Search

Net sediment transport is predominantly seaward in fluvial‐dominated estuaries worldwide. However, a distributary branch in the Changjiang Estuary, the North Branch, undergoes net landward sediment transport, which leads to severe channel aggradation. Its controlling mechanism and the role of human activities remain insufficiently understood, although such knowledge is necessary for better management and restoration opportunities. In this study we revisit the centennial hydro‐morphodynamic evolution of the North Branch based on historical maps, field data, and satellite images and provide a synthesis of the regime change from ebb to flood dominance. The North Branch was once a major river and ebb‐dominant distributary channel. Within which alternative meandering channels and sand bars developed. Deposition of river‐borne sediment leads to infilling of the branch, while tidal flat embankment reduces the bankfull width and modifies the channel configuration, resulting in a profound decline in the sub‐tidal flow partition rate. The North Branch then becomes tide‐dominant with an occurrence of tidal bores and elongated sand ridges. Once tidal dominance is established, extensive tidal flat reclamation enhances the funnel‐shaped planform, amplifying the incoming tides and initiating a positive feedback process that links tidal flat loss, sediment import, and channel aggradation. Overall, the shift in branch dominance is a combined result of a natural southeastward realignment of the deltaic distributary channels and extensive reclamation. One management option to mitigate channel aggradation is to stop the aggressive reclamation and allow tidal flats to build up, which might reduce the sediment import and eventually lead to a morphodynamic equilibrium in the longer term. Understanding the impact of tidal flat reclamation is informative for the management of similar tidal systems under strong human interference. [ABSTRACT FROM AUTHOR]

Salt marshes are ecologically and physically essential habitats in coastal intertidal flats. In addition to providing nursery grounds for various wildlife, vegetation in marshes acts as a buffer zone that slows flooding waters. Extensive marsh platforms can also attenuate wave energy to protect coastal communities. Complex geometries make it challenging for models to capture the spatiotemporal variabilities in the transport of lateral water and sediment over tidal creek‐marsh complexes. To study water exchange processes between tidal creeks and marshes, field observations were conducted within tidal creeks in the Chongming Dongtan Shoal of the Changjiang Estuary, an area dominated by Phragmites australis, Scirpus mariqueter, and Spartina alterniflora. A coupled wave‐current‐sediment‐vegetation model that parameterized the three vegetation species was developed and applied to the tidal creek complex of Chongming Dongtan. Validated with the observational data, the modeling results were used to examine the influence of vegetation on flooding/draining processes within tidal creeks and mudflats. The model showed that vegetation not only modified the flow pattern at high tide in the high‐elevation marsh but also significantly intensified the tidal flows in tidal creeks in the low‐elevation regions. Both the observations and model detected significant transport of water and sediment from the marsh to tidal creeks, seaward sediment export over tidal cycles. The simulated salinity exhibited a considerable spatial gradient, matching well with the salt tolerances of the major vegetation types. This model can be potentially used to determine how future scenarios of hydrodynamics could affect vegetation zonation. Key Points: A vegetation model was developed to simulate the impacts of multiple vegetation species on flow, turbulence, waves and sedimentThree vegetation species were parameterized in the module, which was successfully applied to the tidal creek complex of Chongming DongtanModel demonstrated seaward sediment export from creeks, and consistency between the predicted salinity and the vegetation zonation [ABSTRACT FROM AUTHOR]

The morphology of the Yangtze Estuary has changed substantially at decadal time scales in response to natural processes, local human interference and reduced sediment supply. Due to its high sediment load, the morphodynamic response time of the estuary is short, providing a valuable semi‐natural system to evaluate large‐scale estuarine morphodynamic responses to interference. Previous studies primarily addressed local morphologic changes within the estuary, but since an overall sediment balance is missing, it remains unclear whether the estuary as a whole has shifted from sedimentation to erosion in response to reduced riverine sediment supply (e.g. resulting from construction of the Three Gorges Dam). In this paper we examine the morphological changes of two large shoals in the mouth zone (i.e. the Hengsha flat and the Jiuduan shoal) using bathymetric data collected between 1953 and 2016 and a series of satellite images. We observe that the two shoals accreted at different rates before 2010 but reverted to erosion thereafter. Human activities such as dredging and dumping contribute to erosion, masking the impacts of sediment source reduction. The effects of local human intervention (such as the construction of a navigation channel) are instantaneous and are likely to have already resulted in new dynamic equilibrium conditions. The morphodynamic response time of the mouth zone to riverine sediment decrease is further suggested to be >30 years (starting from the mid‐1980s). Accounting for the different adaptation time scales of various human activities is essential when interpreting morphodynamic changes in large‐scale estuaries and deltas. © 2019 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Separation zone and shear face generate most turbulence in confluence junctions. Flume experiment studies show the variation of a separation zone from the water surface to the bottom and impacts of the two circulation cells (secondary flow) on the sediment transportation in a confluence zone. The turbulence generated by the shear face is anisotropic, so it needs to be cautious to choose appropriate turbulence models in the simulation of confluence flow. This article utilizes the standard k − ϵ, RNG k − ϵ, and RSM turbulence models to simulate the secondary flow and separation zone in the confluence junction, and a quantificational comparison with the experiment data is provided. [ABSTRACT FROM AUTHOR]

