Sustainable river management often requires long-term morphological simulations. As the future is unknown, uncertainty needs to be accounted for, which may require probabilistic simulations covering a large parameter domain. Even for one-dimensional models, simulation times can be long. One of the acceleration strategies is simplification of models by neglecting terms in the governing hydrodynamic equations. Examples are the quasi-steady model and the diffusive wave model, both widely used by scientists and practitioners. We established under which conditions these simplified and often more efficient models are accurate.@en

Sustainable river management often requires long-term morphological simulations. As the future is unknown, uncertainty needs to be accounted for, which may require probabilistic simulations covering a large parameter domain. Even for one-dimensional models, simulation times can be long. One of the acceleration strategies is simplification of models by neglecting terms in the governing hydrodynamic equations. Examples are the quasi-steady model and the diffusive wave model, both widely used by scientists and practitioners. Here, we establish under which conditions these simplified models are accurate. Based on results of linear stability analyses of the St. Venant-Exner equations, we assess migration celerities and damping of infinitesimal, but long riverbed perturbations. We did this for the full dynamic model, that is, no terms neglected, as well as for the simplified models. The accuracy of the simplified models was obtained from comparison between the characteristics of the riverbed perturbations for simplified models and the full dynamic model. We executed a spatial-mode and a temporal-mode linear analysis and compared the results with numerical modeling results for the full dynamic and simplified models, for very small and large bed waves. The numerical results match best with the temporal-mode linear analysis. We show that the quasi-steady model is highly accurate for Froude numbers up to 0.7, probably even for long river reaches with large flood wave damping. Although the diffusive wave model accurately predicts flood wave migration and damping, key morphological metrics deviate more than 5% (10%) from the full dynamic model when Froude numbers exceed 0.2 (0.3).@en

Intratidal variability in stratification, referred to as internal tidal asymmetry, affects the residual sediment flux of an estuary by altering sediment transport differently during ebb and flood. Although earlier studies suggest that flood-dominant mixing increases the residual landward sediment flux, the role of ebb-dominant mixing remains largely unknown. Based on field data, we investigate the mechanisms that cause ebb-dominant mixing and its effect on the residual sediment flux in a stratified estuarine channel. Observations based on two tidal cycles show that the pycnocline remains largely intact during flood. Vertical mixing during flood is inhibited by a strong fresh water outflow, confining landward transport of suspended sediment to the bottom layer. During ebb, the pycnocline height decreases until it interacts with the bottom boundary layer, resulting in enhanced vertical mixing and sediment transport extending further to the surface. Thus, ebb-dominant mixing increases the residual sediment flux in seaward direction. The long ebb period in combination with limited bed sediment availability further contributes to the residual ebb-flux. This is noteworthy since a long ebb duration corresponds to flood dominance, which is often associated with a landward residual sediment flux. Although our data represent average conditions and cannot readily be extrapolated to different forcing conditions, we conclude that asymmetries in vertical mixing considerably affect the residual sediment flux under average conditions.

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Recent research highlights the abundance of floccule (flocs) in rivers, formed by aggregation of clay particles with organic matter. These flocs affect the transport and the eventual fate of clay. Flocs exhibit distinct behaviour from the unflocculated sedimentary counterparts: they can deform and break, and have higher settling velocities, which may in turn cause flocs to deposit and possibly interact with the riverbed. Here, we conducted systematic experiments in a laboratory flume to identify the mechanisms by which flocs and bedforms interact. Flocs showed a saltating (bouncing) behaviour, and were incorporated in the sediment bed as single flocs, clusters, or strings, via deposition and burial in the lee of a dune. Dune geometry was negligibly impacted by the presence of flocs. In natural systems, the burial of flocculated clay particles can affect contaminant spreading, aquatic ecology, the interpretation of deposition patterns, and clay transport.

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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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Effects of sea-level rise (SLR) on future peak water levels in tidal deltas and estuaries are largely unknown, despite these areas being densely populated and at high risk of flooding. While the rates of SLR accelerate, many channels simultaneously experience channel deepening for navigation. With globally decreasing sediment supplies, most channels are at risk of becoming deeper when the rate of SLR accelerates and sedimentation cannot keep pace with SLR. These factors potentially favor amplification of the tides and thereby increase flood risk, but the extent to which they will do so is unknown. Here, we introduce and use a validated model for an artificially deepened multi-branch delta to get a mechanistic understanding of non-linear SLR-effects on peak water levels. Results show that, when the current deepened bed level will be maintained, peak water levels do not rise on par with mean sea-level. Thus flood risk increases less than what can be expected from the predictions of the mean sea-level increase. The reason is that SLR causes a proportional reduction in convergence of channel area. This mechanism reduces tidal amplification. Nevertheless, SLR effects extend far beyond the range of present-day seasonal variations, with future low water levels being equal to present-day high water levels, while the tidal range slightly reduces. This will have consequences not only for flood risk, but also for freshwater availability, navigation and ecology.

