Author(s): Dake Chen; Chi Zhang; Dawei Guan; Yuan Li; Jinhai Zheng
Linked Author(s): Jinhai Zheng
Keywords: Sand-mud mixtures; Critical shear stress; Erosion threshold; Fractal dimension; Cohesive sediment
Abstract: In many natural environments, e.g., estuaries, delta, mangrove forests, muddy and silty coasts, non-cohesive sediment and cohesive sediment are not completely separated and often occur as sand-mud mixtures. Existing experiments have shown that sand-mud mixtures behave like cohesive sediments if they contain a sufficient amount of cohesive particles, otherwise like non-cohesive sediments. The surface erosion process of non-cohesive sand-mud mixtures is primarily controlled by the sand component, with individual sand grains being eroded, while in cohesive mixtures, the process is controlled by the mud fraction, typically eroded in agglomerate form. This study investigates the erodibility of sand-mud mixtures, including pure mud. First, the fractal theory and the van der Waals force between particles are employed and the momentum balance analysis is performed to examine the incipient motion of the mud aggregates. A formula of two coefficients for the surface erosion threshold of pure mud is developed. Among the two coefficients, one represents the fractal dimension and the other denotes the cohesion of the sediment. The comparison between the formula and experimental data shows that the fractal dimension of mud is a function of the compactness degree of sediment and the diameter of primary particles. The effect of the particle size on the fraction dimension is rarely mentioned in the research of cohesive soil. For sediments with relatively large particle diameters and low solid volume fractions, the contribution rate of aggregate weight may be significant and cannot be neglected. However, for the most common types of cohesive sediment found in river and marine beds, which typically have relatively high solid volume fractions, the contribution rate of aggregate weight is sufficiently low and can be disregarded. For these sediments, the developed formulas can be further simplified to a simple formula with only one coefficient for practical use. On the basis of the above understanding of erosion threshold of mud, the threshold of surface erosion of sand-mud mixtures is further investigated using the same approach. A formula for the critical shear stress of sand-mud mixtures is derived, which is a function of the primary particle diameter of the sand and mud components, mud content, and dry bulk density. This formula effectively represents the erosion threshold of sand-mud mixtures and simplifies to the classical formula for non-cohesive sediment when the mud content is zero, and to the earlier developed formula for mud when the mud content is 100%. The developed formula suggests that the variation of the critical shear stress of sand-mud mixtures over mud content is mainly caused by the varying dry bulk density of the mud component in the mixture, which is controlled by the network structure of the sand and mud mixture.
Year: 2023