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The erosion and transport of cohesive sediment are more difficult to study than non-cohesive sediment, largely because these processes vary with the salt in the water. Clay minerals are the major components that contribute to the cohesiveness of cohesive sediment because they have significantly larger surface charges and surface area-to-volume ratio than non-cohesive sediment. The electrochemically active clay surfaces can adsorb ions on their surfaces, form an electrical double layer, and cause clay particles to aggregate or form a gel. In this chapter, we first discuss the properties of clay minerals, including the structure of clay primary particles, their surface charge and area, and their interaction with ions in water. The surface charges and surface areas of clay are several orders of magnitude larger than non-cohesive sand, thus predisposing it to interactions with salt in aqueous environments. Second, we summarize studies that reveal the role of salts, specifically salinity and sodium absorption ratio (SAR), on sediment aggregation, stability, and settling speed. An increase in salinity from 0.15 to 1.5 ppt has been shown to increase the erosion threshold of smectite clay by more than 10 times. These findings underscore the crucial role of salt in shaping cohesive sediment transport. 

Abstract Sand‐clay mixtures are common in both freshwater and saltwater environments, yet how they behave under different levels of salinity remains poorly understood. Here, we demonstrate the impact of salinity on the rheological properties and erosion threshold of sand‐clay mixtures through systematically controlled flume experiments and rheological measurements. Mixtures with a representative bentonite‐to‐sand ratio typical of natural estuarine and coastal sediments were prepared at salinities ranging from 0 to 35 parts per thousand (ppt), spanning freshwater to seawater conditions. We measured viscosity, flow‐point stress, and yield stress of the mixtures using a rheometer and determined the critical bed shear stress in a water‐recirculating flume. Our results indicate that as salinity increases from 0 to 35 ppt, the critical bed shear stress decreases by about two orders of magnitude, from about 60 Pa at 0 ppt to less than 1 Pa at 35 ppt. Similarly, both the flow‐point stress and yield stress decreased by over two orders of magnitude with increasing salinity. These changes correspond to a salinity‐induced transition of the sand‐bentonite mixture from a cohesive, strong‐gel state in freshwater (0 ppt), to a weak‐gel state between 3 and 10 ppt, and finally to a fluid‐like state above 10 ppt. Our research highlights the important role of salt in controlling the rheological properties and erosion threshold of fresh, non‐consolidated deposits of sand‐clay mixtures, with implications for predicting coastal landscape evolution and designing erosion‐control strategies.