SOIL ERODIBILITY AND MONITORING AT A GULLY IN SÃO PEDRO
RIVER´S DRAINAGE BASIN, MACAÉ MUNICIPALITY/RIO DE JANEIRO
STATE – BRAZIL
Hugo Alves Soares Loureiro – MSc Student (UFRJ)
hugogeogr@gmail.com
Sara Regina de Araújo Neves – Geography Student (UFRJ)
sara.regina_geo@yahoo.com.br
Raphael Rodrigues Brizzi – Geography Student (UERJ)
rodrigues_brizzi@hotmail.com
Stella Peres Mendes – PhD Student (UFRJ)
stellapmendes@yahoo.com.br
Antonio José Teixeira Guerra – PhD (Oxford); Professor of Geography (UFRJ)
antoniotguerra@gmail.com
Av. Athos da Silveira Ramos, 274. CCMN, Departamento de Geografia, Bloco I, LAGESOLOS - salas
I1-009 e I1-011, Ilha do Fundão. CEP: 21.941.916 – Rio de Janeiro, Brasil
ABSTRACT
Erosion is a natural process acting on landforms, which due to human actions could be
accelerated, with the possibility of bringing disruptions and damages to the environment and
society. São Pedro river´s drainage basin (Macaé/RJ – Brazil) has several signs of degradation
along its slopes. The removal of original vegetation cover through the centuries for different land
uses incorporated to the landscape features like sheet erosion, rills and gullies on slopes, most
“covered” by cattle paths. The drainage basin has a diverse geomorphology, high altimetric
range and remnants of the Mata Atlântica Forest on the upper parts of the basin, and occurrence
of some agricultural areas. This study aims to determine the soil erodibility of a gully and of the
hillslopes surrounding it, analyzing some soil properties; and analyze the initial results of gully´s
dynamics and evolution monitoring. To determine sand, silt and clay content, bulk density,
particles density and porosity, the Manual of Methods for Soil Analyses of EMBRAPA (1997)
was used. Organic matter content was obtained following the adapted method from Ball (1964).
The gully monitoring was based on Guerra (1996), with stakes to measure the retreat of the gully
borders, and erosion pins on gully sidewalls to verify the sediments removal. Results showed that
the gully is active and its dynamic development is related to the rainfall period, when the borders
retreated some centimeters. The erosion pins installed on the walls may be a good method to
show the removal of soil, independently of the retreat of the borders. The laboratorial analyses
showed that the area is susceptible to erosion, with low contents of organic matter and clay. All
this data can be treated together, completing the analysis.
Keywords: Soil Erodibility; Soil Properties; Gully Monitoring; São Pedro River.
INTRODUCTION
The erosion is one of the different processes that cause soil degradation, especially that caused by water
action. On topsoil, rain water can break down and transport particles through the soil surface, by splash
and runoff processes. Under the soil, infiltrated water can disrupt the soil mass stability, causing mass
movements of different proportions, or trough piping processes undermine entire terrains. These
processes of erosion act degrading the landscape, and could bring a diversity of risks for human lives
and activities.
The paper objectives are to verify the soil erodibility of a gully and its surrounding slopes, through soils
physical properties and its organic matter content; and examine the initial results of gully´s dynamic of
development monitoring.
The study area is located in São Pedro river drainage basin (Figure 1), at the Macaé Municipality, Rio
de Janeiro State (Brazil), where the degradation signs are easily noticed and constantly present in the
landscape. With an area about 420 Km², the drainage basin suffered for centuries with direct impacts
(as changes in channels with impoundments and rectifications) and indirect ones (as land use), having
its original vegetation cover of Mata Atlântica removed for implementation of agricultural activities
and pastures, changing the hydrological system.
Figure 1. Location map of the study area (Loureiro, 2011).
The drainage basin geomorphology has since floodplains used mainly by agriculture to Scarped Ranges
with remnants of native forest, and the Hilly Domain, with slopes heavily degraded by cattle ranching.
The soils are predominantly classified as Cambisols, following the Brazilian System of Soil
Classification (EMBRAPA, 2006), that are shallow soils. The rainfall is well marked with a rainy
summer (more than150 mm monthly of average in 25 years) and a dry winter (less than 65 mm
monthly of average in 25 years).
