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INTERNATIONAL SOCIETY FOR
SOIL MECHANICS AND
GEOTECHNICAL ENGINEERING
This paper was downloaded from the Online Library of
the International Society for Soil Mechanics and
Geotechnical Engineering (ISSMGE). The library is
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This is an open-access database that archives thousands
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maintained by the Innovation and Development
Committee of ISSMGE.
The paper was published in the proceedings of the 13th
International Symposium on Landslides and was edited by
Miguel Angel Cabrera, Luis Felipe Prada-Sarmiento and
Juan Montero. The conference was originally scheduled to
be held in Cartagena, Colombia in June 2020, but due to
the SARS-CoV-2 pandemic, it was held online from
February 22nd to February 26th 2021.
SCG-XIII INTERNATIONAL SYMPOSIUM ON LANDSLIDES. CARTAGENA, COLOMBIA- JUNE 15th-19th-2020
Debris flows in a tropical environment: relation between magnitude,
deposits and watershed morphometry
Vivian Cristina Dias¹, Marcelo Fischer Gramani², Rebeca Durço Coelho¹, Helen Cristina
Dias¹, and Bianca Carvalho Vieira¹
¹University of Sao Paulo, Sao Paulo, Brazil;
²Institute of Technological Research of Sao Paulo State, Sao Paulo, Brazil.
Corresponding author: vivian.cristina.dias@usp.br
Abstract
Debris flows are one of the most destructive type of mass movement causing social and economic losses when occur
in areas densely occupied. Serra do Mar is a mountain range that extends through about 1.500 km in southeast coast
of Brazil and is the most susceptible area to the occurrence of debris flows and landslides due its geology,
geomorphology and climate. In recent Brazil history there are several disasters related to those process, highlighting
Caraguatatuba, 1967; Rio de Janeiro, 2011; and Itaoca, 2014. In this way, this study aimed to contribute to the
investigation of debris-flows occurrence in tropical environment focusing in the magnitude and morphological
evaluation of a past event and its relationship with morphometric characteristics of watersheds. To fulfill this
objective, it was selected four watersheds with register of debris flows; the determination of debris-flows deposits,
morphology and magnitude; and the mapping of morphometry of watersheds. The results showed that the morphology
of the deposits presented the main features also identified by literature in other occurrences, majorly in temperate
environment. The magnitude classification and potential consequences of each standard was compatible with real
damages registered in the watersheds. Also, morphometric analysis presented critical values in all areas, however,
the altimetric gradient of longitudinal profile was significantly higher in the watershed classified with higher
magnitudes, showing the importance of this parameter. This study can help in the mitigation action by government
and in the understanding of main triggering factors.
SCG-XIII INTERNATIONAL SYMPOSIUM ON LANDSLIDES. CARTAGENA, COLOMBIA- JUNE 15th-19th-2020
1 INTRODUCTION
Debris flows are among the most destruction
types of mass movements, causing a lot of damage
and casualties when occurs in areas densely
occupied. As major characteristics, debris flows
have its potential to transport a high volume of
different material through long distances, and for
being a viscous plastic saturated flow, which favor
the transport of large boulders. These
characteristics are presented after the occurrences,
in the form of deposits, which shows some features
that indicate if it is a debris flow or other type of
process. Some of these characteristics are inverse
grading, presence of levees and large boulders
(Johnson, 1970; Costa, 1984; Znamensky &
Gramani, 2000; Jakob, 2005; Dias et al., 2019).
