Marsh creeks are perceived as important conduits for transporting water and sediment between mudflats and marshes. In order to advance the understanding of the transport mechanisms in creeks, the source and ultimate sink of sediment which moves between mudflats and marshes through creek channels need further investigation. Therefore, two field campaigns were conducted in two intertidal systems with varying sediment availability. The water depth, flow velocity, suspended sediment concentration, and bed level change were measured simultaneously in a marsh creek and on the adjacent mudflat in Chongming Island (China) and in Paulina Saltmarsh (the Netherlands). Paulina Saltmarsh is much smaller, more frequently flooded, and has lower sediment concentration than Chongming. These contrasting conditions allow for a comparison of transport mechanisms and functioning of the creek. Both systems first show that the high suspended sediment concentration (SSC) measured in marsh creeks is mainly the consequence of sediment advection rather than local erosion. In addition, erosion in marsh creeks is usually limited during ebb tides, reducing the export of sediment through these creeks. However, differences have been observed between two systems. The measured SSC was highly asymmetric between flood and ebb tides in Chongming. Large peaks in SSC during the flood period can be observed for most tidal cycles. The marsh creeks in Chongming therefore function as conduits for sediment import. Additionally, there are distinct overbank and underbank tides in Chongming. Sediment was trapped and retained in creeks during underbank tides, which can then be eroded and transported to the marsh during subsequent overbank tides. We also observed that mudflats in Chongming quickly recovered after erosion. These mechanisms have not been observed in Paulina Saltmarsh, where net sediment export via the marsh creek was observed due to a lack of abundant sediment in suspension during flood tides. Furthermore, the remaining bed surface of mudflats after an erosion event was stronger than before, limiting further erosion in Paulina Saltmarsh. These findings from the two systems indicate that the role of creeks in sediment import/export depends on the availability of sediment from mudflats, shedding light on nourishment strategies for salt marshes.

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The mechanisms controlling the formation of an estuarine turbidity maximum (ETM) in estuaries have been extensively investigated, but one aspect that has received much less scientific attention is the role of high suspended sediment concentrations in combination with tidal asymmetry in ETM formation. Particularly in highly turbid estuaries, sediment suspensions influence ETM development through a combination of horizontal sediment-induced density currents, a reduction in turbulent mixing, and water-bed exchange processes. In this study, we developed a schematic model resembling the Yangtze Estuary where the ETM is controlled by tidal pumping, estuarine circulation, and advection operating simultaneously. Model results suggest that high water slack tide asymmetry with Sediment-induced density effects (SedDE) favors landward migration of the ETM. In addition, without SedDE, stronger flood tidal dominance leads to more pronounced sediment trapping through tidal pumping. Depending on the type of tidal asymmetry, SedDE strengthen ETM growth by increasing estuarine circulation but may also lead to increased or reduced sediment concentration in the ETM due to enhanced or weakened landward tidal pumping, respectively. Higher near-bed sediment concentrations as a result of water-bed exchange processes, in turn, strengthen the effect of estuarine circulation but simultaneously strengthen the divergence of sediment by tidal pumping. Overall, the SedDE and higher near-bed sediment concentration, in combination with tidal asymmetry, play an important role in ETM formation and should be properly accounted for in studies on ETM dynamics in turbid estuaries.

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Estuaries are the core area of land-sea interactions and have significant ecological and economic value. In estuaries, hydrodynamics and sediment dynamics are the crucial processes governing geomorphology, navigability, and primary production. Since the turn of the 20th century, increased human activities (e.g., damming, dredging, and reclamation) have subjected estuaries to significant pressure and prompted changes in hydrodynamics and sediment dynamics. Some estuaries, for example, experienced a transition from low- to hyper-turbidity after channel deepening. This transition is particularly common in tide-dominated estuaries such as the Ems and Loire estuaries. However, the reactions of estuaries with a high runoff (such as the Changjiang/Yangtze Estuary) are unclear, and the transition and underlying processes of such estuaries are further complicated by declined fluvial sediment supply. By integrating acoustic and optical sensors, this PhD dissertation developed a wide-range and high-precision sediment concentration observation system to monitor high sediment concentrations in the Changjiang Estuary. Over the past three decades, observations revealed a transition in suspended sediment concentrations, with increasing concentrations near the bed and decreasing concentrations near the water surface. The drag reduction induced by suspended sediment was assessed by a bottom-mounted tripod system. Moreover, this dissertation clarified the mechanism controlling the formation of concentrated benthic suspensions, namely the positive feedback between stratification, turbulence damping, and hindered setting. The comparison between sediment transport processes before and after the Deep Waterway Project reveals that estuarine circulation is the primary force driving sediment import from the sea, whereas tidal pumping results in the along-estuary extension of the estuarine turbidity maximum zone. These findings enhance our knowledge of the response of estuarine hydrodynamics and sediment dynamics to human interventions and provide a theoretical basis for the effective management of estuarine systems.@en

