Content uploaded by Pedro Santarém Andrade
Author content
All content in this area was uploaded by Pedro Santarém Andrade on Nov 27, 2023
Content may be subject to copyright.
Estudos do Quaternário, 9, APEQ, Braga, 2013, pp. 33-43
http://www.apeq.pt/ojs/index.php/apeq.
33
1. INTRODUCTION
The nature and intensity of exogenous
processes, such as those associated with the
weathering and erosion of rock massifs, slope
instability, sediment deposition and flooding in
river valleys, are determined by climate variables
and the geology (especially the lithology and the
structural features) and relief characteristics of the
regions where they take place. With regard to the
climate variables, rainfall plays a key role. On one
hand, it directly influences weathering and erosion
processes and affects slope dynamics and the flow
regime in river systems. On the other hand, rainfall
determines the geotechnical behaviour of soils and
rocks, which in turn is reflected on the surface
process referred above. Furthermore, the intensity
and seasonal distribution of rainfall determine some
biogeographic parameters of the regions, like the
type and density of vegetation cover, and these
parameters influence the magnitude of the
processes responsible for changes in the landscape.
To achieve a proper interpretation of natural
processes on the Earth's surface it is essential to
have accurate weather data. Unfortunately, in many
parts of the globe, there is not a sufficiently dense
(1) IMAR-CMA Centro do Mar e Ambiente, Departamento de Ciências da Terra da Universidade de Coimbra, Largo Marquês de Pombal. 3000-272 Coimbra,
Portugal, pdinis@dct.uc.pt
(2) IMAR-CMA Centro do Mar e Ambiente
(3) Centro de Geociências, Departamento de Ciências da Terra da Universidade de Coimbra, Largo Marques de Pombal. 3000 -272 Coimbra, Portugal
(4) Serviço Nacional de Protecção Civil e Bombeiros, Delegação Provincial de Benguela, República de Angola
(5) Escola de Formação de Professores do Ensino Médio, Quilengues, República de Angola
(6) IMAR-CMA Centro do Mar e Ambiente, Departamento de Ciências da Terra da Universidade de Coimbra, Largo Marquês de Pombal. 3000-272 Coimbra,
Portugal
CONTRIBUTION OF TRMM RAINFALL DATA TO THE STUDY OF
NATURAL SYSTEMS AND RISK ASSESSMENT. CASES OF APPLICATION
IN SW ANGOLA
P.A. DINIS (1 ), V. MANTAS( 2), P.S. ANDRADE (3 ), J. TONECAS(4) , E. KAPULA(5 ), A. PEREIRA(6 )& F.S.
CARVALHO(2 )
Abstract: Researchers studying exogenous processes in developing countries have to deal with the problem of scarcity of
rainfall data. With satellite measurements, such as those provided by the Tropical Rainfall Measuring Mission
(TRMM), it may be possible to overcome this limitation. In this work we link the TRMM rainfall measure-
ments with two examples of exogenous processes from southwest Angola: slope instability, focused on the
events at the Leba road in early 2011, and flow conditions, based on the water level determined in gauging
stations from rivers Cavaco and Catumbela. It is demonstrated that major mass flow movements occur when
specific TRMM rainfall thresholds are reached. Regarding the flow conditions, the water level in the two
gauging stations is conditioned by other factors beside the rainfall in their watersheds, some related to human
activities, but short term oscillations are closely linked with the rainfall in the proximity. TRMM data is found
to be very useful for the analysis of specific extreme events or the patterns of behavior of natural systems and,
consequently, constitute a valuable tool in natural risks assessment.
Resumo: Contribuição dos dados TRMM para o estudo de sistemas naturais e avaliação de riscos. Casos de apli-
cação no SW de Angola
Os investigadores que estudam processos exógenos nos países em desenvolvimento lidam com o problema da
escassez de dados pluviométricos. Com dados de satélite, como os fornecidos pela Tropical Rainfall Measu-
ring Mission (TRMM), será possível superar essa limitação. Neste trabalho relacionamos os valores de precipi-
tação TRMM com dois exemplos de processos exógenos no sudoeste de Angola: movimentos de massa, foca-
dos nos acontecimentos ocorridos na estrada da Leba no início de 2011, e as condições de escoamento, com
base no nível de água determinado em estações hidrométricas nos rios Cavaco e Catumbela. Demonstra-se que
os maiores movimentos de massa ocorrem quando determinados limiares de precipitação TRMM são alcança-
dos. Em relação às condições de escoamento, o nível de água nas duas estações hidrométricas é fortemente
condicionado por outros fatores para além da precipitação ocorrida nas áreas de drenagem, alguns relacionados
com actividades do Homem, mas as oscilações de curto período estão intimamente ligadas à intensidade da
precipitação que ocorre nas proximidades das referidas estações. Os dados TRMM revelam-se muito úteis para
a análise de eventos extremos específicos ou de padrões de comportamento de sistemas naturais superficiais e,
consequentemente, constituem uma ferramenta valiosa na avaliação de riscos naturais.
