Mapping geological at risk areas in são paulo
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Mapping geological at risk areas in são paulo Document Transcript

  • 1. Proceedings of the Second World Landslide Forum – 3-7 October 2011, Rome Luciana Pascarelli(1), Rafaela Lançone(1), Rodrigo Costa(1), Luiz Pires(1), Eduardo Macedo(2), Fabricio Mirandola(2), Fabiana Checchinato(2), Katia Canil(2) Mapping geological at-risk areas in the city of São Paulo: issues and results from the largest risk survey in Brazil (1) Consultancy Services and Technical Works Department, Sao Paulo City Hall, Rua Libero Badaro, 425, Brazil –lupsantos@prefeitura.sp.gov.br (2) Laboratory Environmental Risks, Environmental Technology Centre, Technological Research Institute, Sao Paulo, Brazil - emacedo@ipt.brAbstract Since the 1980s the risk studies have definitely geological process causes consequences on Humans orbeen unable to follow the growth and the density of their property. This concept involves, besides thefavelas, and some communities have started to trigger the physical process that causes instability (landslides,first records of accidents in areas hitherto stable. The exfoliations, etc), the accident attributed to it.mapping made in 2010 is today the largest geological-risk Although the Brazilian slopes have witnessed somedatabase in the country. Besides technical information, instability phenomena, it was through the advent of soilthe survey has also shown the types of intervention to be mechanics that Brazil has deepened its research. Theimplemented according to the degree of risk and the type improvement of these studies had a breakthrough in theof verified occupation, - vital data to prioritizing the 1950s, when the first works appeared, and each newpublic-authority actions. Out of 1,602 favelas and occurrence generated the study phase, aiming atinformal settlements in the city, 25 percent have understanding the mechanisms of such events (Guidicinicurrently occupied areas with some occurrence and Nieble, 1976).probability of mass-movement processes. Together, the In the City of São Paulo, the first reports ofassessed areas cover about 13.5 square kilometers, located accidents resulting from landslides have had a directin the outskirt regions. Nowadays, about 115,000 people relationship to the urban expansion that has been goingare living in landslide-prone areas in the capital. As a on since the 1930s. According to Nogueira (2002), theresult the low-income intervention projects by the urban sprawl to the city has increased from 355 squaregovernment have been reassessed and adjusted, meters to 1,370 square meters in five decades. The realprioritizing housing, social-educational infrastructure, estate speculation has used a method of the landand basic-sanitation actions in areas of greatest subdivision that has generated “urban voids”, latersusceptibility. occupied by favelas and irregular settlements. Especially since the 1970s, this occupation has alsoKeywords mapping, susceptibility, disaster, urban area begun in areas of major environmental fragility, such as slopes and stream banks. In the late 1980s, the firstRisk scenario in São Paulo City, political and social accident on the slopes happened, and in the mid-1990’s,framework these accidents became more frequent and less localized, revealing the presence of a large population in at-riskThe magnitude of the concept of geological risk is larger occupation areas.than the “natural risk”. Other terms such asenvironmental risk and technological risk are also Instability processes in São Paulointerrelated, and sometimes these concepts partiallyoverlap. The presence of floodplain and watershed areas and Any research and risk assessment should consider reservoirs has characterized the geography of the City ofwhich events affect the social and economic aspects of a São Paulo. An extensive hydrographical networkparticular place and community. The geological composed of streams and rivers has served every area ofoccurrence does not represent a risk in itself if its the municipality, with approximately 3,200 km oftriggering event does not generate material, economic, or watercourses.environmental damages, or losses. Mass movement are Within their administrative boundaries, thenatural processes and they are part of the erosive/ municipality has schematically covered three sets of quitedepositional cycle, responsible for modelling the Earth’s different sectors: the Sedimentary Basin of São Paulo,surface and its landscape setting. However, the concept from the Tertiary age; its granite-gneiss-schist edge,of geological accident arises when the occurrence of a broken in a system of blocks and wedges in steps by a
