Forum Geografi, 31(1), 2017; DOI: 10.23917/forgeo.v31i1.3790
Spatio-temporal distribution of landslides in Java and the Triggering Factors
1 Faculty of Geography, Universitas Gadjah Mada, Yogyakarta, Indonesia
2 Université Paris 1 Panthéon-Sorbonne & Laboratoire de Géographie
Physique, UMR 8591 CNRS, Meudon, France
3 UCR3 - Risk, Resilience & Renewal, Waterways: Centre for Freshwater
Management, College of Science, Department of Geography, University of Canterbury, private bag 4800,
Christchurch 8140, New Zealand
4 Graduate School of Maritime Sciences, Kobe University, Higashinada-ku, Fukae-Minamimachi 5-1-1, 658-0022 Kobe City, Japan
5Meteorological, Climatological
and Geophysical Agency, Jl. Angkasa No. 1-2, Kemayoran, Jakarta, Indonesia
*) Corresponding author (e-mail: hadmoko@ugm.ac.id)
Received: 30 March 2017 /
Accepted: 13 April 2017 / Published: 30 April 2017
Abstract
Java Island, the most
populated island of Indonesia, is prone to landslide disasters. Their
occurrence and impact have increased mainly as the result of natural factors,
aggravated by human imprint. This paper is intended to analyse: (1) the spatio-temporal variation of landslides in Java
during short-term and long-term periods,
and (2) their causative factors such as rainfall,
topography, geology, earthquakes, and land-use. The evaluation spatially and
temporally of historical landslides and
consequences were based on the landslide
database covering the period of 1981 – 2007 in the GIS environment. The database showed that landslides distributed
unevenly between West Java (67%), Central Java (29%) and East Java (4%). Slope
failures were most abundant on the very intensively weathered zone of old
volcanic materials on slope angles of 30° – 40°. Rainfall threshold analysis
showed that shallow landslides and deep-seated
landslides were triggered by rainfall events of 300 – 600 mm and >
600 mm respectively of antecedent rainfall during 30 consecutive days, and many
cases showed that the landslides were not always
initiated by intense rainfall during the landslide day. Human
interference plays an important role in
landslide occurrence through land conversion from natural forest to dryland
agriculture which was the host of most of landslides
in Java. These results and methods can be used as valuable information on the spatio-temporal characteristics of landslides
in Java and their relationship with causative factors, thereby providing a sound
basis for landslide investigation in more detail.
Keywords: landslides, spatio-temporal
distribution, causative factors, Java, Indonesia.
Abstrak
Pulau Jawa, salah
satu pulau dengan kepadatan pendududuk tertinggi di dunia, rawan terhadap
bencana longsorlahan. Kejadian longsorlahan dan dampaknya cenderung
mengalami peningkatan sebagai kombinasi antara faktor alami
maupun faktor manusia. Terkait dengan permasalahan tersebut maka penelitian
ini bertujuan untuk menganalisis: (1) variasi spasiotemporal kejadian longsorlahan di Jawa dalam jangka
pendek (bulanan, tahunan) maupun jangka panjang dan (2) faktor yang berperan terhadap kejadian longsorlahan tersebut antara lain hujan, topografi, geologi, gempabumi dan penggunaan lahan. Evaluasi sejarah kejadian longsorlahan dan dampaknya baik secara keruangan maupun menurut waktu dilakukan dengan periode kejadian antara tahun 1981 – 2007. Analisis ambang batas curah
hujan menunjukkan bahwa tanah longsor
dangkal dan tanah longsor yang mendalam dipicu oleh hujan kumulatif
antara 300 - 600 mm dan
> 600 mm yang terjadi selama
30 hari berturut-turut. Banyak kasus yang menunjukkan bahwa longsorlahan tidak selalu dipicu oleh
hujan dengan intensitas tinggi pada hari kejadian
longsorlahan tersebut. Campur tangan manusia
memainkan peran penting dalam kejadian
longsorlahan melalui konversi lahan dari hutan untuk
lahan pertanian, yang merupakan wilayah dimana sebagian besar longsorlahan terjadi. Hasil mapupun metode yang digunakan dalam penelitian ini merupakan informasi yang berharga dalam mengenali karakter spatio-temporal longsorlahan di Jawa dan faktor
yang mempengaruhi, sebagai informasi dasar dalam investigasi dalam skala yang lebih detail.
