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This paper reviews slope processes associated with mass movements and soil erosion and contributory factors, including physical and human agents. Acting together, these cause diverse geomorphological features. Slope processes are illustrated by reference to case studies from Brazil and the UK. The causes and impacts of erosion are discussed, along with appropriate remedial bioengineering methods and the potential of measures to prevent these types of environmental degradation. Key Words: Mass movements, soil erosion, land degradation, hazards, risks, soil recuperation.
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*Ilha Universitária, Rio de Janeiro, CEP. 21941-972, Brazil, tel: 0055-21 32152106, antoniotguerra@gmail.com.
PEDOSPHERE(土壤圈英文版)
Pedosphere
ISSN 1002-0160/CN 32-1315/P doi:10.1016/S1002-0160(15)600
Slope Processes, Mass Movements and Soil Erosion: A Review
Antônio José Teixeira GUERRA*1, Michael Augustine FULLEN2, Maria do Carmo Oliveira
JORGE1 and José Fernando Rodrigues BEZERRA3
1Department of Geography, Federal University of Rio de Janeiro, Brazil.
2 Faculty of Science and Engineering, University of Wolverhampton, UK.
3 Department de Geography, State University of Maranhão, Brazil.
ABSTRACT
This paper reviews slope processes associated with mass movements and soil erosion and contributory
factors, including physical and human agents. Acting together, these cause diverse geomorphological features.
Slope processes are illustrated by reference to case studies from Brazil and the UK. The causes and impacts of
erosion are discussed, along with appropriate remedial bioengineering methods and the potential of measures to
prevent these types of environmental degradation.
Key Words: Mass movements, soil erosion, land degradation,hazards, risks,soil recuperation.
INTRODUCTION
This article reviews the dominant hillslope processes associated with gravity and running water.
Human activity plays important roles in hillslope processes, due to land use changes and vegetation
clearance, both in rural and urban areas. These processes can be accentuated by climate change
(Varnes, 1978; Trudgill, 1988; Selby, 1993; Goudie, 1995; Cendrero and Dramis, 1996; Cruden and
Varnes, 1996; Goudie and Viles, 1997; Favis-Mortlock and Guerra, 1999; Fullen, 2003; Fullen and
Catt, 2004; Crozier and Glade, 2005; VanWesten et al., 2008; Arbuckle, 2013; Kanungo and
Sharmas, 2014; Shafiq et al., 2014; Agnihotri and Kumar, 2015). The causes and consequences of
both sets of processes are discussed. Furthermore, the importance of monitoring these processes is
considered, in order to understand how they occur and can be prevented (Lascelles et al., 2000;
Valentin et al., 2005; Bochet et al., 2006; Kitutu et al., 2007; Nadal-Romero et al., 2014;
Vanmaercke et al., 2016). In addition, once they do occur, we consider potential recuperation
technologies (Fullen et al., 1995; Fullen and Catt, 2004; Subedi et al., 2009; Bhattacharyya et al.,
2010, 2011; De Baets et al., 2011; Fullen et al., 2011; Subedi et al., 2012; Dhital et al., 2013;
Guerra et al., 2015).
Proactive management of vegetation systems are essential for effective recuperation (Trudgill,
1988; Fullen and Catt, 2004; De Baets et al., 2011; Fullen et al., 2011; Bhattacharyya et al., 2012;
Dhital et al., 2013; Guerra et al., 2015). The role of soil erosion is discussed, considering
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accelerated erosion as one of the greatest problems of land degradation, because it seriously
depletes fertile topsoil. The removal of original vegetation for agricultural purposes is one of the
main factors causing soil erosion. The general forms of soil erosion by water (including sheet, rill
and gully erosion) are discussed and illustrated using Brazilian and British field examples.
A detailed description of the main geo-environmental features of a region and its different
effects on the occurrence of landslides and soil erosion is presented. To reach this objective, several
factors have been addressed, and are illustrated using examples from Brazil and the UK.
Geomorphic activity is usually a critical determinant of damage. Each of the two geomorphic
processes analysed in this article have specific causal factors, such as vegetation clearance, rainfall
intensity, rainfall volume, slope angle, soil properties, land use and land management, which affect
soil erosion and mass movements, both in urban and rural areas. Depending on the frequency and
magnitude of each one of these factors, catastrophic landslides might occur, and some examples are
discussed.
The role of mass movements and associated geomorphological processes are discussed, along
with the diagnostic parameters to recognize different types of mass movement in the field (Varnes,
1978;Brunsden, 1988; Goudie and Viles, 1997; Crozier and Glade, 2005;Morgan, 2005; Lin et al.,
2006; VanWesten et al., 2008;Eeckhaut et al., 2010; Clague and Robert, 2012; Guzzetti et al., 2012;
Kanungo and Sharmas, 2014). Again, Brazilian and British examples are used to illustrate these
features (Selby, 1993; Goudie and Viles, 1997; Fullen and Catt, 2004; Coelho Netto et al., 2007;
Guerra et al., 2007; Graeff et al., 2012; Petruci et al., 2013; Guerra and Jorge, 2014).
Selby (1993) outlined that mass movements (or mass wasting) is the movement of soil and/or
rock downslope, under the influence of gravity, being a collective material movement, without
necessarily being influenced by water or ice. Nevertheless, water or ice may decrease the shear
strength of slopes, thus soils physically behave as plastics or, in very moist conditions, as fluids
(Abrahams, 1986; Brunsden, 1988; Selby, 1993; Goudie and Viles, 1997; Clague and Robert, 2012;
Kanungo and Sharmas, 2014; Guerra and Jorge, 2014). This might, consequently, make mass
movements even more catastrophic, causing destruction and even mortalities.
