Drone application for landslide hazard assessment on the Western Flank of Lawu Volcano, Central Java, Indonesia

Remote sensing technology advancements have opened novel possibilities for precise and effective landslide investigation. Because both regions have rocky topography, this study will employ unmanned aerial vehicles (UAVs) or drones to examine landslide activity on the west side of Mount Lawu, namely in Nglelaka Hamlet, Segorogunung Village, Ngargoyoso District and Guyon Hamlet, Tengklik Village, Tawangmangu District. Landslides are caused by the rocks and their geological conditions. As a result, extensive monitoring is essential to ensure that mitigation activities are carried out exactly. The spatial and descriptive analysis strategy was employed in this study. Data collecting methods used included landslide censuses and field surveys. A Geographic Information System was then used to process the data. According to the research findings, the landslide in Nglelaka Hamlet was of the translational kind, but the landslide in Guyon Hamlet was of the termite type. The findings of this study helped us better understand the dynamics of landslides in diverse locations, particularly in difficult-to-reach areas, and demonstrate how beneficial UAVs can be in gathering data for disaster management and community resilience. Drones for landslide studies offer valuable information to local governments and stakeholders, allowing them to develop preventative measures and emergency response plans, lowering the impact of landslides on residents and infrastructure.


Introduction
Disaster is defined as a significant disruption in the functioning of a community or society on any scale caused by hazardous occurrences interacting with conditions of exposure, susceptibility, and capability, resulting in one or more of the following: Human, material, economic, and environmental losses and consequences [1] [2].Natural or man-made disasters can both occur.A landslide is one sort of natural catastrophe.There are several definitions of landslides.In general, a landslide is the downslope movement of soil, rock, and biological material that occurs on the surface of a rupture, which can be curved (rotational slide) or flat (translational slide), and in which most of the material frequently flows as a coherent or semi-coherent mass under the influence of gravity [3] [4].
Landslides, also known as mass movement of rocks, soil, or a mix of these, are a natural occurrence that frequently happens on natural or non-natural slopes.This is because the structure of the earth's 1314 (2024) 012104 IOP Publishing doi:10.1088/1755-1315/1314/1/012104 2 surface seeks a new balance when disturbances or other factors cause an imbalance between the two forces that grip the slope, called the retaining force and the shear force.These two forces are affected by water content, the weight of the soil mass, the physical properties of the soil, and the slope itself [5] [6].
Landslides are common in Indonesia.According to statistics from the 2022 Landslide Disaster Risk Index in Indonesia, there are 514 districts/cities with the potential for landslides, with 244 districts/cities classified as high risk.When compared to other natural disasters, landslides have the most widespread dispersion [7].The majority of landslides in Indonesia hit as a result of heavy rain or prolonged rain.Karanganyar Regency in Central Java is one of them [8].Landslides are more likely in Karanganyar Regency regions with steep topography with slopes of more over 30%, high rainfall intensity, and agricultural land usage [6].
Landslides are one of the most destructive natural catastrophes, inflicting massive damage and loss of life.Landslides of varying severity occur often in Karanganyar Regency, however not all incidents result in casualties [6].One of them is the landslide that happened at Segorogunung Village on February 15, 2023.There were no casualties, but the route between Nglelaka and Segorogunung villages was cut off.The disaster's losses are estimated to be over 50 million rupiah [9].In the same month, a landslide occurred at Tengklik Village, causing damage to the soil and settlements.
The urgency of study in the Segorogunung and Tengklik Village areas is that these areas have a high level of vulnerability to landslides.In the previous year, at 2014, in Ngargoyoso District there was a landslide which resulted in cutting off access to rural roads connecting between hamlets.At 2007 in Tengklik Village there was a landslide that caused 33 houses to collapse.Not only because there was prone to landslides, the importance of this study in these two areas compared to other areas is there are protected forests with large area.In Segorogung Village the protected forest area is 817.80 Ha, and in Tengklik Village the protected forest area is 697.54Ha.The higher area of protected forest, cause the higher vulnerability to landslides [23].We know that protected forests have some function of protecting life support systems to regulate water management, prevent flooding, control erosion, prevent seawater intrusion, and maintain soil fertility [24].That is why this study is important to do.
However, hazard mitigation for landslides in Karanganyar Regency has been less effective.This is due to a variety of factors, including a partial, sectoral, and less integrated disaster management paradigm, as well as a continued concentration on regional government activities, which are confined to providing physical aid and are only carried out during the emergency period [6].As a result, precise and timely monitoring of landslide movements is required to research and anticipate the potential of landslides, as well as to develop appropriate mitigation measures to decrease their damage [10].This is no exception in areas that are inaccessible because the environment is steep, dangerous and remote [11].Because of its potential to deliver useful and insightful data sets encompassing geographical, temporal, and spectral perspectives, remote sensing technology is increasingly being used in a variety of industries, including landslide mapping [12].The Unmanned Aerial Vehicle (UAV) or drone is a type of remote sensing technology that may be used to map landslide spots quickly and easily [5].
The use of unmanned aerial vehicles (UAVs) for landslide research has several advantages over traditional monitoring approaches, including the ability to capture high-resolution images, access difficult-to-reach regions, and collect data more often and at a cheaper cost [13] [14].Based on the advantages of UAVs, this research aims to analyze landslide activity in Nglelaka Hamlet, Segorogunung Village, Ngargoyoso District on February 15 2023 and in Guyon Hamlet, Tengklik Village, Tawangmangu District on February 2023, using UAVs, because morphologically these two areas is in a hilly area with a steep slope [6] [15], so using UAVs to monitor landslide activity in these two locations is the best option.