  • 2. Pascarelli, L et al. – Mapping geological at-risk areas in the city of São Paulo…reactivated the Precambrian old fault system; and the Sliding on Slopesquaternary colluvial and alluvial cover. It consists of gravitational mass movement (soil, rock, or Sand deposits, clays, and subordinate conglomerates debris). The loss of stability of the slope material canpredominate in the sedimentary basin. The alluvial occur through infiltration and water concentration. Thedeposits have their occurrence along the floodplains of increase in the weight by the flooding, the loss of internalrivers and streams in the city, especially the plains of the cohesion, and the geometry of the slope (steepness,Tietê, Pinheiros, and Tamanduateí, though human morphology, height, etc) lead to destabilization andactivity has intensely reshaped them, through channel triggers the slides. In São Paulo, there is a higherrectification, floodplain grounding, etc. These are areas occurrence of induced landslides, whose outbreak is dueprone to flooding and differential settling by the presence to the implementation of cuts and inadequate landfills,of shallow groundwater. concentration of storm water and served water, as well as In the granitic unit, the biggest instability problems the removal of vegetation.correspond to boulder rolling, especially in areas near the In the City of São Paulo, the PLANAR slides aremountains. usually the most common types of sliding, common in The oldest rocks in the area of São Paulo are gneiss, thin soils and rocks with a weakness plane, and themica-schist, and quartzite, covered by diverse CIRCULAR slides, which occur in homogeneous thickmetasediments and basic metavolcanics. The main soils and fractured rocks. They feature variable geometryproblems associated with massive land occupation of this and material compound, where soil, gravel and, mostly, aunit are landslides of altered mica schist with low degree mixture of these two components represent theof soil compaction. There are also intense erosions in mobilized materials. This type of mixed material is todayheavy cuts (bare soil) and schist-phyllite dumped the worst kind of soil if we think of stability, since itslandfills, exfoliation of massive quartz, and low capacity porosity and permeability indexes are extremely high,of soil support due to the presence of expanded clay. compared to natural soil. This geologic framework has conditioned themorphology of the region, reflecting the existence of a Erosions on Stream Bankshilly topography, with floodplains and terraces of theTietê and Pinheiros Rivers and tributaries, where lies its They correspond to the active erosion on marginal slopesconsolidated urban core, surrounded by the most salient of the streams/ rivers. There is often a correlationforms of elevations, and sustained by granitic bodies between them and the high water and/or flood processes.(Serra da Cantareira) and resistant metasediments. The presence of landfill and pouring of water directly on This natural landscape, associated with illegal the banks have enhanced them.human occupation, promotes the development of theprocesses associated with natural disasters. Modification Rock-block Exfoliation and Fallingof soil characteristics happens in non-ordered areas Gravity on the rock blocks causes exfoliation and falling,where the stable areas can become highly susceptible to associated with erosion and/or anthropogenic activities.occurrences of these processes, even if that interference It creates movement and subsequent break and fall alonghappens to be punctual. The main events in the urban the discontinuities or weakness planes.area are mass movements (landslides/slumps), stream-bank erosion, and sliding and falling boulders. Aspects to define in at-risk areas The causes can be natural, enhanced by theaccumulation of rainwater, or induced by human When examining an at-risk area, it is necessary tointervention through the modification of natural understand the geological processes, their occurrenceconditions of the terrain, such as cuts, landfills, and probability, and their potential to cause harm.pouring and concentration of the water on the hillsides, Based on technical information on a land in a givenetc. The highest number of accidents takes place during region, it is possible to identify the scenarios where suchthe months of highest rainfall – November to March in events may occur. Only from information on the actualthe Southeast region. However, the records show use and occupation specific to each area of the city, oneoccurrences in the “dry seasons”, as the water from can measure the magnitude of these events. For example,sewage or households contributes to the processes. if in a particular site, there has been some identification There have been increasingly common occurrences of a geological EVENT possibility that can generateof destabilization in areas of uncontrolled deposits of “social and/or economic harm”, one can admit that this islitter and/or debris, directly affecting nearby occupations. an AT-RISK AREA. Conceptually, there is only risk whenThese situations can be more critical where high there is some prospect of loss or damage.permeability and porosity of this material promote rapid These areas have generally corresponded to low-saturation and, therefore, its collapse by overweight. income housing units (settlements) in the context of Brazilian cities.
