Estuarine Turbidity Maxima (ETM) arise from various sediment transport processes, each responding differently to anthropogenic and climatic pressures. This study makes a step toward constructing an explanatory parameter space for sediment trapping in estuaries by systematically investigating ETM formation across a wide range of idealized, M2-dominated estuarine configurations using a width-averaged numerical model. The sensitivity of sediment transport processes to four key estuarine parameters - estuary length, bed roughness, river discharge, and tidal amplitude - is examined. Additional simulations explore the influence of bed slope, width convergence, and settling velocity. Through decomposition of sediment transport contributions in the model results, four distinct sediment transport regimes are identified, each named after its dominant import process: the Baroclinic Regime, the Dispersive Regime, the Sediment Advection Regime, and the Internally Generated Tidal Asymmetry Regime. This regime-based classification represents a novel framework for linking estuarine parameter configurations to dominant sediment dynamics. In a schematic straight, flat channel configuration, only the Baroclinic and Sediment advection regimes can govern ETM formation. The Internally Generated Tidal Asymmetry regime becomes dominant in ETM formation only when the channel is sloping or converging. The Dispersive regime cannot produce an ETM. While the individual balances, or regimes, have been discussed, this study newly identifies the region of parameter space where each regime dominates. Despite its limitations, the proposed parameter space successfully captures the dominant transport processes in the Loire and Scheldt Estuaries. This process-based characterization of ETM dynamics offers insight into the physical conditions that favor high sediment concentrations in tidal estuaries.

The salinity in estuaries continuously adapts to varying forcing for example, by discharge and tidal conditions. The changes in salinity lag behind the changes in forcing. Previous work has mostly related this delay to the adjustment time, which depends on an average background state of the estuary. Payo-Payo et al. (2022), https://doi.org/10.1029/2021jc017523 showed that adjustment time however cannot explain the actually observed delays for a realistic salinity and forcing signal. Inspired by this, this study aims to develop relations between delay time and forcing variations and background state of the estuary. To this end, I first propose a definition of the actual delay of salinity based on wavelet analysis, applicable to observed or modeled salinity signals. To compare delay to estuarine parameters, I use a linear 1D model, but qualitative results carry over to the general case. Using model experiments with harmonic and peaked variations in the forcing, the delay time depends not only on the adjustment time, but also on the timescale of the forcing variation. Even for forcing timescales that are up to a factor 100 longer than the adjustment time, both forcing timescale and adjustment time are important for the delay. A second novel finding is that the delay depends strongly on the position along the estuary where the delay is observed. As verification, model experiments with realistically varying forcing were done, roughly inspired by the Modaomen Estuary (China). Although delay times showed a complicated and scattered dependency on model variables in this case, the above qualitative conclusions were confirmed.

The Loire estuary (France) was extensively deepened during the 20th century. Coincidentally, suspended sediment concentrations increased drastically from ∼0.1 g/l to ∼1–5 g/l at the surface and the estuarine turbidity maximum (ETM) moved upstream. In this study we, for the first time, brought together a century of observations of estuary bed level, tidal amplitude, and sediment concentration to demonstrate these large changes. Next, we analyzed a minimal set of physical mechanisms that explain the dramatic increase in sediment concentration. To this end, we used the iFlow model representing dynamic equilibrium conditions in the Loire. Novel in the model is that it dynamically resolves salt stratification and corresponding damping of turbulence. For conditions representing the year 2000, high sediment concentrations were found with satisfactory correspondence to observations. Low sediment concentrations were found when using the year 1900 bed level but keeping all other model parameters the same. Varying the bed level gradually between these two extremes, the equilibrium solution suddenly increases for intermediate bed level, constituting an abrupt regime shift. Robustness of this result was established in an extensive sensitivity study featuring 13,200 model experiments. The regime shift is enabled by a feedback between increasing sediment concentration, reducing turbulence due to sediment and salt stratification, and increasing sediment importing capacity of the estuary. The essential sediment importing mechanisms in this feedback are related to the tidal asymmetry and gravitational circulation. This is the first time gravitational circulation and salt stratification are shown to be important factors in a transition to hyperturbidity.