An estuarine turbidity maximum (ETM) is a region of elevated suspended sediment concentration (SSC) resulting from residual transport mechanisms driven by river flow, tides, and salinity-induced density gradients (SalDG). However, in energetic and highly turbid environments such as the Yangtze Estuary, SedDG may also substantially contribute to the formation and maintenance of the ETM. Since this mechanism is relatively poorly understood, we develop a three-dimensional model to explore the effect of SedDG on tidal dynamics and sediment transport. By running sensitivity simulations considering SalDG and/or SedDG, we conclude that the longitudinal SedDG leads to degeneration and landward movement of the ETM. Moreover, two effects of the vertical SedDG are identified to be responsible for sediment trapping: One by enhancing the vertical sediment concentration gradients, and another by additionally affecting hydrodynamics including the water levels, velocities and salinities. The longitudinal and vertical SedDG leads to seasonal and spring-neap variations of upstream migration of the salt wedge: Vertical SedDG is more pronounced at neap tides in the wet season due to stronger stratification effects, whereas longitudinal SedDG is more pronounced at intermediate tides in the dry season due to weaker mixing and limited deposition. These findings imply that the SedDG contributes substantially to channel siltation and salt intrusion in highly turbid systems, and need to be accounted for when numerically modeling such phenomena.

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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 (<1 km) as the CBS is concentrated in the Deepwater Navigational Channel. The movements of the CBS pool, combined with tidal asymmetry (e.g., slack-water asymmetry and lateral flow asymmetry), results in sediment trapping in the middle reaches and on the south flank of the channel. Observations by a bottom tripod system show the response of friction/drag coefficient to sediment concentration: (1) nearly linear decrease within low SSC (<10 kg/m³); (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.

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Accurate measurement of suspended sediment concentration (SSC) in highly turbid environments has been a problem due to optical or acoustic signal saturation and attenuation. The saturation returns a limited measurement range, and the attenuation raises an ambiguity problem that a low optical or acoustic output could mean a low or a high SSC. In this study, an integrated optic and acoustic (IOA) approach is proposed to (i) overcome the ambiguity problem; (ii) increase the measurement range to high SSC values; and (iii) obtain high-resolution SSC profiles. The IOA approach is a combination of Argus Suspension Meter (ASM), Optical Backscatter Sensor (OBS) and Acoustic Doppler Velocimeter (ADV). In this approach, the ASM-derived SSC is preferred because of its lowest relative error, followed by OBS and ADV. The ASM can produce high-resolution (1 cm interval) SSC profiles when it is not saturated (usually SSC < 9 g/L). When ASM is saturated, the SSC is recovered by OBS. Since the ambiguity problem is solved, the measurement range of OBS and ADV can be extended up to 300 g/L. The best way to use an ADV, however, is to have a rough estimation first and assist in the OBS conversion, because its estimates contain large uncertainty. To further mitigate the impact of sediment particle size on SSC retrieval, we suggest the usage of in-situ sediment samples for sensor calibration. The IOA approach was verified in the Yangtze Estuary which is a highly turbid system. Comparison of the IOA approach outputs against water sampling results demonstrates the reliability of the IOA approach with a relative error of 17–34%. The observed high SSCs were up to 63 g/L. The field data show that high SSCs were confined in the benthic layer (within 2 m above the bed) in the wet season under a high river discharge, whereas the suspension was better mixed throughout the water column in the dry season.

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