Kata kunci: longsorlahan,
distribusi spasiotemporal, faktor penyebab, Jawa, Indonesia.
Introduction
Landslides are known to be very hazardous phenomena,
especially in the mountainous area of the
humid tropics. They are also a cause of high
risks, in terms of economic losses, casualties, and environmental problems (Iida, 1999 ; Schuster and Highland, 2001; Dai et al., 2002; Spiker and Gori, 2003; Zhou et al., 2003; Petely et al., 2007, Hadmoko et al.,
2010; Hadmoko et al,
2017). The
understanding of spatio-temporal
characteristics of landslides such as frequency, magnitude and human consequences has recently become a topic
of major interest for geoscientists, as well as for governments,
non-governmental organisations and local
administrations in many regions of the tropics (Paudel et al.,
2007; Devoli et al., 2007). These data are vital for quantitative estimation of landslide
hazards, in order to calibrate predictive
models and to validate temporal
predictions (Brunsden et al., 1995, Samia et al., 2017).
In this contribution, we analysed the relations between landslides occurrence and their
triggering factors over a long period (27
years) in Java. With 120 million inhabitants and
a population density of 900 inhabitant/km2, Java (which is located in
the centre of the Indonesian archipelago)
(Figure 1), is the most populated Indonesian Island. It represents 60% of the
Indonesian population on only 6.9% of the country’s surface (BPS, 2005). This population settlements’ disequilibrium within the physical
environment occurring on steep and unstable slopes is the source of numerous
problems on landslides.
Figure 1. Java Island in Indonesian Archipelago
As a result of its geological
setting, topographical characteristics, and climatic characteristics, Java is the region most
exposed to mass movements in Indonesia, due to several factors: (1) located on a subduction zone, 60% of Java is
mountainous (Hadmoko, 2006), with volcano-tectonic mountain chains and 36 active volcanoes out of
the 129 in Indonesia, and these volcanic materials are intensively weathered
(2) Java Island was shaken by more than 200 earthquakes ranging from 5 to 9 on
the Richter scale from the 19th
century to the mid-1980’s (SEASEE, 1985); (3) Java is under a humid tropical climate associated with heavy rainfall during the rainy season
from October to April. Most
of the landslides occur during this period (Ayala, 2004; Gabet et al., 2004). In
addition, seismic activities are believed to be potential triggering factor in
this area (Saputra et al., 2016). On top of these “natural” conditions, the
human activity is an additional factor of landslide occurrence, driven by a
high demographic density.
Within this setting, the
goals of our study were twofold: (1) to collect and analyse spatial and temporal data of landslide occurrence for the
period 1981-2007; (2) to evaluate the main landslide causative factors, i.e.,
rainfall, topography, geology, earthquakes, and land-use. Our paper provides a new insight into our understanding of landslide
mechanisms in the humid tropics, and a basis for
predicting future landslides and assessing related hazards at a regional scale
(Zhou et al. 2002). The results from this paper should be
valuable for decision makers and communities with
respect to future landslide risk reduction programs.
Methods
Database on landslides
events
For the period 1981-2007, we compiled data on
landslide occurrence from various sources: the Indonesian Centre of Volcanology and Geological Hazard Mitigation
(CVGHM) and the Research Centre for Disasters at Universitas
Gadjah Mada (PSBA-UGM)
provided data for 1216 landslide events. On top of these official sources, we
added 87 additional data from internal reports, monographs, bulletins and
national and local newspapers (Kompas,
Jawa Pos, Kedaulatan Rakyat, Republika, Bernas, etc).
As a whole, our database documents 1303 landslide events in Java. Data include
the date of occurrence, the type and magnitude of the event, the number of
victims, the infrastructure affected, and the economic losses for each event.
We plotted these data on a base map and compiled all tabular data by using
Geographic Information System (GIS). In order to
analyze
the spatial distribution of landslides by provinces, we then plotted the location of landslide events in a
1: 100,000 scale GIS divided into 3 regions, i.e. West Java (Banten Province,
DKI Jakarta Province and West Java Province), Central Java Region (Central Java
Province and Yogyakarta Special Province) and East Java Region (Est Java
Province).