Received: 13 September 2013; Accepted: 6 November, 2013
Palavras-chave: Idade do Bronze, contextos e práticas funerárias, monumento sob tumulus, cista
“megalítica”, oferendas cerâmicas.
Palavras-chave: Dados de precipitação TRMM, Avaliação de riscos, Deslizamento, Nível da água no canal,
Angola
P.A. Dinis, V. Mantas, P.S. Andrade, J. Tonecas, E. Kapula, A. Pereira & F.S. Carvalho
34
network of meteorological stations with long time
series to understand weather patterns and their
spatial variability. The need for data is particularly
important when natural processes are potentially
hazardous. The issue becomes also urgent when
dealing with poorly known regions where the
population has been rapidly growing and in a
disordered way (e.g. ALCÁNTARA-AYALA 2002), as
is the case for many areas of southwestern Angola.
This paper deals with an analysis of rainfall
data obtained through the Tropical Rainfall
Measuring Mission (TRMM - http://pmm.nasa.gov/
node/158). The pattern of rainfall is expected to
play a key role in two cases of natural processes,
associated with slope dynamics and fluvial
behavior, observed in the southwest of Angola (Fig.
1). The slope dynamic case study refers to the
landslide events that took place in early 2011 in the
Leba hills (GOMES-LEAL & TEIXEIRA-PINTO 2012;
KAPULA 2012). The assessment of river flow
conditions is based on a set of water level
measurements obtained in gauging stations installed
in rivers Cavaco and Catumbela (TONECAS 2012).
In both cases we attempt to establish relationships
between the exogenous processes and the coeval
rainfall intensities in the areas of influence.
Fig. 1. Map showing key locations of the presented case studies from southwest Angola. It is represented the location of Leba road
affected by mass flow events, watersheds of rivers Catumbela and Cavaco, gauging stations in these rivers and TRMM cells used
for the acquisition of rainfall data.
Fig.1. Mapa mostrando locais-chave dos estudos de caso apresentados para o sudoeste de Angola. É representada a localização da estrada
da Leba afetada por processos de instabilidade de taludes, as bacias hidrográficas dos rios Catumbela e Cavaco, as estações h idrométricas
nestes rios e as células TRMM utilizados para a aquisição de dados de precipitação.
Contribution of TRMM rainfall data to the study of natural systems and risk assessment. Cases of application in SW Angola
35
2. TRMM RAINFALL DATA
Given the scarcity of in situ rainfall
measurements, remote sensing data could provide
valuable information for the study of exogenous
processes. TRMM is a Japanese-American joint
venture launched in 1997 that provides researchers
with regular rainfall estimates from a suite of several
instruments (LIU et al. 2007). These data are
available at the Goddard Space Flight Center from
the North America's Space Agency (NASA).
The TRMM datasets were reinforced with the
introduction of multi-mission products (3B42 and
3B43) which are available for a significant portion
of the globe (50º N-S) (HUFFMAN et al. 2007). The
Multi-satellite Precipitation Analysis (TMPA)
products are available at the TRMM Online
Visualization and Analysis System (TOVAS) (LIU
et al. 2012), which is one of the instances available
at NASA's Giovanni data access portal (BERRICK et
al. 2009). The datasets can be obtained from
archives for different intervals and as near real time
versions. For the two study areas, daily data from the
archived products 3B42 were acquired.
2.1 Cases of application in southwestern
Angola
Two examples of natural processes strongly
influenced by rainfall from southwestern Angola
were selected for this work. These are:
1) Hazardous slope dynamics in the hills of
Leba, near the border between Huíla highlands and
the lower elevation Namibe province. Particular
attention was given to the events that occurred at the
beginning of the year 2011 leading to the closure of
an important road (National Highway 280 – EN 280)
that links the coastal region with the inland. Leal-
Gomes and Teixeira-Pinto (2012) and Kapula (2012)
had already described the landslides associated with
those mass flow processes.
2) Hydrologic flow conditions in Cavaco and
Catumbela rivers, whose outlets are located near
Benguela, based on a time series of water level
measurements from two gauging stations run by the
Angolan Civil Protection and Fire Services. The
periodic floods of these rivers have caused
significant damages to the local population,
including human casualties, houses destruction and
crop losses. The water level data used in this work
was obtained between January 2008 and March
2010 and previously presented by Tonecas (2012).
The Cavaco and Catumbela drainage basins
were delineated from a Digital Elevation Model
(DEM), using standard spatial analysis techniques.