Soil erosion and land degradation are global problems and pose major issues in many countries,
including Brazil. The hazards affect both urban and rural areas within the extensive national
territory (8,547,403 km2). In turn, these problems have serious environmental and socio-economic
impacts (Guerra et al., 2014). It is important that soils be conserved, for present and future
generations. Although erosion is a natural phenomenon, often human activity accelerates erosion
processes. Erosion may occur naturally, due to slope angle and rainfall. Some surveys exemplify
this, often based on stratigraphical and archaeological evidence within valley floor deposits. For
instance, natural soil erosion has been reconstructed in North Germany from the early Holocene,
when soil developed under natural woodlands, up to the early Middle Ages, when erosion rates
were still very low (Bork, 1989). Furthermore, Dotterweich (2009) and Dreibrodt et al. (2010) have
discussed soil erosion during the Holocene. During the Neolithic (~7,500 years BP) “many areas of
central European soil has been washed downslope by soil erosion and gullies have incised, leading
to the development of colluvial and alluvial deposits” (Dotterweich, 2009). Soil erosion on US
agricultural soils causes the loss of an average of 30 tonnes per hectare per year; some eight times
greater than rates of soil formation. A survey by EMBRAPA (Brazilian Agricultural Research
Corporation) suggested the situation in Brazil is often worse, reaching 60 tonnes per hectare per
year in south-eastern Brazil (EMBRAPA, 2002). According to Goudie and Boardman (2010) “it is
quite clear that the major areas of intense erosion are associated with both human and natural
factors.” Boardman (2006) suggested the following countries and regions are global erosion
‘hotspots:’ the Loess Plateau of China, Ethiopia, Swaziland and Lesotho, the Andes, South and East
Asia, the Mediterranean basin, Iceland, Madagascar, the Himalayas, the Sahel of West Africa, the
Caribbean and Central America. We propose Brazil is also an erosion ‘hotspot’ (Silva et al., 2005;
Gurgel et al., 2013; Guerra et al., 2014; Nacinovic et al., 2014).
Although both soil erosion and mass movements are two forms of land degradation, and humans
play important roles in these geomorphological processes, they present different modes of
occurrence and consequently different ways of being identified, monitored and they present diverse
features (Varnes, 1978;Small and Clark, 1982; Abrahams, 1986; Brunsden, 1988; Gerrard, 1992;
Evans, 1993; Selby, 1993; Guerra, 1994; Goudie and Viles, 1997; Favis-Mortlock and Guerra, 1999;
Fullen and Catt, 2004;Crozier and Glade, 2005; Morgan, 2005; Lin et al., 2006; Van Beek et al.,
2008; VanWesten et al., 2008;Goudie and Boardman, 2010; Vente et al., 2011; Boardman and
Favis-Mortlock, 2013; Kanungo and Sharmas, 2014; Oluwagbenga and Orimoogunje, 2014; Guerra
et al., 2015; Monsieurs et al., 2015; Vanmaercke et al., 2016). Nevertheless, the best way to avoid
both forms of land degradation is acting preventively, that means to understand the risks of soil
erosion and/or mass movements, in order to avoid them. In this respect, Oluwagbenga and
Orimoogunje (2014) stated that conservationism emphasizes the need to guarantee a sustainable
supply of productive land resource for future generations. Preservationism seeks to protect scenery
and ecosystems in a state as little affected by humans as possible.
MATERIALS AND METHODS
Firstly, an extensive literature survey of over 100 publications was completed. Studied aspects
included soil erosion, including soil erodibility, erosivity, soil properties, soil types, slope angle,
length and shape, vegetation cover, vegetation clearance, climate change and land use and
management.Climate regime plays an important role for both soil erosion and mass movements.
With regard to rainfall, over a long period, most erosion occurs during events of moderate
frequency and magnitude, because catastrophic events are not so frequent so as to cause a great
amount of net erosion. This this is a short-term perspective; when high magnitude events occur soil
loss is much higher than during moderate rainfalls; heavy precipitation events are the main reason
for the incision of gullies in many landscapes (Dreibrodt and Wiethold, 2015). The same applies to
mass movements, taking into account the main factors, which affect this geomorphological process
(i.e. slope angle and shape, soil properties, vegetation cover, soil depth, the interface between soil
and the underlying rock, vegetation clearance, human factors, such as slope talus cuts, lack of soil
drainage and sewage and unpaved roads). At a long temporal scale, the relationship between
landslide activity and triggering mechanisms can be established from the temporal clustering of
dated landslides (Borgatti and Soldati, 2010). The comprehensive literature survey article is based
on articles, mainly from 2000-2015, but also some pre-2000 literature.
DISCUSSION
Mass movements
Soil erosion and mass movements have attracted thousands of studies across the world.
Although both processes constitute forms of land degradation, for this review article we describe
them separately. Selby (1993) concisely described mass movements, or mass wasting of soils as
the movement of soil and/or rock, downslope, under gravity, of collective materials, without
necessarily water or ice action. Varnes (1978) developed a mass movement classification based on
the material (mud, soil, earth, rock and debris) and movement type (falls, topples, slides (rotational
and translational), lateral spreads and flows). Varnes also proposed a further type, which he named
complex, which is a combination of two or more principal types of movement. These movements
are outlined in this article, and some examples are presented. When there is the action of water
and/or ice, the agents may decrease soil shear strength and, consequently, contribute to the plastic
or liquid behaviour of soil, making mass movements even more catastrophic (Varnes, 1978;
Brunsden, 1988; Goudie, 1995; Cendrero and Dramis, 1996; Goudie and Viles, 1997; Crozier and
Glade, 2005; Van Beek et al., 2008; VanWesten et al., 2008; Fell et al., 2012; Graeff et al., 2012;
Korup, 2012; Petruci et al., 2013; Guerra and Jorge, 2014; Kanungo and Sharmas, 2014). Surveys
of mass movements have different aims, including predicting their occurrence, which depends on
several factors. Therefore, care is required with the interpretation of site characteristics.
