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Aggregation is used to represent the real world in a model at an appropriate level of abstraction. We used the convection-diffusion equation toexamine the implications of aggregation progressing from a three-dimensional (3D) spatial description to a model representing a system as asingle box that exchanges sediment with the adjacent environment. We highlight how all models depend on some forms of parametric closure,which need to be chosen to suit the scale of aggregation adopted in the model. All such models are therefore aggregated and make use of someempirical relationships to deal with sub-scale processes. One such appropriately aggregated model, the model for the aggregated scalemorphological interaction between tidal basin and adjacent coast (ASMITA), is examined in more detail and used to illustrate the insight that thislevel of aggregation can bring to a problem by considering how tidal inlets and estuaries are impacted by sea level rise.

Global climate changes have accelerated sea-level rise (SLR), which exacerbates the risks of coastal flooding and erosion. It is of practical interest to understand the long-term hydro-morphodynamic adaptation of coastal systems to SLR at a century time scale. In this work we use a numerical model to explore morphodynamic evolution of a schematized tidal basin in response to SLR of 0.25–2.0 m over 100 years with special emphasis on the impact of lateral basin expansion. Starting from a sloped initial bed, morphodynamic development of the system leads to the formation of alternating bars and meandering channels inside the tidal basin and an ebb-tidal delta extending seaward from the basin. Imposing rising sea level causes progressive inundation of the low-lying floodplains, found along the basin margins, inducing an increase in basin plain area and tidal prism, as well as intertidal area and storage volume. Although the overall channel-shoal structure persists under SLR, lateral shoreline expansion alters the basin hypsometry, leading to enhanced sediment export. The newly-submerged floodplains partly erode, supplying sediment to the system for spatial redistribution, hence buffering the impact of SLR. The vertical accretion rate of the tidal flats inside the tidal basin lags behind the rate of SLR. However, lateral shoreline migration under SLR creates new intertidal flats, compensating intertidal flat loss in the original basin. In contrast, a constrained tidal basin without low-lying floodplains is subject to profound drowning and tidal flat losses under SLR. Overall, the model results suggest that an unconstrained tidal system allowing lateral shoreline migration has buffering capacity for alleviating the drowning impact of SLR by evolving new intertidal areas, sediment redistribution and morphodynamic adjustment. These findings suggest that preserving tidal flats located along the margins of tidal basins (instead of reclaiming them) sustains the system's resilience to SLR.

Lateral flow significantly contributes to the near-bottom mass transport of salinity in a channel-shoal system. In this study, an integrated tripod system was deployed in the transition zone of a channel-shoal system of the Changjiang Estuary (CE), China, to observe the near-bottom physics with high temporal/spatial resolution, particularly focusing on the lateral-flow-induced mass transport. These in situ observations revealed a small-scale salinity fluctuation around low water slack during moderate and spring tidal conditions. A simultaneous strong lateral current was also observed, which was responsible for this small-scale fluctuation. A high-resolution unstructured-grid Finite-Volume Community Ocean Model has been applied for the CE to better understand the mechanism of this lateral flow and its impact on salinity transport. The model results indicate that a significant southward near-bed shoal-to-channel current is generated by the salinity-driven baroclinic pressure gradient. This lateral current affects the salinity transport pattern and the residual current in the cross-channel direction. Cross-channel residual current shows a two-layer structure in the vertical, especially in the intermediate tide when the lateral flow notably occurred. Both observation and model results indicate that near-bottom residual transport of water moved consistently southward (shoal to channel). Mechanisms for this intratidal salinity variation and its implications can be extended to other estuaries with similar channel-shoal features.

The radial sand ridges (denoted as “RSRs” hereafter) in the Southern Yellow Sea, China, are morphologically striking because of the remarkable size and radial planar orientation, standing out as a unique coastal geomorphology among the worldwide sand ridge systems. The formation of this giant fan‐shaped geomorphic feature requires delicate conditions and awaits in‐depth investigation. Using an idealized morphodynamic model, this study unravels the governing factors for the formation of the unique large‐scale RSRs, in comparison with other types of sand ridge systems over the world. The effects of the M2 tidal constituent, the Coriolisforcing, the bed resistance, and the initial water depths on the morphodynamic behavior of the RSRs are explored. Numerical results indicate that the tidal regime, characterized by rotational and progressive current action associated with the tidal bulge, is dependent on the eastern coastline of China as well as latitudinal effects. Through the comparison between the simulated and the measured morphology, this tidal regime is demonstrated to be the key driverin forming and maintaining the present‐day RSRs. The runs with different parameters further suggest that the asymmetric pattern of the RSRs, which shows larger northern sand ridges than the southern ones, results from both the asymmetric distribution of current activity caused by the tidal bulge and unequal sediment supply. Overall, this study highlights the delicate condition, predominantly represented by the particular currents set up by the tidal wave system and the sediment supply, required to shape the striking large‐scale RSRs in the Southern Yellow Sea.

Tidal waves traveling into estuaries are modulated in amplitude and shape due to bottom friction, funneling planform and river discharge. The role of river discharge on damping incident tides has been well-documented, whereas our understanding of the impact on overtide is incomplete. Inspired by findings from tidal data analysis, in this study we use a schematized estuary model to explore the variability of overtide under varying river discharge. Model results reveal significant M4 overtide generated inside the estuary. Its absolute amplitude decreases and increases in the upper and lower parts of the estuary, respectively, with increasing river discharge. The total energy of the M4 tide integrated throughout the estuary reaches a transitional maximum when the river discharge to tidal mean discharge (R2T) ratio is close to unity. We further identify that the quadratic bottom stress plays a dominant role in governing the M4 variations through strong river-tide interaction. River flow enhances the effective bottom stress and dissipation of the principal tides, and reinforces energy transfer from principal tide to overtide. The two-fold effects explain the nonlinear M4 variations and the intermediate maximum threshold. The model results are consistent with data analysis in the Changjiang and Amazon River estuaries and highlight distinctive tidal behaviors between upstream tidal rivers and downstream tidal estuaries. The new findings inform study of compound flooding risk, tidal asymmetry, and sediment transport in river estuaries.

Lateral flows redistribute sediment and influence the morphodynamics of channel-shoal systems. However, our understanding of lateral transport of suspended sediment during high and low water slack is still fairly limited, especially in engineered estuaries. Human interventions such as dike-groyne structures influence lateral exchange mechanisms. The present study aims to unravel these mechanisms in a heavily engineered, turbid channel-shoal system in the Changjiang Estuary, using a high-resolution unstructured-grid three-dimensional model and in situ observations. Analysis of model results reveals two typical transport patterns during slack-water conditions, that is, shoal-to-channel transport during low water slack and channel-to-shoal transport during high water slack. A momentum balance analysis is carried out to explain mechanisms driving the lateral transport of suspended sediment during high water slack, revealing the importance of lateral pressure gradients, Coriolis force, and the curvature-induced term. Groyne fields play a crucial role in sediment transport, especially during low water slack. A model scenario in which one groyne is removed reveals that groyne fields strongly influence lateral sediment transport. The decomposition of the sediment transport flux reveals that the turbidity maximum is shaped by a balance between seaward advection by residual flows, and landward transport by tidal pumping and gravitational circulation. Within the turbidity maximum, sediment is laterally redistributed by lateral flows during slack-water conditions, greatly influencing estuarine channel morphology.

