Channel Deepening Research Articles

Fine sediment transport into the hyper-turbid lower Ems River: the role of channel deepening and sediment-induced drag reduction

Deepening of estuarine tidal channels often leads to tidal amplification and increasing fine sediment import. Increasing fine sediment import, in turn, may lower the hydraulic drag (due to a smoother muddy bed and/or sediment-induced damping of turbulence), and therefore, further strengthen tidal amplification, setting in motion a process in which the sediment concentration progressively increases until the river becomes hyper-turbid (Winterwerp and Wang, Ocean Dyn 63(11–12):1279–1292, 2013). To advance our understanding of the relative role of bed roughness and bed topography on sediment import mechanisms and sediment concentration, a Delft3D numerical model has been setup for an estuary which has been deepened and as a consequence experienced a strong increase in suspended sediment concentration: the lower Ems River. This model is calibrated against present-day hydrodynamic and sedimentary observations, and reproduces the basic sediment transport dynamics despite simplified sedimentological formulations. Historic model scenarios are semi-quantitatively calibrated against historic high and low water observations, revealing that changes in hydraulic roughness and deepening are probably equally important for the observed tidal amplification. This model is subsequently used to better understand historic changes in the hydrodynamic and sediment transport processes in the lower Ems River. Import of fine sediment has increased because of larger tidal transport, even though the degree of tidal asymmetry may not have significantly changed. The resulting rise in suspended sediment concentration reduced hydraulic drag, amplifying the tidal range. Export of fine sediment became less because the river-induced residual flow velocity decreased with deepening of the channel.

Effects of human alterations on the hydrodynamics and sediment transport in the Sacramento-San Joaquin Delta, California

Abstract. The Sacramento-San Joaquin Delta, California, (Delta) has been significantly altered since the mid-nineteenth century. Many existing channels have been widened or deepened and new channels have been created for navigation and water conveyance. Tidal marshes have been drained and leveed to form islands that have subsided, some of which have permanently flooded. To understand how these alterations have affected hydrodynamics and sediment transport in the Delta, we analysed measurements from 27 sites, along with other spatial data, and previous literature. Results show that: (a) the permanent flooding of islands results in an increase in the shear velocity of channels downstream, (b) artificial widening and deepening of channels generally results in a decrease in shear velocity except when the channel is also located downstream of a flooded island, (c) 1.5 Mt/year of sediment was deposited in the Delta (1997–2010), and of this deposited sediment, 0.31 Mt/year (21%) was removed through dredging.

The impact of channel deepening and dredging on estuarine sediment concentration

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.

Water quality of a coastal Louisiana swamp and how dredging is undermining restoration efforts

The Bayou Boeuf Basin (BBB), a sub-basin of the Barataria Basin estuary in coastal Louisiana, consists of forested and floating wetlands receiving drainage from surrounding agricultural fields and urban watersheds. We characterized surface water quality in the BBB, and determined through hydrologic modeling if a series of levee breaks along major drainage channels would significantly improve water quality by allowing flow into surrounding wetlands. Surface water monitoring found surrounding sugarcane farm fields to be major sources of nutrient and sediment loading. Hydrological modeling indicated that levee breaks would increase N reduction from the current 21.4% to only 29.2%, which is much lower than the anticipated 90–100% removal rate. This was due to several factors, one them being dredging of main drainage channels to such a degree that water levels do not rise much above the surrounding wetland elevation even during severe storms, so only a very small fraction of the stormwater carried in the channel is exposed to wetlands. These unexpected results provide insight into an undoubtedly pervasive problem in human dominated wetland systems; that of decreased flooding during storm events due to channel deepening by dredging activities. Additional water quality management practices should be implemented at the farm field level, prior to water entering major drainage canals.

Small estuary, big port – progress in the management of the Stour-Orwell Estuary system

Management of port development is increasingly challenging because of the competitive requirement for deeper channels and because of the need to preserve important coastal wetlands which function as both habitat and flood defence. This paper describes the management of the Stour/Orwell Estuary system, Eastern England, an estuary system which has experienced considerable development and morphological change. The estuary is internationally important for its wetland bird populations and the intertidal areas of the estuary system are protected under European legislation. It is also the location of the Port of Felixstowe. In 1998/2000 the approach channel to the Port of Felixstowe was deepened from −12.5 mCD to −14.5 mCD. This paper describes the effects of the approach channel deepening, the approach taken to identifying the potential impact to intertidal habitat resulting from the deepening, the sediment recycling implemented as mitigation to prevent increased loss of habitat and the subsequent response of the estuary system to this intervention.

