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Slowing the Flow:
Grass Hedges To
Catch Runaway Soil
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In a flume, grass hedges are tested
at increasing water discharge rates
until inundated and no longer effective.
(K9629-1)
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Narrow, parallel strips of
stiff-stemmed grass, planted along the contour of steep hills, can successfully
halt soil erosion caused by flowing water.
"Hedges cause sediment to be deposited upstream of them and reduce erosion
and the amount of sediment transported by the flow downstream," says
Darrel M. Temple. He is a hydraulic engineer at the
ARS Hydraulic Engineering Research
Laboratory in Stillwater, Oklahoma. "This reduces sediment delivery to the
stream and river system."
Temple, working with ARS agronomist Seth M. Dabney at the National
Sedimentation Laboratory in Oxford, Mississippi, is evaluating the
effectiveness of two types of stiff-grass hedges in controlling this type of
soil erosion. |
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Windows in the flume’s side wall
allow observation of flow patterns
and effects of debris on the flow
through the grass hedges.
(K9629-2)
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"Stiff-grass hedges form a
porous barrier to flow that lets water pond upstream. They also diffuse the
water over a large area as it flows through them," says Dabney. "Over
time, soil deposited above contour hedges may form stable terraces."
Dabney has assessed the beneficial role of grass hedges in controlling erosion
from croplands for several years.
"Although stiff-grass hedges are a valuable conservation tool, little data
on their hydraulic behavior is available. That's because much of the research
on flow resistance of grasses doesn't apply to stiff grasses," says
Dabney. "Stiff-grass hedges are designed to function with water flowing
through them rather than over them."
In the field and at flume facilities at Stillwater, the two scientists decided
to study the hydraulic behavior of two types of stiff-grass hedges:
switchgrass, Panicum virgatum, and eastern gamagrass, Tripsacum
dactyloides. They wanted to determine the maximum depth of water upstream
that different hedges could withstand without being bent over and overtopped.
First, in a test area of the outdoor laboratory, they established 16 hedges in
one-, two-, three-, and four-row configurations. For 2 years, they fertilized
and irrigated the hedges but didn't cut them.
They then put the hedges to the test, discharging more and more water until
grasses bent and broke under the flow. Switchgrass proved better adapted to the
test environment than gamagrass. It stood erect and held back sediment and
water until the water ponded upstream to a depth of 1.4 to 1.6 feet. One- and
two-row hedges grew the best and were as resistant as the wider hedges to
overtopping, while occupying less area.
After the testing, the two scientists cut half of the hedges to remove damaged
stems and left the rest undisturbed. While all hedges recovered by the next
growing season, clipping or mowing the damaged grass hastened its recovery.
"The next 2 years of study will focus on the ability of switchgrass hedges
to protect soil from concentrated waterflow on very steep slopes that are
already eroded," says Temple. "Our future research will provide more
details on how well these hedges perform."—By Hank Becker,
formerly with ARS.
This research is part of Water Quality and Management, an ARS National
Program (#201) described on the World Wide Web at
http://www.nps.ars.usda.gov.
Darrel M. Temple is with
the USDA-ARS Hydraulic
Engineering Research Laboratory, 1301 N. Western St., Stillwater, OK
74075-2714; phone (405) 624-4135, ext. 226, fax (405) 624-4136.
Seth M. Dabney is with the
USDA-ARS National
Sedimentation Laboratory, 598 McElroy Dr., Oxford, MS 38655; phone (662)
232-2975, fax (662) 232-2915. |
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"Slowing the Flow: Grass Hedges To Catch Runaway
Soil " was published in the
October 2001
issue of Agricultural Research magazine.
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