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The bed stability of an estuary is determined by the net import or export of sediment, which in turn is controlled by multiple processes. Apart from the upstream riverine sediment supply, the net sediment flux is largely controlled by tidal hydrodynamics and the associated sediment exchange with the sea. In general, flood dominance causes landward residual sediment transport (sediment import from the sea), and ebb dominance causes seaward residual sediment export to the sea (Guo et al., 2014). In the New Waterway, The Netherlands, , residual fluxes are mainly associated with upstream and downstream advective transport in the salt wedge and in the fresh water layer, respectively. The associated processes have been well documented (De Nijs et al., 2010; Dronkers, 2017), which result in accumulation of sediment near the tip of the salt wedge (De Nijs et al., 2010). While it is known that mixing between freshwater and saltwater layers plays an important role in the residual salinity flux, little is known however about the exchange of sediment between both layers. We aim to quantify and understand the exchange of sediment across the freshwater-saltwater interface based on field data in a stratified tidal channel.@en

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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Sustainable river management can be supported by models predicting long-term morphological developments. Even for one-dimensional morphological models, run times can be up to several days for simulations over multiple decades. Alternatively, analytical tools yield metrics that allow estimation of migration celerity and damping of bed waves, which have potential for being used as rapid assessment tools to explore future morphological developments. We evaluate the use of analytical relations based on linear stability analyses of the St. Venant-Exner equations, which apply to bed waves with spatial scales much larger than the water depth. With a one-dimensional numerical morphological model, we assess the validity range of the analytical approach. The comparison shows that the propagation of small bed perturbations is well-described by the analytical approach. For Froude numbers over 0.3, diffusion becomes important and bed perturbation celerities reduce in time. A spatial-mode linear stability analysis predicts an upper limit for the bed perturbation celerity. For longer and higher bed perturbations, the dimensions relative to the water depth and the backwater curve length determine whether the analytical approach yields realistic results. For higher bed wave amplitudes, non-linearity becomes important. For Froude numbers ≤ 0.3, the celerity of bed waves is increasingly underestimated by the analytical approach. The degree of underestimation is proportional to the ratio of bed wave amplitude to water depth and the Froude number. For Froude numbers exceeding 0.3, the net impact on the celerity depends on the balance between the decrease due to damping and the increase due to non-linear interaction.@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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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

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

An idealized width-averaged model is employed to study the influence of wind stress on subtidal salt intrusion and stratification in well-mixed and partially stratified estuaries. We show that even in mild conditions, wind forcing can influence the estuarine salinity structure in a substantial way. By studying the role of wind forcing on dominant salt transport balances and associated salt transport regimes, we unify and clarify ambiguous observations from previous authors regarding the influence of wind stress: the response of the estuarine salinity structure to wind forcing is different depending on the underlying dominant salt transport balance, which in turn was found to determine whether wind-induced salinity shear, wind-induced modulation of the longitudinal salt distribution, or wind-induced mixing dominates. SIGNIFICANCE STATEMENT: The purpose of this idealized study is to better understand how wind influences the salinity distribution in estuaries on large time scales. This is important because a change in winds can move saline water further inland, threatening freshwater availability and the natural balance of delicate ecosystems. We clarify the sometimes ambiguous observations regarding the influence of wind on the salt distribution and highlight the importance of including average wind forcing in analyses of estuarine dynamics on large time scales.

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Conceptually, tidal rivers are seen as narrow channels along which the cross-section geometry remains constant and the bed is horizontal. As tidal waves propagate along such a channel, they decrease exponentially in height. The more rapid the decrease, the stronger the river flow. Near the coast, the tidally averaged width and depth change little throughout the year, even if the river discharge varies strongly between the seasons. However, further upstream, the water depth varies considerably with the river discharge. Recent observations from the Kapuas River, Indonesia, show that the water surface forms a backwater profile when the river flow is low. In this case, the depth converges, i.e. it gradually decreases between the river mouth and the point where the bed reaches sea level. This effect distinctly influences how tidal waves propagate up river so that their wave height does not decrease exponentially any more. We present a theoretical analysis of this phenomenon, which reveals several so far overlooked aspects of river tides. These aspects are particularly relevant to low river flow. Along the downstream part of the tidal river, depth convergence counteracts frictional damping so that the tidal range is higher than expected. Along the upstream parts of the tidal river, the low depth increases the damping so that the tide more rapidly attenuates. The point where the bed reaches sea level effectively limits the tidal intrusion, which carries over to the overtide and the subtidal water level set-up.@en