Influences of Channel Deepening and Widening on the Tidal and Nontidal Circulations of Tampa Bay

A high-resolution, three-dimensional coastal ocean circulation model is applied to Tampa Bay to determine the effects that channel deepening and widening may have on the estuary’s tidal and nontidal circulations. The model domain includes all of Tampa Bay, Sarasota Bay, the Intra-coastal Waterway, and the adjoining rivers. With resolution as high as 20 m, the model resolves the main shipping channels, the bridge causeways, and all important inlet mass conveyances. Model evaluations are made against in situ data and a previously published, relatively lower-resolution version. By deepening and widening the shipping channels for the purpose of accommodating larger vessels, our results suggest an increase in tidal range and a decrease in tidal phase from the mouth to the head of the bay. Along with these tidal effects, our results also suggest an increase in the nontidal circulation, resulting in an increase in salinity and attendant consequences to the estuary’s ecology.

On the Flushing of Tampa Bay

A high-resolution, three-dimensional coastal ocean circulation model is used to estimate the bulk residence time distributions for the entirety of Tampa Bay and for five of its sub-regions using an Eulerian passive tracer technique. With a resolution as high as 20 m, the model domain includes all of the important mass conveyances by the channels and inlets, plus the impediments to flow by bridge causeways and natural constrictions. The non-tidal circulation is found to be the primary agent for the flushing of Tampa Bay. Tides alone have a minor effect. Exceptions pertain to within a tidal excursion from the mouth of the bay and sub-regions with multiple inlets, where differential flows related to tidal phase differences aid in flushing. By resolving the main shipping channels, we also address the effect of channel deepening and widening on the flushing of Tampa Bay. Bulk residence time is found to decrease due to an increase in the non-tidal circulation.

The influence of channel deepening on estuarine turbidity levels and dynamics, as exemplified by the Ems estuary

Active deepening of tidal channels usually results in the alteration of the vertical and the horizontal tide. This may lead to concurrent significant increases in mean suspended matter concentrations (SPM) in coastal plain estuaries, the turbidity maximum (ETM) included. This is exemplified by an extensive analysis of the Ems estuary, a prototypical heavily stressed estuary in the Dutch-German border area. Measurements show that the SPM concentrations in the lower reaches of the estuary have increased an average of 2- to 3-fold between 1954 and 2005, with a 10-fold increase observed in the upper estuary (tidal river). Longitudinal profiles of surface SPM demonstrate that the ETM has moved upstream by up to 25 km and has broadened into a zone 30 km in length which extends into the freshwater tidal river. On an annual scale, variations in freshwater discharge significantly influence the formation and breakdown of the ETM: during low river discharge the ETM approaches equilibrium over 2–3 months, whilst elevated river discharges relocate the ETM downstream over several weeks.An exploratory, semi-analytical model is calibrated to simulate the equilibrium SPM distribution in the upper estuary during five time periods from 1965 to 2005, using archival bathymetric and tidal data. Results suggest that the deepening of tidal channels and a reduction in hydraulic drag have most likely resulted in a landward shift of the SPM trapping location. The measured increase in SPM concentrations and the development of fluid mud around the 1990s likely contributed to reduced mixing and bottom drag, creating a feedback loop that further altered tidal and SPM dynamics. It is argued that the removal of some non-erodible (consolidated) layers in the lower reaches of the estuary has created new internal sediment sources that may be responsible for feeding the observed high SPM concentrations, rather than increased sediment input from the boundaries. All findings are based on and supported by measured short-term seasonal fluctuations, as well as long-term developments of yearly averaged concentrations in the longitudinal SPM distribution.

