Clay settling in fresh and salt water

Abstract

To gain insight into the process of sedimentation occurring when clay-laden estuaries and deltas enter marine water, we perform laboratory experiments to measure the settling rate of initially unflocculated kaolin clay in fresh and salt water. In fresh water, sedimentation is a slow process with the clay particle concentration gradually decreasing nearly uniformly over hours, consistent with the time-scale expected for particles falling at the Stokes settling speed. The dynamics are dramatically different for clay setting in salt water with salinities between $S=10$ and 70 psu. Within minutes the clay particles flocculate and a sharp concentration-front between clear water (above) and water with clay in suspension (below) forms near the surface. After formation the concentration-front descends at a near constant speed until the effects of hindered settling become important. When the concentration-front forms in saline fluid, the $10$ cm deep tank is cleared of particles in tens of minutes instead of tens of hours as is the case for settling in fresh water ($S=0$). The initial speed of descent of the front, $w$, depends weakly upon salinity, $S$, with virtually no dependence upon $S$ provided $S\gtrsim 20$ psu. However, the descent speed, $w$, depends strongly upon clay concentration, $C$, with $w$ decreasing as $C$ increases according to a power law: $w \propto C^{-1.7}$. The results are consistent with observations of relatively quiescent sediment-laden estuaries and deltas where they empty into the ocean.