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A chenier is a beach ridge, consisting of sand and/or shells, overlying a muddy substrate. In this paper, we explore the cross-shore dynamics of cheniers in their ‘active’ phase, i.e. the phase between their formation and their landing on the shore and can no longer be reached by daily wave and tidal influences. While cheniers described in literature are known to only migrate onshore until they reach a stable position with their crest level above tidal influences, observations in Demak suggest the existence of an alternative stable state, highly dynamic on the short term, but stable on the longer term. To explore this alternative stable state, we developed an idealised chenier model to investigate cross-shore chenier dynamics under daily wave and tidal influences. The model is able to predict both onshore and offshore migration; onshore migration is mainly driven by wave action, while offshore migration is induced by a tidal phase lag, or the effect of the storm season. For certain combinations of waves, tide (incl. phase lag) and a storm season effect, the model predicts a dynamically stable chenier. In absence of a phase lag and storm season effect, the model yields a ‘classic’ stable chenier that welds onto the shoreline by onshore migration.

Deltas worldwide are at risk of elevation loss and drowning due to relative sea-level rise. Management strategies to restore or enhance sedimentation on delta plains, Sedimentation-Enhancing Strategies (hereafter SES), are now being pursued in many deltas but there has been limited cross-disciplinary and cross-delta review. Here we compare 21 existing and planned SES, synthesizing their physical characteristics, funding, governance arrangements, stakeholder engagement, process of implementation, environmental impact, land use change, and potential for upscaling. Strategies exist at various scales, from ~0.05 km2 - 500 km2. 79% of strategies are capable of outpacing high rates of sea-level rise. Cheaper strategies are limited to short term impacts and small spatial scales, while more expensive strategies can have longer lifetimes. Most strategies create wetlands and flood water storage. Some create opportunities for agriculture, aquaculture, housing, or recreational land use. Combinations of SES will likely be the most effective and sustainable method for maintaining elevation in river deltas.

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.

Human interventions and climate change can heavily influence the large-scale morphological development of tidal basins. This has implications on sediment management strategies, as well as ecological and recreational purposes. Examples of heavily impacted tidal basins are those in the Western Dutch Wadden Sea. The closure of a large sub-basin in 1932 triggered a shift in the sediment budgets of the remaining basins, leading to sediment infilling that is still ongoing. This paper presents a quantitative analysis of the post-closure sediment volumes, differentiating between sand and mud. Analysis of historical sediment composition data combined with bathymetry data revealed that the intervention caused a redistribution of sand and mud sedimentation. The responses of both sediment types differ spatially and temporally. The total infilling of the basins over the last century was substantially caused by mud (~32%, which is much larger than the average mud content in the bed). Initially, large mud volumes accreted in abandoned channels. At present, mud sedimentation along the mainland coast is still ongoing with nearly constant sedimentation rates over the past century, while the net import of sand significantly decreased over time and has been fluctuating around 0 over the last two decades. This research shows the importance of distinguishing between the response of sandy and muddy sediments when analysing the morphodynamic impact of an intervention, since they operate on different time and spatial scales. Sea level rise is currently a major threat for the existence of the Wadden Sea; its future fate will depend on whether the tidal flats are able to keep pace. Our results show that the supply of mud is sufficient to keep pace with the current sea level rise rates.

Cheniers are important for stabilising mud-dominated coastlines. A chenier is a body of wave-reworked, coarse-grained sediment consisting of sand and shells overlying a muddy substrate. In this paper we present and analyse a week of field observations of the dynamics of a single chenier along the coast of Demak, Indonesia. Despite relatively calm hydrodynamics during the one-week observational period, the chenier migrated surprisingly fast in the landward direction. The role of the tide and waves on the cross-shore chenier dynamics is explored using velocity moments as a proxy for the sediment transport. This approach shows that both tide and waves are capable of transporting the sediment of the chenier system. During calm conditions (representative for the south-east monsoon season), the tides generate a landward-directed sediment transport when the chenier crest is high relative to mean sea level. Waves only generate substantial sediment transport (direct, via skewness, and indirect, via stirring) when the chenier is submerged during periods with higher waves. The cross-shore chenier dynamics are very sensitive to the timing of tide and waves: most transport takes place when high water levels coincide with (relatively) high waves.

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.

Despite availability of a large amount of observational data and modelling studies, the mechanisms maintaining the Turbidity Maximum in the Belgian-Dutch coastal zone around the port of Zeebrugge (Belgium) are insufficiently understood. In order to better understand the dynamics of this turbidity maximum we examine the role of baroclinic (salinity and sediment-induced) processes and local sediment sources on the formation and persistence of the turbidity maximum through two different numerical model approaches. One model approach allows erosion of the highly compacted muddy seabed, serving as a sediment source, in line with observations of bed level change over several decades. The other approach reduces the exchange between the bed and the water column, to mimic the formation of highly concentrated near-bed suspensions with concentrations of several g/l observed around the port of Zeebrugge. Both model approaches are calibrated to various sources of available data (in situ sediment concentration observations, satellite image, bed level changes, mud content and dredging data), which they reproduce comparably well. However, reducing the water-bed exchange strengthens sediment convergence in the turbidity maximum, whereas the sediment source leads to sediment export. With the available data, it is difficult to determine which of the approaches is more realistic. Apparently, the lack of knowledge on near-bed exchange processes introduces an important source of uncertainty which cannot be adequately addressed with currently available observations. This work therefore shows that more quantitative knowledge on water-bed exchange processes in turbid marine environments is needed. It is further hypothesized that the large-scale erosion of the muddy seabed following the extension the port of Zeebrugge in the early 1980's brought such a large amount of sediment in suspension (50–100 million ton) that sediment convergence was strengthened. This increasing sediment convergence introduces a positive feedback mechanism that maintains sediment in the Turbidity Maximum, or even strengthens it. The high sediment concentrations observed today may therefore be a long-term effect of port construction carried out decades earlier.