Building with Nature in Marine Infrastructure: Toward an Innovative Project Arrangement in the Melbourne Channel Deepening Project

Globally, there is an increasing pressure on coastal and marine ecosystems. The development of marine infrastructure is causing additional ecological impacts on these systems. To reduce ecological impacts innovative approaches of marine infrastructure development are being developed. These approaches, such as Building with Nature, aim to integrate ecological and social dynamics in the creation of coastal and marine infrastructure. This integration has implications for the development and management of these projects. To understand opportunities for Building with Nature approaches, this article analyzes the development of the Melbourne Channel Deepening project in Australia that developed from a conventional, state-led project toward a Building with Nature arrangement. This article shows that a Building with Nature project arrangement differs not just substantially in its ecological focus and primacy from a conventional project arrangement, but also in its leading discourse, its actor coalition, its power and resources applied, and the rules of the game through which the project is designed and implemented. We conclude that developing a Building with Nature project requires a specific arrangement that allows for an early involvement of both ecological and social dynamics.

Effects of the anthropogenic activities on the morphological evolution of the Modaomen Estuary, Pearl River Delta, China

Owing to the intensive human activities, the Modaomen Estuary has been significantly modified since 1950s, which has resulted in considerable changes of hydrodynamics and morphodynamics in the area. In this paper, the effects of the anthropogenic activities on the hydrodynamics and morphological evolution in the estuary at different stages are systematically assessed based on the detailed bathymetric data and field survey. The results show that the human activities have caused the channelization of the enclosed sea area in the Modamen Estuary; fast seaward movement of the mouth bar with high siltation; expansion of the channel volume due to channel deepening. The paper also highlights the main hydrodynamic changes in the estuary, including the rise of the water level; the distinguishing changes of tidal range before and after the 1990s (decrease and increase respectively); as well as the increase of the divided flow ratio. It is found that reclamation is the main factor promoting the transition of nature of the estuary from runoff dominant to runoff and wave dominant, and sand mining activities are mainly to strengthen the tidal dynamic and to low the water level. The results provide useful guidance for better planning of the future developments in the estuary and further research in the area.

Influence of morphological changes in a lagoon flooding extension: case study of Ria de Aveiro (Portugal)

Lopes, C.L., Plecha, S., Silva, P.A., and Dias, J.M., 2013. Influence of morphological changes in a lagoon flooding extension: case study of Ria de Aveiro (Portugal) In: Conley, D.C., Masselink, G., Russell, P.E. and O'Hare, T.J. (eds.)Coastal lagoons are interface zones where land, water and atmosphere interact in a dynamic balance that is constantly being changed by natural and human influence. The hydrodynamics and morphology of these coastal systems continuously adapt to changes in the forcing agents and show a complex interconnection. Worldwide studies show that tidal propagation is strongly dependent on local morphologic features. Attending to these concerns, the present study aims to assess the changes in flooding extension at Ria de Aveiro, as well as in the tidal prism, induced by local morphological modifications. The analysis of the topo-hydrography available for the Ria de Aveiro, comprising two general surveys carried out in 1987/88 and 2011 and two updates of the inlet bathymetry performed in 2001 and 2012, shows the deepening of the lagoon main channels and of the inlet evolving region. A flooding assessment of the lagoon margins and the determination of the tidal prism were performed applying the hydrodynamic model ELCIRC considering the four morphologic configurations described and different tidal conditions. The numerical results show the importance of lagoon morphological changes, which induce an increase of about 16% between 1987/88 and 2012 in the lagoon flooded area for spring tide. The new flooded regions correspond to the margins of S.Jacinto channel head and lagoon central areas. Additionally, the tidal prism results evidence a redistribution of the water volume flowing into the lagoon among their main channels during the period under analysis.

ALTERATION IN SALINITY IN THE WESER ESTUARY AFTER 1998: QUANTIFICATION BY ARTIFICIAL NEURAL NETWORKS

In Germany major harbours are located in the river estu-aries. Because of the growth of container ships and the corresponding draft of these ships, the depth of the chan-nels in the estuaries had to be increased several times. This results in consequences for nature and agriculture. One of these effects might be that the brackish water zone is moving upstream and the surrounding farmers can't provide fresh water to their animals or plants any more. The study presented on the poster will show the effects on salinity in the Weser estuary after the channel deepening in 1998-2000 by using artificial neural networks.

