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Wave- and current-supported fluid mud on gently sloped continental shelves represents a type of sediment gravity flow capable of rapidly transporting substantial sediment over short periods, significantly contributing to coastal geomorphic evolution. To investigate the dynamics of intertidal fluid mud events, in-situ observations were conducted from May 9 to May 18, 2017, using a seabed tripod system at the lower intertidal flat of the central Jiangsu coast, China. Fluid mud was observed following a medium wind-wave event, with a maximum significant wave height of 0.42 m. The liquefied seabed, loosened by liquefaction, facilitated bed erosion and sediment resuspension. Fluid mud layers, with mean suspended sediment concentrations (SSCs) exceeding 10 g/L, periodically formed during high slack tide, early flood, and late ebb phases. These layers varied in thickness from 4 cm to 20 cm and exhibited strong stratification caused by suspended sediment. Fluid mud disappeared when the bottom turbulence kinetic energy exceeded a threshold of 0.00045 m²/s² due to an increase in current velocity. The downslope movement of intertidal fluid mud was estimated using a theoretical buoyancy-friction model and validated by observed offshore-directed velocity jets at 0.1 m above the seabed during high slack tide phases. Additionally, onshore winds favored fluid mud formation during early flood phases, while offshore winds favored it during late ebb phases. These observations suggest that fluid mud can form on intertidal flats under conditions where tidal flows and winds align, contributing to a deeper understanding of the formation mechanisms of shallow gravity flows and the improvement of sediment transport models.

Soil organic matter (SOM) is an important part of marine soft soil, and its role in the physico-mechanical properties of the soil needs to be studied urgently. The Holocene marine soft soil in the Lianyungang area is taken as an example, and a hundred groups of SOM tests and physico-mechanical tests are carried out. The results show that the average SOM content is 0.98% in the study area. The SOM content is most widely distributed in the interval of 0.90%−1.00%. The SOM content is unevenly distributed in space. The SOM content of the parallel coastline has a smaller change than that of the vertical coastline. The top-down change rule is complicated, but the overall trend of the SOM content increases with the increasing depth. SOM content is significantly positively correlated with natural moisture content, plastic limit and liquid limit at the level of 0.01. The SOM content has a significant negative correlation with the natural density, dry density and specific gravity in the soil at the level of 0.01. The SOM content and the powder content in the soil are significantly positively correlated at the 0.05 level. The SOM content are poorly correlated with clay content. The SOM content is significantly positively correlated with the void ratio at the stage when the consolidation pressure is less than or equal to 400 kPa at the level of 0.01. The correlation between the void ratio and the SOM content becomes worse when the consolidation pressure is greater than 400 kPa, which is related to the destruction of the complex formed by the SOM content. Marine soft clay minerals have high salinity and water content. SOM and clay minerals are mostly combined through cationic bond bridges in alkaline environments to form organic complexes, which may participate in the millennium and 10 000-year scale carbon cycle. The maximum buried depth of the studied soil is 30 m, which is in line with the evolution law of deep burial of SOM. The maximum deposition time of the studied soil is only about 8 000 years. It is speculated that the SOM has not reached the equilibrium state in the study area, and it is still in the process of further cyclic evolution. The relevant research results of this study are of certain reference value for the engineering construction in the marine soft soil distribution areas.

Slope instability is the result of the continuous evolution of the interaction of geological environment conditions in a certain area, so many factors should be considered when evaluating the stability of a slope. In this paper, the influence factors and evaluation methods of Xiashu loess slope stability in Zhenjiang are taken as the main research objects. Using the attribute reduction of rough set theory, six influencing factors are obtained. The influence degree, importance degree and weight value of influencing factors interaction on slope stability are received by establishing an interaction matrix of influencing factors. Finally, the evaluation index and quantitative standard of slope stability were established, and the calculation method of SII index and the corresponding slope stability classification were determined. As a result, a comprehensive evaluation method for the stability of the Xiashu loess slope in Zhenjiang area is developed, which provide a basis for the slope stability evaluation in this area.