Estuarine tidal dynamics are influenced by changes in morphology and friction. In this work, we quantified changes in tidal damping in the Yangtze Estuary and explored the impact of morphology and friction using a numerical model. In-depth analyses of tidal data reveal a strong reduction in tidal damping from 1990 to 2010, followed by a slightly enhanced damping from 2010 to 2020 in the South Branch. The reduced tidal damping in the South Branch from 1990 to 2010 is controlled by sediment decline which induces an increase in water depth (erosion), thereby strongly amplifying tides. However, the effective bottom roughness (Manning coefficient) is increased by 60%, which is probably related to the (Formula presented.) 80% decrease in the suspended sediment concentration (SSC). Such an effect may enhance tidal damping, which counteracts the contribution of water depth increase on amplifying tides by (Formula presented.) 75%. From 2010 to 2020, the tides in the South Branch became more damped, suggesting a dominance of the decrease in SSC over the morphological changes. In the mouth zone, tidal dissipation is enhanced from 1997 to 2010, which is mainly caused by an overall increase in effective bottom roughness. Local structures dominate the increase in effective bottom roughness; however, fluid mud formation may contribute to a decrease after 2010. Overall, we argue that estuarine morphological and sedimentary changes in response to riverine sediment decline and local engineering works control the tidal evolution in the Yangtze Estuary, which is important for evaluation of human activities and estuarine management.

Channel deepening often triggers positive feedback between tidal deformation, sediment import and drag reduction, which leads to the regime shift in estuaries from low-turbid to hyper-turbid state. In this study, a transition in profiles of suspended sediment concentration (SSC) is hypothesised by including a positive feedback loop of vertical mixing and settling. Such a hypothesis is validated by the historical observations in the North Passage of Changjiang (Yangtze River) Estuary, with decreasing SSC in mid-lower layers and increasing SSC near the bed after the deepening. A mobile pool of concentrated benthic suspensions (CBS) develops in the North Passage, with a tidally averaged length of ~20 km and a mean thickness of ~4 m. The width of the CBS pool is limited (3); (2) constant and minimum coefficient (with drag reduction up to 60–80%) in the presence of CBS (10–80 kg/m3). An empirical relationship was derived, which can be used to predict the friction coefficient and the magnitude of drag reduction for sediment transport studies, particularly for modelling regime shifts in estuaries.

Multiple tidal inlet systems like the Wadden Sea have long been considered as separated basins, bordered by so-called tidal divides. Recently, it was however shown that fluxes of water and sediment occur over the borders of these basins, especially during wind events. In this paper, the wind-driven fluxes over these borders and the residual flow of water through the main inlet are studied. The study is based on flow measurements at the tidal divides and in the main inlet of the Ameland Inlet system in the Dutch Wadden Sea and on numerical modelling. The measurements were carried out during 40 days in the fall of 2017, including both calm conditions and storm events. Numerical simulations of a full year have been used for upscaling results from the measurements to system scale exchange flows, and to unravel the effects of several mechanisms. The wind-driven variability in exchange flows between back-barrier basins at tidal divides was measured in the field and reproduced by the numerical model. Water level set up increases the water depth and thus the conveyance capacity at tidal divides, such that the exchange flows increase in magnitude. The flow conditions due to wind forcing are similar for both tidal divides of the Ameland Basin. The conveyance capacity and therefore the total volume exchange are however different. This leads to a residual compensation flow through the main inlet, which is directed outward (i.e., in the ebb direction) during winds from the prevailing southwestern wind direction. The net discharge through the main inlet is therefore a consequence of the residual flows over the tidal divides.

Quantifying and characterizing suspended sediment is essential to successful monitoring and management of estuaries and coastal environments. To quantify suspended sediment, optical and acoustic backscatter instruments are often used. Optical backscatter systems are more sensitive to mud particles ( < 63μm) and flocs, whereas acoustic backscatter systems are more responsive to larger sand grains ( > 63μm). It is thus challenging to estimate the relative proportion of sand or mud in environments where both types of sediment are present. The suspended sediment concentration measured by these devices depends on the composition of that sediment, thus it is also difficult to confidently measure concentration with a single instrument when the composition varies and extensive calibration is not possible. The objective of this paper is to develop a methodology for characterizing the relative proportions of sand and mud in mixed sediment suspensions by comparing the response of simultaneous optical and acoustic measurements. We derive a sediment composition index (SCI) that is used to directly predict the relative fraction of sand in suspension. Here we verify the theoretical response of these optical and acoustic instruments in laboratory experiments, and successfully apply this approach to field measurements from Ameland ebb-tidal delta (the Netherlands). Increasing sand content decreases SCI, which was verified in laboratory experiments. A reduction in SCI appears during more energetic conditions when sand resuspension is expected. Conversely, the SCI increases in calmer conditions when sand settles out, leaving behind mud. This approach provides crucial knowledge of suspended sediment composition in mixed sediment environments. Plain Language Summary Sand and mud particles are the building blocks of our coastlines. Counting and describing sand and mud particles floating through the water is essential to managing coasts. We commonly do this with devices that send out a sound (acoustic) or light (optical) signal into the water. The sensors measure the strength of the signal reflecting back off of any sand and mud particles passing by. Optical instruments are better at “seeing” mud than sand, and acoustic instruments are better at “hearing” sand than mud. If both sand and mud are present, a single instrument will not accurately estimate the total amount of sediment because of these different sensitivities. Instead, we can use both types of instrument together and compare what we “see” with what we “hear”. This comparison allows us to estimate whether there are more sand or mud particles floating through the water. The relationship between “seeing” and “hearing” can be described in a single number, the sediment composition index (SCI). We successfully tested this approach in laboratory experiments and then applied it to a site on the coast of the Netherlands. This approach gives us a new way to understand environments that are both sandy and muddy.

Sediment tracer studies use uniquely identifiable particles to track the pathways and fate of individual sand or silt grains in marine environments. These techniques are best applied to assess connectivity between potential sediment sources and sinks, such as between a sand nourishment and an ecologically sensitive area. Significant challenges exist when applying sediment tracing techniques to further understanding of systems with complicated hydrodynamic, sediment, and morphological regimes. Ebb-tidal deltas are highly dynamic coastal environments shaped by the complex interplay of waves and tides, but have been under-explored. In this study, we use dual signature (fluorescent and ferrimagnetic) sediment tracers to simulate the dispersal of dredged sediment placed as a sand nourishment on an energetic ebb-tidal delta (at Ameland Inlet, the Netherlands). After deployment, sediment dispersal and grain size sorting behaviour were monitored via the collection of seabed grab samples and magnetic sampling of sediment transported in suspension. The tracer content within collected samples were put in context with hydrodynamic conditions observed during the study period. Here we show that the use of such dual signature tracers, in addition to novel tracer recovery and analysis techniques, enables the dispersal of sediment to be monitored even in such complex settings and energetic conditions as an ebb-tidal delta. Our observations show that tracers transported in suspension are significantly finer than tracers that accumulated in the seabed. These suggest that preferential transport as a function of grain size is a key process in shaping the morphology of ebb-tidal deltas and thus governing the dispersal of sand nourishments there. The findings of this study and the approach used here provide valuable tools for assessing the baseline conditions of complex coastal environments today, and for planning the interventions which may be necessary in future responses to climate change. Lessons learned from the application of sediment tracers in this study are provided to assist future researchers and practitioners in applying this technique in dynamic coastal environments.

Intertidal shoals are pronounced morphological features found in many estuaries worldwide. Apart from maintaining an ecologically unique intertidal environment, shoals also protect adjacent dyke systems by attenuating waves. The fate of sandy shoals under anticipated sea level rise (SLR) scenarios is underexplored. The current research investigates the long-term morphodynamic evolution of estuarine sandy shoals under forcing by short fetch, locally generated wind-waves, tides, and SLR by means of a numerical, process-based model (Delft3D). The focus lies on a sheltered shoal complex in the Western Scheldt, the Netherlands. Starting from the initial, 1963 bathymetry, we model 50-year morphodynamic development with schematized wind-wave forcing. We analyze in detail the impact of locally generated wind-waves on shoal formation. Finally, we impose regional SLR of 1.10 m and 1.95 m for 100 years. Model results show that, on the spatial scale of intertidal flats, small, locally generated wind-waves lower and widen the shoals while the adjacent channels deepen. However, on the estuarine system scale, wind-waves do not lead to fundamentally different channel–shoal patterns and morphodynamic evolution trends. This suggests that channel–shoal formation is mainly due to tide residual sediment transports, with wind-waves playing a secondary role. SLR leads to a notable intertidal area loss, despite a continuous heightening of the shoals, implying that morphodynamic adaptation lags behind SLR. The inclusion of wind-waves does not fundamentally change the reaction of the estuarine shoal to SLR. Future research may focus on exploring the impact of including multiple sediment classes.

