Introduction
Soil erosion is regarded as one of the most critical environmental problems
worldwide (e.g. Meadows, 2003; Le Roux et al., 2007, 2008; Schönbrodt-Stitt et al., 2013;
Ma et al., 2014; Wei et al., 2007). It mainly occurs in the form of sheet, rill and/or gully erosion
(Morgan, 2005; Le Roux et al., 2008). Amongst the three
forms, rill erosion remains the main cause for concern since it is a
precursor of gully erosion. Rill erosion mainly occurs as a result of
concentrated overland flow of water leading to the development of small
well-defined channels (Haile and Fetene, 2012). These channels act as
sediment sources and transport passages, leading to soil loss (Wirtz et
al., 2012). Although soil erosion is a natural process, it has been
accelerated by human impact on the landscape due to agriculture,
grazing, mining and fire (García-Orenes et al., 2009;
Giménez-Morera et al., 2010; Leh et al., 2013; Lieskovský and
Kenderessy, 2012; Mandal and Sharda, 2013; Zhao et al., 2013; Ziadat and
Taimeh, 2013). The construction of roads, railways and other infrastructures also result in soil
degradation and changes in the landforms (Cao et al., 2013; Cerdà,
2007; Cheng et al., 2013; Jimenez et al., 2013; Lee et al., 2013; Villarreal
et al., 2014).
The study of soil erosion, particularly in South Africa, has, however, been
limited to agricultural and pastoral land, and research investigating
road-related soil erosion is scarce, despite much literature having been
produced on combating soil erosion per se. Roads result in the permanent
alteration of the geomorphic and hydrological settings of the landscape,
leading to increased soil erosion (Ramos-Scharron and Macdonald,
2007). Previous studies have shown that roads result in the creation of
roadcuts that contribute to runoff and high sediment production that cause
extreme land degradation (e.g. Arnáez et al., 2004; Megahan et al.,
2001; Xu et al., 2009). Arnáez et al. (2004) recorded a significant
generation of runoff and sediment from roadcuts in the Iberian Range, Spain,
and this was attributed to steep gradients and low vegetation cover.
Megahan et al. (2001) evaluated the effects of slope gradient, slope
length, slope aspect, rainfall erosivity and ground cover density on erosion
on roadcuts in Idaho, USA. The results of multiple regression analysis
demonstrated that the slope gradient was the most significant of all site
variables in affecting erosion on the roadcuts. Moreover, Xu et
al. (2009) evaluated the effects of rainfall and slope length on runoff and
soil loss on the Qinghai–Tibet highway sideslopes in China and found that
rainfall intensity correlated with sediment concentration and soil loss,
while soil loss decreased with increasing slope length. In summary, these
studies highlight that slope properties (viz. slope gradient and length,
vegetation cover and soil properties, particularly soil texture) of the
roadcuts are critical in determining the degree of soil erosion along these
areas. However, to the best of our knowledge, no study has investigated why
certain roadcuts are eroded while others are not, and none has explored the
relationship between the roadcut slope characteristics and the dimensions of
the rills. Moreover, most of the studies of erosion on roadcuts have been
conducted outside southern Africa.
Construction of roads in South Africa has resulted in the creation of
roadcuts, some of which have developed extensive rills and fluting (or
incipient gullies). Soil erosion on roadcuts is significant, since soil loss
can reach magnitudes of 247.6 t ha-1 yr-1 (Megahan et al., 2001). Moreover,
roadcuts have been regarded as the main source of erosion than other parts
of the road system since they account for 70–90 % of soil loss
(Grace III, 2000). The off-site loss of sediment material may lead to
river and reservoir siltation where sediment is deposited (Cerdà,
2007; Zhao et al., 2013). This can exacerbate water management problems,
particularly in a semi-arid region such as South Africa, where water
scarcity is frequent (Marker and Sidorchuk, 2003). Moreover, erosion
on roadcuts may cause roadside slope instability (De Ona et al.,
2009; Osorio and De Ona, 2006). At present, a large volume of soil is lost
annually through water erosion in South Africa. It is estimated that South
Africa loses approximately 400 million tons of soil per year, of which
roadcut erosion is a major contributor (Dlamini et al., 2011).