  • 3. Proceedings of the Second World Landslide Forum – 3-7 October 2011, Rome Favelas and informal settlements (SEHAB, 2006) Figure 1: Records of deaths from landslide in Brazil and location of favelas in São Paulo City. Note that most events occurred in crowded areas of the metropolitan region of São PauloThe product of two or more terms represents most of the Risk analysis should consider information on theat-risk equations that different authors have proposed geological and geomorphological and morphometricthat relate the probability of a process or phenomenon to features of the terrain as height and steepness of thethe magnitude of the socially harmful consequences that slopes, soil/rock composition observations, andit may cause [1]. structures that might constrain movements analyses (e.g. foliation, fractures, and other geological discontinuities), R (hazard) = P (probability) x C (consequences) on the possible geological processes. Besides that, it [1] should also consider which conditions could trigger them, as well as their consequences. The term “probability” relates to specific risk or Considering that our study has focused on denselyhazard that should be at least qualified for the intended populated areas or on the occupational expansion, theuse of the equation. The probability of a landslide in a human level interference has a large weight in the riskgiven area is certainly different from the one of flooding, qualification process. Also, one can say that changing thefor example. Another element to consider is the original setting configuration could also change the“vulnerability of a given area or community”. Similar dynamics of the processes that act upon it, that is,processes of the same intensity can frequently generate processes that might gradually have occurred over theintense damage if the village housings are poor. On the years would occur suddenly. People have made theseother hand, damages are barely noticeable if the affected changes in order to build houses or to expand thearea consists of more consolidated and more resistant existing ones. People have made cuts or landfills withoutbuildings. any appropriated criteria, which may favour The risk equation has already included the presence destabilization, as well as the vegetation removal, which,of an effective management system administered by by exposing the soil, has favoured erosion and reducedpublic officials or by the community. It shows that the the soil capacity for absorbing water.adoption of policies to prevent accidents, although Besides that, the growing of certain kinds of plantdifficult to quantify, directly and positively influences the species can be harmful, as in the case of the “bananapolicy of reducing risks. trees”, which favour the water accumulation on their roots, increasing the terrain weight and reducing theThe importance of field work in mapping vulnerable areas internal cohesion by soil saturation. Water is the main triggering agent of mass-Through the knowledge of the areas naturally susceptible movement The absence of a drainage system can maketo movement-process occurrences, it is possible to assess the area susceptible to instability. In the field, onebehaviour of a certain place, from the data on geology, should check for points of upwelling, infiltration, andgeotechnical engineering, geomorphology, and hydrology concentration of the surface water, observing thecollected from previous studies and analyses of aerial rainwater direction, and identifying the sewage/photographs, low-altitude satellite images, and radar. wastewater treatment systems from the houses.
  • 4. Pascarelli, L et al. – Mapping geological at-risk areas in the city of São Paulo…Table 1: Risk qualitative classification (Brasil, 2006 – simplified) Note that the definition of risk sectors based on judgment of the professionals have been responsible for Low Risk Medium Risk risk mapping and for assigning to each sector a degree of (R1) (R2) occurrence probability of the destabilizing process. Gently sloping (<17º), stable soils There are some evidence of No evidence of instability. incipient instability. There is a This is a qualitative analysis recognized by the IUGS Less critical condition. reduced possibility of destructive Working Group – Committee on Risk Assessment events during episodes of heavy and (Cruden and Fell, 1997), which allows the information prolonged rainfall. refinement, and specially, the ranking of the areas High Risk Very High Risk needing preventive or emergency management. (R3) (R4) It is worth to mention that, according to Nardocci Inclined slopes > 30° Low stability soils. (1999), even if the event-probability calculation is High potential for landslides High conditions of instability accurate, this is only a probability. It is not possible to process. Great deal of instability (garbage, poor drainage, no evidence (cracks in the ground, vegetation) say for sure if the event will occur, neither to define whensteps abatement). It is possible the Lot of evidences of instability. it happens. occurrence of destructive events Recent records of landslides. It is the during episodes of heavy and most critical condition. prolonged rainfall. The São Paulo City’s case From September 2009 to December 2010, the São Paulo Because most mass-movement processes occur City held a survey entitled “Análise e mapeamento degradually and continuously, it is possible to identify riscos associados a escorregamentos em áreas de encostasevidences of movement even before the event starts. e a solapamentos de margens de córregos em favelas doSome evidences are