This paper describes the effects of anthropogenic deepening of tidal rivers in a conceptual way, with focus on tidal distortion and the residual transport of coarse sediment, driven by asymmetries in peak velocity. The rivers under consideration are fairly small, with small river discharge, and may have irregular hypsometry, with substantial intertidal area, or not. Residual sediment transport is driven by asymmetries in tidal velocity (horizontal tide), which is, however, difficult to establish in general. This paper discusses how and under which cases asymmetries in tidal elevations (vertical tide) can provide appropriate information on residual sediment transport. It is argued that deepening may induce a competition between an increase in tidal amplitude by amplification and a reduction in the asymmetry itself. Linear analysis shows that tidal asymmetry may show irregular behavior locally even for regular river configurations. It is therefore expected that these irregularities become larger in natural and engineered rivers. Analysis of local asymmetries may therefore be misleading in assessing the river's response to deepening with respect to the overall residual sediment transport and the river's morphology. Thus analysis of the overall morphodynamic response of a tidal river to tidal asymmetry, as affected by deepening, requires integration of the nonlinear effects along the entire river. It is argued that tidal asymmetry can be quantified by determining the difference in travel times of the high and low waters at any location within the river. This also implies that tidal water level variations and their asymmetries are governed by the entire tidal volume up-river of the cross section under consideration. River discharge further complicates the analyses by affecting residual water flows, effective hydraulic drag, tidal asymmetry, and mean water level. These effects reduce in response to deepening. However, salinity intrusion and gravitational circulation increase with deepening. We believe that assessing the (long-term) effects of deepening a fairway in a tidal river or estuary requires the use of process-based numerical models to account for all these nonlinear interactions, next to appropriate data collection. The current paper may help in analyzing and interpreting the numerical results.

Identifying the pattern of delta morphological change under decreasing sediment flux due to dam construction is essential for sustainable management in such densely populated coastal areas. In this study, we investigated the morphological processes of the Yangtze mouth bar and prodelta based on bathymetric data on a decadal-interannual scale (1958, 1978, 1997, 2002, 2007, 2010, 2013 and 2015). We found that strong accretion (205.1 Mm3 yr−1) occurred during 1958–1978, when a high sediment load (465 Mt yr−1) was supplied by the Yangtze. Afterwards, the net accumulation rate decreased to 31.9 Mm3 yr−1 in 1978–1997 and 114.6 Mm3 yr−1 in 1997–2002 as a result of riverine sediment loads decreasing to 390 Mt yr−1 and 314 Mt yr−1, respectively. Surprisingly, the net accumulation rate increased to 130.8 Mm3 yr−1 in 2002–2007, though the sediment load sharply decreased to 177 Mt yr−1. This anomaly was attributed to the construction of training walls within the mouth bar area, which induced significant accretion in groyne-sheltered areas and nearby regions. Along with a further decrease in sediment load, the entire study area converted to net erosion of −200.4 Mm3 yr−1 in 2007–2010 and −152.2 Mm3 yr−1 in 2010–2013. Stronger erosion in the former period was partly caused by intensive dredging activities in the mouth bar area. The critical sediment discharge for the Yangtze mouth bar and prodelta to retain net accretion was estimated to be ca. 218 Mt yr−1. If deducting the impacts of estuarine engineering projects on accretion/erosion during 1997–2010, the critical sediment discharge is adjusted to ca. 234 Mt yr−1. In combination with previously reported accretion-erosion conversion elsewhere in the Yangtze Delta, we inferred that most portion of the subaqueous delta has most likely converted from net accretion to net erosion in response to fluvial sediment decline, and the mouth bar area showed the latest conversion among portions of the delta. Integrated assessment and adaptive strategies are urgently required for the Yangtze Delta to survive the coming erosional stage.

Tidal waves traveling into estuaries are modified by channel geometry and river flow. The damping effect of river flow on incident astronomical tides is well documented, whereas its impact on low‐frequency tides like MSf and Mm is poorly understood. In this contribution, we employ a numerical model to explore low‐frequency tidal behavior under varying river flow. MSf and Mm are locally generated by frictional mechanisms inside an estuary, and they are larger in amplitude far upstream in tidal rivers and persist landward of the point of tidal extinction. Increasing river flow nonlinearly modulates the longitudinal variations of MSf and Mm amplitudes. This is dynamically explained by flow‐enhanced asymmetry in subtidal friction over the spring‐neap (MSf) and perigee‐apogee (Mm) cycles, respectively. Estuaries act as frequency filters, where low‐frequency waves decay at a smaller rate and propagate more inland than high‐frequency waves. Strong inland penetration of low‐frequency tides informs compound flood management.

China's continuous and rapid economic growth has led to the reclamation of large sections of the intertidal mud coast in combination with port construction, such as that of the proposed Tongzhou Bay port on the Jiangsu coast. These reclamations threaten the local ecosystem services. An ecotope distribution map was created and a hydrodynamic numerical model of Tongzhou Bay was set up to quantify the impacts of reclamation on the ecosystem. Based on the field data and model results, several abiotic features were classified into 11 ecotopes and visualized in an ecotope map of the Tongzhou Bay ecosystem. Validation with spatial distributions of two threatened shorebird species (bar-tailed godwit and great knot) showed confirmation with the mid-range and low-range littoral zones (inundated from 40% to 100% of a tidal cycle), indicating the importance of the areas with these conditions to these populations. Overlaying the ecotope map with recent and proposed land reclamation schemes revealed a loss of ecotopes, composed of the high-range (42%), mid-range (48%), and low-range (38%) littoral habitats, corresponding to a 44%–45% loss of the most important ecotopes for bar-tailed godwit and great knot (mid-range and low-range littoral zones). These results confirm the applicability of the novel ecotope assessment approach in practice.

The fast economic development of the People&rsquo, s Republic of China has created an increasing demand for usable land, resulting in large-scale land reclamations along the coastal zone. One of these regions is Tongzhou Bay (Jiangsu coast), a region characterized by large intertidal mudflats and deep tidal channels with potential for the development of agri-aquaculture and the construction of a deep-sea port. However, these intertidal mudflats also provide vital ecosystem services and support many wildlife species, including several endangered migratory shorebirds within the East Asian&ndash, Australasian Flyway. With increasing realization of the importance of maintaining such ecological values, a more integrated coastal development strategy is needed. This study aims to develop a sustainable integrated design for the Tongzhou Bay port, following a &ldquo, Building with Nature&rdquo, approach. We use a morphodynamic model to compute habitat suitability for two shorebird species (Great Knot Calidris tenuirostris and Bar-tailed Godwit Limosa lapponica). Several port configurations were developed on the basis of three design criteria: (1) create area for future port development, whilst (2) preserving existing high-value ecotopes for shorebirds and (3) enhance the natural accretion rate of such ecotopes. Simulation results showed a clear difference in siltation patterns, preservation and enhancement of preferred ecotopes. This work therefore demonstrates the potential and importance of morphological and habitat suitability modelling when designing large-scale reclamations and port constructions, especially in dynamic areas such as Tongzhou Bay.