Undoubtedly, any judgement, on mass movement hazards will be subjective and it is strongly
advised that local expertise is consulted, as distinct conditions may be important for the initiation
and reactivation of mass movement in a given region(Van Beek et al., 2008). Therefore, the
geomorphological investigation of mass movements may provide a framework, in order to describe
and map surface landslide processes, and to predict future process behaviour (Brunsden, 1988;
Selby, 1993; Griffiths and Whitworth, 2012; Kanungo and Sharmas, 2014). Guzzetti et al. (2012)
recommended that to prepare a landslide map, a legend is required. The legend must meet the
project goals, must be capable of portraying relevant geomorphological characteristics, and must
be compatible with the technique used to capture the information.”
Several authors have monitored mass movement dynamics (Goudie, 1995). In this respect,
Petley (1984) described the main objectives of surveys of mass movements:
1. To understand the development of natural slopes and the processes that contribute to the
formation of new features.
2. To make it possible to stabilize slopes under different conditions.
3. To determine landslide risk, or other forms of mass movements, including on natural and
artificial slopes.
4. To facilitate recuperation on slopes which have experienced mass movements and to plan
land-uses which include preventive measures, so that those geomorphological processes do not
recur.
5. To analyse the various types of mass movements and assess the causes and consequences of
these processes.
6. To know how to respond to external factors influencing slope stability, such as earthquakes,
which also play important roles in triggering mass movements.
Many authors have addressed the important issues of mass movement hazard and risk.
Crozier and Glade (2005) highlighted that the level of risk is the combination of the likelihood of
adverse occurrences and the consequences if it does. The level of risk results from the intersection
of hazard with the value of the elements at risk by way of their vulnerability.
There are different types of mass movement. Therefore, the different definitions used and
the physical principles which underlie mass movements must be explained and the diagnostic
parameters to explain how to recognize different types of mass movements in the field are
fundamental. The main types of mass movement are falls, slides and flows (Varnes, 1978; Brunsden,
1988; Selby, 1993; Van Beek et al., 2008; Clague and Robert, 2012). They have many causes,
including deforestation, adverse hydrological conditions and slope undercutting, climate
(precipitation/thawing of ice), geology (water impermeable layers and swelling clays), earthquakes
and volcanic eruptions. In addition, meteorological events, such as heavy rainstorms, inducing
water infiltration and increased pore water pressure; and increased air temperatures, inducing the
melting of glacial or ground ice (Cendrero and Dramis, 1996).
The most common and catastrophic mass movements are landslides. According to Clague
and Robert (2012) each year, landslides are responsible for hundreds of millions of dollars’worth
of damage and, on average, claim more than 1,000 lives around the world. Although most common
in mountainous areas, landslides can occur anywhere with enough local relief to generate
gravitational stresses capable of causing rock or soil to fail(Figs. 1 and 2). They may be one of
the most damaging and deadly of the natural hazards in the world, and the data available from the
Centre for Research on the Epidemiology of Disasters (CRED), located in Leuven, Belgium,
suggest that landslides were responsible for over 10,000 deaths and left 2.5 million people homeless
between 2001-2010 (CRED, 2011). Another useful definition of landslides is proposed by Korup
(2012), who stated landslides are the downhill and outward movement of slope-forming materials
under the influence of gravity and also, in most cases, water. Mostly triggered by earthquakes,
rainstorms, snowmelt, and slope undercutting, they are among the prime producers of sediment and
major agents of denudation. In fact, although there are different types of mass movement, it is very
common amongst many authors view landslides as synonymous with mass movements.
Fig. 1 A landslide scar formed due to heavy rainstorms in April 2013 in Petrópolis Municipality,
Brazil. The house was condemned on safety grounds (photo by Antonio Jose Teixeira Guerra).
Fig. 2 House destroyed by a landslide in April 2013 in Petrópolis Municipality. This caused the
death of three people (photo by Antonio Jose Teixeira Guerra).
Mass movements have been surveyed by many disciplines, including geologists,
geomorphologists and engineers. They have used different approaches, but all of them are
concerned with understanding the processes, in order to be able to propose ways to assess and,
consequently, to avoid them (Morgan, 2005). The amount of sediments transported by mass
movements to rivers is much greater than that transported by rills and gullies (Morgan, 2005). It is
extremely important to predict mass movements. An initial step is to construct accurate and reliable
maps that can be used to assist the prediction of landslide hazards and risks in a specific area. It is
crucial to have insights into the spatial and temporal frequency of landslides, and therefore each
landslide hazard or risk study should start by making a landslide inventory that is as complete as
possible, both in space and time (VanWesten et al., 2008). Consequently, by mapping and dating
the phenomena present in the landscape, we become able to: a) outline hazardous zones (mapping
and comparison with geological and relief data), and b) consider recurrence intervals and relevant
processes (such as dating and comparison with palaeoclimatic data, palaeovegetation data and
historical land use data).
In a survey in Ubatuba Municipality (São Paulo State, Brazil) deforested steep slopes were
the necessary preconditions for mass movements, which were then triggered by heavy rainstorms
(Guerra and Oliveira, 2009). In Ubatuba, these natural conditions can be accentuated by unplanned
settlements. Urban expansion was accelerated after the construction of the Rio-Santos Highway,
attracting many tourists to this area and, consequently, promoting rapid construction of houses and
resort buildings, without respecting environmental risks (Fig. 3).
House construction has tended to move beyond the densely settled coastal plains onto
adjacent hillslopes, which are often steep. This poses problems to both residents and tourists (Souza
and Suguio, 2003; Ferreira et al., 2005; Guerra and Oliveira, 2009; Mendes and Valerio Filho,
2015). Ubatuba is infamous for landslides. In general, the major natural constrains that are
responsible for translational landslides in the study area include high slope steepness (usually over
30o), which is associated with morphology (concave and/or linear geometry), and the presence of
seasonal “apparent”cohesion, which results from saturated soil profiles and high rainfall
(cumulative and/or hourly rainfall intensity) (Mendes and Valerio Filho, 2015).
Fig. 3 Shallow landslide scar on Rio-Santos Highway, in Ubatuba Municipality, São Paulo State,
Brazil, December 2009 (photo by Maria do Carmo Oliveira Jorge).