The water motion computed using 3D and 2DH models in tidally dominated shallow waters can, in some cases, differ significantly. In 2DH models, bed friction is typically parametrised in terms of the depth-averaged velocity, whereas in 3D models, typically the near-bed velocity is used. This difference causes the bed shear stress in 2DH models to point towards the depth-averaged velocity, whereas in 3D models, it points towards the near-bed velocity, which are not necessarily the same. Focussing on linearised barotropic models, we derive an exact friction parametrisation for 2DH models such that the same depth-averaged dynamics are described as in the corresponding 3D model. The result is a convolutional friction formulation where the instantaneous friction depends on the present and past velocities, thus modifying the traditional 2DH friction formulation that only depends on the present depth-averaged velocity. In the case of harmonic (tidal) waves, this parametrisation has a clear physical interpretation and shows that the near-bed velocity should be parametrised as a rotated, deformed and phase shifted variant of the depth-averaged velocity. We demonstrate that in certain regions of the parameter space, it may be impossible to calibrate a 2DH model that uses a traditional friction law to reproduce the water levels from a 3D model, showing that the 3D friction formulation can be crucial to capture the 3D dynamics within a depth-averaged model. This phenomenon is explored in detail in a narrow well-mixed estuary.

Estuaries often show regions in which Chlorophyll-a (Chl-a) accumulates. The location and magnitude corresponding to such accumulation result from a complex interplay between processes such as river flushing, salinity, nutrients, grazing on phytoplankton, and the light climate in the water column. An example is the multi-annual evolution of the estuary-scale Chl-a distribution in the Scheldt estuary (Belgium/Netherlands) in spring. From 2004–2007, we observed a limited spring bloom in the brackish region (km 60–90 from the mouth, salinity ∼ 1–10 ppt). This bloom intensified in 2008–2014 and disappeared after 2015. This multi-annual evolution of Chl-a has been hypothesized to be linked to simultaneous multi-annual trends in the suspended particulate matter (SPM) distribution in summer and winter between 1995–2015 and the improvement of the water quality (e.g., reduction of ammonium), which affects grazing on phytoplankton by zooplankton. However, this hypothesis has not been systematically investigated. In this contribution, we apply a modeling approach in which observations are the core. We first analyze multi-annual in situ observations covering the full estuary. These observations include the SPM concentration, zooplankton abundance, and other variables affecting the Chl-a concentration. They show a multi-annual estuary-scale evolution not only in the SPM distribution but also in zooplankton abundance, freshwater discharge, and phytoplankon photosynthetic characteristics. Next, we apply a model approach that consists of an extensive sensitivity study and four model scenarios that are supported by these observations to constrain the processes and corresponding parameter variability that may have caused the observed change in Chl-a. Our results suggest that a change in SPM alone cannot explain the Chl-a observations. Instead, a multi-annual change in mortality rate, which we can attribute to both grazing by zooplankton and phytoplankton community (i.e., mortality dependence on salinity), may explain the multi-annual estuary-scale evolution of Chl-a in spring. Different model parameter choices may thus lead to similar model results (equifinality). Our results highlight that insight into the zooplankton dynamics and phytoplankton community characteristics is essential to understand the phytoplankton (cf. Chl-a) dynamics in the Scheldt estuary and that additional data regarding mortality and grazing rates is required to further constrain the model parameters.

Tidally averaged transport of salt in estuaries is controlled by various subtidal and tidal processes. In this study, we show the relative importance of various subtidal and tidal transport processes in a width-averaged sense. This is done for a large range of forcing and geometric parameters, which describe well-mixed to salt wedge estuaries. To this end, we develop a width-averaged process-based model aimed at conducting and analyzing a large number of experiments (∼40,000). We find that the salt transport is dominated by one of seven salt transport balances, or regimes. Four of these regimes are dominated by subtidal processes, while the other three are dominated by tidal processes. Which regime occurs in a part of an estuary depends on four dimensionless parameters, representing local geometry, and forcing conditions. One of the regimes features salt import by correlations between the depth-averaged tidal velocity and salinity. While this mechanism was previously only associated with along-channel geometric variations, we find it can also be a dominant mechanism in a significant part of the parameter space due to river-induced tidal asymmetry, independent of river geometry. We apply our classification to a case study of part of the Dutch Rhine delta and compare to decomposition results of a fully realistic three-dimensional model. We find the estuary features two regimes, with import dominated by subtidal shear transport in the seaward part of the estuary and by depth-averaged tidal correlations in the landward part of the estuary.