The tidal variation in Singapore's coastal waters is influenced by large-scale, complex tidal dynamics (by interaction of the Indian Ocean and the South China Sea) as well as monsoon-driven low frequency variations, requiring a model with large spatial coverage. Close to the shores, the complex topography, influenced by headlands and small islands, requires a high resolution model to simulate tidal dynamics. This can be achieved through direct nesting or multi-scale nesting, involving multiple model grids. In this paper, we investigate the effect of grid resolution and multi-scale nesting on the tidal dynamics in Singapore's coastal waters, by comparing model results with observations using different statistical techniques. The results reveal that the intermediate-scale model is generally sufficiently accurate (equal to or better than the most refined model), but also that the most refined model is only more accurate when nested in the intermediate scale model (requiring multi-scale nesting). This latter is the result of the complex tidal dynamics around Singapore, where the dominantly diurnal tidal currents are decoupled from the semi-diurnal water level variations. Furthermore, different techniques to quantify model accuracy (harmonic analysis, basic statistics and more complex statistics) are inconsistent in determining which model is more accurate.

Many estuaries worldwide are modified, primarily driven by economic gain or safety. These works, combined with global climate changes heavily influence the morphologic development of estuaries. In this paper, we analyze the impact of human activities on the morphodynamic developments of the Scheldt Estuary and the Wadden Sea basins in the Netherlands and the Yangtze Estuary in China at various spatial scales, and identify mechanisms responsible for their change. Human activities in these systems include engineering works and dredging activities for improving and maintaining the navigation channels, engineering works for flood protection, and shoreline management activities such as land reclamations. The Yangtze Estuary is influenced by human activities in the upstream river basin as well, especially through the construction of many dams. The tidal basins in the Netherlands are also influenced by human activities along the adjacent coasts. Furthermore, all these systems are influenced by global changes through (accelerated) sea-level rise and changing weather patterns. We show that the cumulative impacts of these human activities and global changes may lead to exceeding thresholds beyond which the morphology of the tidal basins significantly changes, and loses its natural characteristics. A threshold is called tipping point when the changes are even irreversible. Knowledge on such thresholds or tipping points is important for the sustainable management of these systems. We have identified and quantified various examples of such thresholds and/or tipping points for the morphodynamic developments at various spatial and temporal scales. At the largest scale (mega-scale) we consider the sediment budget of a tidal basin as a whole. A smaller scale (macro-scale) is the development of channel structures in an estuary, especially the development of two competing channels. At the smallest scale (meso-scale) we analyze the developments of tidal flats and the connecting channels.

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 downstream peak discharge increase during hyperconcentrated floods in the Yellow River has been attributed to bed erosion, roughness reduction and floodplain effects. While great improvements have been made on the nderstandings of the roles of bed erosion and roughness reduction, the effects of floodplain remain poorly understood. Here, as a first step to reveal the floodplain effects, we present a numerical experimental study on how the channel-floodplain system reacts to a hyperconcentrated flood process. For this purpose, schematized channel-floodplain systems are designed and the classical 1992 flood record data is prescribed at the upstream boundary. By applying a fully coupled morphodynamic model, numerical experiments are conducted for a comprehensive analysis on the effects of bed erodibility, floodplain width, bed roughness variation, symmetry and longitudinal variability of geomorphology. Our results show two distinct trends for the response of channel- floodplain system depending on bed erodibility. For a small bed erodibility, both channel and floodplain experience erosion. For a moderate/large bed erodibility, only the channel experiences erosion whereas deposition occurs on the floodplain. The variation of the floodplain width does not affect these erosion-deposition behaviors while changing the magnitude and patterns of floodplain deposition. Thelongitudinally discontinuous channel-floodplain divided by either water storage areas or housing/farming banks diminishes the floodplain deposition at the discontinuous locations. The present numerical experiments do not show an obvious peak discharge increase, nonetheless, the recognized erosion-deposition characteristics would help further study of the floodplain effects on the peak of hyperconcentrated floods.