The economic costs associated with the negative impacts of erosion are
significant. For instance, it is estimated that soil erosion costs
approximately USD 200 million annually, including the off-site
costs of purification of silted dam water in South Africa (Le Roux et al., 2008).
Additionally, slope instability could create excessive
maintenance costs (Robichaud et al., 2001), and in extreme cases requires
re-grading or reconstruction of the site (Persyn et al.,
2005). In the light of the above, understanding the relationship between the
characteristics of roadcuts and rill erosion can be important for
sustainable future road construction and soil erosion control. The present
study therefore aims to assess the characteristics (gradient, length and
vegetation cover) of degraded and non-degraded roadcuts to understand why
rills are present on some roadcuts but not others, and to investigate the
relationship between the characteristics of the roadcuts and the dimensions
(width and depth) of the rills in the south eastern region of South Africa.
Materials and methods
Site description
The roadcuts used in this study are located in the south eastern part of
South Africa within the KwaZulu-Natal (KZN) Province and the former Transkei
region of the Eastern Cape Province (Fig. 1). In this study, roadcuts are
defined as roadslopes that result from excavation of high areas. The study
area is characterised by a high level of erosion (Hoffman and
Todd, 2000; Le Roux et al., 2007) and road construction has provided
roadcuts that could exacerbate the problem. The terrain of the area is
undulating; it consists of a series of dissected steps that rise from a
relatively flat coastal plain in the east of South Africa, to the
Drakensberg mountains which reach over 3000 metres above sea level and form
the western boundary of the region (Beckedahl, 1996).
The location of the studied roadcuts in the south eastern
region of South Africa.
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Schematic representation of slope angle and length
measurements on the roadcuts.
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KZN has a subtropical climate characterised by high humidity, high temperatures
and high rainfall (900–1200 mm) (Fairbanks and Benn, 2000). Summers are warm
and wet, while winters are cool and dry. The climate changes gradually from
the coast to the westerly plateau. On the other hand, the greater part of
the Transkei is characterised by a sub-humid warm climate with summer-dominant rainfall (Jeschke et al., 1990). Annual rainfall varies between
500 and 1400 mm, with a mean temperature of 20 ∘C
(Madikizela, 2000). This region has among the highest values of
rainfall erosivity index (EI30) (∼ 300 MJ mm ha-1 h-1 yr-1)
in southern Africa (Beckedahl, 1996). The EI30 shows the potential ability
for rainfall to cause soil erosion (da Silva, 2004). It is the product of the total storm kinetic energy and the maximum
30 minutes rainfall intensity (Le Roux et al., 2008). The
biomes of KZN and Transkei range from coastal tropical forest along the
coast and inland along the riverine gorges, to temperate transitional forest
and scrub to grassveld. Geology of the study area consists mainly of
sandstones and mudstones of Beaufort and Ecca groups (Beckedahl, 1996).
The geology has minor exposures of the Natal Group sandstones. The soil
types vary from podzolic and duplex soils of the midlands and coastal belt
(Beckedahl, 1996).
Field data collection
Identification of roadcuts
Roadcuts of interest were identified by first traversing main and regional
roads in the south eastern region of South Africa on Google Earth. Following
the above procedure, field inspection was conducted on identified sites to
assess the actual condition of the roadcuts. Roadcuts were then numbered and
random samples selected using random number tables, to get actual sizes for
detailed investigation. The roadcuts were then categorised into degraded and
non-degraded. For the purpose of this study, the degraded were those with
the presence of either rills or flutes, whereas non-degraded roadcuts were
those with no apparent rilling. This resulted in twenty nine degraded and
twenty non-degraded roadcuts. The degraded roadcuts were further classified
into three erosion categories based on the mean percentage cover of rills
per square metre plots established on the roadcuts: (1) slight: less than
25 %; (2) moderate: between 25 and 50 %; (3) extensive: between
50 and 75 %; and (4) very extensive: above 75 %. The selected
roadcuts did not receive any form of treatment after construction (e.g.