Turning the tide: experimental creation of tidal channel networks and ebb deltas

AbstractTidal channel networks, estuaries and ebb deltas are usually formed over a period longer than observations cover. Much is known about their characteristics and formation from linear stability analyses, numerical modelling and field observations. However, experiments are rare whilst these can provide data-rich descriptions of morphological evolution in fully controlled boundary and initial conditions. Our objective is to ascertain whether tidal basins can be formed in experiments, what the possible scale effects are, and whether morphological equilibrium of such systems exists.We experimentally created tidal basins with simple channel networks and ebb deltas in a 1.2 by 1.2 m square basin with either a fixed or self-formed tidal inlet and initially flat sediment bed in the tidal basin raised above the bed of the sea. Rather than create tides by varying water level, we tilted the entire basin over the diagonal. The advantage of this novel method is that the bed surface slopes in downstream direction both during flood and ebb phases, resulting in significant transport and morphological change in the flood phase as well as the ebb phase. This overcomes the major problem of earlier experiments which were entirely ebb-dominated, and reduces the experiment time by an order of magnitude.Ebb deltas formed in sand were entirely bedload dominated whereas the lightweight plastic sediment was intermittently suspended. Channels bifurcated during channel deepening and backward erosion to form a network of up to four orders. For initially dry tidal plains, the tidal prism increased as more sediment eroded from basin to ebb delta, so that evolution accelerated initially. The rate of change, the size of the channels and the final length of channels and delta were very sensitive to the tidal amplitude, tidal period and initial water depth in the basin. Most experiments with sand terminated with all sediment below the threshold for motion, whilst lightweight sediment remained mobile in the inlet region and firstorder channels, suggesting that sustained morphodynamics are feasible in experiments. We discuss how this novel experimental setup can be extended to produce tidal deltas, estuaries and other tidal systems and study their dynamics as a function of their forcing.

Impact of Back-Barrier Dams on the Development of the Ebb-Tidal Delta of the Eastern Scheldt

Elekema, M.; Wang, Z.B., and Stive, M.J.F., 2012. Impact of back-barrier dams on the development of the ebb-tidal delta of the Eastern Scheldt.The Eastern Scheldt tidal basin in the SW part of the Netherlands has seen dramatic changes due to human intervention in the past few decades. The construction of two back-barrier dams in 1965 and 1969 had a significant impact on tidal hydrodynamics and sediment transport. The effects of these interventions were still ongoing when the hydrodynamic regime was altered again by the construction of the storm surge barrier between 1983 and 1986.This research describes the hydrodynamic and morphological evolution of the Eastern Scheldt ebb-tidal delta between 1960 and 1983, before construction of the storm surge barrier started. The analysis focuses on how the back-barrier dams changed the tidal flow on the ebb-tidal delta and how these altered hydrodynamics influenced sediment transport and morphology. It starts with a description of the observed hydrodynamic and bathymetric changes. These observations are used as input for a process-based numerical model (Delft3D FLOW), which is used to simulate situations before and after channel closures.In the decades before 1965, the Eastern Scheldt exported large quantities of sediment towards the sea through its inlet. The implementation of the dams caused a significant increase in tidal prism, which was observable in the response of the bathymetry; the export and the rates of channel deepening and ebb-tidal delta growth both increased. Analysis of tidal-flow measurements and model output show a persistent trend for sediment transport towards and out of the Eastern Scheldt inlet. The construction of the back-barrier dams only amplified this export by increasing the tidal prism and pushing the basin out of equilibrium even more than it apparently already was.

Spatial variability in sedimentation rates and artificial radionuclide storage in alluvial banks of the lower Rhône River

This paper analyzes the relationship between bank sediment storage and radionuclide content in six alluvial sites located in different geomorphic contexts along the lower Rhone River. The 137Cs, 238Pu, 239+240Pu, 241Am and 210Pb profiles show different patterns, which indicates a differential storage of contaminated sediment in the banks. Three sites record historical nuclear releases in the river and give evidence for long-term retention of particle-reactive long-lived radionuclides. Two sites record only atmospheric global fallout. Only one site, connected to the river groundwater, provides some evidence for desorption of particle-bound contaminants, with a low and constant 137Cs activity profile. The history of the releases from the Marcoule spent-fuel reprocessing plant—the main source of artificial radioactivity—provides a reliable chronology of the last 50 years. Sediment grain size and bank topography are important factors in determining where artificial radionuclides are stored, but these two parameters cannot be used alone to determine variations in high concentrations of radionuclides. The chronology of fluvial geomorphic “metamorphosis” during the twentieth Century, especially after 1960, is also a critical factor affecting the spatial variability in sedimentation rates and artificial radionuclide storage; the timing of channel deepening and bank sedimentary accretion interfere with the chronology of major floods and the short period of low discharge during the height of contamination from nuclear liquid effluents. The reach-scale adjustment described in this paper can contribute to determining what the local history may have been. This result has important implications for river management decisions.