In this paper, we analyse the behaviour of fine sediments in the hyper-turbid Lower Ems River, with focus on the river’s upper reaches, a stretch of about 25 km up-estuary of Terborg. Our analysis is based on long records of suspended particulate matter (SPM) from optical backscatter (OBS) measurements close to the bed at seven stations along the river, records of salinity and water level measurements at these stations, acoustic measurements on the vertical mud structure just up-estuary of Terborg and oxygen profiles in the lower 3 m of the water column close to Leerort and Terborg. Further, we use cross-sectionally averaged velocities computed with a calibrated numerical model. Distinction is made between four timescales, i.e. the semi-diurnal tidal timescale, the spring–neap tidal timescale, a timescale around an isolated peak in river flow (i.e. about 3 weeks) and a seasonal timescale. Thedata suggest that a pool of fluid/soft mud is present in these upper reaches, from up-estuary of Papenburg to a bit downestuaryof Terborg. Between Terborg and Gandersum, SPM values drop rapidly but remain high at a few gram per litre. The pool of fluid/soft mud is entrained/mobilized at the onset of flood, yielding SPM values of many tens gram per litre. This suspension is transported up-estuary with the flood. Around high water slack, part of the suspension settles, being remixed during ebb, while migrating down-estuary, but likely not much further than Terborg. Around low water slack, a large fraction of the sediment settles, reforming the pool of fluid mud. The rapid entrainment from the fluid mud layer after low water slack is only possible when the peak flood velocity exceeds a critical value of around 1 m/s, i.e. when the stratified water column seems to become internally supercritical. If the peak flood velocity does not reach this critical value, f.i. during neap tide, fluid mud is not entrained up to the OBS sensors. Thus, it is not classical tidal asymmetry, but the peak flood velocity itself which governs the hyper-turbid statein the Lower Ems River. The crucial role of river flow and river floods is in reducing these peak flood velocities. During elongated periods of high river flow, in e.g. wintertime, SPM concentrations reduce, and the soft mud deposits consolidate and possibly become locally armoured aswell by sand washed in from the river. We have no observations that sediments are washed out of the hyper-turbid zone. Down-estuary of Terborg, where SPM values do not reach hyper-turbid conditions, the SPMdynamics are governed by classical tidal asymmetry and estuarine circulation. Hence, nowhere in the river, sediments are flushed from the upper reaches of the river into the Ems-Dollard estuary during high river flow events. However, exchange of sediment between river and estuary should occur because of tide-induced dispersion.

In past decades, the turbidity in Singapore's coastal waters has been increased. This has led to reduced visibility and increased siltation rates, detrimental for the coral reefs and other sensitive ecosystems around Singapore. The reasons for this increased turbidity are poorly known because little quantitative information exists on sediment dynamics around Singapore, on changes in sediment sources, and on the physical environment. Therefore we set out to quantify the effect of fluvial contributions on changes in sediment dynamics, using a combination of numerical models, satellite images, and hydrodynamic and sedimentary data. Results indicate that the main fluvial source enters an ebb-dominant estuary, with sediment export primarily balanced by settling/scour lags rather than estuarine circulation. A large part of the sediment load enters the Singapore Strait, where the large-scale marine currents effectively transport most sediment towards the coral reefs. However, mixing with marine water masses in both the estuary and the adjacent Singapore Strait sufficiently dilutes this fluvial sediment source to have a negligible impact on Singapore's coral reefs.

High-resolution morphological modeling of fluvial processes with complex, rapidly varying flows has been limited so far by model accuracy or computational efficiency. One of the most widely used numerical algorithms is based on the total variation diminishing method, solved by either upwind or centered approaches. An upwind scheme preserves high accuracy but is complex and computationally demanding, whereas the simplicity and efficiency of a centered approach compromise the accuracy. The present paper extends a recent upwind-biased centered scheme originally developed for clear water and scalar transport over a rigid bed, to sediment-laden flows over an erodible bed. It does so by developing a fully coupled 2-D mathematical model using a finite volume method for structured grids. The complete set of noncapacity-based governing equations, involving the effects of bed deformation and sediment density variation, as well as the influences of turbulence and sediment diffusion, and the temporal and spatial scales needed for sediment adaptation, is solved at one time to obtain synchronous solutions for the entire computational domain. For stability, a two-stage splitting approach together with a second-order Runge-Kutta method is employed for the source terms. The model is verified in a number of tests covering a wide range of complex (sediment-laden) flows. The model is demonstrated to accurately simulate shock waves and reflection waves, but also rapid bed deformations at high sediment transport rates. The combination of high numerical accuracy and computational efficiency makes the model an important tool to forecast flood events in morphologically complex areas.

It is well known that landward transport of fine sediments on tidal flats is caused by lag effects, of which the scour lag and the settling lag are the best known. These lag effects result from a combination of sediment properties and hydrodynamic asymmetries. However, it is not well-understood how, in a quantitative way, these lag effects depend on the sediment properties and the hydrodynamic forcing, and what the relative importance of these sediment properties and hydrodynamics is. As a result, it is not known which lag effect is more important under which conditions. We therefore set out to explore the relative importance of hydrodynamics and sediment properties on tidal-flat sedimentation using a schematized 2DV cross-shore profile model with a tidal range of 6 m and an intertidal width of ∼5 km. The effect of hydrodynamics is parameterized through a varying offshore tidal asymmetry and four different cross-shore profiles (a horizontal, convex, linear and concave profile). The effect of sediment properties is examined by evaluating a range of settling velocities w s and critical bed shear stresses for erosion τ cr . The main findings of this work are that (1) conditions of maximum sediment deposition rates exist for w s ∼0.5 mm/s and τ cr ∼0.1 Pa, (2) deposition rates due to slack tide asymmetry are comparable to symmetric tides, while peak flow asymmetry produces the greatest deposition rates, (3) tidal flat deposition rates are greatest for concave profiles and least for convex profiles mainly due to the horizontal velocity gradient, and (4) the type of lag effect dominating landward transport varies spatially.