Guerra et al. (2007) conducted a comparable survey and analysed mass movements in
Petrópolis Municipality, where 50 people died in 2001, due to landslides caused by ~200 mm of
rain in 24 hours. In 2011 another heavy rainfall of 240 mm in 24 hours caused landslides that
resulted in the deaths of 71 people in the same Municipality (Graeff et al., 2012). In both surveys,
the authors arrived at very similar conclusions. The main causes of these catastrophic
geomorphological processes were both natural (i.e. heavy rainstorms and steep slopes) and human
factors (i.e. unplanned settlement, vegetation clearance, unpaved roads and lack of appropriate
sewage systems and rain-water conduits). These findings agree with Trudgill (1988), who outlined
that mass movements can be seen from the perspective of their relationships between natural
components and responses to slope perturbations. Trudgill (1988) identified that mass movements
usually start with vegetation clearance, although in some cases they might occur on vegetated
slopes. Furthermore, soil and vegetation systems are complex, and one of the main associated
problems is the application of the stability concept. Some subcomponents of the system will
experience more changes than others (Brunsden, 1988; Gerrard, 1992; Selby, 1993; Goudie and
Viles, 1997; Morgan, 2005; Van Beek et al., 2008; Clague and Robert, 2012; Fell et al., 2012;
Korup, 2012; Brunetti et al., 2014).
The Rio de Janeiro-Ubatuba Highway, which connects Rio de Janeiro and São Paulo States,
has attracted many people, who often build their houses on steep slopes. This type of urban
settlement on these steep slopes has been responsible for many landslides, especially in recent years
(Fig. 4). They have caused the death of dozens of people and severe material losses (Ferreira et al.,
2005; Guerra and Jorge, 2009; Guerra et al., 2013; Mendes and Valério Filho, 2015).
Often there are time-lags between deforestation of steep slopes and the onset of mass
movements. On forest clearance, tree roots remain largely intact and thus maintain slope stability.
Roots can act as environmental nails which retain soil in place. However, tree roots will undergo
decomposition processes and these processes are usually rapid in the humid tropics. Thus, after
about two years the environmental nail effect is lost and slopes enter a precarious phase of
potential instability (Goudie and Viles, 1997; Brunetti et al., 2014; Nadal-Romero et al., 2014).
The response of slopes to the different ways they are occupied depends on several factors,
including the existing soils, slope angle, shape and human intervention. In recent decades in Brazil,
and several other countries, there has been an increased frequency and magnitude of mass
movements, partly due to physical environmental variables, but mainly due to the way constructions
are built without taking into account the risks posed by the natural triggers at each site.
Fig. 4 Landslide scar in Angra dos Reis Municipality. During this event over 40 people died within the
Municipality, due to landslides associated with ~200 mm of rain in 24 hours, December 2009 (photo by Antonio
Jose Teixeira Guerra).
Brunsden (1988) pointed out that planners need to know the risks to slopes, due to the kind
of occupation. This is also emphasized by Small and Clark (1982), who outlined the role of humans
when they alter the landscape, and they called this process the production of artificial slopes, which
is particularly important on a local scale.
Local governments should obtain detailed information from scientists (geographers,
geomorphologists, civil engineers, architects, planners, ecologists, soil scientists and geologists), in
order to avoid the occurrence of mass movements, and consequently, loss of lives and property
(Brunsden, 1988; Trudgill, 1988; Goudie and Viles, 1997; Guerra et al., 2007; Van Beek et al.,
2008; Graeff et al., 2012; Guerra and Jorge, 2014; Fell et al., 2012; Korup, 2012). Furthermore,
Brunsden (1988) stated that in cases of subsequent mass movements, local authorities should be
responsible for authorizing the construction of roads and buildings. That is one of the reasons to
produce environmental surveys, including slope assessment, before these areas are occupied, so that
mass movement risks may be evaluated. In order to assess slope hazards and risks, it is also
important to evaluate the rainfall threshold for landslides to occur. Therefore, Kanungo and
Sharmas (2014) outlined that a threshold may define the rainfall, soil moisture or hydrological
conditions that when reached or exceeded, are likely to trigger landslides. This combination of
environmental and human variables has to be taken into account, to predict mass movements and,
therefore, try to avoid them.
SOIL EROSION
Selby (1993) classified soil erosion in his classic book Hillslope Materials and Processes
as a geomorphological process which occurs on hillslopes, carried out by flowing water and splash
processes. Selby (1993) termed this erosion on hillslopes by raindrops and flowing water. Selby
outlined the role of water in removing and transporting sediments, which he described as wash, a
term adopted by many authors (Gerrard, 1992; Evans, 1993; Goudie and Viles, 1997; Poesen et al.,
2006; Goudie and Boardman, 2010; Guerra et al., 2014).
It is important to outline the difference between natural soil erosion and accelerated soil
erosion. The first one is what we can also call geological erosion, which is water flowing on the
soil surface, possibly transporting sediments and, consequently reducing soil thickness, but over a
long period of time, and usually very slowly. In this case, weathering, which occurs on the rocks
underneath the soil, can compensate for the eroded soil. Accelerated soil erosion usually occurs on
agricultural fields and bare soils and depends on several factors, which are discussed in this paper.
Therefore, other concepts may be introduced to differ natural and accelerated soil erosion; the first
one is with respect to soil loss tolerance, and whether this exceeds a limit, causing land degradation,
as rates of soil formation are usually much less than soil loss.