The DEM applied in this analysis was a subset of the
research grade imagery provided by the Shuttle
Topography Radar Mission (SRTM) 90-meter
product acquired from the Earth Explorer website of
the United States Geological Survey (USGS). A
careful definition of the limits of the Cavaco and
Catumbela drainage basins was conducted and fine
scale corrections introduced where possible,
especially in the regions closest to the shore where
multiple watersheds were automatically erroneously
generated due to the reduced slope and because of
errors known to exist in the SRTM data.
Rainfall data were retrieved from NASA’s
TOVAS data access portal. Different products are
available at this instance of Giovanni, including the
TMPA. The TMPA was selected since it provides
rainfall estimates using a combination of different
sensors, thus increasing accuracy, coverage and
resolution (HUFFMAN et al. 2007). These products
are generated in a four-step process which
encompasses the calibration and merge of
microwave precipitation estimates, infrared (IR)
precipitation retrievals, merging of microwave and
IR estimates and, finally, incorporation of rain gauge
data for non-real time datasets.
The rainfall estimates thus created and made
available at Giovanni, are distributed with a spatial
resolution of 0.25º, while covering most of the
globe. The data, from the period of interest, were
acquired for each cell containing sections of the
Cavaco and Catumbela drainage basins and later
combined for analysis. The cells from which data
were retrieved are displayed in figure 1. Prior to
basin-wide analysis, the 3-hourly rainfall estimates
were summed to generate a daily product,
compatible with the river gauge information. The
daily rainfall estimates, in mm/day, were combined
for each drainage basin. The values for each cell
were summed (without taking into account the area
of watershed covered by each TOVAS cell) and a
standard statistical analysis was applied to the output
results.
For the second study area, at Leba, the same
procedure was adopted, including the retrieval of
3B42 Accumulated Rainfall, but instead of a
segmented acquisition, a single block was defined
for a 1 x 1o cell (Fig. 1). The available data was
extracted and analyzed in the same way as for the
Cavaco and Catumbela watersheds, with the
standard statistical parameters being calculated for
the area of interest.
3. GEOLOGICAL AND GEOMORPHOLOGICAL
SETTING
A striking aspect of the southwestern area of
Angola is the presence of flattened surfaces
separated by more or less steep steps, which have
been used to delimit the major geomorphological
units (Jessen, in FEIO 1946; FEIO 1981; DINIZ
2006). The Surface of Humpata (level V of Jessen)
is the highest surface, reaching 2000 to 2300 meters
P.A. Dinis, V. Mantas, P.S. Andrade, J. Tonecas, E. Kapula, A. Pereira & F.S. Carvalho
36
are affected by several joint systems, part of them
sub-vertical, and faults. The units display open folds
and flexures, although, in general, stratification dip
angles are low.
In the westernmost fringe of southwestern
Angola outcrop Mesocenozoic sedimentary rocks
belonging to the Benguela sub-basin, which has
been considered the southern part of the Kwanza
Basin (PINTO & CARVALHO 2010), and the Namibe
Basin. In general, the Mesocenozoic units overlay
metamorphic rocks of the Paleoproterozoic or
Archean complex (CARVALHO 1983). A clear
topographic step that, in places, reaches 100 meters
is observed at the transition between the Namibe-
Benguela basins and the pre-Mesozoic basement
(FEIO 1981; GUIRAUD et al. 2010).
Broadly speaking, the climate of Angola is
characterized by an interchange of a wet season and
a dry season. The wet season, during which most
rainfall takes place, is also marked by higher
temperatures. This season usually runs from
September to April, depending, however, on the
geographical area. The dry season is colder and
usually runs from May to September. According to
Koppen's classification, in the area of Benguela, the
climate type is BSh (hot semi-arid), evolving
towards the interior to Cwa (humid sub-tropical) and
Cwb (temperate highland tropical with dry winters),
and towards the south to BWh (hot desert) (PEEL et
al. 2007). At Leba the climate is classified as Cwb.
According to Thornthwaite classification, the
climate is arid in the western margin, with the
highest dryness in southern locations, changing to
mesothermal and humid or sub-humid in innermost
positions of the center-west of Angola (DINIZ 2006).
From the climate classification of southwestern
Angola, it is expected that rainfall is reduced along
the coast, increasing significantly inland.
Some general features of the areas related to
the case studies presented in this work are
summarized in Table 1. The Cavaco river catchment
is located entirely in the sub-plateau area, draining
rocks of the Benguela Basin and the old
metamorphic-magmatic complex; the Catumbela
catchment extends into the Central Plateau where
Eburnean magmatic rocks outcrop throughout a
large area. The region of Leba is located at the
in elevation. It is surrounded, to the east, by the
Main Plateau (level IV of Jessen), and to the west by
a much lower surface, which is usually more than
1000 meters below (FEIO 1981). An extensive region
of Angola that is more than 1500 meters high, which
integrates the Surface of Humpata and the Main
Plateau, has been called the Central Plateau.