The sediment load of the Yangtze River (China) is decreasing because of construction of dams, of which the Three Gorges Dam (TGD) is the best known example. The rate of the decline in sediment load is well known, but changes in the sediment grain size distribution have not been given much attention. The TGD mostly traps sand and silt while clay is flushed through the reservoir. A large amount of sand is available in the Yangtze River downstream of the reservoir, and therefore the pre-dam sand concentration is not substantially reduced. The availability of silt on the Yangtze River bed is limited, and it is expected that most silt will be removed from the riverbed within one to two decades. In order to evaluate the impact of the change in grain size distribution on the tidal flats of the Yangtze Estuary, a highly schematized tidal flat model is setup. This model broadly reveals that the observed deposition rates are exceptionally large because of the high sediment concentration, the abundance of silt, the seasonal dominance of waves (shaping a concave profile), and the offshore tidal asymmetry. The model further suggests that deposition rates will be limitedly influenced by reductions in clay or fine silt but strongly impacted by reductions in median to coarse silt. The response of the downstream morphology to reservoir sedimentation therefore strongly depends on the type of trapped sediment. As a consequence, silt-dominated rivers, such as the Yangtze River and the Yellow River may be more strongly impacted than sand-dominated systems.

Sediment found in China’s Yangtze and Yellow River systems is characterized by large silt fractions. In contrast to sand and clay, sedimentation and erosion behaviour of silt and silt–clay–sand mixtures is relatively unknown. Therefore, settling and consolidation behaviour of silt-rich sediment from these river systems is analysed under laboratory conditions in specially designed settling columns. Results show that a transition in consolidation behaviour occurs around clay contents of about 10 %, which is in analogy with the transition from non-cohesive to cohesive erosion behaviour. Above this threshold, sediment mixtures consolidate in a cohesive way, whereas for smaller clay percentages only weak cohesive behaviour occurs. The settling behaviour of silt-rich sediment is found to be in analogy with granular material at concentration below 150 g/l. Above 150–200 g/l, the material settles in a hindered settling regime where segregation is limited or even prevented. The results indicate that for modelling purposes, multiple sediment fractions need to be assessed in order to produce accurate modelling results.

This paper describes the results of a study on potential wave damping in the Dutch Wadden Sea by viscous wave energy dissipation within soft and/or liquefied mud layers. The results are relevant for wave height boundary conditions for levee design along the mainland coast. From a site inspection, we observed that the mud fields along the Wadden Sea coast are quite dynamic, and seem to migrate in alongshore direction during storm conditions. A site along the Wadden Sea coast with extensive soft mud deposits, known as Zwarte Haan, was selected for more detailed laboratory and model analyses. Both field and laboratory measurements on sediment cores indicated strengths of a few kPa, i.e. slightly above the Liquid Limit. To assess the sensitivity of the mud deposits to liquefaction induced by incoming waves, we carried out cyclical triaxial tests in the laboratory on a series of samples taken from the field. The stresses prescribed were established from the wave heights and periods expected during storm conditions. However, even at stresses well beyond these values, the triaxial tests did not indicate full liquefaction of the mud samples; only some frictional losses were monitored. These results are in contradiction with laboratory studies, where liquefaction has been measured frequently. However, our observations corroborate with other recent observations in literature, indicating that wave damping does not occur during a storm, but afterwards. It is hypothesized that fluid mud, dissipating wave energy, is formed by so-called auto-saturation and/or from deposition of mud eroded during that storm, and not from liquefaction during the storm. Wave damping then occurs through viscous dissipation in the fluid mud layers until these mud layers regain their initial strength through consolidation. Next, we assessed the degree of wave damping to be expected at the beginning of a storm, when the bed is still unaffected and during/after storm conditions, when fluid mud is formed. We used the measured mud properties and those typical for fluid mud, as found in the literature. This assessment was done through simulations with a new version of SWAN, referred to as SWAN-mud. These simulations suggest significant wave damping under fluid mud conditions, but only 10% damping at the beginning of the storm; moreover, the degree of wave damping over a muddy bed may decrease during the storm, as the soft mud deposits may be eroded by the waves. Our analyses suggest that wave cannot liquefy soft mud beds under natural conditions, and that fluid mud layers, damping incoming waves by viscous dissipation, are formed from mud eroded by the shear flow induced by the waves' orbital motion. Note that in-situ pore water pressure measurements are required to corroborate this hypothesis.