Sediments transported by running water usually pose another environmental problem; that is
the off-site effects of soil erosion. This is becoming a recurrent problem in the UK, and therefore,
Boardman and Vandaele (2010) outlined that muddy flooding is caused by runoff carrying soil
from bare or relatively bare agricultural fields. Documentation of muddy flooding exists for
several other European countries, including Belgium, France, The Netherlands, Poland, Slovakia,
Germany, Spain and Italy (Boardman and Vandaele, 2010). This is a good example of off-site
effects from agricultural fields damaging property, roads and water bodies (rivers, reservoirs and
lakes). Evrard et al. (2010) reported that in the European loess belt, water flowing from agricultural
fields frequently carry large quantities of soil as suspended sediment. These geomorphological
processes cause muddy floods in settlements downstream and are generally triggered on silty and
loamy soils, which are prone to surface sealing (Boardman et al., 2006). Nevertheless, the best
option is in order to prevent (ephemeral) gullies from developing in cropland, all possible
measures leading to an increase in rain inltration, to a reduction in Hortonian overland ow
discharge and hence also to a reduction of ow shear stress need to be applied (Poesen et al.,
2006). Consequently, there will be less risk of both on-site and off-site effects. In the European
context, most concern is expressed over the damaging off-site effects of soil erosion on water
quality and the costs associated with subsequent water purification for water supply systems (Fullen,
2003). Soil erosion has different classifications, according to the region where it occurs, soil types,
precipitation regime, soil properties, slope characteristics, land-use and management. Nevertheless,
most authors agree that this process can cause three main features, depending on causal factors and
on its evolution. These are sheet, rill and gully erosion (Abrahams, 1986; Selby, 1993; Goudie,
1995; Fullen and Catt, 2004; Morgan, 2005; Valentin et al., 2005; Boardman, 2006; Van Beek et al.,
2008; Goudie and Boardman, 2010; Vanmaercke et al., 2012; Monsieurs et al., 2015; Vanmaercke
et al., 2016). Although the three erosion processes cause land degradation, wherever they occur
recent field-based studies indicate that: (1) gully erosion is an important soil degradation process
in a range of European environments, causing considerable soil losses and producing large
volumes of sediment, and (2) (ephemeral) gully development increases the sediment connectivity in
the landscape and hence also the sediment delivery to lowlands and permanent water courses
where gullies aggravate off-site effects of water erosion (Poesen et al., 2006). This is another
good example how off-site effects are usually at least as important as on-site effects in soil erosion
surveys. Fullen and Catt (2004) outlined that when rainfall intensity exceeds soil infiltration
capacity, runoff begins, thus provoking soil erosion. They also stated that the process initiates as
sheet erosion, tending to concentrate in minor incisions, forming rills (Fig. 5), which may evolve
into gullies (Figs. 6 and 7), as they widen and incise into the soil. Fullen and Catt (2004) admitted
that this theme might be polemic. Therefore, they stated that while rills tend to incise mainly into
the A horizon, gullies reach easily the B and even C horizons. Sometimes they even reach bedrock,
depending on the magnitude of erosive processes. This is agreed by several authors (Thornes, 1990;
Gerrard, 1992; Selby, 1993; Favis-Mortlock and Guerra, 1999; Morgan, 2005; Valentin et al., 2005;
Boardman and Poesen, 2006; Evans, 2006; Goudie and Boardman, 2010; Guerra et al., 2014;
Guerra et al., 2015; Labriere et al., 2015; Vanmaercke et al., 2016).
Surface runoff is produced due to several factors, including vegetation clearance, agriculture
without conservation practises and rainfall regime. When rainfall intensity exceeds infiltration
capacity, the excess rain forms surface runoff. This process causes sheet erosion, which might
evolve into rill and gully erosion (Gerrard, 1992; Selby, 1993; Fullen and Catt, 2004; Morgan, 2005;
Goudie and Boardman, 2010; Guerra et al., 2014; Labriere et al., 2015). As erosion processes at
field level are dominated by concentrated rills, these linear erosion features can widen and deepen
and cut into the subsoil, thus creating gullies. In addition, depending on the size of the agricultural
fields, erosion may produce less soil loss, but the larger the fields, the larger the runoff collection in
a catchment. As a result one has to consider different scales, both for surveying and for estimating
damage. Enters (1998) reviewed this issue in detail, when he discussed scaling-up from fields to
national levels. From this perspective, Enters (1998) outlined the on-site impacts of soil erosion, at
several hierarchical scales, and occurrences at one scale usually influence outcomes at other scales.
Furthermore, Izac and Swift (1994) defined five hierarchical levels for measuring soil erosion:
cropping system, farming system, catchment system, regional system and supra-regional system.
Soil erosion is a natural phenomenon (i.e. it varies naturally with climate, soils and topography).
Therefore, all landscapes which have slopes >~ 3o may experience some form of erosion (Gerrard,
1992; Selby, 1993; Ashman and Puri, 2002; Fullen and Catt, 2004; Morgan 2005; Evans, 2006;
Gumiere et al., 2009; Liu et al., 2014; Sensoy and Kara, 2014). Nevertheless, in Europe, during the
Holocene, there was relatively little natural erosion once vegetation cover developed, except for
early Holocene climate anomalies. According to Dreibrodt et al. (2010) the general pattern is
clearly reected by the slope deposit record. At a closer look, there are different phases of
variability within the record, and additional deposits are suspected to have been deposited during
the Early Holocene. However, in some areas erosion and consequent deposition is fundamental for
natural soil fertility maintenance, such as the Nile Delta, which receives sediments originating from
Ethiopia. These natural processes have maintained soil fertility for centuries, but dam construction
to control the Nile regime has disturbed this equilibrium (Ashman and Puri, 2002). Sediment from
the Yellow River in northern China is also important for the maintenance of soil fertility on the
adjacent floodplain (Fullen et al., 1995).Currently, synthetic fertilizers can maintain soil fertility,
and river floods pose serious risks to people on the alluvial plain. The more crucial recent problem
is shoreline erosion at the Nile Delta mouth, which can cause severe problems for coastal
settlements.
Fig. 5 Rill erosion along cultivation lines in east Shropshire, England, UK (photo by Michael Augustine Fullen).
Fig. 6 Rill and gully erosion due to low intensity (1.8 mm/h) rainfall on snowmelt saturated soils at Hilton, east
Shropshire. Note the shoulder at ~20 cm depth, due to subsoil compaction (see Fullen, 1985; photo by Michael
Augustine Fullen).
