INTRODUCTION
Indonesia is a country with a very high potential for disaster. Large
archipelago Indonesia is frequently affected by climate and weather-related
disasters such as landslides, volcanic eruptions, and tsunamis (Burrows et al., 2021). This is supported
by the existence of Indonesia which is surrounded by three world plates, namely
the Eurasian plate, the Indian-Australian plate and the Pacific plate (Suharini et al., 2020).
The intensive interaction of the three plates makes Indonesia passed by the
ring of fire, thus forming reliefs and topography in Indonesia to be diverse.
Climate and human factors are very influential in forming new topographical
results with conditions that have differences in elevation. As a result, the
intensity of soil and rocks becomes unstable due to the difference in elevation
produced, and disasters can occur in the form of land movements, especially in
the form of landslides. Landslides are a type of movement of soil or rock masses
that generally occur at a slope of 20�-40� with moving masses in the form of
residual soil, coluvial deposits and weathered
volcanic rocks (Storie et al., 2018).
Moreover, due to global climate change and demographic pressure, the likelihood
of landslides is growing (Nugraha et al., 2015).
Thus, Rough soil will be more at risk of landslides because the land has low
soil aggregate cohesion.�
This situation has similarities with the research area located in the Banjarnegara area, Central Java. Banjarnegera is one of the districts that has historically experienced landslides (Hidayat, 2020).
Data and information on landslide disaster events in Banjarnegara
Regency for example, from January to September 2016, there have been 12
landslide disaster events with 7 deaths, 7 injuries �and 1,237 displaced people (Dyah Susanti et al., 2017). In addition,
landslide conditioning factors that affect soil vulnerability include slope,
elevation, slope direction, land cover, soil type, fault distance, and
geological rocks (Nugroho & Nugroho, 2020). Based on this
condition, research is needed to analyze landslide-prone areas. This research
was conducted as an effort to provide information on the level of influence of
the class and conditioning factors of landslides, and a picture of areas that
have vulnerability to landslides and as an effort to provide information to
related parties, such as local governments, which have the authority to provide
planning advice in the construction of houses or residential areas.
One method that can be used to determine the vulnerability zone of ground
movement is the Frequency Ratio method.�
Frequency Ratio method is a method based on the assumption that, the
occurrence of soil movement in the future will occur in conditions similar to
the conditions of soil movement in the past (Yan et al., 2019).
Frequency ratio is the relationship between the occurrence of ground movement
and the parameters that control the occurrence of soil movement. The
results of this method will be obtained a map of the potential of land movement
which is the result of calculations from the parameter map. In addition,
through these results, a socialization and training were carried out to the
community to prevent land movement.
METHOD
Research was conducted for two weeks, starting from September 1-4, 2021 in
the Banjarnegara Regency area. This research was
conducted by geological mapping and remote sensing using DEM (Digital Elevation
Model) and landsat-8 as field support data and visualization of the condition
of the research area. DEM is a digital representation of the earth's
surface elevation (Hidayat et al., 2016). In this study, various data are
needed such as slope data from DEMNAS data, rainfall data from CHIRPS data in
2017 - 2021, fault fracture density data (FFD), land use processed data from
Landsat imagery with guided classification maximum likelihood method processed
from Landsat imagery, NDVI processed from Landsat imagery,� disaster point events from geological
mapping.�
The method used is frequency ratio (FR).�
The frequency ratio method is based on the assumption that the future
land movement events will occur under the similar condition to the past
landslide movement (Darminto et al., 2021; Silaban, 2021).� The parameters used in this study are slope, lithology, land use, vegetation index, rainfall,
and fault fracture density. The parameters are then processed using ArcGIS
into a thematic map, then calculated the number of ground movement points in
each class of each parameter to get a frequency ratio (FR) value (Sukristiyanti et al., 2020).
After the mapping results are obtained, it is necessary to prepare a
mitigation strategy. The mitigation is in accordance with mitigation theory
which divides mitigation into 2 forms, namely: Structural mitigation, in the form
of making infrastructure as a driver of impact minimization and the use of
technological approaches and non-structural itigation,
in the form of spatial management and training to increase community
capacity.�� The research flow is
presented in figure 1.
Figure 1. Research
Flow Diagram
�
RESULTS
AND DISCUSSION
A. Landslide
Vulnerability
Banjarnegara is one of the regencies in Central
Java Province with news of land movements frequently appearing. Moreover, there
were 149 landslides in Banjarnegara Regency in 2018 (Kuntjorowati, 2020). Therefore, the purpose of using the
method is to determine the potential distribution of ground motion. In this FRM
processing, several parameters are used. These parameters include lithology,
slope, lineament density, rainfall, land use, and vegetation cover.
