I began working on environmental suspensions during my PhD research, nearly ten years ago at the University of Illinois. I was surprised to find that the physics of how fine particles move in a natural (stratified and turbulent) suspension was far from a solved problem. Unlike coarser sediment (i.e., sand), fine material has a strong affinity to behave as a “continuum.” That is, fine suspensions often behave like a single fluid of a slightly higher density than water by itself. Fine particles also have a tendency to flocculate, or stick together. The interaction of these processes is not understood, even in the simplest laboratory experiments. Natural complexities make the situation even worse. Biofilms, chemical variability and the large variety of motions present in the oceans all affect these underlying physical processes. Despite our limited understanding, the description of how particles (particularly fine particles) move in the environment is essential for virtually every numerical model of earth surface processes used today. Fine particles are important for everything from the carbon cycle to the transport of adsorbed contaminants. The mode of sediment transport also affects the way geologists interpret the past to identify ancient climatic episodes. In sum, it is an exciting time for this new type of physical research.

My early work on environmental suspensions investigated double-diffusive sedimentation (DDS). DDS is a relatively well-understood phenomenon. It results when temperature diffuses faster the sediment in suspension. The result is that areas of the fluid that become denser than their surroundings and initiate convection, just like warm water in your soup pot. A picture of what that looks like is shown above (the sediment is lit from a laser sheet – bright spots correspond to small sediment-laden plumes, whereas in the surface plume appears dark because there is a lot of sediment there). In all of my experiments (just as in nature), sediment moves from the upper parts of the water column to the bed. We were surprised to discover that convection could initiate even in the relative absence of these effects. This motivated us to do more experiments.

The discovery that convection almost always occurred (regardless of thermal gradients) caused us to step back and examine particle settling in a more simplified setting. Seeking to eliminate turbulence as a potential cause of the convection, we performed experiments in a tank that could produce a quiescent interface of sediment-laden water overlying brine (salty water). The result was a new type of convection; we called “leaking.” Leaking convection is a result of sediment falling out of the upper layer and collecting at the interface between the dirty fluid above and the clean fluid underneath. Eventually so much sediment gets emplaced on the interface that it breaks and leaks the sediment into the lower layer. Again, the bright spots in the picture above are areas of high sediment concentration. If you have any other questions about this work, please download a copy of our paper….

Parsons, J. D., Bush, J. W. M., and Syvitski, J. W. M. 2001. Hyperpycnal plumes with small sediment concentrations. Sedimentology 48 465-478. (pdf)

Most recently, as part of his MS thesis research, Wayne McCool performed a series of experiments where we could tightly constrain the turbulence being produced at the interface between sediment-laden fluid (above) and salty fluid below. Turbulence is an important property of most natural flows. In his experiments, Wayne found that the vertical flux of sediment depended on both the intensity of turbulence and the sediment concentration. He also found that turbulence INCREASED the ability of the water column to convect and rapidly settle particles. Interestingly enough, the vertical sediment flux was substantial under conditions typical of energetic river plumes around the world (Papua New Guinea, northern California, who knows where else???). This work is summarized in:

McCool, W. M. and Parsons, J. D. 2004. Sedimentation from buoyant fine-grained suspensions. Continental Shelf Research 24 1129-1142. (pdf)

However, probably the most interesting thing that we discovered was that the finest material was settling out first. Wayne attributed it to an effect called preferential concentration, or PC. In fact, PC happens so often you can see it in many of the heavily sediment-laden rivers in the Northwest. PC causes the suspension to appear speckled – owing to the variability in local sediment concentration. The picture below was taken in the infamous Toutle River draining Mt. St. Helens in southwest Washington.

Jeff Parsons’ Research UW-Oceanography