Geological soil erosion does not usually cause major environmental problems,because it takes
place under natural conditions (i.e. without human disturbance), but accelerated soil erosion often
does (Thornes, 1990; Gerrard, 1992; Selby, 1993; Morgan, 2005; Boardman, 2006; Boardman and
Favis-Mortlock, 2013; Lopez-Vicente et al., 2013; Almagro and Martinez-Mena, 2014; Labriere et
al., 2015).
Fig. 7 Gully within Bacanga State Park, São Luis, Maranhão State, Brazil, in January 2015 (photo by José
Fernando Rodrigues Bezerra).
The problems related to soil erosion are more evident when soil loss exceeds natural or geologic
levels, usually due to the lack of soil conservation practises, which is called accelerated erosion
(Selby, 1993). If we consider the Holocene, under natural vegetation cover, usually little soil
erosion occurs. In most landscapes, while the vegetation is not removed, the export of matter occurs
in the form of ions, with ground-water migrating into rivers, and then transferring to the sea.
Vegetation might be removed naturally, by the ageing and dying of trees in forests, but this results
in transfers of soil particles within distances of only several metres. Additional natural triggers are
natural forest fires, from which one might expect a transfer of soil particles in the dimension of the
specific slope. Severe climate changes (e.g. glaciation) might result in deforestation, triggering
erosion processes, on a hemispheric to global scale. Earthquakes can also trigger local soil erosion.
In addition, humans often clear forests for economic purposes (usually for agriculture or timber
extraction), which encourages soil erosion, since precipitation and snow-melt can then produce
runoff and, in turn, detach and transport soil particles. One could describe such erosion as
Anthropocene soil erosion (i.e. the human imprint on the global environment is now so active that
it rivals some of the great forces of Nature in its impacts on the Earth system). In discussing the
Holocene in Germany, Dreibrodt et al. (2010) commented the comparison of the data from
colluvial layers reects the known settlement and land use history and testies to the strong human
impact on the geomorphologic system.
In the tropics, where rain-storms may be very intense, the signs of erosion are obvious, when
the rivers become full of sediments, causing siltation (Selby, 1993; Fullen et al., 1995; Goudie and
Viles, 1997; Ashman and Puri, 2002; Fullen and Catt, 2004; Morgan, 2005; Boardman and Poesen,
2006; Guerra et al., 2015; Labriere et al., 2015). Labriere et al. (2015) pointed out that soil control
is still provided in the humid tropics, to a certain extent, for crop and grass-dominated land uses,
but is alarmingly depleted in bare soils, with dramatic consequences on soil loss. Even in
temperate climatic zones, where heavy precipitation events are not usually as intense and
concentrated as in the tropics, soil loss usually occurs at lower intensity, but also causes damage to
agro-pastoral lands (Small and Clark, 1982; Abrahams, 1986; Parsons, 1988; Selby, 1993; Goudie,
1990, 1995; Guerra, 1994; Goudie and Viles, 1997; Ashman and Puri, 2002; Fullen, 2003; Fullen
and Catt, 2004; Morgan, 2005; Boardman and Poesen, 2006; Evans, 2006; Poesen et al., 2006;
Plaza-Bonilla et al., 2013; Labirere et al., 2015). In addition, snow-melt, often over frozen and thus
impermeable soil, can cause soil erosion in temperate zones.
Soil erosion also causes off-site problems, such as silting and pollution of areas where
sediments are deposited, such as in rivers, reservoirs and lakes (Thornes, 1990; Wild, 1993; Goudie
and Viles, 1997; Mosaddeghi et al., 2009; Boardman and Favis-Mortlock, 2013; Nacinovic et al.,
2014; Guo, et al., 2015). According to Boardman and Favis-Mortlock (2013) the period when the
soil is inadequately protected represents a ‘window of opportunity’for erosion. Thus, actual
erosion in any year depends on: 1. the timing, amount and intensity of rainfall in that year, 2. the
start date and duration of the ‘window of opportunity’and 3. the soil and morphological
characteristics of the site.
The need for monitoring soil erosion with the use of experimental stations is very important,
because through monitoring soil loss and runoff, erosion processes can be better understood. These
important monitoring programmes produce short-term data (sometimes for decades). Rare events
(extreme erosion-low frequency, high magnitude precipitation events) might not be measured
within such temporal limitations. The data set could be extended immensely by the
geomorphological study of historical soil erosion landscapes (Figs. 8 and 9). There are several ways
to monitor and investigate soil erosion, in order to determine soil loss from fields and catchments.
To do this, it is possible to use aerial photographs, over different months and/or years, to monitor
rill and gully growth. When field and laboratory data are available, such as bulk density and the
total eroded area, it is possible to calculate the amount of soil loss. This procedure is becoming
more common, particularly when detailed scale aerial photographs are available. The use of remote
sensing is another tool, which makes erosion studies more accurate and detailed, when combined
with field and laboratory data. To achieve this aim, it is necessary to have good ground resolution,
(i.e. 10 m). According to Morgan (2005) the studies of temporal changes in vegetation and soil
conditions indicate that with further research it should be feasible to use remote sensing for
continuous monitoring to identify in advance, when there is a high risk of erosion, so that
appropriate protection measures can be implemented. In addition, depending on field conditions,
monitoring programmes and laboratory analyses, it should be possible to accurately determine how
much soil is being lost from a specific field and/or catchment area.
*Ilha Universitária, Rio de Janeiro, CEP. 21941-972, Brazil, tel: 0055-21 32152106, antoniotguerra@gmail.com.
Fig. 8 Aerial view of the Hilton Experimental Site, East Shropshire, where soil loss and runoff have been
monitored since 1982 (see Fullen, 1992; photo by Gill Barrett).
Fig. 9 Runoff plots at the Hilton Experimental Site (photo by Michael Augustine Fullen).
In 1960 the United States Department of Agriculture (USDA) established the maximum
permissible value of 5 tonnes per ha per year of soil loss for the USA, which became known as the
Tolerable value (or T value) (Schertz, 1983). It is estimated that ~80% of the worlds agricultural
soils are subject to some form of erosion (Ashman and Puri, 2002). On average, soils form at a rate
of ~1 tonne per ha per year, and according to these authors, in Africa, Asia and South America, soil
loss can exceed 30 t/ha/y. In Europe, where rains are not usually so intense, erosion rates can
exceed 17 t/ha/y (Boardman and Poesen, 2006).