The chosen area for the studies has great prominence on the landscape. Located on the foot slope of the
Pico do Frade (a local Peak), a postcard site, it has two huge gullies, with approximately four years of
rapid and continuous growth (Figure 2a); and a smaller gully with a good level of development,
especially on its head, on the left side, being this smaller the gully chosen for monitoring (Figure 2b).
(a)
(b)
Figure 2. (a) In blue, the two huge gullies, and in red, the smaller one, chosen for monitoring
(Loureiro, 2011); (b) The monitored gully, with active head (Jorge, 2011).
THEORETICAL FOUNDATION
Erosion is a natural process sculpturing the relief that can be aggravated by human actions upon it,
causing environmental and social impacts, affecting human activities and space organizations, leading
to huge or irreversible damages (Goudie & Viles, 1997; Mendes et. al., 2009). For Araujo et al. (2009),
rills and gullies are extreme erosion forms, that deform lands, and the risks they bring depends on
natural conditions and land use. Because of that, study the erosion conditioner factors, such as soil
erodibility, and monitor the erosive process, analyzing its space and time evolution, is important to
achieve erosion control mechanisms. According to Suertegaray & Nunes (2001), the geographic
science, trough geomorphology, aims to develop repairing techniques for the damage caused by human
acts on the environment.
To study soil erodibility their physical and chemical properties are essential (Gerrard, 1992; Guerra,
2007). The higher or lower erosion susceptibility depends on the soil size fractions contents. Silt and
fine sand fractions are the easiest removable ones, while clay fraction has a more difficult removal
because of its better aggregation capacity (Guerra, 2005; Morgan, 2005). The bulk density could
indicate soil compaction, which is increased by the reduction of organic matter content, by the use of
agricultural machinery, constant animals trampling, and splash action – breaking soil particles creating
topsoil crusts (Selby, 1993; Fullen & Catt, 2004; Guerra, 2007). Morgan (2005) considers 1.50g/cm³ as
a limit value between low and high bulk density. Related with it, the total porosity is important for
water percolation and to air and root soil penetration. When bulk density increases, porosity decreases
(Guerra, 2007). In pastures with inadequate management the continuous cattle trampling compacts the
soil, creating terracetes, which act as knickpoints on slopes (Guerra, 2005). Soil organic matter input
more aggregate stability, increases infiltration and reduce erosive processes, such as splash and runoff
(Guerra, 2005; Morgan, 2005). Organic matter is also important with high levels of silt on soil (Fullen
& Catt, 2004; Guerra, 2007). Organic matter contents bellow 3.5% means less than 2.0% of carbon
(Morgan, 2005), and represent instability and greater soil erosion susceptibility (Greenland et al., 1975
in Guerra, 2007; De Ploey & Poesen, 1985 in Guerra, 2007).
The erosion by water initiates by splash, acting on soil surface detaching its particles and transporting
them through runoff, that is initially diffuse and become concentrated in small channels that arise.
These channels are called microrills, which could develop into rills and gullies (Morgan, 2005; Guerra,
2005 and 2007). Gullies are defined by international literature as erosive features with width and depth
greater than 0.5 meters (Guerra, 2005; Morgan, 2005). Selby (1993) and Guerra (2007) agree that these
features could become permanent in the landscape, due to their difficult control and recover when these
achieve great proportions. According to Araujo et al. (2009), gullies are common at pasture areas and it
is the main erosive form on drainage basins. Morgan (2005) affirms that is common that a gully have
an extreme activity on its heads and a stable base, which is this paper´s case, but the inverse way is
possible.
Descroix et al. (2004) relates gully erosion to changes in land use. The authors pointed a big water shed
area; relative high silt content (upon 20%, representing a regional high index, which have soils with
sand contents higher than 80%); and large areas of cattle trampling and cultivated areas on the upper
part of a drainage basin, contributing to runoff concentration, as previous problematic and
considerations about the conditions to gullies formation at Sierra Madre. The results in Descroix et al.
(2004) attribute gullies formation to the effect of the rain intensity on topsoil and “classically” related
to runoff. The proportion of exposed soil, the topsoil crusts, and the silt content increases the soil loss
indexes, due to the over grazing that exceed soil capacity to pasture activities; and due to the
deforestation, that transformed 60% of forest area to agriculture area during 30 years. It facilitates the
occurrence and effect of runoff in the erosive features formation, depending on the function of the
erosion control factors.