Hydrodynamic forces on intertidal flats vary over a range of temporal and spatial scales. These spatiotemporal inhomogeneities have implications for intertidal flat morphodynamics and ecology. We determine whether storm events are capable of altering the long-term morphological evolution of intertidal flats, and unravel the contributions of tidal flow, wind-driven flow, waves, and water depth on inhomogeneities in bed level dynamics (bed level changes over ~days) across these areas. We complement decades of bed level measurements on eight intertidal flats in two estuaries in the Netherlands with an extensive 1-month field campaign on one of those flats. Across this intertidal flat, the hydrodynamics and morphodynamics of a storm event were captured, including the post-storm recovery. We show that individual events can persistently alter the morphological evolution of intertidal flats; magnitudes of some bed level changes are even comparable to years of continuous evolution. The morphological impacts of events are largely controlled by the relative timing of the forcing processes, and not solely by their magnitudes. Spatiotemporal variations in bed level dynamics of intertidal flats are driven by a combination of: (1) the inhomogeneous distributions of the hydrodynamic forcing processes (including the under-explored role of the wind); and (2) the linear proportionality between bed level dynamics and the local bed slope.

River longitudinal profile, a key morphological characteristic of the river channel, is subject to river mouth progradation. Given the increasing influence of human activities and climate change on this critical downstream control, understanding its effects on the evolution of the longitudinal profile is imperative. A general theoretical framework is proposed to quantify the relevant effects, which is tested by numerical experiment and compared with field, numerical and laboratory data from the literature. The results suggest the existence of a critical ratio of accommodation space to sediment supply of approximately 0.5, above which the typical concave upward profile tends to form. Further analyses show that sea level rise tends to increase the concavity of the longitudinal profile of a river with a relatively low equilibrium bed slope and progradation rate.

Connectivity provides a framework for analyzing coastal sediment transport pathways, building on conceptual advances in graph theory from other scientific disciplines. Connectivity schematizes sediment pathways as a directed graph (i.e., a set of nodes and links). This study presents a novel application of graph theory and connectivity metrics like modularity and centrality to coastal sediment dynamics, exemplified here using Ameland Inlet in the Netherlands. We divide the study site into geomorphic cells (i.e., nodes) and then quantify sediment transport between these cells (i.e., links) using a numerical model. The system of cells and fluxes between them is then schematized in a network described by an adjacency matrix. Network metrics like link density, asymmetry, and modularity quantify system-wide connectivity. The degree, strength, and centrality of individual nodes identify key locations and pathways throughout the system. For instance, these metrics indicate that under strictly tidal forcing, sand originating near shore predominantly bypasses Ameland Inlet via the inlet channels, whereas sand on the deeper foreshore mainly bypasses the inlet via the outer delta shoals. Connectivity analysis can also inform practical management decisions about where to place sand nourishments, the fate of nourishment sand, or how to monitor locations vulnerable to perturbations. There are still open challenges associated with quantifying connectivity at varying space and time scales and the development of connectivity metrics specific to coastal systems. Nonetheless, connectivity provides a promising technique for predicting the response of our coasts to climate change and the human adaptations it provokes.

It is widely recognized that waves inhibit river mouth progradation and reduce the avulsiontimescale of deltaic channels. Nevertheless, those effects may not apply to downdrift‐deflected channels. In this study, we developed a coupled model to explore the effects of wave climate asymmetry and alongshore sediment bypassing on shoreline‐channel morphodynamics. The shoreline position and channel trajectory are simulated using a “shoreline” module which drives the evolution of the river profile in a “channel” module by updating the position of river mouth boundary, whereas the channel module provides the sediment load to river mouth for the “shoreline” module. The numerical results show that regional alongshore sediment transport driven by an asymmetric wave climate can enhance the progradation of deltaic channels if sediment bypassing of the river mouth is limited, which is different from the common assumption that waves inhibit delta progradation. As such, waves can have a trade‐off effect on river mouth progradation that can further influence riverbed aggradation and channel avulsion. This trade‐off effect of waves is dictated by the net alongshore sediment transport, sediment bypassing at the river mouth, and wave diffusivity. Based on the numerical results, we further propose a dimensionless parameter that includes fluvial and alongshore sediment supply relative to wave diffusivity to predict the progradation and aggradation rates and avulsion timescale of deltaic channels. The improved understanding of progradation, aggradation, and avulsion timescale of deltaic channels has important implications forengineering and predicting deltaic wetland creation, particularly under changing water and sediment input to deltaic systems.

Dredging of navigation channels in estuaries affects estuarine morphology and ecosystems. In the Western Scheldt, a two-channel estuary in the Netherlands, the navigation channel is deepened and the sediment is relocated to other parts of the estuary. We analyzed the response of an intertidal flat to sediment disposals in its adjacent channel. Decades of high-frequency monitoring data from the intertidal flat show a shift from erosion toward accretion and reveal a sequence of cascading eco-morphological consequences. We document significant morphological changes not only at the disposal sites, but also at the nearby intertidal flats. Disposals influence channel bank migration, driving changes in the evolution of the intertidal flat hydrodynamics, morphology, and grain sizes. The analyzed disposals related to an expansion of the channel bank, an increase in bed level of the intertidal flat, a decrease in flow velocities on this higher elevated flat, and locally a decrease in grain sizes. These changes in turn affect intertidal flat benthic communities (increased in quantity in this case) and the evolution of the adjacent salt marsh (retreated less or even expanded in this case). The shifts in evolution may occur years after dredged disposal begins, especially in zones of the flats farther away from the disposal locations. The consequences of sediment disposals that we identify stress the urgency of managing such interventions with integrated strategies on a system scale.

Morphodynamics of world's river deltas are increasingly affected by human activities, which are of great ecological, economic and social implications. However, impacts of human interventions in deltaic regions are insufficientlyunderstood, especially superimposed upon diminishing sediment supplies. This study uses the heavily interfered Yangtze River delta as an example to address this issue. The morphodynamic impacts of the Deepwater Navigation Channel Project (DNCP) during 1997–2013 are investigated through process-basedmodeling approach (Delft3D) and bathymetric data analysis. The DNCP was implemented in the mouth bar area of the Yangtze River delta including the twin dikes and 19 groynes with the total length of 132.0 km. Hydrodynamic simulations indicate that the training walls resulted in weaker tidal flow and longer slack period at the East Hengsha Shoal (EHS) and stronger tidal flow at the subaqueous delta. Thus, the EHS is characterized as a sediment accumulation zone after the completion of the training walls. Subsequently, morphologicalmodeling shows enhanced accretion at the EHS and enhanced erosion at the subaqueous delta when the training walls are taken into account. Numerical experiments further demonstrate that the above changes are mainly attributed to the seaward half of the northern training walls constructed in 2002–2005. This is probably the reason for the observed accretion peak of the EHS in 2002–2007 and the gradual increase in the erosion rate of the subaqueous delta after 2002. The schematized paths of sediment transport after the DNCP indicate thatsediment eroded from the subaqueous delta serves as an important source for accretion of the mouth bar area. It is suggested that siltation promoting projects within the mouth bar area increased shallow shoal accretion and aggravated erosion at the subaqueous delta. With the overall erosion of the Yangtze River delta due to river sediment reduction, large-scale estuarine engineering projects substantially increase the complicacy of its morphodynamic pattern, which merits close attention for sustainable delta management.