A new container dock in the Port of Antwerp, the Deurganckdok, was recently constructed in a location close to the Estuarine Turbidity Maximum (ETM) of the Scheldt River. In order to minimise sedimentation in the new dock, a Current Deflecting Wall (CDW) will be constructed. The aim of this paper is to assess the effect of the CDW on exchange flows and sediment fluxes. Determining this effect requires (1) understanding and reproduction of the sediment dynamics close to the Deurganckdok, and (2) accurate reproduction of the exchange flows between the dock and the Scheldt, and the effect of the CDW thereon. The sediment dynamics close to the Deurganckdok are dominated by a high concentration suspension (near-bed concentrations around 1 g/l) which is advected along the dock by the tidal currents. The sediment concentration is limited by the supply of sediment rather than the flow velocity. Most sediment will temporarily settle from suspension around slack tide. Hence, flood and ebb sediment concentrations are comparable, although the ebb and flood velocities are asymmetric. The peak sediment concentration occurs at the left or right bank of the Scheldt River, depending on the clockwise and counter-clockwise residual cross-channel currents. Exchange flows between the dock and the Scheldt are mainly a combination of horizontal eddies and salinity-driven flows, with velocities around 0.4 m/s. Most sediment enters the dock in a near-bed horizontal eddy, occurring around HW. The main effect of a CDW is a deflection of this sediment patch towards the river, reducing the sediment influx into the dock. The estimated reduction of siltation by the CDW is 18%. Alternative CDW configurations also reduce sedimentation in the dock, but less than computed with the design CDW.

The high sediment load of the Yellow River results in rapid infilling of its reservoirs when sediment is not regularly flushed. Simultaneously, the downstream reaches of the Yellow River experience extremely high siltation rates, which are reduced when sediment is retained in its reservoirs. To minimize siltation in the reservoirs and the downstream river bed, water and sediment are released from the reservoir in a controlled way through flushing experiments. In this paper, we analyze the effect of such a flushing event on the downstream river bed through data analysis and numerical modeling. Sedimentation may be minimized by relating the amount of sediment released from the reservoir to the sediment available for release through operational monitoring and by releasing relatively clear water after turbid water. Despite this flushing of sediment, the reservoir will eventually fill up, and more sediment released again into the lower Yellow River. The change in discharge magnitude and frequency brought about by the reservoir will then probably result in increased siltation rates in the lower Yellow River compared to the predam situation.

Density-driven exchange flows between estuaries and harbour docks are influenced by the length of the dock. As a result, increasing dock size through its lengthening, not necessarily results in an increase in sedimentation rates. The propagation of a low-salinity surface patch into the dock is blocked at the head of a relatively short dock, resulting in a reversal of density-driven flows, and a reduction of the hydrostatic pressure gradients in the entrance of the dock. A reduced hydrostatic pressure in the dock, in turn, promotes near-bed inflow. When this increased near-bed inflow coincides with a high sediment supply on the adjacent river, the sediment transport into the dock increases. This has been tested with an extensively validated high-resolution numerical model developed for the Deurganckdok in the Port of Antwerp. In the Deurganckdok, siltation rates are expected to decrease when the dock is fully excavated compared to the present half-opened dock. Whether exchange flows between estuaries and harbour docks are influenced by the length of the dock, depends on the tidal variation in salinity. For small tidal density variations (around 0.5 kg/m3), the dock length is expected to influence exchange flows in a short dock (approximately 1 km), whereas the dock should be much longer (4 km) when the tidal density variation is higher (around 5 kg/m3). Whether these changing exchange flow result in a lowering or increase of sediment import, depends on the phase difference between sediment concentration peaks on the adjacent river/estuary and the salinity variation, and on the vertical distribution of sediment.

The Yellow River in China carries large amounts of sediments in suspension at concentrations up to several hundreds of kilograms per cubic metre; the sediment is composed mainly of silt. These high sediment concentrations influence the hydrodynamics (flow velocity and turbulence) which, in turn, determine the sediment concentration profile, whereas both the high sediment concentrations and pseudo-cohesive properties of silt determine the morphodynamics of the Yellow River. The effect of sediment on the hydrodynamics is analysed using the Richardson number and the Reynolds number to provide a framework to differentiate between various flow regimes in the Yellow River, which is calibrated and validated with Yellow River data. The flow may be sub-saturated (stable flow), super-saturated (unstable flow characterized by high deposition rates, caused by collapse of turbulence), or hyperconcentrated sub-saturated (stable flow because of hindered settling effects), depending on the Richardson number. Independent of this, the flow may be turbulent, transitional or laminar, depending on the Reynolds number. Analysis of these flow types improves understanding of the flow regimes and morphodynamics of the Yellow River. The morphodynamics of the Yellow River are also affected by pseudo-cohesive behaviour caused by shear dilatance, which results in increasing critical shear stress for erosion at decreasing grain-size. This pseudo-cohesive behaviour may be partly responsible not only for the high deposition rates which characterize the lower Yellow River, but also for mass erosion during river floods.

