Methodological procedures to landslide risk mapping in brazilian slums
- 1. Methodological procedures to landslide risk mapping in Brazilian slums Presenter Luciana Pascarelli Santos Authors Eduardo Soares de Macedo(1), Kátia Canil(1), Fabrício Araujo Mirandola(1), Fabiana Checchinato Silva(1), Marcelo Fischer Gramani(1), Agostinho Tadashi Ogura(1), Alessandra Cristina Corsi(1), Luciana Pascarelli Santos(2) (1) Institute for Technological Research – IPT , São Paulo (SP), Brazil (2) São Paulo City Council, São Paulo (SP), Brazil
- 2. Description of mapping methodology In this methodology, the factors that make assessment and risk analysis are simplified, grouped and analyzed qualitatively, from direct observation in the field. In this sense we have evaluated the following factors, considered essential to risk analysis: • typology of the expected process and its probability or possibility of occurrence; • vulnerability of urban settlements; and • potential damage.
- 3. Stages of the mapping methodology In accordance with the requirements, the specific objectives of the performed activities were the following: • to obtain low-altitude oblique photos of the areas at risk from helicopter overflights; • identify the sectors with different degrees of probability of occurrence of destructive processes (landslides on slopes and/or washouts of stream banks) and enclose them in the oblique photos; • indicate the number of threatened houses or buildings that could be affected due to the occurrence of the destructive processes mentioned above; • present alternative measures to control the identified risks; • design a database containing the results of the mapping.
- 4. Description of mapping methodology In the areas selected by the council were performed: • a survey of the area occupation characteristics and its infrastructure CATEGORY OF CHARACTERISTICS OCCUPATION Densely occupied areas, with basic Consolidated area infrastructure. Areas in process of settlement, adjacent to areas of established Partially consolidated occupation. Density of occupation area ranging from 30% a 90%. Reasonable basic infrastructure. Expansion areas, remote and distant from urban nucleus. Low density of Split area occupation (up to 30%). Devoid of basic infrastructure. In such cases, characterize the area Mixed area with regard to occupation density and implementation of basic infrastructure
- 5. Description of mapping methodology • risk mapping procedures using geological and geotechnical surface investigations in order to identify the types of gravitational mass movements CHARACTERISTICS OF THE PROCESSES MOVEMENT/MATERIAL/GEOMETRY - several (internal) displacement planes / - very low to low (cm/year) velocities that decrease with depth / - constant, seasonal CREEP or intermittent movements /- soil, deposits, altered/fractured rock /- undefined geometry - few (external) displacement planes / - medium (m/h) to high (m/s) velocities / - small to large volumes of material - variable geometry and material / PLANAR: SLIDES thin soils, soils and rocks with one weakness plane / CIRCULAR: homogeneous thick soils and very fractures rocks / WEDGE: soils and rocks with two weakness planes - no displacement planes / - free fall- or inclined plane-type movement / - very high (several m/s) velocities FALLS - rocky material / - small to medium volumes / - variable geometry: flakes, plates, blocks, etc. / BOULDER ROLLING / TOPPLING - many displacement surfaces (internal and external to the moving mass) / - movement similar to that of a viscous liquid / - development along drainage / - medium to FLOWS high velocities / - soil, rock, debris and water mobilization / - large volumes of material / - extensive range, even in flat areas
- 6. Description of mapping methodology • as well as the constraints of the destabilizing processes, evidence of instability and indication of the development of destructive processes. Thus the prevailing destabilizing processes will be identified, delimiting and characterizing the risk sectors. MOVEMENT SITE CHARACTERIZATION EVIDENCE - Natural slope - Presence of blocks - House - Cut slope of rock/boulders/rock cracks/landfill - Slope height walls - Trees/poles/walls - Compacted fill - Presence of inclination - hauled fill trash/debris - Etch-plains - Distance from - Amphitheater - Landslide scars home Landfill - Swollen walls - Declivity - Occupation of - Erosion features - Unfavorable headwaters - Stream margins structures in undermining soil/rock PRESENCE OF PRESENCE OF WATER VEGETATION - Surface concentration of stormwater - Trees - Release of wastewater into surface - Undergrowth - Presence of septic tanks/sewage/water - Deforested area mains - Farming area - Upwelling water - Leakings
- 7. Field activities The materials to be used to obtain the support information and product to the risk mapping are: • vertical aerial photos, with appropriate scale, up to 1:5.000; • copies of the plano-altimetric maps, basis for the indicated mapping areas; • information on risk areas, such as development records and/or occurrences of instability processes, copies of technical reports and expert opinions previously prepared, any intervention guidelines proposed (implemented or not) etc.; • technical publications, geological, geotechnical, geomorphological and slope angle maps, with information on areas indicated for risk mapping; • copy of the street guide page on which is located the chosen mapping area.
