quantitative data-based understanding of this interaction is not yet available. In this work, we collected a series of field data to quantify the sediment gain and loss in the river-lake system in the middle-lower Yangtze River, and evaluate the lake’s response to the reduction in riverine sediment supply. We find that Dongting Lake and Poyang Lake shifted from net sedimentation to erosion in 2006 and 2000, and back to a sedimentation regime again after 2017 and 2018, respectively. Natural morphodynamic adaptation and sand mining play an important role in the regime changes in the Dongting Lake whereas sand mining dominates the abrupt changes in the Poyang Lake. The Dongting and Poyang Lake contributed maximum by 38% (2015) and 17% (2006) (respectively) to the sediment recovery in the erosion regime, whereas the riverbed erosion dominates the main sediment source. These changes in the relative contribution of sediment sources also indicates a response time of ~ 20 years in the lakes towards a new equilibrium state. It is noteworthy that the lakes’ buffer effects may be overestimated as the supplied sediment from the lakes is rather small compared to the significant dam trapping in the upstream basin and sediment source from downstream degradation. The results imply that river management and restoration should take into account of the river-lake interactions and feedback impact at decadal time scales.

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Intertidal ecosystems are threatened by sea level rise and anthropogenic pressures. Understanding the processes controlling the morphodynamic developments of tidal flats is crucial for sustainable management of these systems. Analysis of three extensive fieldwork campaigns carried out on two adjacent mudflats fringing the Dutch Western Wadden Sea (from 2016 to 2018) provides important new insights into the conditions controlling a permanent increase of tidal flat elevation (‘accretion’), in which the wind and consolidation processes play a pivotal role. Sediment temporarily settles (‘deposition’) on the flats during a period of high suspended sediment availability and water level setup (often following a storm). A tidal flat accretes when a new layer of sediment over-consolidates: a state in which the bed strength is much larger than it would attain during inundated conditions, due to high stresses experienced during prolonged drying. This happens when a phase of sediment deposition is followed by a sufficiently long period with a low ambient water table (phreatic level) and aerial exposure. The chronological order of sediment deposition and over-consolidation provides a window of opportunity for tidal flat accretion. Such a window of opportunity depends on the hydrodynamic forcing (tides, waves, wind), on the consolidation state of the bed, and on sediment availability. Wind plays a crucial role in creating the conditions for tidal flat accretion because the wind direction influences the duration of a low water table and aerial exposure and therefore (over-)consolidation rates, which we refer to as the ‘winds of opportunity’. An abundance of sediment may even limit tidal flat accretion, because large deposition rates substantially increase consolidation timescales.

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Existing knowledge about groyne-induced effects is primarily based on riverine or coastal environments where salinity gradients are absent or limited. However, in estuaries, salinity gradients drive physical processes such as longitudinal and lateral residual flows. The effect of groynes is much more complex because they can modulate channel hydrodynamics and directly affect lateral salinity gradients. In this study, an idealized model is applied to investigate the effects of groyne layouts in estuarine environments, including effects on (1) channel hydrodynamics, (2) lateral water exchange, (3) Coriolis effects, and (4) saltwater intrusion. Model results show that the aspect ratio (the width of groyne fields to the length of groynes) of groyne fields plays an important role. Groynes also induce asymmetry of lateral flows, for example, increasing near-bottom shoal-to-channel flows during low water slack. The aspect ratio has opposite effects on horizontal and vertical components of water exchange. A large aspect ratio strengthens horizontal exchange and weakens density-driven currents. For a large-scale groyne field (several kilometers), Coriolis effects introduce a substantial difference in exchange mechanisms along the north and south banks. A medium range of aspect ratio (2.0-3.0) leads to the strongest saltwater intrusion during both neap and spring tides.

