Landslide potential mapping in Pituruh District of Purworejo by using geographic information system

Located in Purworejo Regency, Pituruh is an area that prone to a landslide. Disaster mapping was a strategy that has been done so far by the government institutions and academia. According to its topography, Purworejo has been dominated by slope with more than 30 degree. The creation of landslide vulnerability map was carried out through several stages, namely, calculating rainfall at 6 stations located in Pituruh district, then producing average rainfall data to be used as a reference to create a rainfall map, then overlay process. Referring to the 4 parameters used, slope became an important parameter to determine the level of landslide vulnerability in Pituruh. The development of Geographic Information Systems can provide various types of spatial data related to the earth’s surface, and has the ability to perform accurate spatial analysis. Mitigation aims to reduce risks that have the potential to become disasters or reduce the effects of disasters. This study represents the level of vulnerability in Pituruh district by classifying the existing level. The results of this study have 5 classifications with different areas, namely highly vulnerable that covers 240.27 ha, moderately vulnerable for about 2237.81 ha, moderate with 3322.68 ha, vulnerable by 3040.14 ha, and not vulnerable area of 24.72 ha.


Introduction
Indonesia is a country that has the highest potential for natural disasters, especially hydrometeorological disasters which have become the most frequent disasters in the last ten years.Hydrometeorological disasters are disasters whose triggers come from meteorological parameters such as rain, wind or temperature.The most common types of hydrometeorological disasters in Indonesia are floods, droughts, landslides, forest/land fires, hurricanes, and tidal waves.
The coastal area of the Purworejo regency is likely to be affected by a major tsunami as it includes the southern coast of the island of Java, which faces seismic faults capable of generating earthquakes.large land in the east in the future [1] Natural disaster is a phenomenon that can be experienced at anytime, anywhere and anytime, causing risks to human life, both property losses or human casualties [2].Landslides are natural geological disasters that can cause casualties and huge material losses, for example siltation, disruption of traffic routes, damage to agricultural land, settlements, bridges, irrigation channels and other community infrastructure.One of the many disasters experienced during the rainy season is landslides.
Landslide based on (SNI 13-7124-2005), is "the process of moving various types of slope-forming materials that move down or out of the slope".Landslides can be found in areas that have hills and tropical climate.Landslide is a type of movement of soil or rock mass or also a mixture of both down the slope due to disruption of soil stability, landslides in addition to changing landforms, loss of fertile soil surface layers also cause material losses and casualties.Factors that influence landslides are divided into two: natural or internal factors, and external or artificial factors.Natural factors are due to the physical and meteorological conditions of the area, and artificial factors are due to human activities in terms of land use or management.
Susceptibility is the physical characteristics of disaster-prone areas.The term susceptibility itself is a pre-event phase [3].Geographic Information Systems (GIS) are computer-based tools for geographic data analysis and spatial visualization.It provides theme-oriented databases and information that support multiple representations and information.The ability to process, analyze and visualize spatial data, calculate distances between zones accurately is the function of Geographic Information Systems as a tool.The existence of Geographic Information Systems can provide various conveniences including the availability of data on various objects on the earth's surface quickly, and being able to conduct accurate analysis.By utilizing Geographic Information System applications, various efforts can be made to mitigate disasters so as to minimize negative impacts both on the environment and society.
Purworejo Regency topographically is an area of lowland areas and partly in the form of mountains with a slope of > 30º, the rest of the weathered passive loamy soil above the impermeable rock in the form of andesite and andesite breccia (Andesite, Breccia Formation, Dacite and From.Old Andesite/Form.Bemelan, Peniron Formation), with an unconsidered cropping system such as the siring era cropping pattern on slopes that have a high slope level, resulting in Pituruh Sub-district having the potential for landslides.Considering the disaster that occurred in Wonosido Village, Pituruh Sub-district on October 7, 2022 which resulted in several houses being victimized by the landslide that occurred, not only that the disaster also resulted in the main access road of Wonosido Village being buried by landslides.Landslide-related dams are a major threat in mountainous areas of the world, with the potential for sudden dam failures, which in turn could lead to downstream flooding [4].
Previous research conducted by Fakhryza et al, 2019 shows the need for detailed identification of disaster risk, an area that is threatened by disaster does not necessarily have the same level of disaster risk in each community.Mapping can be done by clustering or by identifying each development.Prone areas based on the level of landslide disaster.
This study aims to map potential landslide prone areas in Pituruh Sub-district, Purworejo Regency and find out which areas or villages are potentially prone to landslides.The benefit of this study is expected that with the map of potential landslide prone areas in Pituruh sub-district, quick action can be taken through a comprehensive processing approach to both affected and unaffected areas.

