A Positive Feedback Between Sediment Deposition and Tidal Prism May Affect the Morphodynamic Evolution of Tidal Deltas

Tidal deltas are fragile systems whose morphology can be easily impacted by variations in water and sediment fluxes caused by natural and human processes. Here we explore the relation between tidal prism and sediment dynamics in tidal deltas using the recent evolution of the Yangtze River estuary, China, as an example. Using the numerical model Delft3D, we examine how changes in delta morphology can trigger variations in tidal signal, suspended and bed load transport, and how these could ultimately cause additional morphological changes. Our results show that a positive feedback between sediment deposition and tidal prism dominates the morphodynamic evolution of the delta. Accretion of the shoals in the delta front increases the dissipation of tides and decreases the tidal prism leading to weaker tidal flows. This reduction in tidal currents lowers the sediment flushing capacity of the system, promoting deposition on the shoals and tidal dissipation. This positive feedback potentially traps more sediment in the delta topset and possibly offsets the decrease in sediment load triggered by the construction of upstream dams, often present in tidal deltas

Human Intervention–Induced Changes in the Characteristics of the Turbidity Maximum Zone and Associated Mouth Bars in the Yangtze Estuary

In the past two decades, the dynamic sedimentation process of the Yangtze Estuary has been seriously disturbed by coupled human interventions from the river basin to the estuary, especially the impoundment of the Three Gorges Dam in 2003 and the large-scale Deep-water Navigational Channel (DNC) regulation project in 1998–2010. This study investigated the changes in sedimentary dynamic and geomorphological processes in the turbidity maximum zone (TMZ) by analyzing the historical and present data for current, salinity, suspended sediment, and bathymetry. The results show that the decreased riverine sediment input caused a lagging decrease in suspended sediment concentration in the TMZ during the flood seasons. The DNC caused changes in the flow structure, sediment transport, and geometry of the TMZ in the North Passage (NP) and the South Passage (SP). In the NP, decreased ebb transport in the upper reaches led to landward migration of the TMZ during low discharges, while increased ebb transport in the middle and lower reaches caused the seaward migration of the TMZ during high discharges. As the associated topography of the TMZ, the mouth bar in the NP was mostly removed by channel dredging. However, rapid deposition at the location of the previous mouth bar indicates the formation of an incipient bar. In the SP, increased ebb transport after the DNC-induced disappearance of the TMZ and the mouth bar in the upper reaches and the seaward migration of the TMZ in the middle and lower reaches. Therefore, we found that the construction of dams and large-scale estuarine projects changed the sediment dynamics and geomorphological processes of the TMZ and even affected the long-term evolution of the estuary. Construction regulation projects in the TMZ, intended to narrow the cross-section and enhance seaward sediment transport, may produce the opposite effect. Before and after engineering projects, their impacts on estuarine processes need to be carefully estimated

Human Intervention–Induced Changes in the Characteristics of the Turbidity Maximum Zone and Associated Mouth Bars in the Yangtze Estuary

In the past two decades, the dynamic sedimentation process of the Yangtze Estuary has been seriously disturbed by coupled human interventions from the river basin to the estuary, especially the impoundment of the Three Gorges Dam in 2003 and the large-scale Deep-water Navigational Channel (DNC) regulation project in 1998–2010. This study investigated the changes in sedimentary dynamic and geomorphological processes in the turbidity maximum zone (TMZ) by analyzing the historical and present data for current, salinity, suspended sediment, and bathymetry. The results show that the decreased riverine sediment input caused a lagging decrease in suspended sediment concentration in the TMZ during the flood seasons. The DNC caused changes in the flow structure, sediment transport, and geometry of the TMZ in the North Passage (NP) and the South Passage (SP). In the NP, decreased ebb transport in the upper reaches led to landward migration of the TMZ during low discharges, while increased ebb transport in the middle and lower reaches caused the seaward migration of the TMZ during high discharges. As the associated topography of the TMZ, the mouth bar in the NP was mostly removed by channel dredging. However, rapid deposition at the location of the previous mouth bar indicates the formation of an incipient bar. In the SP, increased ebb transport after the DNC-induced disappearance of the TMZ and the mouth bar in the upper reaches and the seaward migration of the TMZ in the middle and lower reaches. Therefore, we found that the construction of dams and large-scale estuarine projects changed the sediment dynamics and geomorphological processes of the TMZ and even affected the long-term evolution of the estuary. Construction regulation projects in the TMZ, intended to narrow the cross-section and enhance seaward sediment transport, may produce the opposite effect. Before and after engineering projects, their impacts on estuarine processes need to be carefully estimated

Effects of navigational works on morphological changes in the bar area of the Yangtze Estuary

To improve navigability of the major access channel into Shanghai Harbor, a large-scale Deep Waterway Project was carried out in the North Passage (NP) of the Yangtze Estuary. In this paper, we investigate how the navigational works affected morphological changes of this channel, as well as those of the adjacent North Channel (NC), Hengsha East Shoal (HES), Jiuduansha Shoal (JDS), and the South Passage (SP). Morphological changes were assessed by analyzing digitized bathymetric data of this area prior to and after execution of the engineering works. The qualitative relations between these changes, along with the hydrodynamic changes as a result of the construction of engineering works, were subsequently investigated. The results reveal that the construction of training walls, groins and jetties resulted in decreased ebb transport in the upper reach of the NP and increased ebb transport in the upper reach of the SP. In turn, this led to intensive upstream erosion with large amounts of sediment transported seaward in the SP and associated sedimentation in the downstream area. At the same time, intense siltation occurred in the upper reach of the NP, while the main channel of the NP mainly experienced erosion caused by the construction of training walls and groins that concentrated ebb flow in the main channel. The waterway deepened significantly in previously shallow areas. Small tidal channels in the HES, which used to connect the NP and the NC, were cut off by the northern training wall. Consequently, residual flow in the middle reach of the NC flowed directly toward the northern bank of the HES, leading to local erosion. Flow obstruction by the southern training wall reduced the upstream propagation of the flood tide in the SP. As a result, a collection of small flood channels evolved under the flushing action of flood currents in the JDS, preventing the natural trend of horizontal extension of the JDS. However, the flow obstruction enhanced the vertical accretion of the JDS. -------------------------------------------------------------------------------

A positive feedback between sediment deposition and tidal prism may affect the morphodynamic evolution of tidal deltas

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Stabilization of Fluidic Silty Sands with Cement and Steel Slag

Fluidic silty sand is often difficult to use directly in engineering construction because of its low strength and plasticity index. This study employed steel slag to replace part of the cement in silty sand stabilization to broaden the feasibility of resource recycling and to reduce the construction cost and carbon emissions in engineering practices. A series of indoor tests investigated the influences of the cement/steel slag ratio, initial water content, curing age, and temperature on the compressive strength of cement- and steel slag-stabilized fluidic silty sands (CSFSSs). Their stabilization mechanism was discussed via microstructural observation and spectral analysis. The results showed that the most economical cement/steel slag ratio could be 9:6, saving 40% of cement and not changing with the initial water content. The compressive strength of the CSFSSs decreased with the initial water content and increased rapidly and then slowly over the curing age. The curing temperature had a positive impact on their strength growth. The microstructure characteristics and spectral analysis showed that adding steel slag indeed affected the formation of gels in the cement-stabilized fluidic silty sands. This study could reference the application of CSFSSs in engineering practices