Marques (1966, 1977) assigns the term Marginal
Mountains Range (Cordilheira Marginal de
Montanhas) to an area that is broadly oriented
parallel to the Atlantic coast and marked by the
presence of the highest flattened surfaces of Angola.
The boundary between the drainage basins of the
rivers that flow directly to the west (eg., the rivers
Catumbela and Cavaco) and others facing south and
east, integrating the drainage basins of the Kunene
and Cubango rivers, is located at this mountain
range.
Along the Main Plateau outcrop mainly
Paleoproterozoic igneous rocks, associated with the
Eburnean igneous cycle, which intrude an older
complex unit with metamorphic and igneous rocks
that may already belong to the Archean (PEREIRA et
al. 2011). The Eburnean magmatic units are covered
by a stratified succession formed by the Chela
Group and the Leba Formation that is well
represented in the Surface of Humpata (CARVALHO
1981). At the zone of contact between the Eburnean
magmatic rocks and the overlying succession the
former are usually deeply weathered.
The Chela Group and the Leba Formation
were described in detail by Correia (1976).
According to this author, the Chela Group consists
of four formations, namely: Tundavala Fm. (mainly
detrital, comprising conglomerates, sandstones and
quartzites); Humpata Fm. (volcano-sedimentary
rocks, associated with acid volcanism, sometimes
with interlayers of intrusive rocks); the Bruco Fm.
(mainly detrital, distinguished from the previous
units by the scarcity of volcanic rocks and coarser
sediment grain size); and the Cangalongue Fm.
(consisting essentially of fine-grained sedimentary
rocks and carbonates). The Leba Fm., which stands
on the Chela Group, consists mainly of silicified and
dolomitic limestones (CORREIA 1976). One of the
most relevant aspects of this unit is the presence of
well-preserved stromatolitic structures. Both units
Case study
Leba Cavaco
(drainage basin)
Catumbela
(drainage basin)
Latitude -15,06 to -15,08 -12,56 to -13,38 -12.37 to -13,79
Longitude 13,22 to 13,24 13,41 to 14,26 13,47 to 15,44
Area (Km2) - 3512 11641
Elevation (m) 1300-1900 0-2570 0-1592
Climate (Koppen) Cwb BSh, Cwa and Cwb BSh, Cwa and Cwb
Table 1. General features of the study cases
Tabela 1. Características gerais dos casos de estudo.
Contribution of TRMM rainfall data to the study of natural systems and risk assessment. Cases of application in SW Angola
37
the outcropping units tends to be high. Hence, driving
forces can overcome resistance forces, such as
cohesion and friction, and originate slope
movements. These features of the topography and
geological structure, coupled with the periodically
high rainfall intensity, are likely to promote
landslides along the slopes of Leba hills (Fig. 2).
Slope movements occur periodically along road EN
208 even when the weather conditions are not
marked by severe rainfall. However, during these
events usually minor volumes are displaced and the
consequences for the road circulation tend to be limited.
outstanding transition zone between the Central
Plateau and a lower elevation region being
characterized by the presence of very high and steep
slopes. In this area occur mainly Eburnean magmatic
rocks covered by the Chela Group and the Leba Fm.
4. SLOPE INSTABILITY ON LEBA HILLS
4.1. Instability processes
Both natural and cut slopes along Leba hills are
frequently very steep and the weathering degree of
Fig.2. Examples of mass movement in the Leba road and surrounding area. Rock falls and wedge failure (A); planar failure (B)
affecting stratified units of the Chela Group. Photomosaic (C) and image interpretation (D) of an area affected by mass flow. Black
patches represent mass flow sites.
Fig.2. Exemplos de movimentos de massa na estrada Leba e área circundante. Queda de blocos e deslizamentos em cunha (A) e desliza-
mento planar (B ) afetando unidades estratificadas do Grupo da Chela. Fotomosaico (C ) e imagem de interpretação (D) de uma área afeta-
da por fluxos de massa. Manchas pretas representam locais com fluxos de massa.
P.A. Dinis, V. Mantas, P.S. Andrade, J. Tonecas, E. Kapula, A. Pereira & F.S. Carvalho
38
Rock falls, translational rockslides and
toppling failures occurred at the beginning of 2011
in several slopes of road EN 280 where rocks of
the Chela Group outcrop (Fig. 2). Rock falls and
translational rockslides, consisting of planar and
wedge failures, are frequent in cut slopes affected
by discontinuities of variable orientation and
spacing. Rock falls correspond to blocks detached
from slopes, and are a common and very rapid type
of slope instability along the study area. The rough
topography and high slopes, the close joint spacing
and its orientation have a clear influence on
triggering rock fall movements. Planar rockslides
are associated with three major joint sets. Wedge
failures occur at main discontinuities intersections.