hydroseeding) and were characterised by natural herbaceous vegetation
cover. Additionally, the selected roadcuts were located along roads that
were constructed at the same period to minimise the effects of the roadcuts'
age on erosion. Moreover, these roadcuts were chosen because precipitation
across the study region did not vary significantly; hence it was assumed
that the selected roadcuts received approximately the same amount of
rainfall.
Measurement of the roadcut characteristics
The gradient, length, percentage of vegetation cover and soil texture (i.e.
percentage of sand, silt and clay content) were measured on the degraded and
non-degraded roadcuts identified in the south eastern region of South
Africa. Slope profile measurements were done along three cross-profile
transects on each roadcut by using an Abney level, a ranging rod and a
measuring tape. Transects were established from the top to the bottom of the
roadcuts, with the first transect running along the maximum slope length.
The next two transects were located on both sides of the first transect and
halfway to the end of the roadcut width (Fig. 2). Slope profiles were
measured by recording a series of measured lengths along a transect and
corresponding series of measured angles. The slope gradient for each
roadcut was calculated as the average of averages for each transect. The
maximum lengths of the roadcuts were then considered as overall lengths of
the roadcuts.
Schematic representation of rill survey plots on the
roadcuts.
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Descriptive statistics for slope characteristics.
Degraded roadcuts
Non-degraded roadcuts
Slope characteristics
min
max
mean
SD
min
max
mean
SD
Gradient (∘)
24.5
78.3
52.5
13.1
13.2
42.9
28.2
9.5
Length (m)
5.1
20.0
10.7
4.0
5.7
14
6.4
3.3
Veg. cover ( %)
0.0
45.5
24.1
24.5
50.4
100
91.7
14.0
Sand ( %)
44
78
66
9.73
6
84
39.5
26.4
Silt ( %)
8
47
22
11.4
2
60
20.4
16.1
Clay ( %)
6
12
8.7
1.9
8
70
39.1
22
SD = standard deviation
Percentage of vegetation cover was measured by demarcating transects made of
1 m long and 4 m wide plots which were then numbered. Random samples were
selected from the numbered plots using random number tables, to get actual
sizes for detailed investigation. This resulted in selection of more than 70 %
of the plots on each roadcut, of which the number of plots on each
roadcut was determined by the surface area. In each plot, a 4 m string
attached to two metal pins was placed at 0.5 m length of a plot. Vegetation
cover was calculated as the total vegetated distance of the string to the
total length of the string, and recorded as a percentage (Kercher et
al., 2003). Total percentage of vegetation cover for the entire roadcut was
then calculated as the mean of all plots percentage covers (Bochet and
García-Fayos, 2004).
Soil samples obtained from the rill complex of the roadcuts were placed in
labelled sample bags. All sample bags were stored in dry conditions until
they were transported to the laboratory for determination of the soil texture
(i.e. percentage sand, silt and clay content). Soil texture was determined
by the pipette/hydrometer method for the fraction of particles with a
diameter less than 2 µm (clay fraction) by sieving for particles
between 200 and 2000 µm (coarse sand) and between 20 and 200 µm
(fine sand), while the fraction between 2 and 20 µm (silt) was obtained
by difference (Mesquita et al., 2005).
The measurement of rill dimensions
Measurements of rill dimensions were made from 4 m2 plots located
upslope, midslope and downslope of the roadcuts (Fig. 3). The widths and
depths of the rill were measured using a measuring tape and a 30 cm ruler
respectively, at regular intervals (i.e. 0.01 m) along the sinuous length of
the rill, and the averages were calculated (Hagmann, 1996; Sidle et al.,
2004).