Pressures on soils exerted by human activities is one of the main causes of erosion (Wild, 1993).
The world population is large and growing, and totalled 7,401,421,170 according to the World
Population Clock (12/02/2016). The exact world population is unknown; this is the best estimate we
have, based on the integration of several demographic models. Moreover, people understandably
aspire to higher living standards, placing yet more pressure on soil resources. These demographic
processes require larger areas to cultivate, graze cattle and provide timber for fuel and construction.
Together these activities clear permanent natural vegetation and expose soils to the erosive
processes of wind and water. Although soil erosion occurs in different parts of the world, there is a
difference between small fields, used by local farmers for subsistence agriculture, and the large
fields of agro-industrial monocultures, since the latter usually use large connected fields. These
often have poor soil structures and low soil organic matter contents, which produce much more
runoff, and consequently, much more erosion. This is the Brazilian case, where these conditions
produce total soil losses >50 t ha-1 y-1, and sometimes >100 t ha-1 y-1 (EMBRAPA, 2002; Guerra et
al., 2014). Wild (1993) summarized the main causes of erosion:
1. Vegetation clearance, leaving soils unprotected.
2. Agriculture and cattle ranges, without conservation practises.
3. Cultivation and cattle ranges on slopes, sometimes >45o, without conservation practises.
4. Trails caused by animals and humans, compacting soils and thus increasing surface water flow.
5. Highway construction, with inadequate environmental planning, which increases surface water
flow and thus generates rills and gullies.
6. Different types of mineral quarries and other economic activities, leaving soils unprotected, and
without proper rehabilitation, during and at the end of these activities.
Many authors agree with Wild (1993) (e.g. Selby, 1993: Goudie, 1995; Goudie and Viles, 1997;
Favis-Mortlock and Guerra, 1999; Ashman and Puri, 2002; Fullen and Catt, 2004; Valentin et al.,
2005; Evans, 2006; Morgan, 2005; Boardman, 2006; Boardman and Favis-Mortlock, 2013;
Monsieurs et al., 2015).
Despite being a typical geomorphological process from rural areas, Guerra and Hoffmann (2006)
outlined that for two Brazilian cities (São Luis, Maranhão State and Palmas, Tocantins State),
although they were founded in different periods (São Luis was established in the 17th Century and
Palmas, in the 20th Century), and although they have different locations, histories and climates,
both cities are experiencing an increasing problem of gully erosion, especially within the city
limits(Guerra and Hoffmann, 2006). This is due to similar factors, including vegetation clearance,
lack of urban planning, inadequate rain and sewage systems and unpaved roads, especially on the
city outskirts. This is agreed by several authors, who have discussed gully erosion within urban
areas (e.g. Selby, 1993; Goudie and Viles, 1997; Favis-Mortlock and Guerra, 1999; Poesen, 2003;
Morgan, 2005; Boardman, 2006; Evrard et al., 2010; Graeff et al., 2012; Monsieurs et al., 2015).
Besides the need to implement soil conservation practises (Mishra et al., 2015) to avoid damage
to both the soil and environment, it is necessary to apply different techniques to recuperate soils
once they become degraded (Fullen et al., 1995; Fullen and Catt, 2004; Bhattacharyya et al., 2009;
De Baets et al., 2011; Fullen et al., 2011; Bhattacharyya et al., 2012; Dhital et al., 2013; Guerra et
al., 2015). Some parts of the world produce the highest erosion rates due to soil mismanagement
practises, such as slash and burn, and the absence of appropriate soil conservation techniques (e.g.
terracing, contour cultivation and crop rotation) (Fullen and Catt, 2004; Morgan, 2005; Labriere et
al., 2015). Consequently, long-term spatial variations in erosion occur in relation to changes in land
cover, i.e. soil use and management.According to Bhattcharyya et al. (2012) vegetation growth on
problematic slopes often encounters problems, such as the absence of initial binding material in the
soils prone to erosion by water.
Biological geotextiles constructed from different materials, such as Buriti (Mauritia flexuosa L.)
in Brazil, are readily available in São Luis Municipality, and are simple and cost-effective to
manufacture and provide immediate erosion control (Guerra et al., 2015) (Figs. 10 and 11). Most
examples of soil recuperation are very localized and have short-term data. Hence, reports regarding
long-term data are still needed, showing the effectiveness of land recuperation at the drainage basin
scale (Kerr, 1998; Fullen and Catt, 2004; Morgan, 2005; Bhattacharyya et al., 2009, 2012; Guerra et
al., 2014). Very good results have been obtained in São Luis using buriti leaves, which is a typical
palm tree from Maranhão State, where the geotextiles plus vegetation cover have decreased runoff
and erosion, consequently promoting water circulation within the soil profile (Guerra et al., 2015).
The runoff which forms the gully is produced completely within the catchment area above the gully
head, and on this specific site the catchment area is very small, because the local authorities have
made major urban works to decrease this area. Consequently, little runoff is now generated.
Therefore, the recuperation using buriti leaves has worked very well. Although there are many
examples of soil recuperation, most of them are considered on a local scale and one has to also
consider erosion on a global scale. Even considering soil erosion as a global problem of
Anthropocene erosion, local studies might contribute, in the long-term, to solving this problem, or
at least to decreasing soil loss and promoting sustainable agriculture. This is the case of many
countries, but especially those where agricultural production is crucial to development and the
majority of the rural people base their livelihood strategies on the primary sector (Kerr, 1998).