MATERIALS AND METHODS
The methodology for the gully monitoring follows the proposed orientations by Guerra (1996). To
monitor the retreat of the gully´s borders 18 stakes were placed around it, numbered of 1A and 1B to
9A and 9B (Figure 3), measuring 20cm of height, 2cm of thickness and 3cm of width, for the effective
measurements. These stakes have as an initial reference other 9 stakes with same size, numbered of 1 to
9, functioning as zero point of the measurements, they are positioned about 10 meters from the borders.
Other 17 stakes, numbered of 1a and 1b to 9a and 9b, with 10cm of height (2a was not necessary) were
placed closer the borders, in order to be a reference for the measurement of the 18 stakes (Figure 4).
The loss of the smaller stakes due to the retreat of the borders is not a problem because their function is
only being a reference for the correct measurement. This placement of the stakes closer to the borders
is a methodological adaptation judged necessary during the field work.
Figure 3. Staking around the monitored gully. Stake 1A (Jorge, 2011).
Figure 4. Diagram illustrating the position of each stake for monitoring the retreat of the borders
(Loureiro, 2011).
Another part of the gully monitoring represented an innovation in the methodology about the erosion
pins. According to Guerra (1996), monitoring with erosion pins consists of measuring the height of the
pins upon soil´s surface. With the removal of soil material by erosion, the pins will be increasingly
exposed. This appears to be quite useful to verify sheet erosion. The innovative adaptation is the
placement of erosion pins (made of iron rods, measuring about 40cm length) at the gully walls, with the
same aim of measure the material removal, in this case of the walls, and it could be measured in
centimeters and/or millimeters. Seven areas on the gully walls were selected for the distribution of the
pins. Its installation respected some rules: (a) each first pin of a column is installed at 0.5m and 1.0m
from the top of soil, alternately; (b) the vertical distance between the pins of a same column is of 1.0m;
(c) the distance between the columns of erosion pins is of 2.0m. For four columns was necessary to
adapt rules (a) and (b) because of the height of the wall. A total of 56 pins were installed, but only 23
will be discussed in this paper (Figure 5).
Figure 5. Representative diagram of the position of the pins at the head of the gully (Loureiro, 2011).
The collection of deformed samples from the gully walls, to analyze soil particles size distribution and
texture, bulk density, particles density and organic matter content were made from 0 to 2.0 meter of
depth, with intervals of 0.5m. There were 4 samples from the right head (RH), 4 from the left head
(LH), 4 from the left sidewall (LW), 4 from the right sidewall near the exit (RWE), 4 from the left
sidewall near the exit (LWE), and two 0-5cm deep, 1 from the right side floor of the exit (RFE) and 1
from the left side floor of the exit (LFE), totaling 22 deformed samples. Three volumetric samples were
collected around the gully on the topsoil to analyze bulk density. Besides all those samples, 4 points
next to the largest gully were collected (GE, G1, G2 and G3), using a Dutch Auger, at 0-40cm of depth,
and with volumetric collector, superficially.
In the laboratory the analysis of particle size distribution, bulk density, particle density and porosity
were made according to the methods of the Manual of Methods of Soil Analysis of EMBRAPA (1997).
The particle size distribution analysis followed the pipette method, based on the speed of falling
components of soil particles, in a water suspension with chemical dispersant.
The analysis of bulk density consists on determine the weight (g/cm³) of the sample collected with a
ring of determined volume, that have in this study 100 cm³. The particles density is also given on
g/cm³, and it is made by determining the amount of alcohol it takes to complete the capacity of a flask,
so that the soil inside it does not present any spaces between the particles. Total porosity expresses the
volume of soil pores occupied by water and/or air. It is obtained by a mathematical expression that uses
the values of bulk density and particles density: Porosity = 100 x (Bulk D. – Particles D.) ÷ Particles D.
The organic matter content followed the methodology of Ball (1964), which consists in getting this
content (%) by burning it in a muffle oven.
RESULTS AND DISCUSSION
The most significant results of the borders retreat monitoring are associated with a rainy season, as
expected. The black highlighted results in Table 1 shows that happened a development of the gully,
especially in its most active part, the head, represented by 4B, 5A e 5B stakes.