Database on triggering
factors
Rainfall data were
collected from 1034 rain
gauges distributed all over Java. These data were
obtained from several governmental institutions such as the
Meteorological, Climatological and Geophysical Agency (BMG), the Directorate of
Water Resources of Ministry of Public Works (Puslitbang
Air DPU), and the Ministry of Forestry. We also used
unpublished data from the hydrological and meteorological laboratory in Faculty
of Geography, Gadjah Mada
University. All rain gauges were plotted
in a GIS in order to relate their
location to the closest landslide. Unfortunately, the hourly rainfall data is
not available in our archive database. Therefore we only used the daily
rainfall to calculate rough rainfall thresholds of landslide occurrence.
Topographical data at the scale of the whole Java
Island were obtained through a DEM that
we built using SRTM data at a 90 m resolution. We used this DEM to analyse the landslides distribution general
trends at a regional scale. We also calculated slopes angles with ILWIS®, and grouped them into seven classes ranging from 0° to > 60°.
Then we calculated the number of landslides for each class.
Seismic data were collected for the period 1833 - 2006 at
the BMKG and CVGHM offices. This set of data only concerns earthquakes that
caused infrastructure damages and casualties. We organised the data attributes in
order to provide automatic plotting for our GIS, and find quantitative relations between seismic activities and
landslides. We also compared the landslide activity with low magnitude (< 4
on the Richter scale) earthquakes data for the period 1990 – 2002.
Land use and vegetation cover data were collected at the Ministry of Forestry,
and, further, digitalized into a GIS. We completed these data through a
combination of aerial photographs, panchromatic SPOT images and Landsat TM
images.
After this
initial compilation and the preliminary data processing, we carried out a multi-temporal
analysis of the behavior of landslides and their causative factors at three
different scales: (1) on a yearly basis, we analyzed the general trend of
landslide events over 27 years; (2) on a monthly basis we analyzed the
relations between landslides and time inferred factors, such as cumulative
rainfalls, earthquakes, etc.; (3) and on a daily basis, we worked on landslides
triggering rainfalls.
Result and Interpretation
Temporal distribution of landslides
Inter-annual
variation
During the period
1981-2007, the annual landslide frequency varies significantly from 13 to 90
events per year, with an average of 49 events per year. We can observe three main trends in the landslides occurrence since 1981:
(1) a first period from 1982 to 2000 when the number of landslides increases
significantly, with a maximum of 90 events recorded in 2000, and (2) a second
period with fewer landslides – 29 in average - with a decline from 2000 to 2004
and (3) third period with the landslide increasing from 2004 to 2006 (Figure 2).
Figure 2. The chart of the annual
distribution of recorded landslide events in the period of 1981 – 2007 (red
line, right-hand scale) and their impacts
on human losses (bar, left-hand scale).
The fewer number of landslides during
1981 – 1990 compared with those other periods was
mainly associated with lower of demographic pressures, and less
development activities. Human intervention in the landscape was relatively low
due to the reduced population numbers in Java. Development activities in
several sectors such as infrastructures, settlements and roads were not
intensive.
However, since 1990s
landslides occurrences increased rapidly due to the rapid increase of population and the economic development.
Over exploitation of natural and land resources
occurred in order to support human needs and development. Excavating
hillslopes for road development and settlement construction, as well as
deforestation on many parts of volcanic slopes for agriculture areas increased
rapidly. These activities extend into the upper part of mountainous areas due to the full land occupation on the lower part of the island.
An obvious
example is given by the
flanks of the Sumbing and Sindoro
volcanoes in Central Java. Since 1997, high deforestation rates (600 ha/year)
result from the upslope expansion of tobacco fields by local farmers. As a
result, subsistence crops have also been progressively pushed upslope towards
the steeper areas, with poor terracing or shelter planting systems and consequent exposure to increased landslide
hazards. The trend is similar on the volcanic slopes of East Java where the
forest areas drastically decreased between 1990 and 2000 (Lavigne and Gunnell, 2006).