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An extensive field campaign, the Ems-Dollard Measurements (EDoM), was executed in the Ems Estuary, bordering the Netherlands and Germany, aimed at better understanding the mechanisms that drive the exchange of water and sediments between a relatively exposed outer estuary and a hyper-turbid tidal river. More specifically, the reasons for the large up-estuary sediment accumulation rates and the role of the tidal river on the turbidity in the outer estuary were insufficiently understood. The campaign was designed to unravel the hydrodynamic and sedimentary exchange mechanisms, comprising two hydrographic surveys during contrasting environmental conditions using eight concurrently operating ships and 10 moorings measuring for at least one spring–neap tidal cycle. All survey locations were equipped with sensors measuring flow velocity, salinity, and turbidity (and with stationary ship surveys taking water samples), while some of the survey ships also measured turbulence and sediment settling properties. These observations have provided important new insights into horizontal sediment fluxes and density-driven exchange flows, both laterally and longitudinally. An integral analysis of these observations suggests that large-scale residual transport is surprisingly similar during periods of high and low discharge, with higher river discharge resulting in both higher seaward-directed fluxes near the surface and landward-directed fluxes near the bed. Sediment exchange seems to be strongly influenced by a previously undocumented lateral circulation cell driving residual transport. Vertical density-driven flows in the outer estuary are influenced by variations in river discharge, with a near-bed landward flow being most pronounced in the days following a period with elevated river discharge. The study site is more turbid during winter conditions, when the estuarine turbidity maximum (ETM) is pushed seaward by river flow, resulting in a more pronounced impact of suspended sediments on hydrodynamics. All data collected during the EDoM campaign, but also standard monitoring data (waves, water levels, discharge, turbidity, and salinity) collected by Dutch and German authorities are made publicly available at 4TU Centre for Research Data (https://doi.org/10.4121/c.6056564.v3; van Maren et al., 2022).@en

A decline of the fluvial sediment supply leads to coastal erosion and land loss. However, the fluvial sediment load may influence not only coastal morphodynamics but also estuarine hydrodynamics and associated saltwater intrusion. Previous studies revealed that suspended sediments influence estuarine hydrodynamics through various flow–sediment interactions. In this contribution, we systematically investigate how changes in fluvial sediment load and other climate-change-induced environmental change influence estuarine hydrodynamics and sediment dynamics. For this purpose, we utilize a well-calibrated fully coupled model in which hydrodynamics, saltwater intrusion, and sediment transport interact with each other, to explore saltwater intrusion in the Yangtze Estuary in response to a decline in the sediment load, modified discharge, and sea-level rise. Model results suggest that a 70% decline in the suspended sediment load weakens the impact of sediments on salinity-induced stratification and thereby reducing saltwater intrusion. Sea-level rise or discharge peak reduction increases saltwater intrusion. However, a fully coupled model accounting for sediment effects predicts a much larger increase in saltwater intrusion compared to noncoupled models. Whether this effect is important depends on estuarine sediment concentrations and therefore the potential role of sediments should be carefully investigated before applying a noncoupled model. This work highlights not only the relevance of a suspended sediment decline but also the use of fully coupled models for predicting saltwater intrusion in turbid estuaries and has broad implications for freshwater resource management in turbid estuarine systems influenced by human interventions and climate change.

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The mechanisms controlling the formation of an estuarine turbidity maximum (ETM) in estuaries have been extensively investigated, but one aspect that has received much less scientific attention is the role of high suspended sediment concentrations in combination with tidal asymmetry in ETM formation. Particularly in highly turbid estuaries, sediment suspensions influence ETM development through a combination of horizontal sediment-induced density currents, a reduction in turbulent mixing, and water-bed exchange processes. In this study, we developed a schematic model resembling the Yangtze Estuary where the ETM is controlled by tidal pumping, estuarine circulation, and advection operating simultaneously. Model results suggest that high water slack tide asymmetry with Sediment-induced density effects (SedDE) favors landward migration of the ETM. In addition, without SedDE, stronger flood tidal dominance leads to more pronounced sediment trapping through tidal pumping. Depending on the type of tidal asymmetry, SedDE strengthen ETM growth by increasing estuarine circulation but may also lead to increased or reduced sediment concentration in the ETM due to enhanced or weakened landward tidal pumping, respectively. Higher near-bed sediment concentrations as a result of water-bed exchange processes, in turn, strengthen the effect of estuarine circulation but simultaneously strengthen the divergence of sediment by tidal pumping. Overall, the SedDE and higher near-bed sediment concentration, in combination with tidal asymmetry, play an important role in ETM formation and should be properly accounted for in studies on ETM dynamics in turbid estuaries.

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