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 arj@psu.edu
Contacts:
A'ndrea Elyse Messer (814) 865-9481 (office)
aem1@psuvm.psu.edu or 76520.3240@compuserve.com
Vicki Fong (814) 865-9481 (office) vyf1@psu.edu