The Wadden Sea is a unique coastal wetland containing an uninterrupted stretch of tidal flats that span a distance of nearly 500km along the North Sea coast from the Netherlands to Denmark. The development of this system is under pressure of climate change and especially the associated acceleration in sea-level rise (SLR). Sustainable management of the system to ensure safety against flooding of the hinterland, to protect the environmental value and to optimise the economic activities in the area requires predictions of the future morphological development.The Dutch Wadden Sea has been accreting by importing sediment from the ebb-tidal deltas and the North Sea coasts of the barrier islands. The average accretion rate since 1926 has been higher than that of the local relative SLR. The large sediment imports are predominantly caused by the damming of the Zuiderzee and Lauwerszee rather than due to response to this rise in sea level. The intertidal flats in all tidal basins increased in height to compensate for SLR.The barrier islands, the ebb-tidal deltas and the tidal basins that comprise tidal channels and flats together form a sediment-sharing system. The residual sediment transport between a tidal basin and its ebb-tidal delta through the tidal inlet is influenced by different processes and mechanisms. In the Dutch Wadden Sea, residual flow, tidal asymmetry and dispersion are dominant. The interaction between tidal channels and tidal flats is governed by both tides and waves. The height of the tidal flats is the result of the balance between sand supply by the tide and resuspension by waves.At present, long-term modelling for evaluating the effects of accelerated SLR mainly relies on aggregated models. These models are used to evaluate the maximum rates of sediment import into the tidal basins in the Dutch Wadden Sea. These maximum rates are compared to the combined scenarios of SLR and extraction-induced subsidence, in order to explore the future state of the Dutch Wadden Sea.For the near future, up to 2030, the effect of accelerated SLR will be limited and hardly noticeable. Over the long term, by the year 2100, the effect depends on the SLR scenarios. According to the low-end scenario, there will be hardly any effect due to SLR until 2100, whereas according to the high-end scenario the effect will be noticeable already in 2050.

Chu A.; Wang Z.B.; de Vriend H.J., and Tai J.A., 2018. Parallel morphodynamic modelling for the Yangtze Estuary. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 641–645. Coconut Creek (Florida), ISSN 0749-0208. Model input reduction and morphological acceleration techniques to bridge the large time scale of morphological evolution and the small time scale of hydrodynamically driven sediment transport is investigated for the Yangtze Estuary morphodynamic model. The discharge series of the river is schematized into a set of multi-discharge levels. Each discharge level coupled with the spring-neap tide forms one morphodynamic model for the estuary with bed stratigraphy approach applied. These models are used to simulate the bathymetry change of the estuary in parallel with the results merged together base on the Mor-Merge (MM) method. The morphological f...

Many alluvial estuaries worldwide include an inside bar system, a large sediment deposit deeply stretched into the estuary. A good example of such a system is the large sediment deposit in the Qiantang Estuary, China. Its length and height reach 130 km and 10 m, respectively. Bathymetrical comparison reveals that the large bar has moved seaward by around 15 km over the last decades, probably related to the large-scale coastal embankment project. This motivated a quantitative investigation of the impact of estuarine planform on the inside bar development. The bar morphology is reproduced by means of an idealized 1-D morphodynamic model. Model results suggest that the bar movement is related to a decreasing tidal prism, increasing flood dominance in the lower reach and enhanced ebb currents in the upper reach, in response to the embankment. The timescale of the morphological response is only several years. The rapid response is related to the strong tidal currents and large sediment fluxes within the estuary. Sensitivity experiments show that the location and dimensions of the bar are related to the convergence length of the estuary. A decrease of the convergence length causes seaward movement and shortening and lowering of the bar. The bar dimensions also depend on the ratio between river and tidal discharges. When the ratio increases, the bar apex moves seaward and the elevation decreases. The bar movement has significantly influenced the tidal bore in the Qiantang Estuary.

The variation of the floc population in the Changjiang Estuary has been studied for both winter and summer season as a function of the presence of living (micro)algae. The influence of algae has been characterized through the use of the chlorophyll-a concentration to suspended sediment concentration (CC/SSC) ratio. Two whole tidal cycle sampling campaigns were carried out and a full set of parameters (particle size distribution, particle concentration, salinity, velocities, chlorophyll-a concentration) was recorded as function of time for 6 vertical depths. It is found that the floc population can be described by three particle classes. The two most dynamic classes (microflocs and macroflocs) co-exist in the water column. It was nonetheless found, due to the correlation between CC/SSC and particle sizes that the system is at steady state, both in summer and in winter. This can be explained by the limited flocculation ability between the classes due to their segregation in the water column. In winter, macroflocs are found at the top of the water column but their amount and size are very reduced with a mean CC/SSC value of 13 ± 11 μg g−1. In summer, algae-rich macroflocs are abundant at the top of the water column with a mean CC/SSC value of 21 ± 18 μg g−1, especially at flood tide. Microflocs, on the other hand, have a higher density and are generally found deeper in the water column. At high water slack, both macroflocs and microflocs will settle but will never catch-up. The fact that the flocs are at steady-state in terms of flocculation is of importance for sediment transport modelling.

We investigate the morphological responses of the Hangzhou Bay, China, located immediately south of the Changjiang Estuary, to the drastic reduction of the sediment load from the Changjiang River and the large-scale coastal embankment schemes over past decades. The spatial patterns of deposition and erosion, sediment volume changes, and the hydrodynamic and sediment dynamic feedback were analyzed, on the basis of historical bathymetric and hydrographic data. The results show that the sedimentation rates in the bay have generally increased rather than decreased over the past decades, despite bed erosion having occurred in the northern bay-mouth. This observation reveals that the influence of the reduction in the Changjiang River sediment supply on the morphological evolution of Hangzhou Bay has to date been insignificant, mainly due to the buffering effect of existing sediment in the outer Changjiang Estuary. The morphological change is mainly related to the implementation process of the coastal embankment. Sediment accumulation induced by progressive seaward coastal embankment has resulted in seaward aggradation from the Qiantang Estuary towards Hangzhou Bay. Analysis of the annually-averaged high and low tidal levels, and durations of rising and falling tides reveals that flood dominance in the inner bay has been increased, due to the coastal embankment and sediment accumulation. The ratio between annually-averaged rising tide and falling tide durations have decreased from 0.85 to 0.63. The tidal prism at the interface between the inner and outer bay has decreased by about 25% since the 1980s, while the net landward sediment flux has been intensified to a certain extent, which is responsible for the intensifying sedimentation in the inner bay. The local human activities have overwhelm the decreased sediment from the Changjiang River. Although the coastal embankment will cease in the near future, the morphological response to human activities is expected to continue on for a longer time.

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. River deltas are shaped by interactions between fluvial and tidal processes. Tides act to stabilize delta morphology, but sediment depletion due to human activities disrupts the balance and leads to erosion and scour.

Understanding the decadal morphodynamic evolution of estuaries and deltas and their controls is of vital importance regarding management for estuarine function and sustainable development. This work addresses this issue by applying a process-based model system (Delft3D) to hindcast and then forecast the morphodynamic evolution of the Yangtze Estuary at a decadal timescale. Forced by the river and tides, the model considers sand-mud mixture and the variations of river water discharge and sediment discharge. The morphodynamic model is validated against three periods, i.e., an accretion period (1958–1978), an erosion period (1986–1997) and a recent accretion period with human activities (2002 − 2010). Model results show good performance with respect to spatial erosion and deposition patterns, sediment volume changes, and hypsometry curves. The model reveals quite different behaviors for mud transport between the dry and wet seasons, which is subject to the prescription of river boundary conditions and bed composition. We define six scenarios to project evolution to the year 2030 under decreased river inputs and increased relative sea level. The simulations reveal that overwhelming amount of erosion will likely occur in the inner and mouth bar area of the estuary. Particularly, the mouth zone will shift from net deposition before 2010 to net erosion by 2030, mainly because of decreasing sediment supply. Changes in water discharge have minor effects on the projected trend. Net erosion will be considerable when the sediment supply is extremely low (100 Mt yr − 1 ) due to the abundance of erodible modern sediment in the Yangtze Estuary. Erosion within the mouth bar area may be unexpected, including the deepening of the tidal inlet at East Chongming mudflat and the formation of a flood channel on the seaward side of Jiuduansha Shoal. Overall, the model results provide valuable information for sustainable delta management under changing conditions for both the Yangtze system and other similar estuaries and deltas with diminishing sediment supplies.