This is only one example of how geotextiles made from vegetal fibres can be used to recuperate
eroded slopes. They are usually cheaper than synthetic geotextiles and generate income for
impoverished local people. Soil erosion processes are influenced by many factors, including rainfall,
soil properties, land-use and land management. Therefore, decreasing surface runoff in the
catchment area (e.g. by increasing soil infiltration capacity and evapotranspiration rates), results
from a permanent vegetation cover. Consequently, the fixation of gully walls is a real erosion
mitigation action and, in many circumstances, it only attenuates the erosion processes. There are
many conservation soil practises, including mulching, crop rotation systems, no-tillage agriculture,
terracing and contour cultivation. Examples of effective soil conservation practises include:
1. Increased extent and density of vegetal cover.
2. Use of green manure (i.e. the addition and incorporation of undecomposed vegetal biomass on
fallow soils).
3. Good soil management practises, particularly minimum tillage.
4. Maintaining cover on soils, especially retaining harvest residues on topsoils, thus adding organic
matter to soil systems.
5. Improved cattle management systems and optimizing the combination of these systems with
arable cropping systems to minimize soil erosion.
6. Re-afforestation and particular protection of riparian vegetation on erodible soils.
7. Contour cultivation. Experiments in Brazil have shown this can reduce runoff by 30% and soil
loss by 50% (Bertoni and Lombardi Neto, 1990).
8. Vegetative buffers (strips of vegetation) in agricultural areas. These act as physical barriers to
runoff and erosion and encourage infiltration.
9. Strips of stones, where stones in agricultural areas are placed in small channels dug parallel to
contours, to impede surface water flow.
10. Construction of small retention basins in small depressions, between areas of permanent
agriculture.
All these conservation practises have promoted much more sustainable agriculture in several
parts of the world, which improves soil drainage and simultaneously decreases soil erosion.
Nevertheless, in many areas, soil degradation still occurs, due to the use of conventional agricultural
systems and cattle ranching (Fullen and Catt, 2004; Morgan, 2005; Goudie and Boardman, 2010;
Guerra et al., 2014).
Fig. 10 Recuperation work in Sacavém gully, with the application of buriti geotextiles, manure and grass seeds, in
São Luis City, Brazil, February 2008 (photo by José Fernando Rodrigues Bezerra).
Fig. 11 Gully wall completely recuperated, after the application of buriti geotextiles, grass seeds, manure and
nitrogen, phosphorus and potassium (NPK) fertilizers, February 2008 (photo by José Fernando Rodrigues
Bezerra).
CONCLUSIONS
Several aspects related to mass movements have been discussed, taking into account the role of
the main environmental triggers, together with human actions on slopes. Such actions nearly always
accelerate mass movements. The unplanned growth of cities is an important factor triggering mass
movements. When this occurs often damage is severe and may even cause fatalities.
Soil erosion is another form of land degradation on slopes and both the factors which trigger its
occurrence, together with its consequences, have been reviewed. Depending on the interactions of
different erosion factors, including natural ones and soil use and management, different soil features
appear on the soil surface, including sheet, rill and gully erosion. Although these erosion features
tend to be more dramatic in the tropics, in recent decades they have also occurred in temperate
morphoclimatic regimes. Wherever they occur, there is always damage and losses to agriculture and
grazing land, with concomitant financial costs.
The Holocene encompasses the growth and impacts of the human species world-wide, including
all written history, the development of major civilizations, and the overall significant transition
toward urban living in the present. Human impacts on modern-era Earth and its ecosystems are
considered to be of global significance for the future evolution of living species, including
concomitant lithospheric or, more recently, atmospheric evidence of human impacts. Holocene
mass movements and soil erosion are global problems, as they cause damage and deaths. These
problems also they make life more difficult for millions of people, especially the poorest ones, who
suffer most with the effects of catastrophic landslides, in urban areas and soil erosion in agricultural
fields. According to Borgatti and Soldati (2010) establishing links between climate and past
landslides activity is indeed very difficult. This is primarily due to the few records of landslide
events (imprecise dates, incomplete databases) dating back to the last century, to the Little Ice Age
and to the Holocene.
Both mass movements and soil erosion, although two different forms of land degradation,
usually cause severe forms of environmental damage and material loss and even injury and death.
Both cause on-site and off-site effects, causing problems to where they occur (the export zone) and
to places of deposition (import zone). The distance between export and import zones can be many
kilometres. Preventive ways of handling these very destructive geomorphological processes is
always the best way to address them (i.e. prevention is better than cure). When preventing soil
erosion and mass movements, one has to also consider the role of climate change, with most
predictions suggesting more intense and extreme rainfall. Together with population growth, this
can drastically increase landslides and soil erosion, especially in developing countries, where
both population and agricultural pressure on land resources often lead to exploitation of unstable
slopes” (Borgatti and Soldati, 2010). Nevertheless, once such environmental damage occurs, it is
possible to recuperate affected areas. The use of geotextiles has been adopted in many countries,
such as in Brazil, using fibres sourced from indigenous vegetation and local labour and knowledge.
This has potential as a sustainable way of tackling the problems of degraded areas.
ACKNOWLEDGEMENTS
The authors acknowledge the financial support of the Brazilian Research Council (CNPq).
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Global population growth and associated socio-economic development have led to rapid urban expansion worldwide, with management implications for sustainable natural resources and societal resilience. Natural ecosystems and the services they provide are essential for societal mitigation and adaptation to adverse environmental consequences in urban areas. Mapping ecosystem services is a valuable tool in spatial planning for urban development, as it provides a deeper understanding of complex human-natural system interactions. This study analyzed and mapped two ecosystem services (local climate regulation and nutrient regulation), which play a key role in mitigating the impacts of local and global climate change in urban areas and of nutrient loads entering surface waters. The specific cases analyzed (Amsterdam city and the Netherlands as a whole) provided insights into opportunity pathways for adaptive development and management of complex urban environments and can support policy and decision-making processes for a sustainable and resilient future.
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Discusses the type and methodology of earth science data required by planners and decision makers. Looking at the experience of other countries especially in Europe the different requirements for geology maps are compared in tabular form. A code of practice is required for the correct procedures for the application of geomorphology to planning containing a methodology for factual maps, data analyses, material properties, hydrology, and special problems, eg karst, subsidence, etc. -after Author