Several natural and human
factors should be analysed in order to explain the wide range of landslide
fluctuation during the years of 2000 and 2007. Decreasing of landslide
occurrence during 2000 – 2004 was probably caused by the decreasing of annual
precipitation in Java and the strict governmental regulation on land
management. Several projects related to
conservation measures have been implemented by the government in order to minimise
environmental degradation such as the encouragement of farmers to substitute
tobacco for coffee trees on the slopes of Mts Sumbing and Sindoro and
reforestation by using teak trees in Cilacap (Lavigne and Gunnell, 2006). However, such programmes are sometimes
unsuccessful, deforestation and over-cultivation are still occurring in Java.
Teak forests were logged and replaced by
young 30-cm-high pine saplings and subsistence crops such as peanuts, cassava,
sweet potatoes and banana plants on steep slopes.
Intra-annual
variation
Within the year, the number
of landslides increases from June to November and decreases significantly from
January to July (Figure 3). Statistically,
both January and November are the most susceptible months for landslide
generation, with respectively nine and seven events on average. This
distribution is closely related to the monthly
rainfall variations. The two maximum of landslide number of occurrence correspond to the bimodal repartition of
monsoon rain showers. Thus rainfall is a more deterministic causative factor on
an intra-annual basis than on an inter-annual basis.
Figure 3. Monthly variation of
landslides in Java during the period 1981 – 2007. January is the most abundant period with an average of nine events,
whereas July has the fewest events (one in average).
The analysis of monthly rainfall and monthly landslides (Figure 3) in Java reveals a strong climatic control
on the triggering of slope failures. The maximum landslide frequency occurs
concomitantly with maximum precipitation
in January, the fewest landslides corresponding to the middle of the dry
season, in August. However, landslides still occur during the driest periods in
July and August, due to the contribution of other
factor such as earthquakes that are frequently occurring in Java. An evident example is given by the big landslide
event in Majalengka, West Java that occurred in July 6th 1990 after a big earthquake of 5.8 Richter in this
area.
Spatial
distribution landslides and causative factors analysis
The landslides are unevenly distributed in Java (Figure 4). Most landslides are concentrated
in West Java Region (67%), followed by Central Java (29%) and East Java (4%). The overall landslide density in Java reached
1 x 10-3 events/km2 with the annual average was 3.6 x 10-4
event/km2/year.
This spatial distribution of landslides is therefore combined with their causative
factors i.e. topography, rainfall, geology, earthquakes and land use. Spatial analysis of each causative
factor was conducted separately in order to
understand the role of each factor to landslides triggering.
Figure 4. Spatial distribution of
landslides in Java during the periods of 1981 – 2007. Red dots indicate the locations of landslide
events.
Landslides
and topography
Java Island is mainly represented by slopes ranging from 0°
to 10° (28.5%). Those that exceed 60° (0.84% only) are located on the upper
volcanic slopes and along mountain escarpments (Figure 5). In general, slope gradient has an
influence on landslides events. The number of landslides increases
significantly from slopes inferior to 10° and from 30 to 40°. We also found the
overall positive correlations between slope angle and landslide density,
although they are not very strong. The highest
density of landslides can be found on
slopes ranging from 40° to 50° (0.021 events/
km2). However, inverse correlation
between landslides events occurs on slope steepness more than 40° (Figure 6) when the
landslide frequency tends to decline with an increasing of slope angle.
Figure 5. Spatial
distribution of landslides (blue dots) based on a slope map (in degree).
The positive correlation
between landslides and topography for the slope range of 30° to 40° is due to
the force influence of slope gradient, combined with the unconsolidated colluvial materials of volcanic origin which can fail
easily, very intensive human occupation and development activities within this
slope range. The steeper slopes (> 40°) can be more stable due to the
existence of resistant bedrocks and consolidated materials. These areas usually
consist of non-arable land leading to the fewer human activities.
Figure
6. a) Areal ratio of each slope class (bar graph, left-hand scale) and cumulative area of slopes; b) Frequency of
landslide in each slope classes (bar graph, left-hand
scale) and cumulative frequency of landslides (line graph, right-hand scale).
Landslides
and rainfall
Figure 7 displays the spatial variation of the rainfall distribution in Java.
