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Sand In Sediment Can Predict River Damage

New method sheds light on a stream's response to land disturbance

A Johns Hopkins University scientist has devised a simple method of estimatinghow quickly excess river sediment--unleashed by natural or man-made landdisturbances--will flow downstream. Peter R. Wilcock's technique, unveiled inthe April 17 issue of the journal Science, may help prevent or correctenvironmental problems that occur when large amounts of fine-grained materialswash into a river.

Forest fires, logging, road construction, urban development and dam operationscan send a rush of sediment into gravel-bed rivers, triggering seriousconsequences, says Wilcock, a professor in the Department of Geography andEnvironmental Engineering. For example, a surplus of sand can degrade thehabitat of fish and other aquatic animals and increase thepotential for flooding along a river.

For this reason, says Wilcock, river managers need to know how quickly thissand will be swept downstream, returning the gravel bed to a healthiercondition. "We want to be able to predict how long it will take for the extrasediment to move through the river," he says.

But predicting how sediment will move has been difficult because the particlescome in many sizes, each moving at a different pace. "The sampling necessaryto 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 practicalamount of field observation."

The Hopkins scientist solved this problem by dividing sediment into just twocategories: sand, with grains smaller than 2 millimeters; and gravel, withgrains larger than 2 millimeters. The percentages of these two sizes can bedetermined by wading along the river, which is much simpler and quicker thanother sampling techniques, Wilcock says.

"You need to know the rate at which the sediment will be transported by thestream," he says, "and you want to know whether the fine material--thesand--will move downstream faster than the gravel."

Wilcock's method demonstrates how fast the water must move in order to firstdislodge the sand and gravel from the river bed and begin then moving thematerial.

"Estimates of transport rate are needed to forecast the extent and duration ofriver impacts caused by land disturbance," Wilcock says. "This information isalso needed to guide efforts to restore rivers that have received excessivesediment loadings."

An interesting consequence of this new way of looking at sediment movementthrough rivers is that it demonstrates that an increase in the proportion ofsand in the bed can increase the rate of transport of both the sand and thegravel. "This is encouraging because it suggests that rivers have a naturalability to increase their rate of transporting sediment in response to anincrease in sediment supply," Wilcock says. "This does not remove thenegative impact of fine-grained sediment loading, but it may reduce andshorten it."

Wilcock's research is supported by the Stream Systems Technology Center of theU.S. Forest Service.

Related Web Site: Johns Hopkins Department of Geography and Environmental Engineering -- http://www.jhu.edu:80/~dogee/

Source: Johns Hopkins University. April 1998.


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