A large amount of the total sediment load in the Chinese Yellow River is transported during hyperconcentrated floods. These floods are characterized by very high suspended sediment concentrations and rapid morphological changes with alternating sedimentation and erosion in the main channel, and persistent sedimentation on the floodplain. However, the physical mechanisms driving these hyperconcentrated floods are still poorly understood. Numerical modelling experiments of these floods reveal that sedimentation is largely caused by large vertical concentration gradients, both in the channel during the rising stage of the flood, as well as on the floodplains, during a later stage of the flood. These vertical concentration gradients are large because the turbulent mixing rates are reduced by the increased sediment-induced density gradients, resulting in a positive feedback mechanism that produces high deposition rates. Erosion prevails when the sediment is largely held in suspension due to hindered settling, and is strengthened by the reduced wetted cross-section caused by massive sedimentation on the floodplain. Observed patterns of erosion and sedimentation during these floods can be qualitatively reproduced with a numerical model in which sediment-induced density effects and hindered settling are included. Copyright © 2009 John Wiley & Sons, Ltd.

This paper evaluates the applicability of the sediment transport methods developed by Engelund and Hansen, Ackers and White, Yang et al., and van Rijn, together with the Wuhan methods developed in China, to the Yellow River, which has highly concentrated and fine-grained sediment. The sediment data includes over 1000 observations from the Yellow River, 32 sets of data from a canal, and 266 sets of data from laboratory flumes. The best predictions were obtained by the Yang 1996 method, the Wuhan method, and the modified Wuhan method by Wu and Long, while reasonably good predictions were also provided by the van Rijn 2004 method. The Engelund and Hansen, the Ackers and White, and the van Rijn 1984 methods in their original forms are not applicable to the Yellow River. The predicted results for total load concentrations were as good as for bed-material concentrations, even though the total load includes a large portion of wash load.

The Yellow River in China is characterized by channel erosion and a meandering channel pattern at both low sediment concentrations and at hyperconcentrations, whereas deposition and braiding tends to dominate at intermediate sediment concentrations. A 3D numerical model (Delft3D) is used to analyze the effect of sediment-induced density effects on the formation of channel patterns using a highly simplified river geometry. Results show that the effect of vertical sediment density stratification and hindered settling significantly influence river channel patterns. At some critical concentration (the saturation concentration), vertical mixing is totally damped by the density gradients at the lutocline (a strong vertical gradient in sediment concentration). This increases the secondary circulation in the river channel significantly, which in turn promotes meander development. When the flow is not close to saturation, the main channel-forming process seems to be channel avulsion through blocking of old channels and subsequent channel cut-off. When the concentrations are so high that sediment is primarily held in suspension by hindered settling, sedimentation is so much reduced that channels are no longer blocked and therefore the river course becomes confined to a single meandering channel.

Sediment particles smaller than 64 μm, with a noncohesive base mineral, are referred to as silt. The settling velocity of suspended sediment decreases with increasing concentration due to hindered settling effects. Compared with clay and sand, hindered settling of silt is poorly documented and understood. A formulation is proposed for the hindered settling of silt-water mixtures, derived from physical reasoning. The effect of silt on the mixture’s viscosity is addressed in particular. The formulation compares favorably with laboratory experiments on the settling of silt-sized sediment. This study shows that effective settling velocities can be derived from a single concentration time series measured within the settling mixture, provided that the sediment is initially homogeneously distributed.

An extensive barrier-spit system 20 to 30 km long developed on the delta front platform of the Ba lat Delta within a relatively short period (10–20 years). However, it was not evident whether the formation was related to river floods, typhoons or calm weather conditions, or to cross-shore or alongshore sediment transport mechanisms. Therefore a process-based numerical model that simulates sediment transport and morphologic changes by waves, tides, wind, and river flow, is used to analyze the processes that govern this barrier formation. The mechanism that initiates the barrier formation is concluded to be onshore transport by wave asymmetry of sediment previously transported seaward in the buoyant river plume. The main mechanism that counteracts the barrier formation is probably destruction of the barrier by river flow. During the subsequent transformation from a subaqueous barrier into a sub-aerial barrier-spit system, overwashes and alongshore sediment transport become increasingly important. This spit system therefore appears to be dominated by alongshore processes, even though its initial formation is more strongly regulated by cross-shore processes. The importance of cross-shore processes results in a minor contribution of typhoons to the initial formation of these barriers. Typhoons are not very important to destruction of barriers as well, because wave energy is gradually dissipated over the gentle offshore profile, which in turn results from river plume sedimentation.

The Red River, annually transporting 100 million tons of sediment, flows into a shallow shelf sea where it rapidly deposits most of its sediment on a prograding delta front. Oceanographic cruises were carried out in February–March and July–August 2000 to determine the vertical structure of the Ba Lat river plume and sediment transport patterns on the delta front. The surface waters in the coastal zone were strongly stratified with a low density and high sediment concentration during the larger part of the wet season, caused by low mixing rates of river plumes with ambient water. The river plume is advected to the south by a well-developed coastal current which originates from the river plumes that enter the Gulf of Tonkin North of the Ba Lat and are deflected southward by the Coriolis force. Sediment predominantly leaves the surface plume by settling from suspension and less by mixing of fresh and marine water. A one-dimensional model for plume deposition valid for fair weather conditions indicates that most sediment is deposited within 10 km and southward of the river mouth. Of prime importance for this depositional pattern is the phase relation between river outflow and tidal currents, in combination with the southward surface flow; alongshore advection is very low during outflow of the turbid river plume. The agreement of modeled plume sedimentation patterns with long-term bathymetric changes strongly suggests that fair weather depositional processes determine delta front development. This may be related to the fact that reworking of sediment mainly occurs several months after the peak deposition period; in the meantime sediment compaction and consolidation have increased the shear strength of deposited sediments.