cracks and steps on ground surface município de São Paulo” (Analysis and mapping of riskstilting of trees, utility poles and linear structures, and associated with slopes landslides and stream-bankcracks on walls and houses that may indicate local or washouts in the favelas of São Paulo city), through theregional terrain destabilization. One should evaluate Instituto de Pesquisas Tecnológicas do Estado de Sãothese signs for its scope (localized or not) and magnitude Paulo (Institute for Technological Research) - IPT. The(low or high), and use them as movement speed work involved the reassessment of the areas mapped inindicators of unstable terrain. Once compiled the previous years, as well as the survey of new sites ofinformation obtained in the field and those gathered in geological risk in the city. More than a geotechnicalmaps, images, photographs, one can classify the areas survey, the mapping had its focus turned to areas whereaccording to Table 01. the population vulnerability was the highest one. Border of área Border of probability sector VP-07-01 (R3) VP: Area code (neighborhood name) 07: Area number 01: Sector number R3: High risk Orthophotos 1:5000 (São Paulo) EMPLASA 2007 Figure 2: Representation of sector in air photo with probability degree indication
  • 5. Proceedings of the Second World Landslide Forum – 3-7 October 2011, Rome In these locations, any type of event, even the smallones, could cause significant damage to the community,given its low risk-perception and its low capacity forrapid management within the proposed period ofimplementation. Furthermore, the survey needed to establishresponse and recovery Initially, more than 400 assessment areas wereassigned for assessment, which required a different planfor the organization of work within the proposed periodof implementation. Moreover, it was necessary to laydown guidelines that would ensure the objectiveeffectiveness of the work the staff should performed. Inaddition to 20 technical experts responsible fordeveloping the work from IPT and São Paulo City Hall,other 52 servers have actively participated in this Figure 3 Oblique image of risk area. View of homes in specificmapping, from the indication of the areas to the risk sectors.monitoring of the final field works. This is the greatestrisk mapping Brazil has done, considering the amount of Quantified results obtainedanalyzed areas and the professionals directly or indirectly São Paulo municipality has hired the IPT to do theinvolved in this activity. The work consisted of the mapping of at risk-areas and they have identified 407following 5 phases, implemented in groups for each stream-bank and slope areas subject to landslide andregion of the city, that is, Northern, Southern/Western erosion processes. Such areas comprise around 13.5and Eastern Zones: square meters representing 0.9 percent of the total Phase one - Appointment and selection of areas the municipality area.staff should evaluate, after the official consultation to Every risk survey has the a technical-report format,each district. There were meetings to explain about the containing location, delimitation, photographicsurvey and introduce the team, as well as for the documentation, and fieldwork technical data in digitaldesignation of the places to assess. and printed media, making available the easy access to Phase two – Field Screening: visits to the areas listed needed information that will lead to planning of actionsin “Phase 1” for the GPS data and for the over flight plans. and decision-making. Phase three – Flying over region for picturingoblique image: the staff made low-altitude oblique (A) (B)photos to identify large-size risk signs or constraints,primary sectoring, and housing quantification. Phase four –Fieldwork: based on previous data, IPTteam checked the specific characteristics of occupations,behavior of water on the terrain, presence of movementsigns, and other information that enabled the neededrefinement for the risk sectoring. Phase five – Delivering Results: final report,including the information on occurrences of destructiveprocesses, potential consequences on household Figure 4 Distribution of areas by risk degree (A) and percentagevulnerability, area sectoring, definition of the sector risk of houses in each one (B) in São Paulo City.degree, recommendations of interventions to control risk,and data insertion into geo-referenced database This study has included all the vulnerable areas inintegrated to the São Paulo City system (HABISP/ the city, located in the 26 administrative districts.SEHAB). Considering the details and the obtained information, The team qualitatively analyzed risk estimate from this has been the largest survey ever conducted in Brazilfield observations, by integrating the analysis parameters on the at-risk areas. Among 407 identified areas, 179contained in a risk-assessment form, with the support of were located in the South, 107 in the North, 100 at East,aerial images. In addition to that, the team set four types and 24 in the Western region. Out of 1,179 identified at-of occupations that indicated their consolidation level: risk areas, 57 percent correspond to slopes and 43 percentconsolidated, partially consolidated, subdivided, and to the occupied stream banks.mixed areas. This information has directly affected the The figure 4 shows the distribution number of therisk degree, once higher consolidation levels and better at-risk sectors (A) and the amount of housing (B)basic infrastructure collaborate to increase the security of estimated according to the destabilization probabilitythe areas. degree.