Field data analysis
Statistical analysis was performed using Statistical Package for Social
Sciences (SPSS) version 21 software. The Kolmogorov–Smirnof test was used
to test data normality. A test of proportions was employed to determine
whether there were significant differences between slope characteristics of
the degraded and non-degraded roadcuts. One-way analysis of variance (ANOVA)
at a 95 % confidence level (P < 0.05) was used to determine whether
there were significant differences between slope characteristics of the
slightly, moderately, extensively and very extensively degraded roadcuts.
Pearson correlation was used to evaluate whether there were any associations
between slope characteristics (gradient, length, percentage of vegetation
cover and soil texture) and rill dimensions. Similarly, one-way ANOVA (P < 0.05)
with a Tukey's HSD post-hoc test was used to determine if
there were any significant differences of rill dimensions upslope, midslope
and downslope of the roadcuts.
Results
Characteristics of the roadcuts
The slope characteristics of the roadcuts are presented in Table 1. Results
show that these characteristics ranged widely for the roadcuts. It can be
observed that the mean slope gradient of the degraded roadcuts was higher
(52.51∘) than that of the non-degraded roadcuts (28.24∘).
Similarly, the mean length of degraded roadcuts was higher (10.70 m) when
compared to that of the non-degraded roadcuts (6.38 m). The vegetation cover
for degraded roadcuts was low, with a mean percentage of 24.12, while
non-degraded roadcuts had a higher mean percentage of vegetation cover of
91.71. The mean sand content of degraded roadcuts was 66 % while the
non-degraded had a mean of 39.5 %. Additionally, mean silt contents of
22 and 20.4 % were observed for degraded and non-degraded roadcuts,
respectively. Moreover, the mean clay content for degraded roadcuts was
8.7 %, while the non-degraded roadcuts had a percentage of 39.1.
The results in Fig. 4 show the significant differences of slope gradient,
length, percentage of vegetation cover, and the percentage of sand, silt and
clay content between non-degraded (ND) and degraded (D) roadcuts. It can be
observed that the slope gradient and length of degraded roadcuts are
significantly (p < 0.05) higher than for non-degraded roadcuts.
Moreover, vegetation cover for degraded roadcuts is significantly lower than
that for non-degraded roadcuts. The percentage of clay content was higher
for degraded roadcuts than that of the non-degraded roadcuts, while the
percentage silt and clay contents were not significantly different.
Proportions of slope gradient, length, vegetation cover,
sand, silt and clay for non-degraded (ND) and degraded (D) roadcuts. Bars
represent proportions of different roadcut characteristics and whiskers
represent 95 % confidence intervals.
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On the other hand, the results of ANOVA with post-hoc test showed that
there were no significant differences (p > 0.05) amongst the site
variables (slope length, gradient, percentage of the vegetation cover, sand,
silt and clay) of the slightly, moderately and extensively degraded
roadcuts.
Rill dimensions
The results show that the characteristics of the roadcuts significantly
determine rill dimensions (Table 2). Significant moderate positive
correlations of gradient with both rill width and depth were observed, while
the percentage of vegetation cover had a strong significant negative
correlation with rill depth and width. The rill width and depth, however,
were not significantly influenced by the roadcut length.
Significant (p < 0.05) relationships between site
variables and rill width as well as depth from Pearson correlation results.
Width
Depth
Slope length
Pearson correlation
0.210
0.221
Significance
0.190
0.110
Slope gradient
Pearson correlation
0.371
0.339
Significance
0.018*
0.033*
Vegetation cover ( %)
Pearson correlation
-0.621
-0.637
Significance
0.000*
0.000*
Sand ( %)
Pearson correlation
0.37
0.41
Significance
0.05*
0.03*
Clay ( %)
Pearson correlation
-0.50
-0.46
Significance
0.04*
0.01*
Silt ( %)
Pearson correlation
-0.23
-0.28
Significance
0.23
0.13
* Correlation is significant at 0.05 level.
The mean values for rill dimensions at different roadcut slope positions
(upslope, midslope and downslope) are shown in Table 3.
