The Yangtze River has a huge amount of sediment entering the sea, and its sediment transport process directly affects the development of estuarine deltas and the evolution of shoals and channels. This article is based on the long-term sediment transport data from the Datong Station of the Yangtze River into the sea over the past 70 years (1951~2022), and comprehensively uses Mann Kendall trend test, Pettitt mutation point test, Morlet wavelet analysis, and annual distribution quantification research methods to analyze the trend, mutation, period, and annual distribution characteristics of sediment transport from the Yangtze River into the sea. The results indicate that in the absence of significant trend changes in the inflow of the Yangtze River into the sea, sediment transport has shown a significant downward trend over the past 70 years. The main change in sediment transport is a sudden decrease, which also shows an 18 year periodic stepwise decline pattern. 1968, 1984, and 2002 were the points of abrupt stepwise decline, especially after 2002, the average annual sediment transport decreased by more than 60% compared to before 2002. The distribution of sediment transport within a year was relatively concentrated between the 1980s and 1990s, and relatively uniform in the early 21st century; The main focus of the year is from July to August; The annual variation was the largest in the 1980s and 1990s, but relatively small in the early 21st century. The research results can provide scientific basis for the development and governance of resources in the Yangtze River Delta.

To visualise this residual sediment transport on a longer time scale (years), a sediment balance was calculated for the Flemish part of the Schelde-estuary.The sediment balance is calculated starting from the principle of conservation of mass applied to a simplified schematization of the system. Within each box, changes in sediment volume are explained by (1) an up-estuarine flux of sediment, (2) an down-estuarine flux of sediment and (3) external factors creating a flux of sediment. The changes in volumes are derived from topo-bathymetries for different moments. The volumes are converted to mass fluxes for sand and mud separately by sand-mud bottom characteristics based on 100’s of individual bottom samples, taken over different habitats. At the most up-estuarine boundaries, the fluvial sediment influx is derived from measurements upstream the estuary and its tributaries. The anthropogenic fluxes are derived from registrations. Starting from these known parameters the down-estuarine sediment flux is derived, which is also the up-estuarine flux for the neighboring box. The sand balance for the 3 available periods (2001-2011-2016-2019) shows mainly up-estuarine transport of sand over the entire estuary the sand transport relates to velocity to the power 3 to 5 (eg. Engelund-Hansen formula). The Schelde-estuary is characterised by higher flood velocities, leading to a flood dominance in sand transport. The mud transport has a different pattern, with a down-estuarine transport over most of the estuary. Only at the most downstream location, mud transport is up-estuarine. For mud transport the classic advection-diffusion equation is valid. The tidal asymmetry (increase of ebb period up-estuary) and the increasing importance of fresh water discharge up-estuary, will result in a more ebb-dominant mud transport.

The identification of submarine dunes represents an important role in the study of the seafloor morphology. In this context, the segmentation and characterization of these sedimentary structures allow a better understanding of the dynamism of the seafloor, since the presence of the dunes is related to different environmental conditions. This presentation exposes the results of segmentation and morphological characterization of dunes considering the DBM (Digital Bathymetric Model) derived from measurements made with MBES (MultiBeam EchoSounder) in the Belgian part of North Sea. The approaches considered in this research were developed for the Estuary of the Saint-Lawrence River allowing the automatic extraction of dunes and their morphological characterization from a DBM. The segmentation method relies on a conceptual model in which dunes can be identified from a DBM by their salient features (i.e. crest line, stoss trough, lee trough). Once segmented, the dunes can be characterized by a series of morphological descriptors. Considering a study area of approximately 6km X 4km, 162 dunes were segmented and approximately 95% of these dunes were well segmented with 2.5% of false negative and 2.5% of false positive considering large and very large dunes due to the cell resolution. A morphological characterization was also performed resulting in approximately 88% of dunes well characterized. The morphological descriptors extracted for each sedimentary structure segmented shall be helpful to better understand the relationship between dunes and their hydrodynamic factors.

In order to improve our system understanding and collect datasets for the numerical model validation, field campaigns were executed focusing on the settling and resuspension of (mainly fine) sediments around the moments of slack. During the field campaigns, a survey vessel was put at a fixed position monitoring different parameters. An ADCP was used to measure flow conditions. Both optical and acoustic backscatter, optical transmission and pump sampling were used to measure the suspended sediment concentration. The LISST-100X also provided grain size distributions during the campaigns. Near the moment of slack, flow velocities diminish to almost 0 m/s, followed by a increase of the magnitude in the next tidal phase (from ebb to flood at slack LW and vice versa). The suspended sediment concentration also decreases near the moment of slack, although minimum SSC are reached up to 30-60 minutes later than the moment of slack itself. This is explained by the settling that continues as long as the flow velocities remain below the threshold for resuspension. Further analysis, combining indirect sediment measurements with information from the pump samples, could create more insight in the processes that occur. Also estimating the variation of sediment mass in suspension can provide more information on the settling and resuspension processes, although the advective transport will have an influence on this when measuring on a fixed position.

Topo-bathymetric DEMs are critical products to study coastal dynamics. However, they can be challenging to produce as it requires multiple sensor integration and a precise calibration procedure for custom made bathymetric platform. Cross-validation and data quality assessment are therefore essential for data integration. Multibeam echosounders (MBES) are rarely compared to terrestrial datasets due to the difficult accessibility of the intertidal zone. This study addresses the issue by doing a direct comparison between bathymetric survey and UAV LiDAR/photogrammetric data at a local scale. We used custom designed topo-bathymetric targets, taking advantage of the overlapping area between high and low tide on the survey site. Differential DEMs and point cloud roughness calculation (distance to the local best fitting plane) are used to evaluate surface representation of each sensor. This approach appears to be useful to assess survey quality as it helps to describe each sensor response to environmental variations and survey design. This allows for better use of each dataset in a high resolution surface construction perspective.

The presented study describes the use of a model sediment to study the erosion phenomena of a model mud under different hydrodynamic loads (current and waves). The analysis of these different results in terms of material rheology and flow characteristics will enables us to discuss the similarities between these model material and real mud, as well as the limitations of this approach in erosion studies.