We investigate the evolution of a large-scale sand body, a unique type of sandbars in a convergent estuary. Specifically, we analyze and simulate the sand deposition system (defined as an inside bar) in the Qiantang Estuary (QE) in China. The deposit is 130 km long and up to 10 m thick, and is characterized by a dextral morphology in the lower QE. Numerical simulation is carried out using an idealized horizontal 2-D morphodynamic model mimicking the present QE settings. Our results indicate that the morphological evolution is controlled by the combination of river discharge and tides. The seasonal and inter-annual cycles of river discharges play a major role on the inside bar evolution. The bar is eroding during high river discharge periods but accretion prevails during low river discharge periods. Meanwhile, the highest part of the sand body can move downstream or upstream by several kilometers, modifying the seasonal sediment exchange patterns. We also show that the Coriolis force plays an important role on the dextral morphology patterns in wide, convergent estuaries. It induces a significant lateral water level difference and a large-scale gyre of residual sediment transport. Subsequently, the seaward tail of the inside bar shifts southward to help create a condition for the development of tidal flats in the lower reach of the estuary. The lateral bed level differences induced by Coriolis force are up to several meters. Coriolis effects also modify the behavior of flood and ebb tidal channels.

The research objective is to investigate the variations of sediment transport in outer Changjiang Estuary and adjacent Hangzhou Bay, induced by the decline of Changjiang River sediment discharge and massive land reclamation in the last three decades. A synchronous hydrographic survey was conducted along two transects (at the bay-mouth and outer Changjiang Estuary, respectively) during the spring-neap tides of January and July 2014. The results show that the suspended sediment grain size, current velocity, suspended sediment concentration (SSC), and the water and sediment fluxes varied with the tidal cycles. Quantitative correlations with the tidal range were found for SSC and fluxes. These data have been compared with those at the same hydrographic stations in the summer and winter of the early 1980s. Along the outer Changjiang Estuary transect, the SSCs and sediment fluxes decreased in the winter, but no apparent changes occurred in the summer. The SSCs in the northern Hangzhou Bay decreased in both summer and winter, while the southern bay mouth has evolved from a low SSC region to a high SSC region. The findings clarify that the SSC and sediment flux changes in this area have only an indirect connection to the dramatic riverine sediment decline, because the sediment resuspension by the strong tidal currents provided a major source. At the present stage, the impact of the riverine sediment decline is insignificant for the SSC variation off the Changjiang River mouth. Finally, a sediment flux model is proposed to explain and predict the morphological evolution trends.

Human interventions have a large impact on estuarine morphology. The intertidal flats in the Eastern Scheldt and Western Scheldt estuaries (The Netherlands) have faced substantial morphological changes over the past decades. These changes are thought to be caused by human interventions, such as the construction of the storm surge barrier in the mouth of the Eastern Scheldt, and the deepening of the navigation channels of the Western Scheldt. This paper analyses several datasets and numerical simulations of hydrodynamics, providing an overview of the various morphological characteristics of the intertidal flats in the two estuaries over time and space. Apart from the volume, area and average height of these areas, also the integral steepness of each flat is quantified based on its full geometry. The analyses focus on the intertidal flats surrounded by water, which allows for a robust comparison between the different flats. The intertidal flats in the Western Scheldt appear to be substantially steeper compared to those in the Eastern Scheldt. The data indicates that a larger average height of a flat is related to a larger steepness. Despite variations in the evolution of the different flats, distinct characteristics of both estuaries are observed. An opposed trend is identified over time: the flats in the Western Scheldt have mainly increased in height, whereas the flats in the Eastern Scheldt have lowered after the completion of the storm surge barrier. This opposing development is associated with differences in tidal flow velocities in the estuaries, which are the result of human interventions.

Climate change, and especially the associated acceleration of sea-level rise, forms a serious threat to the Wadden Sea. The Wadden Sea contains the world&rsquo, s largest coherent intertidal flat area and it is known that these flats can drown when the rate of sea-level rise exceeds a critical limit. As a result, the intertidal flats would then be permanently inundated, seriously affecting the ecological functioning of the system. The determination of this critical limit and the modelling of the transient process of how a tidal basin responds to accelerated sea-level rise is of critical importance. In this contribution we revisit the modelling of the response of the Wadden Sea tidal basins to sea-level rise using a basin scale morphological model (aggregated scale morphological interaction between tidal basin and adjacent coast, ASMITA). Analysis using this aggregated scale model shows that the critical rate of sea-level rise is not merely influenced by the morphological equilibrium and the morphological time scale, but also depends on the grain size distribution of sediment in the tidal inlet system. As sea-level rises, there is a lag in the morphological response, which means that the basin will be deeper than the systems morphological equilibrium. However, so long as the rate of sea-level rise is constant and below a critical limit, this offset becomes constant and a dynamic equilibrium is established. This equilibrium deviation as well as the time needed to achieve the dynamic equilibrium increase non-linearly with increasing rates of sea-level rise. As a result, the response of a tidal basin to relatively fast sea-level rise is similar, no matter if the sea-level rise rate is just below, equal or above the critical limit. A tidal basin will experience a long process of &lsquo, drowning&rsquo, when sea-level rise rate exceeds about 80% of the critical limit. The insights from the present study can be used to improve morphodynamic modelling of tidal basin response to accelerating sea-level rise and are useful for sustainable management of tidal inlet systems.

We use the results of a one-dimensional morphodynamic model and the basis of the “Lagrangian equilibrium state” (Maan et al., 2015, https://doi.org/10.1002/2014JF003311) to derive a quantitative relationship between the progradation speed of tidal flats and the suspended sediment concentration in their adjacent waters and show that the speed increases more than linearly with the concentration. We also show that horizontally prograding flats rise vertically with sea level rise at the expense of their horizontal speed via a linear relationship. If accretion rates are insufficient to keep up with sea level rise, however, the intertidal flat submerges and retreats landward at the same time. We apply the obtained relationships to the Yangtze Estuary to estimate the critical sediment concentration level below which a shift from progradation to retreat can be expected.

The records of HW and LW in the most upper part of the Scheldt Estuary since 1971 have been analysed together with the daily river discharge. The tidal range, the hydraulic head and the ratio between the rising tide period to falling tide period have been determined for investigating the tidal amplification, the water surface slope along the river and the tidal asymmetry. The purpose of the investigation is to find out if a regime shift to high turbidity and strong tidal amplification is developing in the system. The results of the analysis show that both the hydraulic head and the tidal amplification have increased over time. The tide in this part of the estuary is flood-dominant, but the flood-dominancy is decreasing in time. These developments of the tide can be plausibly explained by a decrease of the river width followed by gradual deepening in the river. The most upper part of the Scheldt Estuary still behaves normally as a not too muddy system. No decisive answer can be given whether or not a regime shift towards a high-turbid system with strong tidal amplification may develop. How the system will develop depends on the change in capacity of pumping mud towards the upper reaches of the estuary. The increasing tidal amplitude can enhance this capacity although the flood-dominance itself is decreasing. Further study is recommended to better specify the potential danger of a regime shift to high-turbid system.

