Salinity Reduces Yield Stress and Erosion Threshold in Sand-Clay Mixtures: Evidence From Rheometry and Flume Experiments

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.