ENVIRONMENTAL - International Erosion Control Association

(RECP); or left untreated (bare slopes
were allowed to vegetate naturally).
The sediment losses from the side
slope portion of the project were measured
over all eight years of the project.
The input fi les for the WEPP model included
slope, climate, soil, and management
for each treatment on both types
of slopes.
Sediment losses the fi rst year after
establishment of both the native and
exotic vegetation represented from 60
to 90 percent of the total losses from
the cut and the fi ll slopes over the eight
years. On cut slopes, 81 percent of total
sediment losses occurred during the
fi rst year of stand establishment. By
contrast, just 38 percent of total sediment
loss from fi ll slopes was observed
during the fi rst year.
These results reflect different soil
conditions between the two types of
slopes, Grace explains. Because they
represent a mixture of topsoil and subsurface
soil, fi ll slopes tend to have a
looser texture. This improves infi ltration
of water, reducing runoff volume,
and makes it easier for vegetation to
establish compared to cut slopes, where
most, if not all, of the looser texture
topsoil has been removed.
“In this study, the fi ll slopes exhibited
100 percent cover of vegetation after
two years,” he says. “It took more than
a year longer to achieve a 90 percent to
100 percent vegetative cover on the cut
slopes.”
Sediment losses from the control
plots on both the cut and the fi ll slopes
continued at a high rate until the fourth
year after disturbance.
Differences in Efficiency
WEPP predictions of sediment
losses over the eight years for the native
and RECP treatments agreed with the
actual measured losses on both cut and
fi ll slopes, Grace reports.
“The model was most efficient in predicting
soil loss from the native treatment
on both types of slopes and it was
high for the RECP treatments on the
cut and fi ll slopes, too,” he says. “It was
least efficient in predicting sediment
losses from the cut and the fi ll slopes
with both the exotic treatment and the
control,” he says. “This reflects the inability
to characterize the vegetation
mixture in the WEPP management fi le
for the erosion control treatments.
“It’s also significant that the native
vegetation was as effective as the exotic
in controlling erosion of the side slopes
over the eight-year period,” Grace adds.
“For the fi rst two years, sediment losses
from the RECP were less than the other
treatments. However, as time went on,
sediment losses from vegetation treatments
decreased and there was no statistical
difference between them.”
Road Surface Results
The crowned unsurfaced roads
consisted of native sandy loam soil
overlaying a clay loam subsoil. The
Figures 1 – 3 show equipment used to measure runoff from the road section part of the research in the Chattahoochee National Forest in Georgia, USA.
Figure 1: The white trapezoidal flume on the
ground includes a submerged pressure transducer
that measures the depth of runoff from a
section of the forest road. This information is
used to calculate the flow rate. Photo Credit:
Johnny M. Grace, III.
Figure 2: The white pole-mounted box is the
instrumentation hut. It contains a stormwater
sampler, a flow meter and batteries to operate
these instruments. A computer is connected
to these devices to download the information
they’ve collected. Photo Credit: Johnny M.
Grace, III.
Figure 3: Engineering technician Preston
Steele, Jr., measures water collected in a manual
rain gauge. Electronic rain gauges also
were used to measure rainfall. Photo Credit:
Johnny M. Grace, III.
OCTOBER 2007 • 23