Based on the
regional geological map, the plots located in Banjarnegara
Regency are composed of several types of lithology. The lithology is composed
of alluvium, agglomerates, reef limestones, tuffaceous claystone, sandstones, breccias,� polymeric
volcanic breccias, alternating sandstones with flysch, conglomerate, marl,
lava, ophiolite, m�lange, diorite intrusion, diorite/porphyry
intrusion/porphyry gabbro. Based on the lithology, they are grouped into
several types of rock and then assigned a value based on their influence on
soil movement by determining, the following division of class values: alluvium
is worth 1, sedimentary rock is worth 2, and volcanic rock is worth 3.
Figure 2. Litology
Map
Scoring from
the slope map (figure 3), the plot area consists of flat slopes with a slope
percentage of 0% � 8%, gentle slopes with a slope percentage of 8% � 15%, wavy
slopes with a slope percentage of 15% �30%, steep slopes with a slope
percentage of 30% � 45%, and slopes very steep with a slope percentage of more
than 45%. Based on the level of the slope, a value is given based on its effect
on landslides, following the division of class values: flat slopes are worth 1,
gentle slopes are worth 2, wavy slopes are worth 3, steep slopes are worth 4,
and very steep slopes are 5.
Figure 3. Slope Map
Based on the
rainfall map from CHIRPS data processing from January 2017 to divided into
three classes, 3100�3400 mm/year, 3400 � 3700 mm/year, and more than 3700
mm/year. Based on the rainfall, a value is given based on its effect on
landslides, the following is the division of class values: rainfall ranging
from 3100 � 3400 mm./year is worth 1, rainfall ranging from 3400 � 3700 mm/year
is worth 2, and rainfall is around 3700 mm/year is 2.
Figure 4. Rainfall Map
For the
lineament density map of the plot area, there are several levels of lineament
density. The alignment is divided into 5 classes or levels. The levels consist
of very low level (0 � 0.0004), low level (0.0004 � 0.0009), medium level
(0.0009 � 0.0013), high level (0.0013 � 0.0018) and very high level (0.0018 �
0.0022). For the distribution of the straightness/straightening itself, most of
the straightness/straightening is found in the southeast and northwest areas of
the plot area. Based on the straightness density level, a value is given based
on its effect on landslides, following the division of class values: a very low
level is worth 1, a low level is worth 2, the medium level is worth 3, a high
level is worth 4, and very high level is 5.
Figure 5. Lineament Density Map
Based on
Landsat 8 on September 3, 2019, land use classification was carried out using a
guided classification using the maximum likelihood method. Based on this
process, the results were obtained in the form of seven land covers. The land
cover includes forests, gardens, fields, housing, rice fields, shrubs, and
bodies of water. Based on the land use, a value is given based on its effect on
landslides, following the division of class values: water bodies are worth 1,
settlements are worth 2, forests & gardens are worth 3, bushes are worth 4,
and fields & rice fields are worth 5.
Figure 6. Land Use Map
Viewed from
the cover map In terms of vegetation, the plots are
divided into five classes based on the NDVI classification according to Wahyunto (2003) which consists of unvegetated land (-1 �
-0.03), very low vegetation (-0.03 � 0.15), low vegetation (0 .15 � 0.25),
medium vegetation (0.26 � 0.35), and high vegetation (0.36 � 1). Based on the
level of vegetation cover, a value is given based on its effect on landslides:
unvegetated land is worth 1, very low vegetation is worth 2, low vegetation is
worth 3, moderate vegetation is worth 4, and high vegetation is 5.
Figure 7. NDVI Map
Thus, from these parameters then FRM
processing is carried out and the results of the FR calculation are obtained as
follows at Table 1.
The frequency
ratio method is based on the relationship between the distribution of ground
motion points with each controlling factor of ground motion. From this data, it
is known that the RF value will be used in determining the class value of each
parameter. The RF value varies in each class in each parameter. Next, a raster
calculator is used to create a landslide susceptibility map, so that the area
of the vulnerability level and the map is as follows (Silalahi et al., 2019).
Figure 8. Vulnerability Area of Landslide
Based on the results of processing parameters with the
FRM method, the results obtained are maps of ground motion susceptibility.
