Methodological procedures to landslide

risk mapping in Brazilian slums

AuthorsEduardo 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)

Presenter

Luciana Pascarelli Santos

(2) São Paulo City Council, São Paulo (SP), Brazil

(1) Institute for Technological Research – IPT , São Paulo (SP), Brazil

In this methodology, the factors that make assessment and risk analysis are

simplified, grouped and analyzed qualitatively, from direct observation in the

Description of mapping methodology

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.

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.

In the areas selected by the council were performed:

• a survey of the area occupation characteristics and its infrastructure

Description of mapping methodology

CATEGORY OF CATEGORY OF

OCCUPATION CHARACTERISTICS

Consolidated area Densely occupied areas, with basic

infrastructure.

Partially consolidated

area

Areas in process of settlement,

adjacent to areas of established

occupation. Density of occupation

ranging from 30% a 90%. Reasonable

basic infrastructure.

Split area

Expansion areas, remote and distant

from urban nucleus. Low density of

occupation (up to 30%). Devoid of basic

infrastructure.

Mixed area

In such cases, characterize the area

with regard to occupation density and

implementation of basic infrastructure

• risk mapping procedures using geological and geotechnical surface investigations in order

to identify the types of gravitational mass movements

Description of mapping methodology

PROCESSES CHARACTERISTICS OF THE

MOVEMENT/MATERIAL/GEOMETRY

CREEP

- several (internal) displacement planes / -

very low to low (cm/year) velocities that

decrease with depth / - constant, seasonal

or intermittent movements /- soil, deposits,

altered/fractured rock /- undefined

geometry geometry

SLIDES

- few (external) displacement planes / -

medium (m/h) to high (m/s) velocities / -

small to large volumes of material

- variable geometry and material / PLANAR:

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

FALLS

- no displacement planes / - free fall- or

inclined plane-type movement / - very high

(several m/s) velocities

- rocky material / - small to medium

volumes / - variable geometry: flakes,

plates, blocks, etc. / BOULDER ROLLING /

TOPPLING

FLOWS

- many displacement surfaces (internal and

external to the moving mass) / - movement

similar to that of a viscous liquid / -

development along drainage / - medium to

high velocities / - soil, rock, debris and

water mobilization / - large volumes of

material / - extensive range, even in flat

areas

• 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.

Description of mapping methodology

SITE CHARACTERIZATION MOVEMENT

EVIDENCE SITE CHARACTERIZATION

EVIDENCE

- Natural slope

- Cut slope

- Slope height

- Compacted fill

- hauled fill

- Distance from

home

- Declivity

- Unfavorable

structures in

soil/rock

- Presence of blocks

of rock/boulders/rock

walls

- Presence of

trash/debris

- Amphitheater

Landfill

- Occupation of

headwaters

- House

cracks/landfill

- Trees/poles/walls

inclination

- Etch-plains

- Landslide scars

- Swollen walls

- Erosion features

- Stream margins

undermining

PRESENCE OF WATER PRESENCE OF

VEGETATION

- Surface concentration of stormwater

- Release of wastewater into surface

- Presence of septic tanks/sewage/water

mains

- Upwelling water

- Leakings

- Trees

- Undergrowth

- Deforested area

- Farming area

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;• 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.

LogA log is a tool to determine the potential of

accidents through field surveys carried out in order

to identify risk situations. The cards used in field

surveys should contain the aspects to be analyzed

(check list), allowing the professional come to a

conclusion as to the risk of the analyzed area.

FICHA PARA AVALIAÇÃO DE ÁREA DE RISCO

LOCALIZAÇÃO DA ÁREA

Endereço: Área nº:

SITUAÇÃO DA ÁREA (VIDE MODELOS ANEXOS)

( ) Encosta ( ) Margem de córrego ( ) Encosta + Margem de córrego

CIRCUNSTÂNCIAS EM QUE SE DESENVOLVEM OS PROCESSOS

ENCOSTA MARGEM DE CÓRREGO

( ) 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) EFETIVAMENTE ATINGIDAS

HÁ POSSIBILIDADES DE CONVIVÊNCIA 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:

Distância observada:______ m Distância observada:______ m

OverflightOverflights 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.

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.

Description of mapping methodology

PROBABILITY DEGREE

R1 - Low Risk R3 - High Risk

Concerning to evaluated

indicators, the area shows

low risk to develop landslides

processes

Concerning to evaluated

indicators, the area shows high

risk to develop landslides

processes. Relevant indicators

of instability processes can be

observed

R2 - Medium Risk R4 - Very High Risk

Concerning to evaluated

indicators, the area shows

medium risk to develop

landslides processes. Just

some indicators of instability

processes can be observed.

Concerning to evaluated

indicators, the area shows very

high risk to develop landslides

processes. Almost all the

indicators of instability

processes can be observed. It is

the most critical condition.

Representation of sectors in the oblique photosFor 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.

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.