Several conditioning factors are related to the
occurrence of debris flows, highlighting the
watersheds dynamics and its morphometric
characteristics (Thurber Consultants Ltd., 1983;
Vandine, 1985; Slaymaker, 1990; Johnson et al.,
1991; Jakob, 1996; De Scally et al.; 2001; Chen &
Yu, 2011). The morphometry of watershed has
been largely studied in the evaluation of debris
flows in several aspects, such as susceptibility and
magnitude analysis. It may help to evaluate a
watershed predisposition to produce and transport
sediments, for example, which can indicate its high
or low susceptibility, and also the magnitude of
some processes, specially debris flows (Jackson et
al., 1987; Jakob, 1996; De Scally et al., 2001; Kanji
et al., 2001; Wilford et al., 2004; Welsh & Davies,
2010; Chen & Yu, 2011; Zhang et al., 2015; Dias
et al., 2016; Picanço et al. 2016).
In “Serra do Mar”, a mountain range that extend
for about 1.500 km through the south and southeast
coastline region of Brazil, debris-flows occurrence
is quite common due to relief and climate
characteristics (Jones, 1973; De Ploey & Cruz,
1979; Jica, 1991; Kanji et al., 1997; Gramani &
Augusto Filho, 2004; Vieira & Gramani, 2015;
Vieira et al., 2019). The mostly known disasters in
Brazil occurred in Serra do Mar, highlighting
Caraguatatuba and Serra das Araras, in 1967; Rio
de Janeiro Mountain Range Region and Serra da
Prata, in 2011; and Itaoca, in 2014 (Avelar et al.
2011; Brollo et al., 2015; Gramani & Martins,
2016; Picanço et al. 2016) (Figure 1).
Figure 1. Debris-flows occurrences in Brazil. A:
Caraguatatuba, in 1967; B: Serra da Prata, in 2011; and C:
Itaoca, in 2014. Source: O. Cruz (A); GPMorfo (B); and M.
Gramani (C).
Despite its frequency and potential damage,
there are still a lack of studies about debris flows in
Brazil. In this way, this study aimed to fill some
gaps about the process occurrence in a tropical
environment, focusing in the magnitude and
morphological evaluation, and morphometric
characteristics from watersheds. As study areas it
was chosen four watersheds in Caraguatatuba, that
SCG-XIII INTERNATIONAL SYMPOSIUM ON LANDSLIDES. CARTAGENA, COLOMBIA- JUNE 15th-19th-2020
presents register in landscape of debris-flows
occurrence from the 1967 event.
2 STUDY AREA: THE 1967 DISASTER
Caraguatatuba, located in Sao Paulo State, in
southeast coast of Brazil, is one of the many cities
located along the Serra do Mar mountain (Figure
2).
Figure 2. Location of study area.
In March of 1967 rained an average of 946 mm,
with a critical value in days 17 and 18, when rained
586 mm in just 48 hours, triggering landslides and
debris flows (Figure 3). About 2 million of tons of
sediments and boulders were mobilized, reaching
the urban area of the city, in the lowland, causing
120 deaths, the destruction of 400 houses and
damaging the main highway to the city.
Almost all watersheds in the city were affected,
both by landslides and debris flows. Debris flows
were triggered by landslides whose materials (e.g.
soil, boulder, vegetation) hit the channels already
saturated by the large amount of rain, turning into
a viscous plastic flow. One of the most affected
watersheds was Santo Antonio, in which is located
the downtown (Figure 4). At the time, most of the
city was occupied by farms, which contributed to
minor damages compared to the size of the disaster.
Nowadays, most part of the territory it is occupied
by the population, which increased the size of the
urban areas. Also, recent occurrences with less
impact (2017, single landslides without victims;
2020, landslide at Rio-Santos Highway without
victims) highlighted the need for more studies
about the occurrences in the area.
Figure 3. Caraguatatuba Disaster: landslides (A) and debris
flows (B). Source: O. Cruz.
Figure 4. Destruction of a bridge in downtown. Source: Public
Archive of Caraguatatuba.