Study site
Pituruh sub-district is a sub-district in Purworejo district.Pituruh sub-district is in the west of Purworejo district which borders directly with 2 districts, namely Kebumen district and Wonosobo district.Pituruh District area of 71 km² which is divided into 49 villages, Pituruh District distance with the center of Purworejo about 24 kilometers through Kutoarjo.Pituruh sub-district has a highland that is quite risky of landslides.The administrative map below shows that the slope of Pituruh sub-district is quite high.The residential area also looks densely crawling.Using Science Mapping Tool for spatial data visualization [6]

Tools and materials
The tools and materials used in the research of landslide potential mapping in Pituruh sub-district are as follows: Table 1.Tools used in the research No. Name of Tool Functions 1.
Microsoft Excel For processing rainfall data 2.
Software ArcGIS To transform each parameter into a map Rainfall is one of the factors that cause landslides.High rainfall can also cause landslides because there is no ability to absorb the existing water flow.Some areas in Pituruh Subdistrict have high rainfall intensity, which is why the risk or vulnerability of landslides in Pituruh Subdistrict is quite high.The calculation of rainfall value starts from finding the average of 10 years of rainfall data in Pituruh Sub-district.The weighting classification of rainfall can be seen in Table 3.The table above is a parameter obtained from related agencies as a reference for the author in calculating the intensity of rainfall in Pituruh District.There are 6 rainfall stations in Pituruh District, each station contributes data to calculate rainfall in Pituruh District.The intensity of rainfall in Pituruh Subdistrict itself is quite high.

Slope inclination.
Slope is an appearance of the earth's surface that has different heights.Classification of slope class according to BAPEDDA (Local Government Agency) Semarang City but weighting refers to [5] can be seen in Table 4.The slope class above is used to determine the height of the slope in Pituruh sub-district.The classification is useful in the research to determine the level of landslide vulnerability in Pituruh subdistrict.The processed Dem data is then processed into a slope map to identify the slope.

Type of soil.
The soil type parameter was categorized into three categories: high, medium and low.This classification qualitatively refers to the type of soil (LPT, 1969).Soil erodibility is classified into three: high erodibility includes regosol, amdosol, medium erodibility such as andosol, grey humus, mediterranean, and pedsolik and low erodibility includes alluvial, latosol, and grumusol soil types.Soil type is also one of the factors causing landslides.The soil type in Pituruh Sub-district is included in the low score.The classification of soil types can be seen in table 5. Existing land use in Pituruh Subdistrict is used for settlements and some is used for rice fields.The settlements in Pituruh Subdistrict are not spread out, and the forest is also one of the largest land uses.The forest is located in the highlands of Pituruh Subdistrict, quite far from residential areas.

2.3.5.
Scoring.The method of scoring each parameter value to determine its capability level.According to the scoring method, the parameters used for landslide vulnerability potential are soil type, land use, rainfall and slope.The four parameters have different processing methods.Scoring here can be determined from previous research as a reference in order to determine the appropriate score and can be used for the next stage of data processing.According to Taufik Q, Firdaus and Deniyanto, "the factors of landslide occurrence are slope, rainfall, land use, and soil type".The weighting of each parameter used to develop the landslide vulnerability map is shown in Table 7. Furthermore, these parameters are used to determine the level of landslide vulnerability in Pituruh Sub-district from not prone, prone, moderately prone, moderate, and highly prone.

Results and Discussion
The results obtained in this research are 5 classifications of potential landslide vulnerability levels in Pituruh sub-district.Some villages have a high level of vulnerability or unsafe.There are 11 villages which from the level of vulnerability show Very Vulnerable description, those villages are Blekatuk, Grigindo, Kaligintung, Kaligondang, Kalijering, Kedungbatur, Pamriyan, Polowangi, Sawangan, Somogede, Wonosido.Based on the landslide vulnerability area, there are 5 classifications that show the level of vulnerability in Pituruh Sub-district, the category of highly vulnerable is 240.27 ha, moderately vulnerable with 237.81 ha, moderate 332.26 ha, prone 304.41 ha, and not vulnerable 24.72.Slope also affects the level of vulnerability in Pituruh Sub-district.The level of vulnerability will also increase with the presence of buildings or settlements and infrastructure built for human activities in the area.The extent of vulnerability can be seen in table 8. High rainfall in an area prone to landslide potential in the last 10 years of rainfall intensity in Pituruh Sub-district is included in the high intensity, there are 6 rainfall stations located in the Pituruh Subdistrict area from the 6 stations taken data in a period of 10 years from 2011 to 2020.The data can be seen in table 9 below.

Table 2 .
Data used

Table 5 .
[5]ghting classification of soil type parameters[5]The classification of land use in relation to landslide hazard is divided into 6 groups as shown in Table6.

Table 6 .
Weighting The overlay method is to analyze and integrate 2 or more different spatial data.This stage combines several maps that are parameters of landslide vulnerability by using Geographic Information System in ArcGIS 10.2 software.

Table 8 .
Area of vulnerability

Table 9 .
Total Rainfall Pituruh sub-district 6 rainfall stations Figure 3. Landslide vulnerability map of Pituruh sub-district