Table 1. Results of gully´s development monitoring, using stakes. (*) Stakes near the border were lost.
There is an automatic rain gauge in the study area, which was installed at the end of March 2011,
having no records of rainfall before March 21th, which was an intense rainfall period at Rio de Janeiro
State. Even with this lack of data, the record was not deficient. The rain gauge recorded in the last 10
days of March an amount of 136.20mm of rain, while in the entire month of April it recorded
155.18mm.
The highest rainfall records obtained on March marked 40.3mm of rain in just one hour and three
minutes on March 25th, and 55.6mm in four hours and 16 minutes on March 30th. The first date
corresponds to the interval between the first monitoring (02/25) and the second (03/29), which were
recorded 4 of the 6 retreats measured. However, visiting the study area on March 15th, during a heavy
rain, visual differences could be noted at the gully´s head and right sidewall (Figure 6), like the
undermining of the border where were located the 7b stake, lost with the fall of one meter and nine
centimeters of soil (Table 1).
(a)
(b)
Figure 6. (a) On January, before the rains (Jorge, 2011); (b) Picture taken on March 15th, during a
heavy rain storm (Loureiro, 2011).
On March 29th monitoring the changes were confirmed (Figure 7). At gully´s head the retreat was
registered as 54cm at 4B stake, 58cm at 5A stake and thee meters and 38cm at 5B stake, registering the
5b stake loss. About the retreat presented by 5B stake, it is understood that there was an overvaluation
of it, because at field or by photos it does not seen that the retreat had achieved three meters. It is
estimated that the fallow of border´s soil may have been less than one meter, and that this overvaluation
had been occurred because the gully´s head parallel position of 5B and 5b stakes, instead of being in a
transverse position to its head, or because a confusion made by the loss of the 5b stake, that was
reference to the border, changing the measurement position.
The following monitoring happened after the rains of March 30th and April 10th (40.6mm in one hour
and 57 minutes), and recorded two more points of retreating borders. The one referent to 4B retreated
4cm more, totalizing a retreat of 58cm, and the border corresponding to 5A stake retreated 20cmm,
totalizing 78cm of retreat. Looking at Table 1, it is noted some problems, with gray highlights. The 3B,
4A and 8B stakes suffered declines, caused mostly by the passage of the cattle. The 9B stake, for
example, was completely buried into the ground on July monitoring, because of the cattle. In cases like
6A and 9B stakes it is estimated that had occurred an positioning error during some measurements.
Minimal variation cases haven´t been treated, considering a error margin of 2 to 3 centimeters.
(a)
(b)
Figure 7. (a) Gully´s head on February 25th, without recent undermining (Mendes, 2011); (b) gully´s
head on April 26th, with a lot of undermining material during the heavy rains (Loureiro, 2011).
The potential of the erosion pins method to monitor the removal of material from the gully´s walls can
be seen through the figures 7 and 8. In the first, above, it is notice the quantity of walls removed
material. While the second shows a point where water flows “digs” the gully´s head on its left side (2.4
pin), and the imminence of undermining of some monitored parts (RH 1.1 pin). It is possible to affirm
that both monitoring methods – staking and erosion pins on the walls – are complementary, because an
inside gully undermining could happen, without the occurrence of borders retreat.
(a)
Figure 8. (a) 2.4 pin on left head; (b) 1.1 pin on right head (Loureiro, 2011).
(b)
Starting the erosion pins monitoring on May 2011, has not been possible to verify significant results
yet, since the three months analyzed (from May to July) had a low amount of rain, insufficient to be
registered by the rain gauge. However, according to Table 2, in seven of the 23 pins were registered
minimal variations. The biggest were on RH 1.2 and LH 4.2 pins (Figure 9), which could be attributed
to a slight action of rain water carrying detached particles or still detaching slightly some particles.
Table 2. Initial results of erosion pins monitoring.
(a)
(b)
Figure 9. (a) 1.2 pin on right head; (b) 4.2 pin on left head (Loureiro, 2011).
In terms of soil particle size distribution, many observations and analysis can carried out. The silt
content was stable in all the 26 samples, ranging from 17% on LH and 21 to 29% overall. The
exceptions were LW 0.5 to 1.0 and 1.0 to 1.5 meters deep at monitored gully´s sidewall, and three
samples classified as Loamy Sand (Table 3). Both LW samples presented the highest values of silt
content (35.95% and 41.10%), classified as Loam. As a result of these high values, the sum of this soil
size fraction with fine sand fraction was above 50%, showing an elevated susceptibility to erosion on
these points, since the two fractions are the easiest ones to be removed (Guerra, 2005; Morgan, 2005).