Tidal wave deformation and tidal asymmetry widely occur in tidal estuaries and lagoons. Tidal asymmetry has been intensively studied because of its controlling role on residual sediment transport and large‐scale morphological evolution. There are several methods available to characterize tidal asymmetry prompting the need for an overview of their applicability and shortcomings. In this work we provide a brief review and evaluation of two methods, namely, the harmonic method and the statistical method. The latter comprises several statistical measures that estimate the probability density function and various forms of skewness. We find that both the harmonic and statistical methods are effective and have complementary advantages. The harmonic method is applicable to predominantly semidiurnal or diurnal regimes, while the statistical methods can be used in mixed tidal regimes. Assisted by harmonic data, a modified skewness measure can isolate the contribution of different tidal interactions on net tidal asymmetry and also reveal its subtidal variations. The application of the skewness measure to nonstationary river tides reveals stronger tidal asymmetry during spring tides than neap tides, and the nonlinear effects of river discharges on tidal asymmetry in the upper and lower regions of long estuaries.

Sustainable management of barrier islands and tidal inlet systems requires a knowledge of sediment transport pathways throughout the system. This paper places in situ suspended sediment observations (obtained using a LISST) in context with seabed sediment samples and hydrodynamic measurements to identify such pathways. The results indicate two distinct populations of sediment in suspension on the ebb-tidal delta: locally resuspended fine sand and (largely flocculated) mud exported from the Wadden Sea on ebb tide. This reinforces the notion of the strong dependence of sediment pathways on particle size. Future work will combine additional lines of evidence to better distinguish suspended sand from sand-sized flocs and provide a more robust definition of these pathways.

The morphology of the Yangtze Estuary has changed substantially at decadal time scales in response to natural processes, local human interference and reduced sediment supply. Due to its high sediment load, the morphodynamic response time of the estuary is short, providing a valuable semi-natural system to evaluate large-scale estuarine morphodynamic responses to interference. Previous studies primarily addressed local morphologic changes within the estuary, but since an overall sediment balance is missing, it remains unclear whether the estuary as a whole has shifted from sedimentation to erosion in response to reduced riverine sediment supply (e.g. resulting from construction of the Three Gorges Dam). In this paper we examine the morphological changes of two large shoals in the mouth zone (i.e. the Hengsha flat and the Jiuduan shoal) using bathymetric data collected between 1953 and 2016 and a series of satellite images. We observe that the two shoals accreted at different rates before 2010 but reverted to erosion thereafter. Human activities such as dredging and dumping contribute to erosion, masking the impacts of sediment source reduction. The effects of local human intervention (such as the construction of a navigation channel) are instantaneous and are likely to have already resulted in new dynamic equilibrium conditions. The morphodynamic response time of the mouth zone to riverine sediment decrease is further suggested to be >30 years (starting from the mid-1980s). Accounting for the different adaptation time scales of various human activities is essential when interpreting morphodynamic changes in large-scale estuaries and deltas.

Rivers, regardless of their scales and geographic locations, are characterized with natural and human-induced variability in their discharges. While previous studies have established the effects of both interannual and intra-annual variabilities of unsteady river discharge on delta morphological evolution, the long-term cumulative effects of intra-annual unsteadiness on the progradation of delta lobes has remained hitherto elusive. To address this issue, numerical experiments using simplified unsteady discharges were performed in Delft3D and compared with those assuming constant bank-full discharges. A modified box model was further used to explore the effects of varying intra-annual unsteadiness on the progradation of delta lobes at reduced computational cost. While the overall trends of the progradation and the ultimate area created were found to be similar between the unsteady discharge scenarios and their corresponding constant bank-full discharge scenarios, the nuances of intermittent zig-zag variation in natural delta lobe area were well reproduced by model simulations assuming unsteady river discharges. In addition, long-term predictions suggested the potential existence of a tipping point in the area growth trajectory beyond which the delta lobe area declines during periods of low discharge. When confounding factors such as waves and variable sediment capture ratio were further taken into consideration, simulation results for unsteady river discharge scenarios exhibit significant deviations from constant bank-full discharge scenarios. The implications of the modeling results for delta protection and restoration measures, such as the water-sediment regulation scheme in the Yellow River and artificial channel diversions in the Mississippi River Delta, are also discussed.

A key problem in the decadal morphodynamic modelling of tidal inlet system is on the evolution of the inlet channels, especially the unrealistic channel incision. The present study attempts to couple two processes, including the bedform roughness and sediment grain-size sorting, in the two-dimensional, depth-averaged process-based model to solve the problem. Based on the comparison of the modelling results and relevant field observations in the Dutch Wadden Sea, it is suggested that bedform roughness height predictor is a useful tool for determining spatial and temporal heterogeneous bed frictions. Within tidal channels, bedform roughness is dominated by dune roughness. In order to represent “the memory of old bedforms”, a relaxation time method was implemented. The corresponding large relaxation time (2 M2 tidal periods) smooths the tidal variations of dune roughness, and makes it respond to hydrodynamic changes on a longer time scale. When decadal morphological modelling is carried out, the model performance can be significantly improved by introducing either bedform (dune) roughness predictor or sediment sorting processes. The mechanisms are twofold. First, in the deep channels where flow velocities are high, large dunes develop and cause large bed drag coefficients. Second, coarser bed sediments within the channel area induced by sorting prevent the channel incision. If both effects are coupled in the morphodynamic modelling, the inlet channel incision is further controlled, which is not just a result of the linear superposition of both effects, because sorting can further promote the development of dunes within the deep channels.

The Yangtze Estuary in China has been intensively influenced by human activities including altered river and sediment discharges in its catchment and local engineering projects in the estuary over the past half century. River sediment discharge has significantly decreased since the 1980s because of upstream dam construction and water-soil conservation. We analyzed bathymetric data from the Yangtze Estuary between 1958 and 2010 and divided the entire estuary into two sections: inner estuary and mouth bar area. The deposition and erosion pattern exhibited strong temporal and spatial variations. The inner estuary and mouth bar area underwent different changes. The inner estuary was altered from sedimentation to erosion primarily at an intermediate depth (5–15 m) along with river sediment decline. In contrast, the mouth bar area showed continued accretion throughout the study period. The frequent river floods during the 1990s and simultaneously decreasing river sediment probably induced the peak erosion of the inner estuary in 1986–1997. We conclude that both sediment discharge and river flood events played important roles in the decadal morphological evolution of the Yangtze Estuary. Regarding the dredged sediment, the highest net accretion rate occurred in the North Passage where jetties and groins were constructed to regulate the navigation channel in 1997–2010. In this period, the jetties induced enhanced deposition at the East Hengsha Mudflat and the high accretion rate within the mouth bar area was maintained. The impacts of estuarine engineering projects on morphological change extended beyond their sites.

Natural sediment sinks have been removed in many estuaries and tidal basins, and the resulting loss in accommodation space likely led to an increase in the suspended sediment concentration (SSC). The effect of land reclamations and changes in other sediment sinks on SSC has been investigated using the Ems Estuary as a well-documented example. Coastline reconstructions and stratigraphic observations in the Ems Estuary, on the Dutch–German border, have been used to quantify the long-term (centennial) changes in sediment sinks. These data indicate that approximately 2–3 · 10 6 dry weight tons of fine-grained sediments annually accumulated in the estuary from the end of the 16th century up to the beginning of the 20th century. In the 20th century and especially from 1960 to 1994, large amounts of sediment were dredged and removed from the Ems Estuary. Since the 1990s, sediment removal rates through dredging activities sharply decreased, while available monitoring data show that on average SSC in the estuary increased during this period. The potential impact of sediment sinks on SSC has been investigated using a simple mass balance and a complex numerical model. Both methodologies indicate that the reduction in sediment sinks lead to an increase in suspended sediment concentration of several 10s of mg/l averaged over the estuary. The numerical model also resolves the spatial variation in changing suspended sediment concentrations, suggesting that locally the increase may exceed 100 mg/l. This work reveals that a decrease in accommodation space for fine-grained sediments by land reclamations or by reducing fine-grained sediment extraction will lead to an increase in suspended sediment concentrations, and provides methodologies to estimate the impact of interventions on SSC of estuaries and tidal basins.

