Geology Reference
In-Depth Information
and maintenance, including problems with the
terrain surface, steep slope angles and inadequate
drainage. The erosion process on roads starts with
the earthwork of the road itself, and carries on
with the cut and fill talus, the gutters and the
pavement. Erosion rates on the roads are influ-
enced by water flow, hydraulic conductivity and
soil erodibility (Garcia, 2001).
The study was carried out in a reforestation
project, where the main relief characteristics are
a flat surface, on a sandy Oxisol. Total runoff and
sediment yield were monitored under forest road
conditions. Four replicates were made for each of
four treatment combinations consisting of two
slope angles (1% and 7%) and two lengths (20 m
and 40 m), for a total of 16 road segments. Fourteen
rainfall events were measured, ranging between
7 and 79 mm of measured rainfall depth. In each
event, total rainfall, runoff and soil loss were
monitored. The monitoring period was from July
2000 to March 2001, with most runoff and sedi-
ment yield occurring during the rainy season
(December to March).
To measure the amount of eroded material and
runoff, four barrels were placed on the lower part
of each road segment. Furthermore, data regarding
meteorological conditions, rainfall, soil properties,
slope angle and segment length were included.
This procedure aimed to test the model, in order
to adapt it to the Brazilian forest environment.
Monitoring always took place one day after
the rain event. Soil data were obtained by IAC
(Agronomic Institute of Campinas) and rainfall
data were obtained for all of São Paulo State. The
rainfall range between 7 and 19 mm did not cause
significant differences in the total amount of run-
off, when the slope length ranged between 20 and
40 m and the slope steepness ranged between 1%
and 7%. On the other hand, the slope angle signifi-
cantly influenced the runoff amount for the 20 m
slope length for rainfall amounts of 22 and 28 mm,
and on the 40 m length for 60 mm of rainfall. The
variation in slope length did not significantly
influence runoff amount for the 7% slope at 28 mm
rainfall, or for the 1% slope with 79 mm rainfall.
There is a direct relationship between soil loss
and both slope steepness and slope length. If we
take a 7 mm rainfall as an example, for a 20 m
long slope with 7% grade, the total amount of
soil loss was 900% higher than on the 1% slope.
On the 40 m slope segments, total soil losses from
the 7% slopes were 487.5% higher than the val-
ues found for the 1% slope segments.
In all rainfall events, total runoff was higher
from the 40 m slope length segment. The propor-
tional differences were higher when total rainfall
amounts were smaller. The influence of slope
angle on erosion was more evident for smaller
rainfall events. When rainfall was >19 mm, the
slope angle effect was weak or non-existent for
the 20 m slope. For the 40 m slope, the slope angle
influence occurred for rainfall events
9 mm.
When total rainfall was >9 mm, there was no
significant difference between slope angle and slope
length in terms of runoff amount. In some cases,
there was even more runoff from the 1% slope than
from the 7% slope. Garcia (2001) suggested that
this probably happened because of differences in
rainfall intensity, which was not measured. During
low rainfall intensity events there was little runoff,
because most water infiltrated.
The correlation was positive between total
soil loss and total rainfall amount. In all rainfall
events, there was an increase in total soil loss on
slopes between 1% and 7%, except for the 10 mm
rainfall event. According to Garcia (2001), it is
not possible to explain the higher amount of soil
loss for this rainfall event for the smaller slope
gradient, because rainfall intensity was not
measured.
For the 8 mm rainfall event and 1% slope, total
soil loss for the 40 m slope was 159.4% higher
than for the 20 m segment. For the 20 m slope,
total soil loss on the 7% slope was 275% higher
than on the 1% slope.
In terms of total runoff, the highest values
occurred during the 91.8 mm rainfall event on the
20 m long slope at 1% slope. The highest total
soil loss occurred during a 79 mm rainfall event,
for the same slope length and steepness. For the
20 m and 7% slope, most runoff was measured
from the 61.1 mm rainfall event. For the same
slope length and slope steepness the highest soil
loss took place on the 69.7 mm event. On the
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