The effects of roughness on the flow structure in a gravel-bed channel were investigated by using particle image velocimetry. The bed was composed of gravel particles, of which the median diameter, d50, (i.e., the diameter at which 50% are smaller) ranged from 2.5 to 35.0 mm. The flow covered a wide range of Reynolds numbers (R=3,048-69,982). The experimental results showed that the logarithmic-law region of the velocity profile changed with the size of the roughness elements. Under the large-scale roughness condition, that is, h/d50=1.78 (the ratio of flow depth to median diameter), the mean velocity distribution deviated from the logarithmic law in the near-wall region. In addition, a new formula was established to calculate the Darcy-Weisbach friction factor for the gravel bed, expressed for the Reynolds number and the surface geometric parameters. On the basis of the boundary layer theory, the displacement thickness was introduced to estimate the limit of the roughness effect, and it was found to be greatly affected in the range of 35,000

• Storm intensity and sediment availability control tidal flat evolution during a storm. • Storms enhance sediment supply for the studied flats undergoing long-term erosion. • A moderate storm offsets rather than accelerates tidal flat erosion. Accretion rates of estuarine tidal flats have strongly decreased globally due to the combined impacts of sea-level rise, reduction in sediment supply, and human interventions. Storms and associated high-energy activities influence sedimentary processes on tidal flats. A better understanding of the role of storm events on the evolution of tidal flats can facilitate better coastal management. This study assesses the impacts of storms on intertidal sedimentary processes. We monitored tidal water levels, tidal currents, waves, suspended sediments, and morphological changes of the eroding Diaokouhe lobe in the Yellow River Delta during and after a storm event. The estimated bed shear stress during the storm event (average 1.60 N/m2) was much higher than that during fair-weather conditions (average 0.71 N/m2). However, surface erosion of tidal flats was negligible during the storm, which was ascribed to the high critical bed shear stress owing to the compacted bed surface. The tidal flats were inundated for a longer time with a higher water depth and higher suspended sediment concentrations (average 1.13 g/L) during the storm, indicating sediment supply from the subtidal zone and adjacent sea. These conditions, namely moderate storm strength, high sediment compaction, and high sediment availability, led to deposition rather than erosion on the intertidal flats. Overall, the results imply that storm-induced erosion is less likely to occur on tidal flats with rich sediment supply and high erosion resistance. [ABSTRACT FROM AUTHOR]

A tidal creek represents a typical morphologic unit in an intertidal flat. The development and migration of a tidal creek can affect the mass transport, ecological environment, and geomorphologic evolution of the flat. By using field observations, this study links hydrodynamics, sediment transport processes with short-term changes in topography at a typical tidal creek system located at the Chongming Island of the Yangtze River estuary. Hydrodynamic and sediment transport associated with varying tidal cycles across both wet (flood) and dry seasons were measured through the field campaign. The wet and dry seasons represent higher and lower discharges of Yangtze River, respectively. The results indicated that most of the suspended sediment becomes entrained at the beginning of a flood tide. At a fixed point, 7.2 times of suspended sediments, which were entrained out of the creek in wet season, began to be transported along the creek compared to dry season. In the dry season, high flow velocity and shear stress conditions occurred in the tidal creek because the water level was below the top of the flat. In summary, the tidal creeks were found to serve as effective conduits for the transportation of sediments in the wet season, and the secondary flow enhanced the development of tidal meandering. Seasonal variations in creek morphological changes were also continuously monitored over two years at intervals of two months. The change of creek morphology varied from the high level to low level, and tidal meandering was strongly associated with flood and ebb tides. • Water flow and sediment transport were studied in a meso tidal creek system. • The tidal flat tended to receive sediment deposits from April to October, and to be eroded in the other months. • Tidal creek morphology was stable at the high creek level, and it meandered at the low level. • The results contribute to a better understanding of the development mechanism of tidal creeks. [ABSTRACT FROM AUTHOR]

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 × 10 8  m 3 . Spatially, the pan-South Branch was net eroded by 9.7 × 10 8  m 3 whereas the mouth bar zone was net deposited by 18 × 10 8  m 3 , 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. [ABSTRACT FROM AUTHOR]