Translational rockslides are relatively less common
than rock falls along the studied area. Toppling
failures are characteristic of very steep slopes with
tens of meters high that present sub-vertical and
highly penetrative discontinuities. Toppling
failures did not represent a major threat to the road
traffic, since they occurred in sites located several
tens of meters from the EN 280.
Mass flow phenomena, which correspond to
debris and soil flows, occurred in lower elevation
sites along the descent of Leba, always below the
steep slopes found in the stratified formations of
the Chela Group and in locations with a higher
density of vegetation cover that developed on the
weathering mantles of the granitic massifs or
covering colluvial deposits (Fig. 2). The debris
flow deposits integrate a diversified set of
lithological types derived both from the Eburnean
magmatic rocks and the overlying Chela Group.
These slope movements have made huge damages
along the road EN 280, leading to its disruption for
several days in April 2011.
4.2. Relationship between the phenomena of
instability and rainfall
The slope instability events in the Leba hills
during the beginning of 2011 are expected to be
intimately linked with the rainfall intensity. During
this period the number of days without rainfall was
reduced (14 days, 12 % of the record between
January and April 30), although most days had
daily rainfall below 5 mm (55.8 %). But the slope
instability period is marked by significant
frequency of days with rainfall exceeding 20 mm
(13.3 %) and 30 mm (4.2 %), and the occurrence of
2 days with rainfall close to, or even higher than,
50 mm (Fig. 3). When considering the entire
record (5479 days), the class with rainfall below 5
mm is clearly dominant (85.2 %) and the
frequencies of days with rainfall above 20 mm and
30 mm are just 1.9 % and 0.5 %, respectively.
The atmospheric precipitation in the early
Fig.3. Comparison of TRMM rainfall data in the first trimester of 2011 with the entire record for the
selected cell.
Fig.3. Comparação entre os dados de precipitação TRMM no primeiro trimestre de 2011 e a totalidade do
registro para a célula selecionada na região da Leba.
2011 can be grouped into four stages (Fig. 4):
- During the first half of January the rainfall
was high, with distinctive peaks in January 4 (27
mm), 6 (38 mm), 10 (28 mm), 11 (47 mm, the
maximum for this stage) and 13 (24 mm).
- After that, for about two weeks, the rainfall
was always substantially lower (less than 0.1 mm)
and several days occurred without any rain. The
longest period without rain elapsed between 23 and
26 January, inclusive.
- In the following weeks, between early
February and mid-April, there was a succession of
days with high rainfall that ended up on April 12.
Contribution of TRMM rainfall data to the study of natural systems and risk assessment. Cases of application in SW Angola
39
The maximum rainfall in this period occurred on
April 2 (56 mm). Beside April 2, several days had
rainfall exceeding 20 mm.
- The rainfall was substantially lower in the
second half of April, never exceeding 10 mm.
The weeks between the beginning of
February and the end of the first half of April
deserve particular attention. During this period
occurred ten days with rainfall exceeding 20 mm,
most of them in the first week of March and the
first fortnight of April, five in the first week of
April; very few days had no rain, all of them in
mid-March.
Slope movements happened since the
beginning of 2011, but only led to circulation
difficulties along the Leba road during the month
of March when slide and rock fall events caused
safety problems (Fig. 4). The traffic circulation
was significantly disrupted in early April when the
road was fully blocked at several points due to
voluminous mass flows. The phenomena of slope
instability continued on the Leba hills during the
subsequent weeks of April, but did not affected the
EN 280 and had no implications on vehicles
circulation. It thus seems evident that the most
significant landslides, given the mobilized volumes
and effects on traffic circulation, coincide with
Fig. 4. Daily rainfall during the period of landslide events in the Leba road during the early months 2011.
Fig. 4. Valores diários de precipitação durante os eventos de instabilidade de taludes na estrada Leba dos primeiros meses
de 2011.
5. WATER FLOW IN RIVERS CAVACO AND
CATUMBELA
5.1. Drainage conditions
The seasonal rainfall pattern is responsible
for pronounced oscillations in the water level of
the rivers of southwestern Angola. In most of them
the channels are dry during most time and, as the
climate in the western regions is much drier than in
the inland, it is frequent to observe noticeable
water level rises in the littoral regions, more
densely populated, when no precipitation has fallen
in these locations, but just in upstream areas.
The Cavaco river has a strong torrential
character. When it rains, the water level in the
channel network rises suddenly along with the flow
periods of higher rainfall. They were, however,
favored by the intense precipitation that took place
during January and increased the water content
leading to the saturation of soils and rocks (Fig. 4).
Besides the occurrences of early 2011 presented
here, previous slope movements took place during
periods when daily TRMM rainfall data was found
to be close to 40 mm.
Depending on the morphological and
geological characteristics of the terrain, there is a
rainfall threshold above which the slope movement
processes tend to be more severe. For the cases of
the landslides on the Leba hills the most notable
movements took place when the daily rainfall
exceeded 30-40 mm. It seems that the very high
rainfall intensity during January 2011 must have
also contributed to the magnitude of the March and
April events.