The amount of annual precipitation is significantly higher in West Java than
further East, decreasing with a constant W-E gradient. This gradient also
corresponds to the landslides distribution, with 67% of the events located in
West Java. The minimum annual rainfall occurs in the northern part and in far East Java, where few landslides can be spotted (Figure 7). On the other hand downpours
concentrate on relief such as the volcanic upper slopes and the intermountain
basin like in the Bandung Basin- West Java, which is the host of numerous
landslides and heavy rainfall (Figure 7). Landslides are mostly located in areas with
2000 mm to 2500 mm of annual precipitation,
which cover the largest amount of land in Java (Figure 8). The surface area of each rainfall class
decreases from the interval of 2000 mm to 2500 mm with increasing and
decreasing of annual rainfall (Figure 8).
Figure 7. Spatial variation
of annual rainfall in Java Island and landslides occurrence (red points). Bar
charts show a monthly variation of
rainfall in four selected rain gauges with the maximum scale of 600 mm. This map is based on a calculation of average
annual rainfall of 1034 rain gauges distributed in Java during 20 years.
Rainfall intensity was classified into
seven classes with the interval value of 500 mm.
Overlaying the annual
isohyets with the landslides distribution indicates that the landslides
occurrence increases concomitantly with the precipitation until the class 2500
– 3000 mm. However, the numbers of landslides decrease with increasing of
precipitation value on more than 2500 mm. Only a few landslides occur under
these high intensities rainfall. This variation shows that the largest area of
Java Island is rained by 2500 – 3000
mm/year, which covers the middle and upper part of volcanic and structural
mountains and several areas of Bandung
Basin. This area usually consists of weathered materials which can fail easily.
Several factors should be taken into account to
explain the fewer frequencies of landslides on more humid area (> 3000
mm/year) such as: the area of this rainfall interval is smaller than others
that cause smaller probability of landslides, this area is usually covered by
consolidated bedrock and better land management (forest area).
Figure 8. The surface area of each annual rainfall interval (bar graph, left-hand scale) and landslides distribution in each class of annual rainfall in Java (line,
right-hand scale).
On a shorter time-scale, cumulative rainfalls play an important role in landslide triggering.
Most of the shallow landslides can be associated with antecedent rainfall reaching
300 mm for the 30 days that precedes the slope failures, and rainfall superior
to 43 mm on the day of landslide occurrence.
There is an inverse relation between antecedent rainfalls and daily rainfall.
Indeed heavy instantaneous rainfall can produce a landslide with the help of
only low antecedent rainfall. On the contrary we encountered 11 cases of
landslides with no rain on a triggering day, but with important antecedent
rainfalls (Figure 9). For example
the landslides of Tawangmangu Sub-District, East Java, occurred after a
period of heavy antecedent rain (more than 175 mm/day) with no rainfall at the
time of the failure. Thus antecedent rainfalls increase the soil moisture
content and the ground water levels. This phenomenon also plays an important
role for deep-seated
landslides, since nine of them, that occurred in
Menoreh Mountains on November 6th, 2000, are the response of 839
mm antecedent rainfalls during the 30 days that preceded the events.
Figure 9. Chart showing the relation between antecedent rainfalls during 30
consecutive days and daily rainfall during landslides day.
Landslides and seismic activities
Forty catastrophic earthquakes were reported in Java during
1833 – 2006, with magnitudes ranging from 4.8 to
7.2 on the Richter scale (DVGHM, 2006). The distribution of epicentres (Figure 10) shows that most of the earthquakes events occurred in West and Central
Java, and only a few shook East Java. This distribution reflects the occurrence
of fault-line, which are also
concentrated in West and Central Java.
Figure 10. Spatial distribution of earthquakes in Java during 1833 - 2006
Earthquakes are an important
triggering factor for landslides. They rupture the rock materials which were
already fragile. The spatial distribution
of earthquakes in Java corresponds with the concentration of landslides, which
most of them spread out in West Java. Big earthquakes can directly cause
landslides in several part of Java.
Several big landslides occurred in Majalengka District, West Java on July 6th,
1990. These landslides were directly caused by an earthquake (5.8 Richter)
during in the middle of dry season when
the precipitation was zero (DVGHM, 2006). The big
earthquake that occurred in Bantul, Yogyakarta
Province in May 2006 caused a lot of soil
cracks along the southern range south of Yogyakarta. More than 20 points of
landslides were identified along the southern
mountain in Bantul which face to Northwest and
Southwest (Figure 11).