Mean rill width and depth values for different slope
positions on roadcut embankments under study.
Slope position
Width (m)
Depth (m)
Upslope
0.14
0.079
Midslope
0.11
0.064
Downslope
0.08
0.045
The rill dimensions were significantly different at different plot positions
(Table 4), with values decreasing downslope. The results showed that the
rill dimensions had highly significant differences between the upslope and
downslope positions.
The results of ANOVA using a Tukey's HSD post-hoc test for
rill dimensions (width and depth) and different slope positions (upslope,
midslope and downslope) at 95 % confidence level (P < 0.05).
Slope position
Rill width
Rill depth
US vs MS
0.149 ns
0.104 ns
US vs DS
0.000
0.000
MS vs DS
0.024
0.041
US = upslope; MS = midslope; DS = downslope;
ns = non-significant
Discussions
This study aimed at evaluating the characteristics of the degraded and
non-degraded roadcuts as well as assessing the relationship between the rill
dimensions and the roadcut characteristics.
The characteristics of roadcuts
The results of this study have shown that the characteristics of the
degraded roadcuts were significantly different from those of the
non-degraded. For instance, it was noted that degraded roadcuts were
characterised by high slope gradients and lengths, low vegetation cover and
lower clay content percentage, when compared to the non-degraded roadcuts.
These results are comparable with previous studies which indicated that
these conditions increase the vulnerability of roadcuts to erosion
(Arnáez et al., 2004; Bochet and García-Fayos, 2004; Flanagan et
al., 2002). This is true because literature shows that an increase in slope
gradient reduces the infiltration rate, hence increasing runoff
(Arnáez et al., 2004; Manyatsi and Ntshangase, 2008; Megahan et al.,
2001). A study by Arnáez et al. (2004) in the Iberian Range, Spain,
demonstrated a significant positive relationship (r = 0.76;
p = 0.004) between roadcut slope gradients and runoff which could result in a
substantial increase in the formation of rills (Fox and Bryan, 2000).
Formation of rills results from the increased scouring capacity of
concentrated runoff (Haile and Fetene, 2012). Similarly, Jordan and
Martinez-Zavala (2008) recorded a total soil loss of 106 and 17 g m-2
from roadcut and side-cast fills, respectively, in southern Spain.
The highest erosion rate was observed on the roadcuts due to steep slopes.
The results of this study have also demonstrated that the degraded
roadcuts had longer slope lengths compared to the non-degraded. To some
extent, this observation is valid because longer lengths have the ability to
increase runoff velocity, resulting in both increased soil particle
detachment and transport efficiency downslope as compared to shorter slope
lengths. For instance, a study by Chaplot and Le Bissonnais (2003)
indicated that slopes associated with long lengths have the ability to
increase runoff velocity as well as quantity, thereby influencing rill
development. Furthermore, a study by Kinnell (2000) showed that
an increase in slope length increases erosion by water, particularly when
slope gradients exceed 10 %. However, these findings are in contrast with
other studies. For instance, Megahan et al. (2001) concluded that slope
length alone or in interaction with other variables has no detectable
effects on roadcut erosion. Similarly, Luce and Black (1999) found that
roadcut slope length is insignificant in determining erosion by water.
Although the findings from the above two studies illustrate slope
length as having an insignificant effect on runoff and rill erosion
development, this may be due to other soil erosion contributing factors that
do not favour rill development. For instance, areas associated with clay soil
properties are bound to have less rill development despite having long slope
lengths, when compared to those that are characterised by sandy soils.
