Impact of Salinity on Rheology Properties and Erosion Threshold of Sand-Clay Mixture*

Abstract

 

 

Mud, consisting of clay and sand, is ubiquitous on each surface and is critical in land surface processes, such as coastal erosion. However, a fundamental understanding of clay's erosional and rheological properties under varying salinity is currently lacking. Here we investigate the influence of salinity on the rheology properties and erosion threshold of the mixture through systematical laboratory experiments.  First, we developed homogeneous sand-clay mixtures with varying sea salt concentrations to simulate mud in environments with different salinity.  The mixtures were made using a custom-built experimental setup involving a vacuum pump, desiccator, CO2 tank, and DI water. Rheological characteristics of the samples were measured using a Discovery Hybrid Rheometer to estimate flow curves and obtain the storage-loss modulus distribution. We also quantified the yield stress of the samples as the limit of the linear viscoelastic region.

Additionally, we measured the critical shear stress to erode the mud in a flume, using a digital camera and an Acoustic Doppler Velocimeter that measures 3D water velocity fluctuations. Our results show that the stirring or the mixing procedure, which influences the homogeneity of clay-sand mixtures, can impact their yield stress by about a factor of 6. In addition, we show that salinity significantly impacts the viscosity and yield stress of the clay-sand mixture. Our results provide critical insights into the erosion of salinity-responsive sand-clay mixtures, commonly found in wetlands, coastal areas, and estuaries. The dependence of erosion threshold and rheological properties on salinity we identified will facilitate future designs of erosion control structures and prediction of sediment transport in these environments.