Researchers Study How Sediment Basins Stop Silt

August 28, 1996

University Park, Pa. -- Muddy water. That's what you get when a typical summer thunder storm dumps on a construction site and that is why developers in Pennsylvania and many other states are required to dig sediment basins to remove silt from runoff.

"Typically, urban sediment basins are designed so that they can be converted to storm water detention basins when construction is completed," says Dr. Albert R. Jarrett, professor of agricultural and biological engineering at Penn State. "An outflow device, usually a perforated riser, is installed to the spillway of a basin to create a sediment basin during the construction phase and removed when construction is complete." A perforated riser is a vertical pipe with holes in it.

Storm water retention basins can be seen nearly everywhere and are used for dispersed flood control. Most of these storm water retention basins begin life with the aim of removing sediment from construction site runoff.

While the requirement for sediment basins is long-standing, Jarrett found there was little research on how the basins function or on possible improvements. One reason little information existed was that studying sediment basins posed research problems.

"We couldn't ensure sufficient rainfall or control the rainfall and silt loading," said Jarrett.

The solution, devised by a graduate student, was to build their own 7,000 cubic foot sediment basin, the size prescribed for a one acre construction site. The facility was then used to test possible improvements to sediment basin design.

"We tested four sediment removal methods in the basin with 12 simulated runoff events, each equivalent to a two-year, 24- hour storm," Jarrett told attendees at a recent conference of the American Society of Agricultural Engineering. "Our sediment runoff was approximately 1000 pounds of soil."

A two-year, 24-hour storm is a storm expected to occur, on average, only once every two years and is the capacity requirement for sediment basins in Pennsylvania.

The removal methods tested by Jarrett and J. Millen, graduate student, were the perforated riser, a skimmer system, a perforated riser with barriers and the skimmer with barriers. The barriers were thought to slow the water so that sediment entrapment would improve. Sediment basin technology is limited to equipment that has no moving parts. Filling and draining of the basin are aided only by gravity and physical design.

The researchers found that the skimmer allowed the least amount of soil, about 37 pounds, to escape from the basin, but that the addition of barriers in the bottom of the basin did not improve or degrade the skimmer's efficiency. The perforated risers allowed 70 pounds of sediment to escape the basin. The barriers did, however, significantly improve the performance of the perforated risers allowing only 54 pounds of sediment to escape.

The skimmer, which was designed by J. W. Faircloth, of Orange County Planning Department in Hillsborough, N.C., consists of a device that floats on top of the water impoundede in the basin and is probably more efficient because it removes water from the water surface zone, which is the cleanest. The perforated riser, which consists of a pipe with holes in it, removes water from near the bottom.

The researchers are also building a second, experimental basin that will hold only 1,800 cubic feet of water. This basin will help evaluate sizing requirements, which are currently 7,000 cubic feet per acre in Pennsylvania, but much smaller in other states.

Other areas of investigation for the future include alterations to perforated risers with geotextiles and evaluations of the soil that is captured by the basin.

EDITOR: Dr. Jarrett may be reached at (814) 865-5661 or
A'ndrea Elyse Messer (814) 865-9481 (office) or
Vicki Fong (814) 865-9481 (office)

Penn State

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