Judging from the area, areas in Banjarnegara Regency are dominated by the
potential for low ground movement followed by the potential for moderate ground
movement.
The result of landslide map is presented in figure 9.
Figure 9. Landslide Potential
Zone Map
1. Areas with Very High Landslide Potential
Areas with very high ground movement potential are in
Karangkobar and Pandanarum sub-districts with a total area of
0.094589 km� or about 0.02%. This area has a lithology with
volcanic rock types, very low vegetation, and straightness.
2. Areas with High Landslide Potential
Areas with very high ground movement potential are
Karangkobar District, Wanayasa District, Pagentan District, Banjarmangu
District, Madukara District, Punggelan District, Sigaluh District, and
Pagedongan District, with a total area of 7.455772 km� or about
1,87%. This area has a lithology with volcanic rock types, rainfall with an
intensity of 3400 � 3700 mm/year and many alignments are found, and has wavy to
very steep slopes.
3. Areas with Medium Landslide Potential
Areas with moderate soil movement potential are in Bawang
District, Rakit District, Banjarnegara District, Purwanegara District, Wanadadi
District, Karangkobar District, Wanayasa District, Pagedongan District, part of
Sigaluh District, part of District, part of Kalibening District, and part of
Banjarmangu District, with a total area of 71.606354 km� or about 17.94%. The
area has a lithology with sedimentary rock types, rainfall with an intensity of
3400 � 3700 mm/year, low vegetation, and sloping to wavy slopes.
4. Areas with Low Landslide Potential
Areas with low ground movement potential are located in
Banjarmangu District, Punggelan District, Madukara District, Kalibening
District, Karangkobar District, Pandanarum District, Banjarnegara District, and
Rembang District, with the total area of 266.736396 km� or about 66, 83%. The area has
moderate rainfall, there is straightening but not as much as in areas with
moderate potential for soil movement has gentle to wavy slopes, and is widely
used as forest plantations.
5. Areas with Very Low Landslide Potential
Areas with very low ground movement potential are located
in a part of Punggelan District, part of Banjarmangu District, part of
Banjarnegara District, part of Pandanarum District, and Kalibening District.
little housing.
B. Landslide Disaster Mitigation in Banjarnegara
Regency
Disaster
prevention using satellite imagery can be used for landslide inventory or
knowing the characteristics of landslides, determination of landslide-prone
areas, mapping of factors associated with landslides (Zakaria et al., 2018). Factors of occurrence or
landslide parameters include slope, lithology, land use, vegetation index,
rainfall, and fault fracture density.�
The results of the risk assessment showed that Banjarnegara
Regency was at low to moderate risk of landslides. The results of the risk
mapping showed that in Banjarnegara Regency, 17.94%
were classified as areas with moderate landslide risk and 66.83% were at low
risk of landslides.� �������� As for thestrategy
of disaster mitigation of landslides, among others:
- Management of areas
with steep marbles and unstable soil
- Reduction of
population activity in areas with a high risk of disaster
- Determination of
evacuation routes and places
- Structural
mitigation by development is not at the site of high disaster risk
- Non-structural
mitigation by creating rules that can reduce the risk to the medium
disaster risk area
- Carrying out mixed
planting patterns such as agricultural crops and deep-rooted trees
- Provision of
relevant disaster-related information that can be accessed by all
stakeholders
- Preparation of early
warning information systems
- The participation of
disaster communities and communities
- The existence of
socialization of landslide disaster simulations to society
CONCLUSION
Banjarnegara regency is an area
that is vulnerable to landslides. Natural factors such as high slope slopes,
high rainfall, faults, geological conditions and land use are influential
factors in this region. The potential vulnerability of landslides in this
region is increasing with the infrastructure, namely buildings due to
population density in areas that are vulnerable to landslides. Based on the
results of mapping with the frequency ratio method, the percentage of low
insecurity is 66.83%, moderate insecurity is 17.94%, high insecurity is 1.87%,
and very high insecurity is 0.02%. Areas with very high land movement potential
are in karangkobar and pandanarum
districts with a total area of 0.094589 km�. The area has a very high potential
because it has litology with volcanic rock types,
very low vegetation and the presence of straightness. Therefore, various
mitigations are needed both structural and non-structural.
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MAPPING OF LANDSLIDE POTENTIAL ZONE WITH FREQUENCY RATIO METHOD AND MITIGATION EFFORTS IN BANJARNEGARA REGENCY, CENTRAL JAVA, INDONESIA