Salt intrusion in surface waters endangers freshwater availability, influences water quality, and affects estuarine ecosystem services with high economic and social values. Salt transport and the resulting salinity distributions result from the non-linear interactions between salt and water dynamics. Estuaries are often considered under (quasi)-steady assumption or by focusing on specific timescales. Our understanding of their temporal multiscale response to transient forcing is limited, which hinders the implementation of effective mitigation strategies. We apply wavelet analyses to quantify the variability of salt intrusion from hourly to seasonal timescales and unravel the temporal variability of its response across scales. We focus on an estuary that undergoes significant transient forcing, the Modaomen estuary in the Pearl River Delta, and apply the wavelet analyses to year-long data generated by a coastal ocean numerical model. Our results show that this estuary responds to changes in tidal and riverine forcing throughout the year over interwoven timescales. Our results highlight the temporal variability of the salt intrusion response time both within a given regime and for the transition between regimes. They also suggest that tides control the response time more strongly than river discharge, even though river discharge determines the magnitude of the salt intrusion, and thus modulates the evolution of the salt intrusion response time. We propose a broadly applicable framework to calculate response times with simple data. These results can provide a first-order guidance for design and implementation of estuarine management strategies and mitigation measures that ensure water access and facilitate sustainable development.

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.

Net water transport (NWT) in estuaries is important for, for example, salt intrusion and sediment dynamics. While NWT is only determined by river runoff in single channels, in estuarine networks, it results from a complex interplay between tides and residual flows. This study aims to disentangle the various contributions of these physical drivers to NWT in estuarine networks and investigate the sensitivities of NWT to variable forcing conditions, interventions, and sea level rise (SLR). To this end, a processes-based perturbative network model is developed, which accounts for the vertical flow structure to resolve density-driven flow driven by a vertically uniform along-channel salinity gradient. Other identified drivers are river discharge and three tidal rectification processes: Stokes transport and its return flow, momentum advection, and velocity-depth asymmetry. The model is applied to the Yangtze Estuary. NWT due to tidal rectifications and density-driven flow can be comparable to river discharge. Specifically in the North Branch, the direction of NWT may differ from the direction of river discharge. Varying river discharge mainly affects NWT as tide-river interaction is weak and density-driven flow is shown to be insensitive to salt intrusion. Conversely, variations in tidal amplitude strongly affect NWT related to tidal rectification and density-driven flow. The deepening (narrowing) of one channel (Deep Waterway Project), affected the NWT mostly through the density-driven flow (advection). Furthermore, NWT distribution in the Yangtze is insensitive to SLR up to 2 m because the effects of SLR on transport due to different drivers compensate each other.

An estuarine turbidity maximum (ETM) results from various subtidal sediment transport mechanisms related to, e.g., river, tides, and density gradients, which have been extensively analysed in single-channel estuaries. However, ETMs have also been found in estuaries composed of multiple interconnected tidal channels, where the water and suspended fine sediments are exchanged at the junctions with possible occurrence of sediment overspill. The overall aim of this study is to understand the processes that determine the ETM dynamics in such channel networks. Specifically, focusing on the ETMs formation due to sediment transport by river flow and density-driven flow, the dependence of ETM locations in an idealised three-channel network on fluvial sediment input and the local deepening and narrowing of a seaward channel is investigated. It is found that the ETM dynamics in channels of a network is coupled, and hence, changes in one channel affect the ETM pattern in all channels. Sensitivity results show that, keeping river discharge fixed, a larger fluvial sediment input leads to the upstream shift of ETMs and an increase in the overall sediment concentration. Both deepening or narrowing of a seaward channel may influence the ETMs in the entire network. Furthermore, the effect of either deepening or narrowing of a seaward channel on the ETM locations in the network depends on the system geometry and the dominant hydrodynamic conditions. Therefore, the response of the ETM location to local geometric changes is explained by analysing the dominant sediment transport mechanisms. In addition to the convergence of sediment transport mechanisms in single-estuarine channels, ETM dynamics in networks is found to be strongly affected by net exchange of sediment between the branches of a network. We find that considering the sensitivity of net sediment transport to geometric changes is needed to understand the changing ETM dynamics observed in a real estuarine network.