P.A. Dinis, V. Mantas, P.S. Andrade, J. Tonecas, E. Kapula, A. Pereira & F.S. Carvalho
40
velocity and the transport capacity. On the other
hand, during the floods, the Cavaco river carries a
high sediment load that promotes channel silting-
up and, consequently, hinders drainage. Given this
set of features, the Cavaco river is generally
considered hazardous. Furthermore, on the banks
of Cavaco occur dense residential areas that
sometimes even spread into the channel itself,
introducing a high level of vulnerability to flooding
(Figure 5). For these reasons, the flood risk of the
Cavaco river is high and it is usually considered
particularly problematic by the flood prevention
and control authorities in the Benguela province.
The Catumbela river has a more steady
behavior. It is perennial, but still with significant
flow fluctuations between wet and dry seasons.
The alluvial plain of Catumbela and the
downstream areas of the valley are regularly
Fig.5. Satellite images (Google Earth TM) of the downstream sectors of the rivers Cavaco (A) and Catumbela (B). Residential
areas attached to the channel of Cavaco river are highlighted.
Fig. 5. Imagens de satélite (Google Earth TM) dos setores jusante dos rios Cavaco (A) e Catumbela (B). São realçadas as áreas residenciais
encostadas ao canal do rio Cavaco.
Both stations are programmed to provide the height
of the water column, in meters, with intervals of
three hours (00:00, 03:00, 06:00, 09:00, 12:00,
15:00, 18: 00 and 21:00). They are equipped with a
sensor for reading the level reached by the waters,
a buoy and a ruler. With this system, the buoy runs
up and down according to the height of the water
in the channel providing automatically information
on the flowing level in the rivers that is transmitted
to a control station.
The gauging stations in the rivers Catumbela
and Cavaco started to provide water heights data at
the beginning of January 2008 and 2009,
respectively. The logged measurements in both
stations have some interruptions due to operational
problems (Figs. 6 and 7), particularly at the station
in the Cavaco river, where the number of reliable
measurements is quite limited. One notable feature
of the record is the presence of intervals with stable
water that last for a few days. In most cases these
intervals are probably "artifacts" attributed to
trapping of the measuring buoy by river channel
sediments.
In the Cavaco river, a significant part of the
results suggest almost no surface flow and sudden
upsurges to near 7 m high. The water level in the
Catumbela river station is usually below 1 m
during the dry season, rising up to values in the
order of 4 meters during the rainy season, albeit
with pronounced swings. In general, there are
flooded due to channel overflow and the rise of the
water table. Since the floodplain has primarily
agricultural land use (Figure 5), the inundation
process are not considered has dangerous to local
residents as in the Cavaco river.
Regardless of other morphometric features of
the drainage basins of these two rivers, we admit
that the highly torrential character of the Cavaco
river is partially related to the relationship between
the spatial distribution of rainfall and the relief of
its catchment. Unlike Catumbela drainage basin,
where rainfall is high in its intermediate and inner
positions, the utmost rainfall intensities in Cavaco
basin are observed at its headwater region. Thus, in
this river, rainfall is higher in the sectors of higher
gradients, where the infiltration will be lower and
the runoff volumes and velocities tend to be
particularly elevated.
5.2. Water level in rivers Cavaco and
Catumbela
Hydrometric data were obtained from
gauging stations installed in the Catumbela
(Caiave, 72 km of the coastline) and Cavaco
(Guvriri, 58 km of the coastline). The gauging
stations were installed in these places during the
colonial times, but they become fruitless during the
civil war and it was not possible to obtain
measurements from the former operation period.
Contribution of TRMM rainfall data to the study of natural systems and risk assessment. Cases of application in SW Angola
41
Fig. 6. Evolution of the TRMM daily rainfall in Cavaco river drainage basin and water level measurements obtained for the Guviri
gauging station.
Fig. 6. Evolução da precipitação diária TRMM na bacia de drenagem do rio Cavaco e medições de nível da água obtidas para a estação
hidrométrica do Guviri .
Fig. 7. Evolution of the TRMM daily rainfall and water level measurements obtained for the Caiave gauging station (Catumbela
river). (A) Rainfall data from the entire drainage basin (mean cell rainfall within watershed). (B) Rainfall in the cell immediately
upstream of the Caiave gauging station.
Fig. 7. Evolução da precipitação diária TRMM e medições de nível da água obtidas para a estação hidrométrica do Caiave (rio Catumbe-
la ). (A) Dados de precipitação na bacia de drenagem (precipitação média para as células no seio da bacia hidrográfica). (B) Precipitação na
célula imediatamente a montante da estação hidrométrica do Caiave.
