Mapping of risk and susceptibility of shallow-landslide in the city of São Paulo, Brazil
Introduction
Since 2007, half of the world's population has lived in urban areas, and the number of people living in urban areas may double in the next thirty years (Véron, 2007). Growing populations increase the vulnerability of cities, making landslide-risk management more complex, especially in developing countries, where the number of people who inhabit risk areas has grown by seventy to eighty million per year (UN, 2005).
The accelerated urbanization in developing countries has contributed to the construction of housing on steep hillsides, often using inadequate building standards, intensifying the occurrence of landslides and resulting in the expansion of risk areas. Additional factors have contributed to this expansion, including economic and social crises with long-term effects, historically inefficient policies for low-income housing, inefficient soil-use management, and a lack of appropriate legislation for the most susceptible areas and technical support for local populations (Ayala, 2002, Carvalho et al., 2007).
Wijkman and Timberlake (1985), Sidle et al. (1985), Anderson and Decker (1992), Alexander (1993), Amaral (1997), Ayala (2002), and others have verified that in developing countries, the impact of landslides is a major cause of loss of human life in densely populated urban areas, whereas in developed countries, it is primarily associated with economic losses. This pattern can be explained by greater prevention initiatives in developed countries. In the principal cities of poorer countries, landslides assume catastrophic proportions due to widespread cuts in embankments, garbage deposits, deforestation, changes in drainage networks, and other anthropogenic pressures without preventive planning (Brunsden and Prior, 1984, Sidle et al., 1985, Crozier, 1986, Fernandes et al., 2004).
In various languages, the concept of risk includes the possibility that a landslide (or another natural event) will cause significant social or economic damage to a particular population. Thus, risk areas are the places likely to be reached by natural or induced landslides. Susceptibility, in turn, is determined by the set of natural factors that may contribute to triggering these events. In other words, hillsides have a natural predisposition to landslides as a function of their geologic, topographic and climatic characteristics, among others. When sites with naturally high susceptibility are occupied in an inadequately managed manner (for example, by making cuts in the land and vegetation), there may be substantial risks to the population. Thus, landslide-associated accidents that cause significant social damage frequently occur in urban slums, forcing public managers to identify, analyze and manage risk areas to prevent accidents and plan for emergency situations (Arnould, 1976, Varnes, 1985, Augusto Filho, 1994, Wu et al., 1996, Cerri and Amaral, 1998, Fell et al., 1998, Guzzeti et al., 1999, Van Westen et al., 2003, Macedo et al., 2004, Remondo et al., 2008, Korup et al., 2010).
Several studies concerned with preventing and reducing risks associated with landslides have been performed. Some of the most important are the ZERMOS program (Zones Exposed to Soil Movement Risks), a risk-mapping system in France that aims to supply details about potential soil movement and slope instability in particular areas (Antoine, 1977, Humbert, 1977); the methodology developed by the Geotechnical Control Office (CGC), which develops maps to evaluate of land instability in the Hong Kong region (an area of high susceptibility to landslides) (Brand, 1988); and the International Decade for Natural Disaster Reduction (IDNDR; 1990–1999), which was instituted by the United Nations General Assembly in response to the increasing number of accident victims and economic damage due to natural disasters throughout the world (WP/WLI, 1990; UN, 2005). Additional methods have been proposed by Varnes (1985); Einstein, 1988, Einstein, 1997; Terlien et al. (1995); IUGS (1997); Wilson and Jaiko (1997); Hartlén and Viberg (1988); Jibson et al. (1998); Campbell et al. (1999).
In Brazil, the mapping of risk areas began around 1965 in the city of Rio de Janeiro, where slums had been located on elevated hillsides since the end of the nineteenth century. Since then, numerous landslide accidents have occurred. Various risk methodologies have subsequently been developed, implemented, and applied by research institutes and universities in other urban areas in Brazil. These projects include the use of risk maps in urban planning, the implementation of corrective and mitigating infrastructure in risk areas, the creation of emergency-response systems, the improvement of legislation dealing with land-use issues, and the public dissemination of information and training.