The mean percentage of vegetation cover (predominantly herbaceous) for
non-degraded roadcuts was high (91.7 %) when compared to degraded roadcuts
(24.12); hence limited soil erosion was noted. This observation stands
because vegetation cover has been found to stabilise and protect slopes
against erosion since the roots hold soil particles together (Bochet and
García-Fayos, 2004; Mohammad and Adam, 2010). Also, this can be
explained by the ability of vegetation cover to moderate and dissipate the
energy exerted by water (Lal, 2001; Ande et al., 2009). In fact,
vegetation intercepts rainfall, increases infiltration of water, intercepts
runoff and stabilises the soil with roots (Bochet and
García-Fayos, 2004; Loch, 2000). The results of this study are
supported by the work of Cerdan et al. (2002) who observed that the
occurrence of rill erosion on fields was directly a function of vegetation
cover. Similarly, Arnáez et al. (2004) found a negative correlation
(r = 0.60, p = 0.05) between vegetation cover and runoff. According to
Laker (2004), vegetation cover (i.e. herbaceous plants) protects the
soil because of their high basal cover, dense and very fine root systems
that bind the soil.
The higher percentage of clay content for non-degraded roadcuts could be an
indication of the role of clay in reducing soil erosion. An increase in clay
content of the soil has been associated with the increase in aggregate
stability, thereby decreasing soil erodibility (Dlamini et al., 2011).
Haile and Fetene (2012) indicated that fine-textured soils such as clays
are not readily detached because of the strong cohesive forces that keep
them aggregated. Yýlmaz et al. (2008) also observed a higher
susceptibility of soil to erosion where the content of clay was low.
The relationship between slope characteristics and rill dimensions
The roadcut embankment slope characteristics were assessed for their
correlation with the rill dimensions. The results indicate that vegetation
cover was the foremost significant variable in determining rill dimensions
on the roadcuts, while slope length and silt content had no significant
effect. A strong negative correlation between vegetation cover and rill
dimensions suggests that an increase in vegetation cover reduces the cross
sections of the rills. Vegetation cover in a rill catchment reduces runoff
and sediment yield through rainfall interception, infiltration and
resistance to flow (Woo et al., 1997). A significant positive
correlation of slope gradient and rill dimensions indicates that an increase
in slope gradient increases the volume of rills and hence the volume of soil
loss (Berger et al., 2010). However, a moderate correlation of slope
gradient and rill dimensions suggests that rill configuration is more complex
than being merely slope gradient-dependent. Similarly, a moderate negative
correlation between clay content and rill dimensions implies that an
increase in clay content of the soil could reduce the sizes of the rills on
roadcuts. This finding is similar to the study of Marquisee (2010), who
found a negative correlation between clay content and the percentage cover
and number of gully channels.
The dimensions of rills that extended continuously from the top to the
bottom of the roadcuts changed significantly downslope. Previous research has
indicated that significant changes in rill dimensions are determined by soil
detachment and deposition along the length of the rill (Bennett et
al., 2000; Lei and Nearing, 1998). In this study, a decrease in rill depth
downslope suggests that a progressive increase in sediment load downslope
decreases detachment rate (Lei and Nearing, 1998). However, this was
significant between upslope and downslope position, and between midslope and
downslope positions. This suggests that detachment is active between upslope
and midslope, while downslope positions are efficient in transporting the
eroded sediment. The results are comparable with other studies available in
literature (Bennett et al., 2000; Cochrane and Flanagan, 1997; Lei et
al., 2001; Merten et al., 2001). Cochrane and Flanagan (1997) found that
detachment decreases with the introduction of sediment at the top of the
rill. Additionally, Bennett et al. (2000) observed that bed degradation
was high in the upslope section of the channel, while Merten et al. (2001)
reported a decrease in detachment, with an increase in sediment load
along the channel length due to the suspended bed load that reduced the
detachment capacity. In this study, a decrease in rill width downslope
implies that the scouring of the rill side walls decreased as a result of
the limited scouring capacity of flow, due to increase in the sediment load
downslope (Bewket and Sterk, 2003). In addition, Lei et al. (2001)
indicated that sediment load decreases the detachment rates particularly on
slopes greater than 15∘. However, the findings of this study are in
contrast with the study by Okoba and Sterk (2006), who observed a
consistent increase in rill width and depth downslope, and attributed this to
cumulative runoff volume and velocity along the slope.