The salinity structure in estuaries is classically described in terms of the salinity structure as well mixed, partially mixed, or salt wedge. The existing knowledge about the processes that result in such salinity structures comes from highly idealized models that are restricted to either well-mixed and partially mixed cases or subtidal salt wedge estuaries. Hence, there is still little knowledge about the processes driving transitions between these different salinity structures and the estuarine parameters at which such a transition is found. As an important step toward a unified description of the dominant processes driving well-mixed, partially mixed, and salt wedge estuaries, a subtidal width-averaged model applicable to all these salinity structures is developed and systematically analyzed. Using our model, we identify four salinity regimes, resulting from different balances of dominant processes. It is shown that each regime is uniquely determined by two dimensionless parameters: an estuarine Froude and Rayleigh number, representing freshwater discharge and tidal mixing, respectively, resulting in a classification of the regimes in terms of these two parameters. Furthermore, analytical expressions to approximate the salt intrusion length in each regime are developed. These expressions are used to illustrate that the salt intrusion length in different regimes responds in a highly different manner to changes in depth and freshwater discharge. As one of the key results, we show that there are only very weak relations between the process-based regime of an estuary and the salt intrusion length and top-bottom stratification. This implies that the salinity structure of an estuary cannot be uniquely matched to a regime.

Estuaries are often characterised by a complex network of branching channels, in which the water motion is primarily driven by tides and fresh water discharge. For both scientific reasons and management purposes, it is important to gain more fundamental knowledge about the hydrodynamics in such networks, as well as their implications for turbidity and ecological functioning. A generic 2DV estuarine network model is developed to study tides and river water transport and to understand the dependence of their along-channel and vertical structure on forcings, geometry characteristics and sea level changes. The model is subsequently applied to the Yangtze Estuary to investigate tides and the distribution of river water over channels during dry and wet season, spring tide, as well as prior to and after the formation of Hengsha Passage and the construction of the Deep Waterway Project and sea level rise. Increasing river discharge enhances the friction for tides by increasing both internal and bottom stresses. Changes in tidal forcing are correlated with the friction for both tide and river. A shortcut channel reduces the water level difference in adjacent channels, as well as tidal amplitudes difference. Sea level rise results in larger friction parameters and faster propagation of tides. The distribution of river water transport is hardly affected by above mentioned changes. Model results and current vertical structure are consistent with observations.

Many estuaries exhibit seasonality in the estuary-scale distribution of suspended particulate matter (SPM). This SPM distribution depends on various factors, including freshwater discharge, salinity intrusion, erodibility, and the ability of cohesive SPM to flocculate into larger aggregates. Various authors indicate that biotic factors, such as the presence of algae and their excretion of sticky transparent exopolymer particles (TEP), affect the flocculation and erosion processes. Consequently, seasonality in these biotic factors may play a role in the observed seasonality in SPM. Whereas the impact of abiotic factors on seasonality in SPM is well studied, the relative contribution of biotically induced seasonality is largely unknown. In this study, we employ two approaches to assess the aggregated importance of biotically induced seasonality in flocculation and erosion on seasonality in SPM in the Scheldt estuary. In the first approach, we focus on seasonality of in situ observations in the Scheldt estuary of turbidity, floc size, Chlorophyll-a, and TEP, showing that the abiotic parameters show seasonality, while seasonality in TEP is ambiguous. The second approach concerns a reverse engineering method to calibrate biotically affected parameters of a coupled sediment transport-flocculation model to turbidity observations, allowing us to compare the modeled SPM concentrations to the observations. Driven by seasonality in freshwater discharge, the model captures the observed seasonality in SPM without requiring biotically induced seasonality in flocculation and erosion, which is supported by the absence of seasonality in TEP.