P.A. Dinis, V. Mantas, P.S. Andrade, J. Tonecas, E. Kapula, A. Pereira & F.S. Carvalho
42
sudden increases in the water level, often
exceeding 1 meter in a day, and a more gradual
decrease which may extend for periods of 40 to 80
days.
5.3. Relationship between rainfall and the
water level
Given the weaknesses of the logged
measurements in the Cavaco river station, it is
difficult to establish relationships between water
level and rainfall in the drainage basin (Fig. 6).
With regard to the Catumbela river, as expected,
the height of the water column, despite pronounced
fluctuations, tends to be higher during the rainy
season than during the dry season (Fig. 7). In the
weeks following the end or significant decrease of
rainfall, a regular and progressive lowering of the
water level occurs. If the river flow was not
conditioned by human intervention, this
progressive lowering could relate to the fact that
part of the rain water ended up feeding aquifers,
being later released gradually after the decrease of
the precipitation and as the river level lowers.
The greater heights of the water column are
not always synchronous with a succession of rainy
days within the catchment. For example, in the first
half of 2008 and the second quarter of 2009, a few
days with high water level (near 3.5 m and 2 m,
respectively) occurred when the rainfall in the
drainage basin of Catumbela was very low (Fig.
7A). When referring to rainfall data in the cell
immediately upstream of the measurement point, it
is possible to find a better match between the
succession of days of heavy rainfall and the
increases in the water level at the hydrometric
station (Fig. 7B). It is even possible to find
noticeable peaks of water level coinciding with
daily rainfall maximums or taking place in the
following days. On the other hand, the low rainfall
intervals tend to coincide with decreases in water
level. Hence, it appears that the short period
fluctuations in water level reflect variations of
rainfall in areas closer to the Catumbela river
gauging station.
The results of the daily mean water level do
not correlate with coeval rainfall in the drainage
basin (Fig. 8). The lack of correlation is partially
justifiable by a variable gap between the moments
of rainfall in some point of the watershed and the
time when this rain water reached the station
through the channel flow. Also, no correlation
exists between the water level on a given day and
the sum of rainfall in the days that precede the
various level measurements. Under these
conditions, it seems clear that other factors beyond
rainfall intensity have a decisive role in setting the
water level. These factors may be natural (e.g. the
way rainfall is distributed throughout the
watersheds, spatially variable infiltration and
runoff, etc) or associated to human action. The
systematic extraction of water for urban
consumption and irrigation of agricultural fields by
the local populations in the area of both stations
should be able to introduce some shifts in the water
level. Part of the divergences between rainfall and
observed water levels may also be attributed to
retention and discharge of water in reservoirs and
to drainage difficulties due to occasional outflow
obstructions.
Fig. 8. Relation between daily mean water level in Caiave gauging station (Catumbela river) and TRMM rainfall data in Catumbe-
la drainage basin. Rainfall values during each day (A) and accumulated in 7 days preceding water level measurement (B).
Fig. 8. Relação entre o nível da água médio diário na estação hidrométrica do Caiave e os valores de precipitação TRMM na bacia de dre-
nagem do rio Catumbela. São apresentados valores de precipitação durante cada dia (A) e acumulados nos 7 dias que antecederam a medi-
ção do nível da água (B).
6. CONCLUSIONS
The TRMM rainfall data were found to be
reliable and consistent with the surface processes
presented in this work. The analysis of two cases
study, one associated with slope movements during
a specific and relatively short period and the other
with the fluctuations of the river water level
throughout a time series of over two years, yielded
the following conclusions:
Contribution of TRMM rainfall data to the study of natural systems and risk assessment. Cases of application in SW Angola
43
- The processes of instability on the slopes of
Leba in early 2011 are associated with extreme
weather conditions marked by high rainfall
intensity, especially when it exceeded 30-40 mm/
day. The duration of rainfall and the quantity of
water accumulated in the land also influences
major landslide processes, in particular mass flows,
which are very destructive and could cause the
obstruction of the road EN 280.
- Although the water levels in rivers Cavaco
and Catumbela tended to be higher during the rainy
seasons, there is no correlation between these two
variables and irregular lags are observed between
them. The relative independence of the two
variables indicates that other factors have the
ability to condition the general pattern of the rivers
water heights. Superimposed on this general
pattern, in the Catumbela river occur short term
fluctuations in the water level reflecting variations
in the intensity of rain at relatively near areas.
The processes considered in this work are
closely linked to the risks of flooding and
landslide. Risk assessment of surface processes in
regions such as Angola can benefit greatly from
TRMM rainfall measurements. These data may be
very useful for the analysis of specific surface
events, more or less extreme, or the patterns of
behavior of natural systems in areas where the
availability of meteorological data is limited.
ACKNOWLEDGMENTS
Three reviewers are thanked for their thoughtful
comments and suggestions that helped to improve the
present work.