Longitudinal training dams (LTDs) are a promising alternative for river groynes. Here we summarize findings of a recent study focused on the along river transition from a series of river groynes to an LTD, where the flow divides between the fairway and the side channel between the LTD and the river bank. A scale model is setup using lightweight granulates made of polystyrene to create conditions that are dynamically similar to a prototype situation in the River Waal. The key advantage of using lightweight granulates is that both the Shields number and the Froude number are similar in the model and the prototype. A high flow and a low flow experiment were carried out. The bedforms in the physical model have dimensions that correspond to theoretical dune height predictions, and also the channel incision due to width reduction is in accordance with expectations. The scour holes that develop near the tip of the groynes, however, are too deep, which may relate to improper scaling of the local turbulent vortices, initiated at the groynes. The morphodynamic developments in the flow divergence zone are subtle, and are overwhelmed by the mobile bed response to the presence of groynes. Considering that the physical model over-predicts the erosion caused by groynes, this suggests that the LTD configuration subject to study results in a comparatively stable bed morphology.

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Lowlands are vulnerable to flooding due to their mild topography in often densely populated areas with high social and economic value. Moreover, multiple physical processes coincide in lowland areas, such as those involved in river-sea interactions and in merging rivers at confluences. Simultaneous occurrence of such processes can result in amplifying or attenuating effects on water levels. Our aim is to understand the mechanisms behind simultaneous occurrence of discharge waves in a river and its lowland tributaries. Here, we introduce a new way of analyzing lowland discharge and water level dynamics, by tracing individual flood waves based on dynamic time warping. We take the confluence of the Meuse River (∼ 33000km2) with the joining tributaries of the Dommel and Aa rivers as an example, especially because the January 1995 flood at this confluence was the result of the simultaneous occurrence of discharge peaks in the main stream and the tributaries and because independent observations of water levels and discharge are available for a longer period. The analysis shows that the exact timing of the arrival of discharge peaks is of little relevance because of the long duration of the average discharge wave compared to typical time lags between peaks. The discharge waves last on average 9 days, whereas the lag time between discharge peaks in the main river and the tributaries is typically 3 days. This results in backwaters that can rise up to 1.5 m over a distance of 4 km from the confluence. Thus, local measures to reduce the impact of flooding around the confluence should account for the long duration of flood peaks in the main system.@en

The prediction of the morphological evolution of renaturalized streams is important for the success of restoration projects. Riparian vegetation is a key component of the riverine landscape and is therefore essential for the natural rehabilitation of rivers. This complicates the design of morphological interventions, since riparian vegetation is influenced by and influences the river dynamics. Morphodynamic models, useful tools for project planning, should therefore include the interaction between vegetation, water flow and sediment processes. Most restoration projects are carried out in USA and Europe, where rivers are highly intervened and where the climate is temperate and vegetation shows a clear seasonal cycle. Taking into account seasonal variations might therefore be relevant for the prediction of the river morphological adaptation. This study investigates the morphodynamic effects of riparian vegetation on a re-meandered lowland stream in the Netherlands, the Lunterse Beek. The work includes the analysis of field data covering 5years and numerical modelling. The results allow assessment of the performance of a modelling tool in predicting the morphological evolution of the stream and the relevance of including the seasonal variations of vegetation in the computations. After the establishment of herbaceous plants on its banks, the Lunterse Beek did not show any further changes in channel alignment. This is here attributed to the stabilizing effects of plant roots together with the small size of the stream. It is expected that the morphological restoration of similarly small streams may result in important initial morphological adaptation followed by negligible changes after full vegetation establishment.

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River delta degradation has been caused by extraction of natural resources, sediment retention by reservoirs, and sea-level rise. Despite global concerns about these issues, human activity in the world’s largest deltas intensifies. Harbour development, construction of flood defences, sand mining and land reclamation emerge as key contemporary factors that exert an impact on delta morphology. Tides interacting with river discharge can play a crucial role in the morphodynamic development of deltas under pressure. Emerging insights into tidal controls on river delta morphology suggest that—despite the active morphodynamics in tidal channels and mouth bar regions—tidal motion acts to stabilize delta morphology at the landscape scale under the condition that sediment import during low flows largely balances sediment export during high flows. Distributary channels subject to tides show lower migration rates and are less easily flooded by the river because of opposing non-linear interactions between river discharge and the tide. These interactions lead to flow changes within channels, and a more uniform distribution of discharge across channels. Sediment depletion and rigorous human interventions in deltas, including storm surge defence works, disrupt the dynamic morphological equilibrium and can lead to erosion and severe scour at the channel bed, even decades after an intervention.@en