Landslide-prone areas in Makale Selatan with the analytical hierarchy process method

Landslides are one of the most common natural disasters in Indonesia. The Tana Toraja area has not been spared landslides, especially in the Makale Selatan district. Makale Selatan is one of the supply areas for agricultural products in South Sulawesi. Therefore it is necessary to map landslide-prone areas to support landslide disaster mitigation. This study aims to map landslide-prone regions and determine the factors most influencing landslide events in Makale Selatan. This study uses an Analytical Hierarchy Process (AHP) method with a weighting factor. The parameters, namely; slope, rainfall, land use, soil texture, soil type, and rock formations. The results showed that the incidence of landslides in Makale Selatan was strongly influenced by the slope, with the highest weighting factor with a weight of 0.410 (Score of 41%). Rainfall and land use with the same weight of 0.170 (Score of 17%), then the parameters of soil type and soil texture also with the exact weight is 0.096 (score of 9.6%), and the rock formation parameters with a weight of 0.058 (score 5.8%). The level of landslide-prone areas in the study area namely; 12.46% for the not prone area, 51.72% for slightly prone, prone areas 25.72%, and very prone was 10.11% for the total study area. The not-prone village is Tiromanda, and the most prone village is Lembang Randan Batu. The slope parameter is the parameter that most influences landslides in Makale Selatan. Land use for agriculture must be mechanically conserved to reduce the slope angle and prevent landslides.


Introduction
Based on the location of the disaster, BNPB [1] informed that South Sulawesi is the fourth province with the highest number of disaster events after Central Java, East Java, and West Java.Of the total number of disaster events in South Sulawesi, according to BNPB data [2], most disaster events occurred in the Tana Toraja regency.According to information from the Tana Toraja Regional Disaster Management Agency (BPBD), Makale Selatan District is a vulnerable area in Tana Toraja.The Makale Selatan region, with mountainous and hilly topography, has increased the susceptibility to landslides, especially for areas used as agricultural land, plantations, and settlements [3].The disasters that often occurred throughout 2018 in Tana Toraja were 19 landslides, 17 tornadoes, and one flood.
The high level of losses experienced by the community is because landslides are challenging to detect and predict when and how long they will occur [4].Hence, efforts to avoid these disasters experience difficulties in carrying out, in addition to the lack of information obtained by the community about the possibility of disasters occurring around them, and disaster management has not yet emerged 1230 (2023) 012083 IOP Publishing doi:10.1088/1755-1315/1230/1/012083 2 as an essential alternative to achieve this resilience and reduce the impact caused by natural disasters [5].The initial information regarding disaster potential and risks are one media that can be used as primary disaster response education for the community.
Mitigation in landslide disaster management consists of several elements, from compiling a database of potential areas to making disaster zone maps [6][7][8].One of the problems in natural disaster mitigation is the lack of disaster information for decision-makers as a basis for policy-making [9].Therefore, providing maps of disaster-prone areas is one of the things that need to be implemented to provide information to the community and policymakers.This study aims to map landslide-prone areas and identify the factors that most influence the occurrence of landslides in the Makale Selatan District.

Material and methods
The materials used are a digital topographical map of Indonesia (2010) with a scale of 1:50,000, a geological map with a scale of 1:250,000 [10], and annual rainfall data from CHIRPS (Climate Hazards Group InfraRed Precipitation with Station).Digital elevation model (DEM) ALOS Palsar with a resolution of 12.4 m, soil type map 1: 250,000, and SPOT mosaic satellite imagery (2018) with 1.5 meter resolution to update the Makale Selatan land use map.Soil and rock samples were taken representing each village in Makale Selatan (figure 1).Soil samples were taken at 0-50cm depth using a soil drill.Soil texture analysis using the hydrometer method [11].AHP analysis with Expert Choice V11 with ten expert judgments in the landslides field.ArcGIS 10.5 software for spatial analysis.Soil texture maps were made using the Thiessen polygon interpolation method, and rainfall maps using the IDW (Inverse Distance Weighted) interpolation method.The distribution of landslide susceptibility is analyzed by superimposing all parameters that have been given weights and scores.Classifying the level of landslide susceptibility was carried out using the natural breaks-GIS method.

3
Validation is carried out by overlaying the landslide-prone map with landslide occurrence points in the field.

Results
Landslide causal parameters that trigger landslide events in Makale Selatan are influenced by general factors: slope, rainfall, rock, and land use.At the same time, the internal factors used are the type and texture of the soil.

Rainfall
Rainfall in South Makale ranges from <2300->2450 mm/year.Rainfall <2350 mm/year covered 1616.81 ha (22.04%) and was located in the south of the study area.The 2350-2400 mm/year rainfall had 2779.32 ha (38.17%) and was found in the middle of the study area.Class of 2400-2450 mm/year with an area of 1907.07 ha (26.86%).Rainfall >2450 mm/year had located in the north with an area of 948.29 ha (12.93%).The distribution of rainfall area is presented in figure 2.