Methods
Methods of spatial and descriptive analysis were applied in this study.The data was collected using a landslide census approach [16] to detect landslide incidents in the districts of Ngargoyoso and Tawangmangu, as well as to conduct field surveys on slope morphology and other morphodynamic data.Aerial photo data from drones paired with field survey data can be utilized as a source of spatial information following landslides and for spatial analytic procedures [17].Data visualization was performed in the form of a 2D map utilizing a Geographic Information System to provide a full view of landslide activity at the research site, as shown in Figure 1.

Landslide activity in Nglelaka Hamlet, Segorogunung Village, Ngargoyoso District
The landslide that happened on February 15, 2023, at Nglelaka Hamlet, Segorogunung Village, Ngargoyoso District shut down road access in Nglelaka and Segorogunung Hamlets (Figure 2).Because the slope reinforcement structure on the road was not adequate, the landslide happened precisely on the route linking Nglelaka and Segorogunung hamlets.

Figure 2. Map of Landslide locations in Segorogunung Village
Morphologically, areas of the Ngargoyoso District feature difficult topography with high slopes ranging from 10% to 75%.Because this region is on the slopes of Mount Lawu, the contour becomes steeper as you move east.Segorogunung Village is one of the village sections in Ngargoyoso District that is located in the eastern portion, hence Segorogunung Village is prone to landslides [18].According to the findings of a field survey done by Hardjono and Cholil in 2017, the distribution of landslide points in Ngargoyoso District happened mostly in Segorogunung Village (10 points) [6].
The landslide occurred at an elevation of 1,025 -1,125 meters above sea level in Nglelaka Hamlet, Segorogunung Village.The high slope of the slope allows the earth to move readily, resulting in landslides throughout the region.Aside from that, the lack of slope reinforcement structures on road slopes, as well as the high intensity of rainfall over a long period of time, makes landslides more likely in the area.In other words, the heavier the rainfall in a location, the greater the risk of landslides [6] [18].
The Lawu volcanic rocks of Ngargoyoso District are formed of tuff and volcanic breccia interbedded with andesite lava.Because the geological circumstances of the study site are adjacent to active faults or faults, the geological structure is still developing.In addition, the geology of the research site is made up of pyroclastic flows in the form of lava pieces, sand, and tuff.The pyroclastic appearance is characterized by sand to block-sized andesite lava that is flat and sheety and easily separated by a tuffaceous sand matrix.
Because of the presence of a tuffaceous sand matrix, the rocks are uneven and relatively loose.The presence of a weak region in the form of contact between worn soil and more impermeable lava serves as a slip area.It is thin clayey sand with a thickness of 1 meter, dark brown in color, loose and easily separated under worn soil conditions.
Banana plantations dominate land usage on the upper slopes.The central section of the hill is utilized for the road that connects Nglelaka and Segorogunung hamlets, as well as mixed gardens, particularly vegetable gardens.The lower slopes are used for mixed gardening, with bamboo and vegetables dominating.A tributary feed into Kali Jumog from the valley.Surface water runs freely throughout the region, and some of it seeps into the earth.Water is abundant due to the existence of springs in the south, which are channeled by paralon pipes.A poor drainage system allows huge volumes of surface water to soak into the soil through soil pores.
The changes in cross sections before and after the landslide are depicted in Figures 3 and 4. According to this photograph, there have been no substantial changes to the ground surface.This landslide is a sort of translation that happens in fault zones and involves the movement of soil masses outward, or downhill and outward over a steeply sloped surface [3][4] and the kind of rock is weathered lava material that travels as a coherent or semi-coherent mass [5].Aside from slope and rock characteristics, landslides at the research site are impacted by significant rainfall and the buildup of surface water running in impermeable channels, preventing the water from moving swiftly.This renders the slope material water transparent and reduces its shear strength.Similarly, fractures serve as a conduit for surface water (particularly precipitation) to enter cracks and cause ground movement.This situation increases water pressure and decreases the soil's binding capability.

Landslide Activity in Guyon Hamlet, Tengklik Village, Tawangmangu District
On February 17, 2023, a landslide occurred in Guyon Hamlet, Tengklik Village, Tawangmangu District, causing damage to dwellings and agricultural land (Figure 5).Landslides had already occurred in this area, most recently in 2007, due to heavy rainfall (245 millimeters/day).

Figure 5. Map of Landslide Locations in Tengklik Village
According to Figure 5, monitoring data from the Early Warning System, there was a rise in extensometer data accompanied with recorded rainfall data in February 2023, which happened relatively regularly for two days between 13-15 February 2023.The catastrophe location is in a zone with a high potential for landslides, which means that landslides can occur if rainfall exceeds normal levels, and past landslides can reactivate.