Formation and evolution of a tidal channel network within a constructed tidal marsh

The morphogenesis of tidal channel networks that dissect intertidal flats and marshes has been studied especially by morphodynamic modeling, while relatively few empirical data exist on high-resolution field observations. Here we measured the spontaneous formation and evolution of a tidal channel network in a newly constructed tidal marsh (Scheldt estuary, Belgium) over a period of 4years, by high-accuracy topographic surveying with a temporal resolution of 1year at high spatial resolution considering all channels deeper than 0.1m. As a reference, topographic measurements with a similar high resolution were performed in a nearby, mature natural tidal marsh network. Based on the field surveying and additional GIS processing, we derived several geometric and hydraulic parameters (channel width, depth, eroded volumes, cross-sectional area, length profiles, drainage density, mainstream length, tidal discharge and watershed area), and compared the evolution of geometric relationships in the constructed marsh with the natural tidal marsh. In this way we have evaluated how fast an equilibrium state was attained. We found that after 2 to 3years of tidal working the cross-sectional areas of former ditches in the constructed marsh were in equilibrium with the corresponding tidal discharge. Furthermore we observed that the mainstream lengths and the drainage densities for the smaller watershed areas were comparable with the natural tidal marsh, demonstrating the rapid headward growth of newly forming channels and tributary channel formation near the channel heads. Newly formed channels preferentially developed in the low elevation zones of the constructed marsh and channel extension was not significantly influenced by the presence or absence of vegetation. However, the overall channel drainage density and channel cross-sectional areas of the newly formed channels were still lower compared to the natural tidal marsh after 4years. This indicates that further channel network extension and continued channel deepening can be expected in the coming years.

Origin of Stream Flows at the Wildlands-Urban Interface, Santa Monica Mountains, California, U.S.A

The Santa Monica Mountains of southern California have undergone moderate urbanization since 1960. Just a few decades ago, many of the riverine systems in the Santa Monica Mountains were intermittent, but they are now perennial. The transition to perennial streams has led many policy makers to conclude that urban runoff from landscape watering accounts for continuous flows during the lengthy dry season. The transition to perennial flows allows expansion of habitat for exotic and harmful species, raising arguments for controls on urban runoff during the dry season. Most communities in the Santa Monica Mountains depend entirely on State Project water imported from northern California. Stable isotopes of oxygen and hydrogen show that imported water is isotopically distinct from local precipitation, providing a useful tool for tagging source flows in Santa Monica Mountain streams. In this investigation, we perform a detailed analysis of dry weather flows in one of the Santa Monica Mountain9s major streams, Las Virgenes Creek. We also present results of reconnaissance surveys of several other creeks. Las Virgenes Creek accumulates most of its initial flow along an urban part of its stream reach. Chloride and sulfate scatter plots indicate that Las Virgenes Creek is dominantly fed along the urban stream reach by locally sourced, groundwater base flow. Oxygen and deuterium isotopes show that Las Virgenes Creek and other creeks in the Santa Monica Mountains contain small percentages of imported waters (less than 10 percent), indicating little urban runoff during dry weather flows. The data suggest that perennial flow in urban streams results from the removal of riparian vegetation and deepening of channels rather than from urban runoff.