Water diversions along alluvial rivers cause water and sediment loss, and thereby affect morphological development. Assuming spatially continuous diversions along a constant-width channel, previous studies suggest a longitudinally convex bed at the equilibrium state. However, the validity of a convex bed profile for practically discrete diversions in natural channels of longitudinally varying width remains to be justified. Moreover, such equilibrium analysis does not reveal the morphological time scale (MTS) associated with water diversions. To solve these issues, a general theoretical framework is proposed for predicting the equilibrium state of the fluvial system, which is applicable to both continuous and discrete water diversions in a longitudinally width-varying channel. Numerical experiments complement the MTS studies for water diversions. The effects of diversion intensity, diversion placement (discrete and continuous), and diversion scheme (pure water and water-sediment mixture) are also s...

The Singapore Strait connects the South China Sea, where tides are dominantly diurnal, to the dominantly semidiurnal Indian Ocean. At this transition, the tidal water level oscillations are observed to be semidiurnal while the tidal current oscillations are mixed, diurnal to fully diurnal. Due to the interaction of the diurnal constituents with the semidiurnal M2 tide, the tides are strongly asymmetric. Both residual flows and subtidal flows, with periodicities of 2 weeks to 1 year, are strong. In order to analyze and explain the hydrodynamics around Singapore, a well-documented and calibrated regional tidal model application was further improved and validated. Analysis of the results of this model shows that the diurnal tidal wave is primarily standing, with an amphidromic point close to Singapore, explaining the dominantly diurnal current and semidiurnal water level oscillations. Analysis of the model results further indicates that the fortnightly constituents in the subtidal flow are probably compound tides, with a combined amplitude over 10 cm/s. Pronounced yearly and half-yearly cycles in spring tidal current amplitude and asymmetry exist, resulting from interaction of the diurnal and the semidiurnal spring-neap cycles, compound tides, and the monsoon currents. A simple analytical transport formula was applied to determine the relative importance of tidal asymmetry and residual flows, verified with a full sediment transport model. With fine sediment being more sensitive for residual flow and coarser sediment for tidal flow, a pronounced divergence in sediment transport pathways may exist, depending on the grain size.

The Bay of Banten provides an example of a semicircular shallow tidal embayment in a tropical ROFI (Region Of Freshwater Influence), subject to a mixed, mainly diurnal tidal regime and a monsoon-driven residual circulation. A partially inactivated delta shapes the east coast of the bay and constitutes a distributed source of freshwater during the wet season. Measurements of flow velocity, salinity and temperature were taken at two contrasting sites in the bay, to investigate stratification dynamics throughout a spring–neap cycle. Bulk Richardson numbers are shown to vary both at tidal and subtidal frequencies, and indicate that the water column occasionally remains stratified all through a tidal cycle. About half of the variance of the subtidal flow can be explained from wind forcing. The wind-driven flow varies between 1 and 5% of the wind speed, with a pronounced difference between the two locations under study. Stratification was further analyzed by estimating the contributions of advection by the depth–mean flow, tidal straining, vertical advection and heating, to variation in potential energy anomaly (PEA). To do so, horizontal gradients in density and PEA were estimated from multi-linear regression analysis. The obtained results show that spatial gradients of density and PEA can reverse sign frequently in the absence of strong mixing, associated to pulses of freshwater discharge. The freshwater discharge, in turn, is distributed over small streams. The contribution of vertical mixing was found to be relatively small, and none of the remaining terms were clearly dominant. Estimates of the boundary layer thickness of clockwise and anticlockwise rotary components of the tidal flow show the impact of baroclinic processes on velocity profiles.

The present paper comments on a recent publication by Woodworth et al. [Woodworth, P.L., Blackman, D.L., Pugh, D.T., Vassie J.M., 2005. On the role of diurnal tides in contributing to asymmetries in tidal probability distribution functions in areas of predominantly semi-diurnal tide. Estuarine, Coastal and Shelf Science 64 (2–3), 235–240.]. It is shown how tidal asymmetries associated with astronomical tides can be accounted for in an extreme sea level analysis, despite the difficulty of deriving probability distribution functions if a large number of tidal constituents are considered. The use of the term spring–neap cycle in a diurnal tidal regime is discussed.

The relation between tidal flow asymmetry and net transport of sediment in the semidiurnal regime has been extensively described. This study reveals that in the diurnal regime, the direction of long-term net bed-load transport and resulting morphologic changes is partly determined by the phase-angle relationship of O1, K1, and M2. Simple analytical formulations of time-averaged bed-load transport were derived which separate the relative contributions of tidal asymmetry from that of residual flow with tidal stirring. In this particular case, the Red River Delta in Vietnam, transports related to tidal asymmetry are larger than those induced by the monsoon currents, and are an order of magnitude larger than those associated with topographic residual flow around the delta. Tide-induced morphologic changes dominate at water depths between 10 and 25 m, of which the patterns of erosion and deposition overlap with observed bathymetric changes. Additional observed changes that occur in more shallow water cannot be explained by tidal asymmetry and are probably related to wave action and to deposition from the buoyant river plume.