  • 6. Pascarelli, L et al. – Mapping geological at-risk areas in the city of São Paulo…The mapping results use in the municipality Among the main activities in progress is the “Favela andmanagement risk system Watershed Upgrade Program”, which includes basic- sanitation and infrastructure actions, relocation ofFrom 2010, the works on the at-risk areas have counted families to new homes, preservation of natural waterwith a technological tool, enabling the integration of reservoirs (Guarapiranga and Billings Dams).information from all Secretariats and other public The municipality has also conducted the stream-agencies, as well as the joint and continuous intervention dredging action and the implementation of a small-to-planning, from punctual actions to the implementation medium-scale intervention project.of governmental programs. Today, through the HABISP They are as part of the Intervention on the at-RiskSystem, all the collected mapping data are available to all Area Program, as well as some maintenance actions tostaff involved. This system allows identifying, for prevent potential risks. The Linear Park implementationinstance, if the limits of a linear park implementation has restored and expanded green and permeable areas inarea are co-incident with a very high at-risk sector, or if a the city in areas along the rivers and streams previouslygiven road project requires the relocation of the occupied by illegal housing. These integrated actionscommunity families from an at-risk area. have reduced the risk in areas previously considered Through this resource, the São Paulo municipality’s critical, such as Paraisópolis and New Jaguaré Favelas.technicians have been identifying and readjusted the It is worth mentioning that, in addition to theactions undertaken by the different departments allowing infrastructure and resettlement of families, an effectivea multidisciplinary approach and, consequently, more system of risk management needs to count on theefficient work. preventive, emergency, and recuperative local actions. AFTER Figure 5: Example of a risk intervention project in a susceptible area “Favela Paraisópolis” For this reason, São Paulo City Hall administration Nowadays, São Paulo has monitored information onhas defined and has been adopting standard proceeding such proceedings in real time, as well as any ongoingfor the rainy season. Actions as signaling risky areas, occurrence in the city. Central remains linked to the allcommunity orientation, hillside monitoring, and districts for 24 hours a day, providing them withevaluation and removal of material mobilized by information on the level in the “big pools” of water andlandslides are some examples of specific procedures of traffic condition on main roads of the City, besides thethese patronization. Secretariat has also recommend meteorology data that the Emergency Managementtheses actions in case of floods, overflows, and tree falls, Center make available. Some emergency services requirewhich have their high incidence during the summer large numbers of operations or machines. At this point,(From October to March). It has also described each of in order to provide them, Central works together withthese actions in detail to ensure standardization of Centro de Controle de Operações Integrada – CCOIservices performed in each district, and a speedy service (Integrated Operations Control Center), as an integratedat the affected areas, responding to the guidelines of action facilitator among the municipal secretariats andMunicipal Civil Defense Plan. other governmental spheres.
  • 7. Proceedings of the Second World Landslide Forum – 3-7 October 2011, Rome In order to ensure the continuity of riskmanagement actions, São Paulo municipality held thefirst Risk Area Mapping Training Course, for geologists,engineers, architect, Civil Defense, and Fire Departmentteams working in at-risk areas. This course has addressedmapping-related themes, risk management, and technicalreports and examinations based on the municipalitycondition, as well as evaluation of recovering solutionsfor the area, in face of the obtained results. This 40-hourcourse has involved both practice and theory. This program has started the mapping techniques,gone through orientation and training the publicauthorities, and reached the careful selection of at-riskarea interventions. A multidisciplinary perspective hasbeen showing the importance of integrated work of bothgovernment and society in building an effective policy ondisaster reduction.ReferencesBrasil, Ministério das Cidades (2006). Capacitação em Mapeamento e Gerenciamento de Risco. Ministério das Cidades, Brasília, DF, Brazil.CETAE-IPT/ATOS (2010). Análise e mapeamento de riscos associados a escorregamentos em áreas de encostas e a solapamentos de margens de córregos em favelas do município de São Paulo. São Paulo City Hall, São Paulo, Brazil.Cruden D., Fell, R. (1997) Quantitative risk assessment for slopes and landslides. The state of the art. IUGS WORKING GROUP – Committee on Risk Assessment. Proceedings of the International Workshop on Landslide Risk Assessment.Nardocci, A.C. (1999) Risco como instrumento de gestão ambiental. São Paulo, 1999, DR Thesis Departamento de Saúde Ambiental da Faculdade de Saúde Pública, University de São Paulo, São Paulo, Brazil.Guidicini G., Nieble C.M. (1976) Estabilidade de Taludes Naturais e de Escavação. University of São Paulo, São Paulo, Brazil.Nogueira F.R. (2002) Gerenciamento de Riscos Ambientais Associados à Escorregamento: Contribuição às Políticas Municipais Para Áreas de Ocupação Subnormal. DR Thesis, UNESP, Rio Claro, São Paulo, Brazil.SEHAB (2008) HABISP: Information system for social housing in the city of São Paulo (http://www.habisp.inf.br) 7