REFERENCES
ALCÁNTARA-AYALA, I. 2002. Geomorphology, natural
hazards, vulnerability and prevention of natural
disasters in developing countries. Geomorphology,
47: 107 – 124.
BERRICk, S.W.; LEPTOUKH, G.; FARLEY, J.D. & RUI, H.
2009. Giovanni: A Web service workflow-based
data visualization and analysis system. IEEE
Transactions on Geosciences and Remote Sensing,
47: 106-113.
CARVALHO, H. 1983. Notice explicative preliminaire sur la
geologie de l’Angola. Garcia Orta, 6: 15-30.
CORREIA, H. 1976. O grupo Chela e a formação Leba como
novas unidades litoestratigráficas resultantes da
redefinição da formação da Chela na região do
planalto da Humpata : sudoeste de Angola. Boletim
da Sociedade Geológica de Portugal, 20: 65-130.
DINIZ, A. C. 2006. Características Mesológicas de Angola.
IPAD, Lisboa, Portugal.
FEIO, M. 1946. O relevo de Angola, Segundo Jessen.
Boletim de Geologia de Portugal, 5: 295-306.
FEIO, M. 1981. O relevo do Sudoeste de Angola. Estudo de
Geomorfologia. Memórias da Junta de Investigações
Científicas do Ultramar.
GUIRAUD, M.; BUTA-NETO, A. & QUESNE, D. 2010.
Segmentation and differential post-rift uplift at the
Angola margin as recorded by the transform-rifted
Benguela and oblique-to-orthogonal-rifted Kwanza
basins. Marine and Petroleum Geology, 27: 1040-
1068.
HUFFMAN, G.; ADLER, R.; BOLVIN, D.; GU, G.; NELKIN, E.;
BOWMAN, K.; HONG, Y.; STOCKER, E. & WOLFF,
D. 2007. The TRMM Multisatellite Precipitation
Analysis (TMPA): Quasi-Global, Multiyear,
combined-sensor precipitation estimates at fine
scales. Journal of Hydrometeorology, 8: 38 -55.
KAPULA, E. 2012. Instabilidade de taludes na descida da
Leba (Humpata, Angola). Unpublished MsC Thesis,
Universidade de Coimbra.
LEAL-GOMES, M.J.A. & TEIXEIRA-PINTO, A. 2012. Notas
sobre a Estabilidade de Taludes e Ravinamentos na
Estrada da Leba, na Serra da Chela (nota técnica).
Geotecnia, 124: 81-89.
LIU, Z., OSTRENGA, D., TENG, W. & KEMPLER, S. 2012.
Tropical Rainfall Measuring Mission (TRMM)
Precipitation Data and Services for Research and
Applications. Bullet in of th e Am er ican
M e t eo r o lo g ic a l S oc ie t y, d oi : h tt p: / /
dx.doi.org/10.1175/BAMS-D-11-00152.1
LIU, Z.; RUI, H., TENG, W. L., CHIU, L. S., LEPTOUKH, G. G.
& VICENTE, G. A. 2007. Online visualization and
analysis: A new avenue to use satellite data for
weather, climate and interdisciplinary research and
applications. Measuring Precipitation from Space -
EURAINSAT and the future. Advances in Global
Change Research, 28: 549-558.
MAR Q UE S , M. M . 19 66 . Les g ran d es unit és
géomorphologiques d'Angola. Boletim dos Serviços
de Geologia e Minas de Angola, 13: 13-16.
MARQUES, M.M. 1977. Esboço das grandes unidades
geomorfológicas de Angola (2ª aproximação).
Garcia de Orta, 2: 41-44.
PEEL, M.C.; FINLAYSON, B.L. & MCMAHON, T.A. 2007.
Updated world map of the Koppen-Geiger climate
classification. Hydrology and Earth System
Sciences, 11: 1633–1644.
PEREIRA, E.; TASSINARI, C.C.G.; RODRIGUES, J.F. & VAN-
DÚNEM, M.V. 2011. New data on the deposition
age of the volcano-sedimentary Chela Group and its
Eburnean basement: implications to post-Eburnean
crustal evolution of the SW of Angola.
Comunicações Geológicas, 98: 29-40.
PINHO, A.C. & CARVALHO, F.F. 2010. Prospecção, pesquisa
e produção de petróleo em Angola. O papel
desempenhado pelas companhias petrolíferas. In.
J.M. Cotelo Neiva, A. Ribeiro, L. Mendes Victor, F.
Noronha e M. Magalhães Ramalho (Eds.) Ciências
Geológicas – Ensino e Investigação e sua história.
Lisboa: Associação Portuguesa de Geólogos: 61-70.
TONECAS, J. 2012. Cheias nos vales do Cavaco e Catumbela
(Angola): constrangimentos geomorfológicos e
climáticos. Unpublished MsC Thesis, Universidade
de Coimbra.