Figure 11. Shallow landslides occurred along southern
mountains of Yogyakarta after earthquakes on May 28th, 2006.
An example from Menoreh
Mountains in Central Java shows that the trend of seismic activity usually
coincides with the landslides occurrence (Figure 12). However we need to be
circumspect in analysing the data,
because there is a lapse of time between the seismic tremor and the landslide
occurrence. Indeed seismic activity during the dry season will favour
landslides for the next rainy season. For example, we registered 1115 seismic
vibrations in 2000, with only 10
landslides on that year.
Figure
12. Relation between the number of
earthquakes (bar graph, left-hand scale)
and shallow landslides activities (line graph, right-hand
scale) during the period 1990 – 2002 on Menoreh
Mountains, Central Java Indonesia.
Landslides, land use and human
activities
Thirteen of land use types were
mapped in Java Island (Figure 13). Dryland agriculture spread largely on the wide range of topography from lowland to upper part of the mountainous area from West to East while the
artificial forest occupies only the middle and upper
part of volcanic slopes. Paddy field
usually covers the alluvial plain and intermountain basin. This island is
mostly covered by dryland agriculture, followed by paddy field and artificial forest which represent of 47.7%, 21%,
and 13.8% respectively of total surface
of island.
Figure 13. Land use type in Java Island combined with
landslides points
Classification of landslide activities based on land use type shows that most of the landslides occur on dryland agriculture, followed by
paddy fields and artificial forests with respectively 66%, 18% and 7% of
the total number of landslides (Figure 14). These data
indicate that human activities play an important
role on landslide occurrence. Dryland agriculture covers not only the lower part
of land, but also reached middle and upper slopes; with terraces agriculture
that often accelerates landslide triggering (Lavigne and Gunnell, 2006). Most of dryland agriculture and
paddy field were previously forested lands that have been logged (Figure 14). Geomorphic impacts were almost immediate:
the first heavy rainfall following the
land cover change triggered several shallow landslides, and this has recurred
every year during the rainy season since 2000 (Lavigne and Gunnell, 2006). For these classes, the vegetation is not
deeply rooted in the ground, and they are often
accompanied by numerous landslides. However, artificial forest was classified
into 3rd rank for landslides
occurrence. This area was established
with the reforestation conducted by the government
which consists of the young vegetation whose roots unable to penetrate deeply into
the soil.
Figure 14. Land use type of Java and landslides activities
with the percentage of land use type
(circle chart), surface of each type of land use
(in km2) (bar chart) and
distribution of landslide based on land use
types (line chart).
The increase of demographic pressure also forced
people to invest in the least cultivable land extending to the tops of slopes
with the extremely steep slopes. During 1980 – 2005, the Java population
increased from 91 to 128 million, amounting to an increase of 40% during 25
years. Of this population, 45% concentrated in West Java Region (including
Jakarta and Banten Province). This population increase was known as a significant factor
initiating and accelerated landslide in this region mostly by undercutting
and removal of the toe of slopes for the cutting of roads, houses and
agricultural area.
Human impacts
Over 27 years, landslides casualties reached 1848
people and 550 injured people. The casualties’ analysis over time varies
significantly from one year to another. Indeed the landslide death toll
increased significantly from 1999 to 2001 and from 2003 to 2006. The five most
deadly years occurred after 2000 in 2000, 2001, 2005, 2006 and 2007. The
increasing of casualties was probably caused by the increasing of population
staying in the hazardous area. The
highest death toll occurred in West Java,
and the lowest victims were noted in East
Java. The different number of casualties of each region correspond closely with
the number of landslides that occurred.
Landslides also affected
houses, building, roads, agricultural land and irrigation drainage
infrastructure. During 27 years, the
numbers of houses destroyed and damaged by landslides were 2078 and 9821
respectively, and 2078 families had to be
evacuated temporarily. The affected agricultural land and roads were
estimated at approximately 4,229 ha and 12.3 km respectively. It was estimated that 67,000
families had to be evacuated because of
the destruction of their houses and the hundreds of villages were threatened. It can be also estimated that the annual economic losses in Java would
exceed one million Euros. The highest estimated economic losses occurred in the
years of 2002 and 2003, reaching 2,185,400 € and 1,816,900 €
respectively (Hadmoko, 2006).