However, despite the innumerable studies that have been performed, Brazil currently faces a complex challenge. The number of risk areas has grown quickly, and public agencies generally do not possess the technical resources to address this problem. One of the principal management tools, an accurate map of risk areas, is difficult and costly to produce because of the many activities (such as field inspections) necessary to create it. Here, we propose a methodological tool that can be applied to the management of the landslide-risk areas. Traditional risk-mapping approaches could utilize prior knowledge of topographic instability through mathematical models based on physical processes (e.g., the SHALSTAB model). Most of these models evaluate susceptibility through a combination of stability models based on hydrological and infinite-slope theories, independent of previous landslide events. Such models have been widely disseminated in the international literature, especially since the 1990s, when the development and improvement of Geographic Information Systems (GIS) promoted the emergence of new approaches to identifying and evaluating unstable areas, minimizing costs, and facilitating the management of risk areas (Dietrich et al., 1992, Montgomery and Dietrich, 1994, Carrara et al., 1995, Wu and Sidle, 1995, Guzzeti et al., 1999, Iverson, 2000, Morrisey et al., 2001, Pack et al., 2001, Dhakal and Sidle, 2003, Van Westen et al., 2003, Van Westen et al., 2006, Calcaterra et al., 2004, Van Westen, 2004).
Existing methods must be strengthened because landslides are the natural events that cause the most casualties in urban areas in Brazil (Fig. 1). Landslides also cause major economic damage and block highways, especially during the summer, when the largest amounts of rainfall occur. Precariously situated urban areas are the result of a housing deficit of more than seven million homes throughout the country. A framework of social exclusion causes thousands of people to occupy inappropriate sites such as steep hillsides and floodplains of rivers, resulting in the presence of numerous slums. The intensive development of these slums began in Brazil during the 1960s, and they have expanded rapidly. The slums are characterized as illegal occupation of public or private land with inappropriately and densely arranged houses, encompassing areas that lack infrastructure and essential public services.
The state of São Paulo, where this study was performed, has the largest housing deficit in Brazil. While the state has only about 4 million houses, the state capital (the city of São Paulo) has about 11 million inhabitants in an area of a little more than 1500 km². From the 1960s to the 1990s, the city of São Paulo experienced a territorial growth of 40%, resulting in the removal of 31% of the vegetative cover. Forests were replaced with inadequately planned roads that cut into the deep valleys and hillsides, and many slums were built (Meyer et al., 2004). In 2004, 562 landslide-risk areas were identified in the city of São Paulo, of which more than 50% were evaluated as having high or very high rates of risk for the population (SVMA and IPT, 2004). Consequently, deaths and damage triggered by precipitation occur on unstable hillsides every year during the rainy period. In January 2010, for example, about 600 mm of rainfall was recorded in 27 days, triggering landslides that killed 60 people and left more than 20,000 homeless. This was the greatest amount of rainfall measured during the corresponding period in São Paulo since January 1947, when about 480 mm was recorded (IAG, 2010).
One of the most critical and largest basins in the city of São Paulo is that of the Aricanduva River, which is located in the eastern portion of the city and has an area of about 100 km². Significant flooding problems occur along the primary river course, which crosses an important avenue in that part of the city. The area is characterized by intense urbanization and the associated reduction in soil permeability. For years, catastrophic flooding events have been recorded during the summer due to rapid drainage of superficial rainwater to watercourses. The overflowing water volume reaches innumerable houses. Between December 2002 and March 2003, more than ten floods were recorded, caused by rainfall lasting between 30 and 120 minutes and amounts from 60 mm to 80 mm. These floods caused several economic and social damages (Canholi, 2005, IPT (Technology Research Institute of the São Paulo State), 2005).