3 METHODS
3.1 SELECTION OF WATERSHEDS
The selection of watersheds was carried out
through field surveys and using the morphological
map made by Cruz (1974; 1990) in which the
author indicated the areas affected by mass
movement, but without specify the typology of the
processes. In the field, we identified features in
landscape that indicated the occurrence of debris
SCG-XIII INTERNATIONAL SYMPOSIUM ON LANDSLIDES. CARTAGENA, COLOMBIA- JUNE 15th-19th-2020
flows. It was selected four watersheds: Santo
Antonio, Guaxinduba, Ribeirão da Aldeia, and Pau
d’alho, which presented deposits still visible in the
areas, despite increasing of human activities and
occupation since 1967, in comparison to other
watersheds.
3.2 DETERMINATION OF
MORPHOLOGICAL CHARACTERISTICS
OF DEBRIS-FLOWS DEPOSITS
During fields surveys it was identified both the
presence of debris-flows deposits and its
morphology. It was used a record (Figure 5) where
we identified the presence of typical characteristics
of debris-flows deposits, such as levees, inverse
grading, and boulders very large, and other
information. Also, it was verified the size of the
boulders in the deposits, using a classification
accord to the diameter in small, medium, large, and
very large boulders.
Figure 5. Field Record for morphological characterization of
the deposits and size of the boulders.
3.3 DETERMINATION OF DEBRIS-FLOWS
MAGNITUDE
The determination of debris-flows magnitude
was carried out using the method propose by Jakob
(2005) using the inundated area (m²) (Table 1).
Also, it was used the mapping made by Cruz (1974;
1990) after the 1967 event to establish the
inundated area by debris flows. Field surveys was
also carried out in order to validate the previous
mapping by identifying debris-flows deposits in the
affected areas indicated by Cruz (1974; 1990).
Table 1. Magnitude classification of debris flow.
Source: modified from Jakob (2005).
3.4 MAPPING OF MORPHOMETRIC
PARAMETERS
The mapping of the morphometric parameters
was made using a topographic map with 1:50,000
scale due to its availability in digitized format
(source: Brazilian Institute of Geography and
Statistics, from 1974) and SRTM (Shuttle Radar
Topography Mission) from NASA, with 30 meters
of resolution, in the software ArcGIS 10.1. The
parameters were selected due its importance in
literature to the occurrence of debris flows (Costa,
1984; Thurber Consultants Ltd., 1983; Vandine,
Level
Inundated area (m²)
Potential Consequences
1
< 4 x 10²
Very localized damage,
known to have killed
forestry workers in small
gullies, damage small
buildings.
2
4 x 10² - 2 x 10³
Could bury cars, destroy a
small wooden building,
break trees, block culverts,
derail trains.
3
2 x 10³ - 9 x 10³
Could destroy larger
buildings, damage
concrete bridge piers,
block or damage highways
and pipelines.
4
9 x 10³ - 4 x 10⁴
Could destroy parts of
villages, destroy sections
of infrastructure corridors,
bridges, could block
creeks.
5
4 x 10⁴ - 2 x 10⁵
Could destroy parts of
towns, destroy forests of
2km² in size, block creeks
and small rivers.
6
> 2 x 10⁵
Could destroy towns,
obliterate valleys or fans
up several tens of km² in
size, dam rivers.
SCG-XIII INTERNATIONAL SYMPOSIUM ON LANDSLIDES. CARTAGENA, COLOMBIA- JUNE 15th-19th-2020
1985; Slaymaker, 1990; Johnson et al., 1991;
Jakob, 1996; De Scally et al.; 2001; Chen & Yu,
2011; Dias et al., 2016). The parameters selected
were Ruggedness number (RN), Relief ratio (RR),
Drainage density (DD), Area above 25° (A25),
Drainage hierarchy (DH) and Longitudinal profile
(LP) (altimetric gradient of initiation and
transportation/erosion).
4 RESULTS AND DISCUSSIONS
The mapping of debris-flows morphology
showed that all main features related to the
occurrence of debris flows (levees, inverse grading
and large boulders) were founded in the four
watersheds, highlighting Pau d’alho watershed,
which presented deposits more preserved on
riverbanks (Figure 6). These features are like the
description of debris flows in literature from other
climate zones, especially in temperate environment
(Eisbacher & Clague, 1984; Ujueta & Mojica,
1995; Jakob, 2005).