Our data show that the
occurrences of catastrophic landslides in Java tended to increase in recent
years, and in recent times have caused
much higher of casualties than before. The increasing human losses reflects the densification of human occupation
on land mostly in hazardous area both for
settlement or agriculture area. Over-cultivation and houses construction on
upper slopes aggravate landslides occurrences. Indeed, the death toll is much
higher than injured peoples, this
phenomenon was caused by the fact that
landslides occurred extremely fast in terms of velocity during a short period (less than 15 minutes). Moreover, the
majority of the landslides occurred during the
night, and in densely populated villages, consequently
the population has no
enough time to be evacuated.
One of example of catastrophic landslides was in December 26th
2007 when landslide occurred sporadically
at 17 locations of Tawangmangu District on the western slopes of
Lawu Volcano. These landslides occurred in densely
populated mountainous area which killed 71 people and destroyed 15 houses. Others example
of catastrophic landslides events (in terms of number of person killed) are (1)
the debris flow which occurred in Pacet Subdistrict, Mojokerto District, East
Java, on December 11, 2003 and killed 32 persons and (2) Kadungora landslide in
the District of Garut, West Java, on January 28, 2003 and 20 person were
buried.
Discussion
Java Island lacks an
integrated spatio-temporal landslides
investigation which covers overall area of the island during three different
timescales: short, medium and long-term periods. Most previous research
conducted in Java has only focused on the specific areas with a short record of temporal data. Impacts of landslides
in Java are also increasing with time, but until now there has been little or
no quantitative data to support this view,
or to explain the causes of the increases. This research highlights two
dimensions: temporal evolution of landslides and its spatial distribution and
together with factors causing landslides.
This contribution provides
an analysis of relevant information for all Java. We provided a long series of
data consisting of 1303 landslides affecting Java Island between 1981 and 2007
with the high temporal variation. Landslides occurrences are much higher
compared to others parts of the world such as in Himalaya Nepal and Italia. In
Himalaya Nepal, 397 catastrophic landslides were recorded during the periods of
1978 and 2005 (Petley, et al. 2007) while In Italia, there were 996 fatal
landslides recorded for the whole country
during 1410 to 1999 (Guzzetti, 2008). Landslides density in Himalaya is also
much lower compared to Java Island with
the density reached 2.7 x 10-3 event/km2. This comparison
indicates a chronic problem to landslides hazards in Java which has to be minimised.
Temporal variation of
landslides in Java reveals a great
variation in different timescales. In the long-term scale, landslides events
increase with a great yearly fluctuation
and regular seasonal variation. This variation reflects the change of
triggering factors over time such as rainfall, earthquake and human pressure. Long-term occurrence of landslides corresponds
with the annual rainfall which greatly varies from one year to another
influenced by conditions such as El-Nino and La-Nina events. Also, this long-term evolution is greatly
influenced by increasing human pressure in Java. Development processes and
over-cultivation on marginal land by people could be the reason for the
increasing landslides and their casualties. Intra-annual distribution of
landslides has a close relationship with the monthly distribution of rainfall
as an effect of monsoon variation in Java Island which leads two different
seasons i.e. wet and dry season that change regularly during six months in the
year.
Landslide events were
spatially distributed unevenly, being concentrated more in West Java Region and
becoming less in Central Java and East Java (Figure 4). This phenomenon is closely related to the
environmental setting of West Java. Landslides were
distributed on wide ranges of slope gradients with the highest
concentration on slope range 30° – 40°. Most of them were well scattered on all types of terrains on middle and upper
parts of volcanic forms (Figure 4 & Figure 6). These parts are characterised by steep slopes, high relief, and the presence of
pyroclastic materials, volcanic ashes, and andesitic lava flows that are very
intensively weathered and fractured, and therefore deeply eroded. Landslides
occur on all type of land use mostly on dryland agriculture,
artificial forests and paddy field (Figure 12).
We found that most of landslides concentrate on the interval slope of
30° – 40° and decrease both with increasing and decreasing of slope steepness.
This result is slightly similar with those of Zhou et al. (2002) on Lantau Island, Hongkong, who found that
the frequency of failures is highest on moderately steep terrain with gradient ranging from 250 – 350.