The Aricanduva River basin has 22 tributaries, and numerous slums are built on steep hillsides or very close to drainage channels, especially in the upstream areas of the basin. Poorly executed embankments, road cuts, deforestation, and other anthropogenic pressures in this portion of the basin contribute to the production of sediments that are not retained in the alluvial plain because of the absence of a riparian zone. These sediments are transported downstream, aggravating the occurrence of silting and flooding. In other words, the drainage channels are blocked by debris and silting caused by erosion and landslides, resulting in the accumulation of garbage that obstructs the passage of water, further reducing outflow capacity.
We selected the Limoeiro River basin as a study area because of the frequent occurrence of landslides in the slums built on dissected topography there and the resulting socio-economic losses. The Limoeiro River is one of the headwater tributaries of the Aricanduva River basin and is located on the outskirts of the city of São Paulo. In January 2010, the Limoeiro River basin received heavy rainfall (103 mm in 24 hours), triggering shallow landslides that caused six deaths and considerable economic damage, destroying slums and forcing about 300 local residents to evacuate their homes. Moreover, the operation of public transportation was compromised, especially the circulation of trains and automobiles; water and electricity distribution were interrupted; and several parts of the basin were flooded. The general objective of this article is to analyze the probability of risk and susceptibility to shallow landslides in the Limoeiro River basin. Although the events described above were triggered by an extreme rainfall event in January 2010, the damaging soil-use practices and natural conditions in the basin may exacerbate landslides in this area.
Section snippets
Study area
The Limoeiro River basin has an area of 9 km² (Fig. 2) and runs through a rocky plateau called the Paulistano Plateau, which is located more than 700 m above the Serra do Mar escarpment along the coast of the state of São Paulo. The Paulistano Plateau contains a diverse set of metamorphic and igneous rocks from the Precambrian and Eopaleozoic ages, with various forms of topography.
In the study area, the most common types of mass movements are shallow landslides and creep, in addition to fluvial
Materials and methods
To analyze the probability of risk and susceptibility to shallow landslides, we used two distinct methodologies. In a direct field survey, we evaluated landslide risk based on natural and anthropogenic indicators. In an indirect approach, we verified susceptibility through the application of a deterministic mathematical model based on physical forces (Fig. 4).
In the direct analysis, areas of risk were mapped using a cadastral survey form containing a checklist of the triggering factors for
Map of landslide-risk areas
In mapping the areas of risk, four of the fourteen mapped probability sectors were classified as R1 (130 houses), two were classified as R2 (130 houses), two were classified as R3 (35 houses), and six were classified as R4 (390 houses). The areas of these sectors varied from 7000 m² to 75,800 m², and approximately 685 houses were counted within the total area (Fig. 9 and Table 3). Houses were counted in the field and confirmed from field and oblique aerial photographs. In some cases, the close
Discussion: prediction of susceptible areas associated with the probability of landslide risk
We verified that the SHALSTAB model, which has been widely used to predict landslides in areas with low anthropogenic intervention, produces satisfactory results that are consistent with landslide-scar maps. In dense urban areas, however, its use is still uncommon. For example, Dietrich et al. (1998) applied the model in seven hydrographic basins in the Oregon chain near Coos Bay, California (U.S.A.), where they recorded approximately 844 landslides between 1979 and 1996 and found that 94% of
Conclusions
Several Brazilian cities have areas of landslide risk due to the predominance of weak building standards and lack of infrastructure. In regions with severe rainy periods, these areas of risk are frequently subjected to accidents that result in death and economic damage. In many Brazilian cities, the weak environmental land-use legislation tends to be applied more rigorously in districts that are more economically and financially important. In contrast, areas that are farther from the city
Acknowledgements
This research and this paper (figures in print version) were supported by FAPESP (São Paulo State Research Foundation). The authors thank their colleagues for continuing support and discussion: Kátia Canil, Luis Antonio Bittar Venturi, Emerson Galvani, Cristiane Incau Pinto Pimentel, Tulius Dias Nery, Nívia Marcello, researchers of IPT (Technology Research Institute of the São Paulo State) and Civil-Defense of São Mateus District (São Paulo city). This manuscript was significantly improved by
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