AHP Analysis
The slope parameter has a high weight with a final score of 23.58 in slope >45%.The second was influenced by rainfall and land use, with a weight of 0.17.Inceptisols had a score of 5.40 which could trigger a landslide event.The results of AHP weighting of the parameters that influence landslide occurrence in Makale Selatan are presented in table 1.

Landslide-Prone Area Mapping
Based on the results of spatial analysis, namely the effects of overlaying the base maps (slope maps, rainfall maps, land use maps, soil texture maps, rock formation maps, and soil type maps), previously, the base maps have been given a final score, namely the result of multiplying the scores sub-parameters with parameter weights (table 1).The landslide-prone map is then validated with landslide occurrence points in the field so that a map of the distribution of landslide points is obtained on the landslide-prone map, as presented in figure 5.The number of landslide points in each hazard class is shown in table 2.  The prone area has a slightly higher rate of landslide events than the very prone area.This shows that the parameters for assessing landslide events still require several supporting parameters to obtain more accurate data.But the data showed a better validity level, where the incidents and susceptibility levels are close to the real thing in the field.
Simulation for each village showed that Tiromanda Village is a safer area against landslides than Lembang Randan Batu and Sandabalik Village, which is very prone to landslides.

Discussion
The slope at the study site strongly influences the occurrence of landslides in Makale Selatan.The slope reaches a weight of 0.410 (score 41%), the highest compared to the other parameters.[12] explain that slope is one of the factors causing landslides in hilly/mountainous areas.A landslide will occur if the gravitational component is so large that the resistance to soil friction in the sliding plane is exceeded [13].Thus the steeper the slope, the greater the chance of landslides occurring [14,15].Makale Selatan is dominated by areas that are slightly prone to landslides.If we look at the distribution in each village (table 3), it seems that a low level of susceptibility is spread across all villages and dominates in Tiromanda.Thus, the safest village is Tiromanda, and the very prone village is Lembang Randan Batu.Based on current land use, the distribution of susceptibility to landslides in settlement areas is 26.842% spread in landslide-prone areas, and 20.028% are in areas very prone to landslides.It shows that most people carry out development without considering the risk of landslides or the lack of public knowledge about places likely to be prone to landslides [16].
Based on the validation results (table 2), 11 landslide points, there were no landslide points in notprone areas.Meanwhile, 2 points were in slightly prone areas, 5 points were in landslide-prone areas, and 4 points were in very prone areas.With landslide points in areas considered less susceptible, we can conclude that even though they are in areas considered less susceptible, we still have to watch out for them.Decision-making with several parameters is only suitable under certain conditions, and not all are under the location, so it is necessary to detail the data for the results' accuracy [17].The existence of landslides in areas considered less susceptible is usually influenced by human activities such as settlements and the construction of roads that cut the slopes without making embankments on the slopes that have been cut [18].The construction of roads and settlements makes the topsoil unable to withstand the weight above it [19].As a result, the soil mass moves and causes landslides.

Conclusion
The slope is the parameter that most influence landslides in Makale Selatan, with a weight of 0.410 (score 41%).Rainfall and land use parameters with the same weight of 0.170 (score 17%), then the parameters of soil type and soil texture also with the same weight of 0.096 (score 9.6%), and finally the rock formation parameters with a weight of 0.058 (score 5.8%).Landslide-prone areas in Makale Selatan District are divided into four classes: not-prone of 12.46%, slightly prone of 51.72%, prone of 25 .72%,and very prone covering an area of 10.11% of the total area of Makale Selatan District, which is 7334.49Ha.Based on the distribution of susceptibility levels in each village, Tiromanda Village is the area that is considered the safest from landslides.At the same time, Lembang Randan Batu is the area most prone to landslides.Land use for agriculture in Makale Selatan must be mechanically conserved to reduce the slope angle and prevent landslides.

Figure 1 .
Figure 1.Map of soil and rock sampling locations in Makale Selatan District

Figure 2 .
Figure 2. A) Slope map with the dominant class of 25-40% and B) Rainfall map of Makale Selatan District

Figure 3 .
Figure 3. A) The Geological map with two rock formations, and B) six land use types in Makale Selatan District

Figure 4 .
Figure 4. A) Soil types map with two order, and B) soil texture with five classes in Makale Selatan District

Figure 5 .
Figure 5. Landslide-prone area map with four classes of susceptibility

Table 1 .
Weights and scores of parameters and sub-parameters analysis results with AHP

Table 2 .
Number of landslide points for each class of landslide susceptibility