Figure 6. Landslide Early Warning System in Tengklik Village
Tawangmangu District has a morphology of wavy hills with medium to harsh relief.The top slope has a slope of more than 51°.The landslides occurred on steep cliffs with slopes more than 13 degrees and height differences of 30-50 meters, at an elevation of 1,100-1,150 meters above sea level (meters above sea level).
Tawangmangu District's geological condition is Lawu volcanic rock, which is made of tuff and volcanic breccia containing andesite lava.Weathered volcanic rock in the form of pyroclastic flows dominated by sand-sized debris and a few lava pieces, dark brown in color and loose.Weak places include contact between worn soil and volcanic breccia rock, which is impermeable and serves as a sliding area, as well as high to extremely steep slopes, which cause the soil to move readily.
Pine trees and bamboo are the primary land uses.Gardens, mixed gardens, and village roads are located on the middle hill.Meanwhile, the lower slopes are mostly covered with mixed gardens.A branch of the Blumbang River passes through the valley.When it rains, the significant surface flow might be generated by the development of new residential property, which results in poor permeability values, generating landslides [19].
The hydrological system is made up of free-flowing surface water, some of which seeps into the earth.Water is plentiful, with a spring 200 meters distant in the northern half that is carried by paralon pipes utilized by people to suit their requirements.High rainfall and the build-up of surface water flowing in loose channels cause the water to be unable to move fast, saturating the slope material and lowering its shear strength.High rainfall intensity will elevate ground water levels, causing negative pore water pressure values to change to positive pore water pressure values, impacting slope stability [20].
Cracks were still discovered on highways and agricultural area around the research site.These cracks acted as a conduit for surface water (particularly rain) to enter and cause earth movement.This situation increases pore water pressure while decreasing soil binding capacity.Because of the steep slopes and the nature of the soil, which easily crumbles, the earth becomes fast wet and unstable, making landslides more likely to occur.
At the research site, the type of landslide is creep, which is a slow earth flow composed of very slow and steady downward movement of soil or rock forming a slope [3].According to Nuryanto's research, Guyon Hamlet, Tengklik Village is one of the villages in Tawangmangu District that has the potential to encounter creep type landslides, which cause the ground to continue to shift, especially during the rainy season [14].
Creeps create longitudinal cracks in roads and buildings.The termites migrate in a southerly direction, with a crack width of 5-20 cm and a length of 85 meters.Previously, a landslide in early 2007 caused the soil to drop by 20 cm, reaching 260 cm in August 2009 [14], As a result, houses were damaged and roads are cracked.The growth of ground movements at Tengklik Village in February 2023 increased the number of buildings impacted, with 4 houses slightly damaged, 1 mosque significantly damaged, and the connecting road between villages damaged along 85 meters.
The transverse profiles before and after the landslide are depicted in Figures 7 and 8.These two photos demonstrated that the ground surface had changed.The surface changed after a landslide occurred, such as lowering and raising the strain on the ground.The valley's mound of earth had not been totally compacted, unlike the original soil below.As a result, if it rains, the ground surface will settle, resulting in fractures in the earth.Landslides have historically occurred during and after the deposit of volcanic material on particularly steep slopes, or during and after faults in the earth's crust.Landslide occurrences are known to be impacted by various elements, including slope slope, geology, geomorphology, rainfall, and land use, based on the regional circumstances stated at the two research locations [6].The influencing factors in both research locations have similar conditions, especially those related to steep slopes and high rainfall [10].According to studies done by Afif et al. in Kuningan Regency, West Java, landslides can occur in regions with steep slopes and high rainfall intensity.Afif et al. also used UAVs to monitor landslides [5], which is done by recording images of the surface of the research location using a camera installed on the UAV [21].Based on the findings of the recordings, it is clear that using UAV technology in landslide mapping has various advantages, including time and cost efficiency, especially when mapping landslide zones that require rapid response [22] and detecting landslide-affected regions, particularly in difficult-to-reach places [5].However, unlike satellite sensors, which offer a variety of sensor types and spectral resolutions ranging from medium (multi-spectral) to extremely high (hyperspectral), sensors aboard UAVs are still quite restricted and under development [22].

Conclusions
This study used UAVs or drones to investigate landslide activity in two areas: Nglelaka Hamlet, Segorogunung Village, Ngargoyoso District, and Guyon Hamlet, Tengklik Village, Tawangmangu District.The usage of UAVs was done with the goal of fast mapping landslide zones and locations that are difficult to reach owing to high slopes.Landslide incidents in both locations were induced by a variety of causes, the most relevant of which were slope grade and rainfall intensity.However, distinct geological circumstances caused different types of landslides in the two areas, with translational landslides occurring in Nglelaka Hamlet and creep landslides occurring in Guyon Hamlet and Tengklik Village.
.1088/1755-1315/1314/1/012104 3 Landslide census data was collected utilizing a UAV or DJI Go 4 drone to capture information such as the area and kind of landslide.