Paudel et al. (2007) also correlated the landslides and topography on Mt. Aso, Japan and pointed out that slope interval of 30° - 35°
is the most exposed area to landslides. These
results are explained by two factors: (1) the existence of an unweathered
concrete materials covering the steeper slopes on upper part mountains with the
low thickness of soil, slightly prone to landslide generation; and (2) human
pressure on land (agriculture, infrastructure, road network) generally take
place in flatter terrain and up to 30° –
40°. Steeper slopes are relatively
virgin from human activities due to remoteness, and impracticalities of
agricultural development.
Besides these, both cumulative rainfalls during
certain periods and rainfall intensity are
widely recognised as important roles on landslide triggering. However there has been some debate as to the
relative role of antecedent rainfall and daily rainfall. Experiences
from various regions of the world have resulted in different conclusions as to
the significance of antecedent rainfall for slope instability (Raharjo et al., 2001). We pointed out that for most landslides
occurrences, the daily rainfall threshold decrease with increasing rainfall
accumulation (Figure 8). Smaller amounts of daily rainfall are needed to trigger landslides as
increasing of cumulative rainfall (Crozier, 1999; Gabet et al., 2004). However, there were several contradictory results regarding daily and
cumulative rainfalls in Java. Some landslides can be produced with less than
300 mm of antecedent rainfall, without any rain shower during landslide day,
and vice versa. Therefore we should take into account the characteristics of
the land (e.g. soil thickness, cracking on soils, slope steepness, land use type) and the depth of failure plane
in our analysis (Paudel et al. 2007).
Variation of rainfall threshold is also be produced by the depth of failure plane. The
threshold conditions for failure of shallow landslides are not necessarily determined by the development of positive pore
pressures on a potential slip plane. Failure conditions can also occur when, at
a critical depth, the moisture content in the soil becomes close to saturation
triggered smaller amount of rainfall, resulting in a considerable reduction of
soil strength (van Asch et al., 1999). In the other hand, deeper
landslides are in most cases triggered by positive pore pressures on the slip
plane induced by a rising groundwater level. The groundwater rising requires
much more input water in order to
establish infiltration, percolation and soil saturation. This means that deeper landslides need larger
absolute amounts of water for triggering conditions than shallower landslides.
For the cases of Java, most of deep-seated
landslides can be produced by an antecedent rainfall more than 600 mm without
any rain shower during landslide day, and the majority of shallow landslides
were triggered by antecedent rainfall between 225mm and 625mm.
Classified as the most
populated island in the world and the problem of land shortage, Java involves
the human factors on landslide triggering. Indicators that could reflect the
intensity of human intervention on the territory include the high rate of
population increase in Java. As population pressure increases, not only will
the stability of the slopes be reduced, but
people will also be forced to cultivate even more unstable slopes. As a
consequence of both, the risk on damage by slope failure increases. For
the case in Java, this human pressure
aggravates the abrupt land use change
from forested land to dryland agricultural area which is the host of the most
landslides in Java. Other results in several parts of the world confirm that
the human pressure and deforestation play an
important role on landslide triggering (van Beek, 2002; Glade, 2003; Knappen et al., 2006).
Conclusion
Java Island experiences a lot of landslides which cause enormous
casualties and damages. The high recurrence of landslides in Java is controlled
both by natural and human factors which are
compounded in Java. As a wet tropical region, abundant rainfalls
occurring on deep weathered materials
situated in mountainous region triggered
many landslides in Java. Indeed, high human interference also aggravated slope
failures: forest clearance, excavating foothills and cutting hillsides for
increased road and settlement
constructions for agriculture areas has increased
rapidly. However, integrated spatio-temporal
landslide studies to understand this problem are still limited. This paper
contributes an analysis of a set of information for all Java concerning
landslides behaviour and their controlling
factors. This work should be of value to the government mitigation program as
well as being helpful in developing a landslide
hazard and risk assessment model for the area.
Acknowledgement
We acknowledge to
Geological Agency Ministry of Energy and Mineral Resources for providing the
landslide data, Research Centre for Disasters, Faculty of Geography Universitas Gadjah Mada for providing the support during field survey. We wish to thank Prof. Michael James
Crozier (Victoria University of Wellington, New Zealand) for his very useful comments and corrections on an earlier version of
this paper.
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