Climate and human pressures can influence the evolution of estuarine sediment dynamics concurrently, but the understanding and quantification of their cause–effect relationships are still challenging due to the occurrence of complex hydro-morpho-sedimentary feedbacks. The Garonne Tidal River (GTR, upper Gironde Estuary, France) is a clear example of a system stressed by both anthropogenic and climate change, as it has been subject to decreasing river discharges, natural morphological changes, and gravel extraction. To understand the relative effect of each hydrological and geomorphological pressure on the turbidity maximum zone (TMZ), the sediment dynamics in the GTR over the last six decades was evaluated using the width-averaged idealized iFlow model. Model results show a gradual increase in tidal amplitude and currents over the decades that has led to the upstream shift of the landward sediment-transport capacity components (external M₄ tide, spatial settling lag, and tidal return flow). The upstream displacement of the TMZ between the 1950s and the 2010s was estimated to be at least 19 km, of which about three fourth was induced by geomorphological changes and one fourth by hydrological changes. Concerning the geomorphological changes, the natural evolution of the lower Gironde morphology was the main pressure inducing the displacement of the TMZ in the GTR. Anthropogenic and natural changes in morphology and bed roughness in the GTR itself also contributed to this evolution. The natural geomorphological changes were, in turn, probably promoted by the evolution of sediment dynamics, so this study reveals the closed circle that governs the intensification of the TMZ.

Sediment transport in estuaries and the formation of estuarine turbidity maxima (ETM) highly depend on the ability of suspended particulate matter (SPM) to flocculate into larger aggregates. While most literature focuses on the small-scale impact of biological flocculants on the formation of larger aggregates, the influence of the flocculation process on large-scale estuarine SPM profiles is still largely unknown. In this paper, we study the impact of flocculation of SPM on the formation of ETM. For this, a semianalytical width-integrated model called iFlow is utilized and extended by a flocculation model. Starting from a complex one-class flocculation model, we show that flocculation may be described as a linear relation between settling velocity and suspended sediment concentration to capture its leading-order effect on the ETM formation. The model is applied to a winter case in the Scheldt estuary (Belgium, Netherlands) and calibrated to a unique, long-term, two-dimensional set of turbidity (cf. SPM) observations. First, model results with and without the effect of flocculation are compared, showing that the spatial and temporal variations of the settling velocity due to flocculation are essential to reproduce the observed magnitude of the suspended sediment concentrations and its dependence on river discharge. Second, flocculation results in tidally averaged land-inward sediment transport. Third, we conduct a sensitivity analysis of the freshwater discharge and floc breakup parameter, which shows that flocculation can cause additional estuarine turbidity maxima and can prevent flushing of the ETM for high freshwater inflow.

Many estuaries are strongly modified by human interventions, including substantive channel deepening. In the Ems River Estuary (Germany and Netherlands), channel deepening between the 1960s and early 2000s coincided with an increase in the maximum near-bed suspended sediment concentration from moderate (∼1 kg/m ³ ) to high (>10 kg/m ³ ). In this study the observed transition in the suspended sediment concentration in the Ems is qualitatively reproduced by using an idealized width-averaged iFlow model. The model is used to reproduce observations from 1965 and 2005 by only changing the channel depth between the years. Model results show an increase in sediment concentrations from approximately 1–2 kg/m ³ to 20–30 kg/m ³ near the bed between 1965 and 2005 if the river discharge is below 70 m ³ /s, which holds approximately 60% of the time. Thereby, this study for the first time provides strong evidence for earlier published hypotheses that channel deepening was the main driver of the increased sediment concentrations in the Ems. The results are explained using two aspects: sediment transport (longitudinal processes) and local resuspension (vertical processes). The magnitude of the sediment import increased, because a combination of channel deepening and sediment-induced damping of turbulence increased the M ₂ –M ₄ tidal asymmetry. This effect is particularly strong, because the M ₄ tide evolved to a state close to resonance. All imported sediment is kept in suspension when it is assumed that resuspension is sufficiently efficient, which depends on the value of the erosion parameter used and inclusion of hindered settling in the model.

Many estuaries are strongly deepened to improve navigation, with sometimes large and poorly understood consequences to suspended sediment dynamics. To improve understanding of such large changes, we study the Ems River Estuary, where a regime shift from low to high sediment concentrations was observed after deepening. The aim of this study is to improve understanding of the development of the sediment concentration regime over time and estimate the associated time scale. Using the idealized width-averaged iFlow model, we identify the coexistence of two distinct stable equilibrium regimes representing low and high sediment concentrations, qualitatively matching the regimes observed in the Ems. Depending on the river discharge, a critical depth profile is identified at which the regime shifts. By combining the model results and long-term observations of the tidal range, first indications of the regime shift are observed around 1989, taking approximately 6–7 years to develop.