The UK North Sea is mature after 30 years of exploration. The application of new technology to continue the discovery and development of smaller reserve base fields is essential. Advances in 3D seismic acquisition have enabled improved subsurface imaging. Ocean Bottom Cable (OBC) seismic can improve imaging of fields overlain by gas chimneys. PreStack Time and Depth Migration have contributed to better definition of steeply dipping prospects underlying diapirs. Amplitude Variation with Offset (AVO) can reduce exploration risk in a proven play. Quantitative interpretation techniques (e.g. seismic inversion) can be used to image reservoir units in mature areas. High resolution geochemistry can also assist intra-reservoir characterisation. New drilling techniques (e.g. multilateral wells) can unlock fault blocks, layered reservoirs or small outstep accumulations. Extended reach drilling from platforms is also effective in mature areas. Long distance subsea tiebacks, for both high pressure/ high temperature and low energy fluids, can make the development of small and remote hydrocarbon accumulations feasible. The Shell Expro multi-discipline organisation has identified new practices for exploration/keeper well design and equipment procurement which reduce the time between field discovery and first production. Innovative engineering designs are unlocking small field development. New commercial practices are also required to facilitate application of technology in mature areas.

Hasan, G.M.J.; van Maren, D.S., and Hin Fatt, C., 2013. Numerical study on mixing and stratification in the ebb-dominant Johor Estuary. A three-dimensional hydrodynamic model is used to investigate intratidal spring-neap variations of turbulent mixing and stratification in the Strait of Singapore and its adjacent Johor estuary area. The waters in the Johor estuary are ebb dominant because of interaction of the O1–K1–M2 tidal constituents. In most estuaries mixing rates during the flood are larger than during the ebb because of a flood-dominant tidal asymmetry. The Johor estuary is an example where tidal asymmetry supports stronger mixing during ebb tides, whereas similar to other estuaries, tidal straining promotes stable stratification during the ebb. Therefore an analysis of intratidal variation in stratification reveals the relative importance of both mechanisms. Vertical profiles of salinity, flow velocity, and eddy diffusivity show a marked asymmetry between flood and ebb tides. Small change...

Many estuaries worldwide are becoming more urbanised with heavier traffic in the waterways, requiring continuous channel deepening and larger ports, and increasing suspended sediment concentration (SSC). An example of a heavily impacted estuary where SSC levels are rising is the Ems Estuary, located between the Netherlands and Germany. In order to provide larger and larger ships access to three ports and a shipyard, the tidal channels in the Ems Estuary have been substantially deepened by dredging over the past decades. This has led to tidal amplification and hyper concentrated sediment conditions in the upstream tidal river. In the middle and outer reaches of the Ems Estuary, the tidal amplification is limited, and mechanisms responsible for increasing SSC are poorly understood. Most likely, channel and port deepening lead to larger SSC levels because of resulting enhanced siltation rates and therefore an increase in maintenance dredging. Additionally, channel deepening may increase up-estuary suspended sediment transport due to enhanced salinity-induced estuarine circulation. The effect of channel deepening and port construction on SSC levels is investigated using a numerical model of suspended sediment transport forced by tides, waves and salinity. The model satisfactorily reproduces observed water levels, velocity, sediment concentration and port deposition in the estuary, and therefore is subsequently applied to test the impact of channel deepening, historical dredging strategy and port construction on SSCs in the Estuary. These model scenarios suggest that: (1) channel deepening appears to be a main factor for enhancing the transport of sediments up-estuary, due to increased salinity-driven estuarine circulation; (2) sediment extraction strategies from the ports have a large impact on estuarine SSC; and (3) maintenance dredging and disposal influences the spatial distribution of SSC but has a limited effect on average SSC levels.

[1] Tidal current asymmetry is usually associated to nonlinear tidal interactions in shallow water, generating compound tides and overtides. The present paper demonstrates that tidal asymmetry is not only caused by nonlinear tidal interaction, but is also generated by astronomical tides in absence of shallow-water effects. In particular, the K1, O1, and M2 constituents give an asymmetrical periodic flow pattern, with the largest peak velocities persistently in the same direction. A theoretical explanation for this phenomenon is presented, and an inventory is made of other, subordinate constituents that may contribute to repetitive asymmetric flow patterns. Tidal current asymmetry may induce residual transport of sediment. Analytical expressions are derived which quantify the residual transport of sediment due to the K1, O1, and M2 tides, as a function of the phases and amplitudes of these constituents, and the time lag between variation in suspended sediment transport and flow velocity. The residual transport induced by the K1, O1, and M2 tides is compared to the residual transport due to the asymmetry associated with the M2 and M4 tides. The former mechanism is particularly relevant in diurnal, or mixed, mainly diurnal tidal regimes, where the K1 and O1 tides are dominant.