Factors affecting stranding of juvenile salmonids by wakes from ship passage in the Lower Columbia River

AbstractThe effects of deep‐draft vessel traffic in confined riverine channels on shorelines and fish are of widespread concern. In the Pacific Northwest of the United States, wakes and subsequent beach run‐up from ships transiting the Lower Columbia River have been observed to strand juvenile salmon and other fish. As part of a before‐and‐after study to assess stranding effects that may be associated with channel deepening, we measured 19 variables from observations of 126 vessel passages at three low‐slope beaches and used multiple logistic regression to discern the significant factors influencing the frequency of stranding. Subyearling Chinook salmon were 82% of the fish stranded over all sites and seasons. Given a low‐slope beach, stranding frequencies for juvenile salmon were significantly related to river location, salmon density in the shallows, a proxy for ship kinetic energy, tidal height and two interactions. The beach types selected for our study do not include all the beach types along the Lower Columbia River so that the stranding probabilities described here cannot be extrapolated river‐wide. A more sophisticated modelling effort, informed by additional field data, is needed to assess salmon losses by stranding for the entire lower river. Such modelling needs to include river‐scale factors such as beach type, berms, proximity to navigation channel, and perhaps, proximity to tributaries that act as sources of outmigrating juvenile salmon. At both river and beach scales, no one factor produces stranding; rather interactions among several conditions produce a stranding event and give stranding its episodic nature. Published in 2010 by John Wiley & Sons, Ltd.

Analytical and numerical analysis of tides and salinities in estuaries; part II: salinity distributions in prismatic and convergent tidal channels

Estuaries, commonly, are densely populated areas serving the needs of the inhabitants in multiple ways. Often the interests are conflicting and decisions need to be made by the local managers. Intake of fresh water for consumption, agricultural purposes or use by industries may take place within a region not far landward of the limit of salt intrusion. Human interventions (e.g. deepening of the navigation channels) or climate changes (sea level rise, reduction of the river discharge) can bring these intake locations within the reach of saline or brackish water and consequently endanger their function. To support policy and managerial decisions, a profound knowledge of processes associated with the salinity structure in estuaries is required. Although nowadays advanced numerical three-dimensional models are available that are able to cope with the complexity of the physics there is still a need for relatively simple tools for quick-scan actions in a pre-phase of a project or for instructive purposes. The analytical model described in this paper may serve these needs. It computes the maximum salinity distribution using the dispersion coefficient in the mouth as the only model parameter. The model has been calibrated using observational data in a large number of estuaries and experimental data in a tidal flume. The dispersion coefficient was successfully related to geometric and hydrodynamic parameters resulting in an expression that can be used for convergent estuaries as well as prismatic channels, see Eqs. 25a and 25b. Application of the model in a predictive mode showed its promising capabilities. Comparison with three-dimensional numerical models indicates that the channel geometry in the estuary mouth largely influences dispersive processes. The analytical model for salt intrusion may be used in combination with the analytical model for tidal propagation in convergent estuaries and tidal channels by Van Rijn (part I). In this way, input is obtained on the tidal velocity amplitude and the Chezy roughness following calibration of this model on tidal amplitudes along the estuary.

Fine sediment transport by tidal asymmetry in the high-concentrated Ems River: indications for a regime shift in response to channel deepening

This paper describes an analysis of the observed up-river transport of fine sediments in the Ems River, Germany/Netherlands, using a 1DV POINT MODEL, accounting for turbulence-induced flocculation and sediment-induced buoyancy destruction. From this analysis, it is inferred that the net up-river transport is mainly due to an asymmetry in vertical mixing, often referred to as internal tidal asymmetry. It is argued that the large stratification observed during ebb should be attributed to a profound interaction between turbulence-induced flocculation and sediment-induced buoyancy destruction, as a result of which the river became an efficient trap for fine suspended sediment. Moreover, an asymmetry in flocculation processes was found, such that during flood relative large flocs are transported at relative large flow velocity high in the water column, whereas during ebb, the larger flocs are transported at smaller velocities close to the bed—this asymmetry contributes to the large trapping mentioned above. The internal tidal asymmetry and asymmetry in flocculation processes are both driven by the pronounced asymmetry in flow velocities, with flood velocities almost twice the ebb values. It is further argued that this efficient trapping is the result of a continuous deepening of the river, and occurs when concentrations in the river become typically a few hundred mg/l; this was the case during the 1990 survey analyzed in this paper. We also speculate that a second regime shift did occur in the river when fluid mud layers become so thick that net transport rates are directly related to the asymmetry in flow velocity itself, probably still in conjunction with internal asymmetry as well. This would yield an efficient mechanism to transport large amounts of fine sediment far up-river, as currently observed.