- 8. Log LOCALIZAÇÃO DA ÁREA FICHA PARA AVALIAÇÃO DE ÁREA DE RISCO A log is a tool to determine the potential of Endereço: Área nº: accidents through field surveys carried out in order SITUAÇÃO DA ÁREA (VIDE MODELOS ANEXOS) to identify risk situations. The cards used in field ( ) Encosta ( ) Margem de córrego ( ) Encosta + Margem de córrego CIRCUNST ÂNCIAS EM QUE SE DESENVOLVEM OS PROCESSOS surveys should contain the aspects to be analyzed MARGEM DE CÓRREGO ENCOSTA (check list), allowing the professional come to a conclusion as to the risk of the analyzed area. Distância observada:______ m Distância observada:______ m ( ) Concentração ou lançamento de águas em superfície ( ) Pontos de infiltração ou minas ( ) Presença de vegetação. Qual? ________ ( ) Área desmatada ( ) Presença de lixo e entulho ( ) Trechos aterrados ou escavados ( ) Outros. Citar:_______________ EVIDÊNCIAS DE PROCESSO DE INSTABILIZAÇÃO ( ) trincas/rachaduras ( ) degraus de abatimento ( ) muros embarrigados ( ) cicatrizes de escorregamento ( ) árvores/postes inclinados ( ) outros. descrever:___________ Nº DE MORADIAS (A SEREM) HÁ POSSIBILIDADES DE CONVIVÊNCIA EFETIVAMENTE ATINGIDAS COM O RISCO? POR QUÊ? GRAU DE PROBABILIDADE DE OCORRÊNCIA DE PROCESSO DE INSTABILIZAÇÃO ( ) R1 – Baixo ( ) R2 – Médio ( ) R3 – Alto ( ) R4 – Muito Alto RESPONSÁVEL PELAS INFORMAÇÕES:
- 9. Overflight Overflights by helicopter should be preceded by flights planning supported by technical field visits in order to obtain coordinates from the GPS readings. Besides the programming of helicopter overflights, all the technical material will allow the optimization of the field activities, as well as a preliminary analysis of the areas to be mapped, allowing for a fast and efficient mapping.
- 10. Description of mapping methodology The risk sectors will be delimited with the indication of the probable destabilizing processes, that is, landslides on natural slopes or cutting slopes and embankments. The criteria for judging the probability of the destabilizing processes of occupied slopes landslides kind, as well as the parameters analyzed for further procedures. PROBABILITY DEGREE R1 - Low Risk R3 - High Risk Concerning to evaluated Concerning to evaluated indicators, the area shows indicators, the area shows high low risk to develop landslides risk to develop landslides processes processes. Relevant indicators of instability processes can be observed R2 - Medium Risk R4 - Very High Risk Concerning to evaluated Concerning to evaluated indicators, the area shows indicators, the area shows very medium risk to develop high risk to develop landslides landslides processes. Just processes. Almost all the some indicators of instability indicators of instability processes can be observed. processes can be observed. It is the most critical condition.
- 11. Representation of sectors in the oblique photos For the representation of the sectors at low-altitude oblique photos the following procedures, criteria, symbols and conventions were adopted • on the oblique photos, with the delimited risk factors the identification of the houses to be removed is done with the letter “r”; • in cases where the same sector is represented in more than one oblique photo, preference should be given to the photo where the sectors can be better seen and their boundaries more precise.
- 12. Final Comments For The risk mapping allows an assessment of the areas aiming to provide immediate, medium and long term solutions for the slums in the most critical situation. The obtained results will enable the city administration develop a strategic plan of interventions to reduce and control the mapped risks, enabling the cost estimate for the suggested interventions for each sector and the establishment of sequence of priorities for the implementation in accordance with the technical criteria. An example of applying this methodology can be seen in the paper entitled “Mapping of geological at-risk areas in the city of São Paulo: issues and results from the largest survey of risk in Brazil”, present in session "Landslide risk assessment and management" of the Second World Landslide Forum.