The Loamy Sand samples presented silt contents of 7 to 11% and clay contents even lower, about 3 to
6%. Also, in general, the clay content decreases with increasing depth of the sample, occurring the
opposite with sand content, that increases with increasing depth. As the results of Descroix et al. (2004)
showed, 20% of silt content represents high levels of its fraction at Sierra Madre, favoring the
occurrence of erosion. It could be verified on the monitored gully and its surrounding, since the
variation of silt content was slightly above this value. The differences between Sierra Madre and the
monitored gully is that there the soil sand content is about 80%, and in the monitored area the values
changes of 40 to 60% of sand.
Table 3. Particle size distribution results from the monitored gully´s samples and surrounding slope´s
samples.
Analyzing LWE and RWE samples is possible to note the textural classification similarity, and at field,
the stability of these sides of the gully´s walls. The texture is Clay Loam to the first two depths (0 to
1.0 meter), and Sandy Loam of 1.0 to 2.0 meter deep samples. As shows the table 3, the Sandy Loam
textural class is one of the most erodible classes and with these samples fine sand and silt contents
together, ranging between 37 to 54%, its increases with the increasing depth and reducing clay content.
Fullen & Catt (2004) outline that this textural class and Loamy Sand are the most erodible ones.
Therefore, examining LFE, RFE and RH 1.5 to 2.0m samples, it is understood that it has a great
susceptibility because of the lower clay content, that do not permit the particles to be more aggregated,
being predominantly sand fractions, that are carried by rainwater flows. It is highlighted the textural
classification of HR 1.5 to 2.0 sample, varying greatly from the upper samples. It could indicate that
this sample represents soil material that fallow from the walls and had accumulated there, having its silt
and clay fractions carried by water on runoff and other water flows.
But if the erosion easiest removable classes are Sandy Loam, Loam and Loamy Sand, why the greater
activity occurs at the left side of the gully´s head, where accordingly with table 3, the samples were
classified as Sandy Clay Loam and Clay Loam? It could be explained considering that the material
below of the first 2.0 meters is more friable, and could be notice clearly the material removal and the
gully development. That is, associated with the stakes monitoring, with the erosion pins monitoring and
with the particles size distribution analyses and texture, it is possible to think that the left head border is
in a certain way preserved until the soil got saturated by the water infiltration, suffering forces of the
weight of accumulated water and the passage of intense surface flows, which cause the undermining of
the material that held no ground underneath.
The samples collected next to the huge gully (GE, G1, G2 e G3), representing the surrounding slopes of
the monitored area, did not show unexpected data. The sum of fine sand and silt content ranged from
34 to 38% confirming the susceptibility of these soils to erosion, and do not represented extreme values
as might be imagined because its proximity of the huge gullies. The results presented by these samples
to the organic matter content were curious (Table 4). These values were equal or greater than 3.5%
while the monitored gully´s organic matter content were far below this limit, for the majority of the
samples. It indicates a greater erosion susceptibility to that samples that are already more susceptible.
The positive highlight is the sample LWE 0 to 0.5m, Clay Loam that presented the highest organic
matter content with 4.37%. With good clay and organic matter contents, this point tends to be less
susceptible, with higher aggregation between its particles.
Table 4. Organic matter content, bulk density, particle density and total porosity results.
Bulk density and porosity results were quite similar and typical from pasture areas, with the results near
the limit between high and low bulk density. Only G1 and G2 samples, located near the huge gullies,
showed high bulk densities and low porosity indexes, indicating it level of soil compaction.
CONCLUSIONS
1. The monitoring methodologies showed to be complementary to each other, and could be
enriched with new ideas. Taken together with the erodibility analyses, turned the understanding
of the gully development and dynamics clearer;
2. The utilization of rainfall data was fundamental to analyze the monitoring results;
3. More volumetric samples have been collected, because of the clear necessity of a bigger
sampling regarding bulk density and porosity, so important to the erodibility analyses. Samples
to aggregate stability tests were collected too, in order to complete the study a little more.
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