Tidal asymmetry is an important mechanism generating tidal residual sediment transport (TRST) in tidal environments. So far, it is known that a number of tidal interactions (e.g., M2-M4 and M2-O1-K1) can induce tidal asymmetry and associated TRST; however, their variability and morphodynamic impacts are insufficiently explored. Inspired by the river and tidal forcing conditions in the Yangtze River Estuary, we explore the morphodynamic development of a 560 km long estuary under the boundary forcing conditions of varyingly combined tidal constituents and river discharges using a schematized 1-D morphodynamic model for long-term (millennial) simulations. We then employ an analytical scheme which integrates sediment transport as a function of flow velocities to decompose the contribution of different tidal interactions on TRST and to explain how the river and tidal interactions control TRST and associated morphodynamics. Model results display varying equilibrium bed profiles. Analytical results suggest that (1) a series of tidal interactions creates multiple tidal asymmetries and associated TRST, (2) river flow modulates tidal asymmetry nonlinearly in space, and (3) more tidal constituents at the sea boundary persistently enhance the seaward TRST through river-tide interactions. It is the combined effects of multiple tidal asymmetries and river-tide interactions that determine the net TRST and consequent morphodynamic development. It thus suggests that tidal harmonics of significant amplitudes need to be considered properly as boundary conditions for long-term, large-scale morphodynamic modeling.

Examination of large scale, alluvial estuarine morphology and associated time evolution is of particular importance regarding management of channel navigability, ecosystem, etc. In this work, we analyze morphological evolution and changes of the channel-shoal system in the Changjiang Estuary, a river- and tide-controlled coastal plain estuary, based on bathymetric data between 1958 and 2016. We see that its channel-shoal pattern is featured by meandering and bifurcated channels persisting over decades. In the vertical direction, hypsometry curves show that the sand bars and shoals are continuously accreted while the deep channels are eroded, leading to narrower and deeper estuarine channels. Intensive human activities in terms of reclamation, embankment, and dredging play a profound role in controlling the decadal morphological evolution by stabilizing coastlines and narrowing channels. Even though, the present Changjiang Estuary is still a pretty wide and shallow system with channel width-to-depth ratios >1000, much larger than usual fluvial rivers and small estuaries. In-depth analysis suggests that the Changjiang Estuary as a whole exhibited an overall deposition trend over 59 years, i.e., a net deposition volume of 8.3 × 108 m3. Spatially, the pan-South Branch was net eroded by 9.7 × 108 m3 whereas the mouth bar zone was net deposited by 18 × 108 m3, suggesting that the mouth bar zone is a major sediment sink. Over time there is no directional deposition or erosion trend in the interval though riverine sediment supply has decreased by 2/3 since the mid-1980s. We infer that the pan-South Branch is more fluvial-controlled therefore its morphology responds to riverine sediment load reduction fast while the mouth bar zone is more controlled by both river and tides that its morphological response lags to riverine sediment supply changes at a time scale >10 years, which is an issue largely ignored in previous studies. We argue that the time lag effect needs particular consideration in projecting future estuarine morphological changes under a low sediment supply regime and sea-level rise. Overall, the findings in this work can have implications on management of estuarine ecosystem, navigation channel and coastal flooding in general.

The above-ground biomass of Spartina alterniflora salt marsh meadows is influenced by numerous interacting factors, among them elevation, tidal range and inundation duration. Bio-geomorphological models make use of either linear or quadratic equations, but it is important to be aware that the variables are area specific and hence not generic. In order to explore the vegetation growth pattern and its influencing factors along the Jiangsu coast, China, field surveys were conducted in two typical S. alterniflora marshes along the coast of Dafeng and Rudong. To combine the influence of elevation and the effect of tidal range, the inundation ratio (IR) is introduced as a novel parameter, which is the ratio between inundation duration and the duration of the whole tidal period concerned. The relationship between above-ground biomass and IR can be expressed by a quadratic equation. The optimal inundation ratio for S. alterniflora along the Jiangsu coast ranges from 0.21 to 0.26, which is much lower than, for example, that for the marsh of North Inlet (0.35), South Carolina, and the Virginia Coast Reserve(0.41), USA. Tidal range plays a significant role in that a larger tidal range leads to a smaller optimal IR, and that the landward and seaward limits are displaced toward higher ground elevations. In macrotidal regions the submergence depth is larger, which results in enhanced submergence and salinity stress for the entire marsh, causing it to shift toward higher elevations. Tidal range is an important factor influencing the growth pattern of S. alterniflora, but geomorphological factors such as topographic profiles, and the presence of cliffs and tidal creeks must also be taken into account.

The importance of seasonal variations in river discharge on the morphological development of estuaries has been recognized in recent years, yet in situ observations about such variations are rare. Here we report a long-term dataset of bathymetry in the middle reach of the Qiantang Estuary, China, characterized by the presence of a large inner bar. Moreover, a hydrographic survey was carried out in the Yanguan reach where one of the largest tidal bores in the world occurs, covering a spring-neap tidal cycle in 2015. Meanwhile, detailed seasonal bathymetric data together with daily river discharges of 2015 were collected. The bed morphology shows strong seasonal and inter-annual variations. During the high flow season, the river flow erodes the bed and transports a large amount of sediments seaward. A good power-law relationship exists between the high river discharge and the channel volume at the upper estuary. Flood tides dominate under usual river flow condition. In particular, the tidal bore during spring and intermediate tides is characterized by large current velocity and high suspended sediment concentration, and transports a large amount of sediment landward. Over a year, a dynamic morphological equilibrium can be maintained. Moreover, the estuary has also been significantly influenced by the large-scale embankment in recent decades, constraining the lateral thalweg migration, bank erosion and point bar deposition, which usually occur in natural sinuous estuaries.

Estuarine intertidal areas are shaped by combined astronomical and meteorological forces. This paper reveals the relative importance of tide, surge, wind, and waves for the flow and sediment transport on large intertidal shoals. Results of an intensive field campaign have been used to validate a numerical model of the Roggenplaat intertidal shoal in the Eastern Scheldt Estuary, the Netherlands, in order to identify and quantify the importance of each of the processes over time and space. We show that its main tidal creeks are not the cause for the dominant direction of the net flow on the shoal. The tidal flow over the shoal is steered by the water level differences between the surrounding channels. Also during wind events, the tidal flow (enhanced by surge) is dominant in the creeks. In contrast, wind speeds of order 40 times the typical tidal flow velocity are sufficient to completely alter the flow direction and magnitude on an intertidal shoal. This has significant consequences for the sediment transport patterns. Apart from this wind-driven flow dominance during these events, the wind also increases the bed shear stress by waves. For the largest intertidal part of the Roggenplaat, only ∼1–10% of the yearly transport results from the 50% least windy tides, even if the shoal is artificially lowered half the tidal range. This dominance of energetic meteorological conditions in the transports matches with field observations, in which the migration of the creeks and high parts of the shoal are in line with the predominant wind direction.

River mouth bar formation, a key process in fluvial-deltaic morphodynamics, is subject to both river discharge and waves. Given the increasing variability of both forcings under continuous climate change and human interventions, assessing their combined effects on mouth bar formatio n is an imperative issue. In this study, an extensive set of combined high and low river flows coupled with varying wave conditions and sediment grain sizes was assumed for numerical experiments conducted in Delft3D-SWAN. The results suggested that three regimes existed for mouth bar formation, namely, stable, ephemeral, and absent. These regimes corresponded to consistently weak, initially-weak-then-strong, and initially strong relative wave strengths, respectively, during the onset and reworking stages. Suppression of mouth bar formation further led to the inhibition of deltaic distributary networks. These findings have important implications for water and sediment management strategies, such as water diversion and dam regulation, in estuaries and deltas to prevent coastal erosion.