Peter R. Wilcock, a professor in the Department of Geography and Environmental Engineering, has devised a simple method of estimating how quickly excess river sediment--unleashed by natural or man-made land disturbances--will flow downstream. Wilcock's technique, unveiled in the April 17 issue of the journal Science, may help prevent or correct environmental problems that occur when large amounts of fine-grained materials wash into a river.
Forest fires, logging, road construction, urban development and dam operations can send a rush of sediment into gravel-bed rivers, triggering serious consequences, says Wilcock. For example, a surplus of sand can degrade the habitat of fish and other aquatic animals and increase the potential for flooding along a river.
For this reason, says Wilcock, river managers need to know how quickly this sand will be swept downstream, returning the gravel bed to a healthier condition. "We want to be able to predict how long it will take for the extra sediment to move through the river," he says.
But predicting how sediment will move has been difficult because the particles come in many sizes, each moving at a different pace. "The sampling necessary to analyze all of these sizes would be so extensive as to be impossible," Wilcock says. "We needed a theory that could be supported by a practical amount of field observation."
Wilcock solved this problem by dividing sediment into just two categories: sand, with grains smaller than 2 millimeters; and gravel, with grains larger than 2 millimeters. The percentages of these two sizes can be determined by wading along the river, which is much simpler and quicker than other sampling techniques, Wilcock says.
"You need to know the rate at which the sediment will be transported by the stream," he says, "and you want to know whether the fine material--the sand--will move downstream faster than the gravel."
Wilcock's method demonstrates how fast the water must move in order to first dislodge the sand and gravel from the river bed and begin moving the material.
"Estimates of transport rate are needed to forecast the extent and duration of river impacts caused by land disturbance," Wilcock says. "This information is also needed to guide efforts to restore rivers that have received excessive sediment loadings."
An interesting consequence of this new way of looking at sediment movement through rivers is that it demonstrates that an increase in the proportion of sand in the bed can increase the rate of transport of both the sand and the gravel. "This is encouraging because it suggests that rivers have a natural ability to increase their rate of transporting sediment in response to an increase in sediment supply," Wilcock says. "This does not remove the negative impact of fine-grained sediment loading, but it may reduce and shorten it."
Wilcock's research is supported by the Stream Systems Technology Center of the U.S. Forest Service.