Figure 6. Deposit of debris flow Pau d’alho watershed.
The magnitude classification using the Inundated
area (m²) as a main attribute showed different levels
in the four watersheds. Santo Antonio was
classified with the higher magnitude, level 3,
follow by Guaxinduba and Pau d’alho, both
classified as level 2. Ribeirão da Aldeia presented
the lowest classification, level 1.
Concerning to the morphometric characteristics,
all watershed presented critical values to the
occurrence of debris flows, highlighting the
parameters ruggedness number, area above 25° and
drainage hierarchy, as showed in Table 2.
Table 2. Results for the morphometric parameters in each
watershed.
Watersheds Santo Antonio and Guaxinduba,
both classified with the higher levels of magnitude
presented critical values for the parameter
longitudinal profile (LP). In the case of Santo
Antonio and Guaxinduba, the results show an
altimetric gradient of 320 and 326 meters in each
1.000 meters, respectively. Pau d’alho and Ribeirão
da Aldeia presented lowest values to LP, 160 and
140 meters, respectively, almost three times
smaller than Santo Antonio. The values for LP
indicate that the watersheds have a higher
altimetric gradient, which favors the triggering,
entrainment and range of the debris flows, once it
is a process induce by gravity (Costa, 1984; Hungr
et al., 2005).
The magnitude classification proposed by Jakob
(2005) also indicate potential consequences related
to each level of magnitude. Comparing with the
damages registered in the 1967 event the potential
consequences predicate was compatible with the
reality. Santo Antonio, classified as magnitude
level 3, had as main damages the destruction of
bridges and the main highway to the city, which
was very similar to the potential consequences, as
follow “could destroy larger buildings, damage
concrete bridge piers, block or damage highways
and pipelines”. Guaxinduba and Pau d’alho, both
classified as magnitude level 2 registered more
local damage, as indicated by the classification
“could bury cars, destroy a small wooden building,
break trees, block culverts, derail trains”. Lastly,
Ribeirão da Aldeia classified with the lowest
magnitude registered very located damage, also
corroborate the potential consequences forecasted,
as follow “very localized damage, known to have
killed forestry workers in small gullies, damage
small buildings”.
Parameters/
Watershed
RN
RR
(m/Km)
DD
(Km/m)
A25
(%)
DH
LP
(m)
Guaxinduba
3444,1
77,76
3,41
31
52
320
Pau d'alho
2610,6
91,27
2,27
28
42
160
Ribeirão da
Aldeia
2247,42
111,91
2,47
36
43
114
Santo
Antônio
2054,8
94,34
2,2
30
62
326
SCG-XIII INTERNATIONAL SYMPOSIUM ON LANDSLIDES. CARTAGENA, COLOMBIA- JUNE 15th-19th-2020
5 CONCLUSIONS
This study produced results which corroborate
the findings of a great deal of previous work in this
field, highlighting here the deposition
characteristics of debris flows and the potential
consequences from the magnitude classification of
the occurrences in the watersheds.
It was possible to verify a relation between the
morphometry of watersheds and the magnitude of
the occurrences, once the watersheds classified
with higher magnitude showed critical values to
some parameters, indicating its importance.
These findings enhance our understanding of
debris flows and to the current literature, especially
considering the occurrence of the process in
tropical environment. However, more research is
needed to better understand the specificities of
debris-flows occurrence related to this specific
climate zone.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge support for
this project from São Paulo Research Foundation
(FAPESP); Institute for Technological Research of
São Paulo State (IPT); Coordination for the
Improvement of Higher Education Personnel
(CAPES); National Council for Scientific and
Technological Development (CNPq) and the
Graduate Program in Physical Geography from
University of Sao Paulo (USP).
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