A. Scope Area
The study area location at a little part of Purworejo Regency in part of Kaligesing district; Kulon Progo
Regency covering Girimulyo, Pengasih, Nanggulan district, part of Kokap, Samigaluh, Kalibawang,Sentolo Wates, Panjatan, Galur, and Lendah district; Sleman Regency covering part of Moyudan and Minggir district, and Bantul Regency covering part of Sedayu district.
B. Material
1) Elevation Countour
Source: Digital Topographical Indonesia Map, Scale of 1:25,000 Bakosurtanal
Coordinate system: (UTM WGS 84 zone 49S)
2) Streams
Source: Digital Topographical Indonesia Map, Scale of 1:25,000 Bakosurtanal
Coordinate system: (UTM WGS 84 zone 49S)
3) Geological Map
Source : Geological Map of Yogyakarta Sheet, Scale of 1:100,000
Coordinate system: (UTM WGS 84 zone 49S)
4) Administration Boundary
Source: Digital Topographical Indonesia Map, Scale of 1:25,000 Bakosurtanal
Coordinate system: (UTM WGS 84 zone 49S)
5) Landuse Map
Source: Digital Topographical Indonesia Map, Scale of 1:25,000 Bakosurtanal
Coordinate system: (UTM WGS 84 zone 49S)
6) Soil Map
Source: Spatial Planning Laboratory, Faculty of Geography, GMU
Coordinate system: (UTM WGS 84 zone 49S)
7) Average Annual Rainfall Station Data
Source : PUSPICS, Faculty of Geography, GMU
Coordinate system: (UTM WGS 84 zone 49S)
C. Methods
a. Landslide Hazard Determination
Determination of landslide hazard considers several parameters that have influence to
landslide hazard occurence. There are 6 landslide hazard determination parameters:
1. Slope
Slope is the level steepness represented in morphology. In general, the greater level of slopes, the further increase the likelihood of landslides in an area. It is also associated with the force of gravity pulling masses of rock from top to bottom. Higher level of slope will be more easily attracted to the bottom resulting in the occurrence of landslides. The steeper of slope in the area, the more potential of landslides occurence. Slope is the most influential factor to the occurrence of landslides, so it is given a weight value of 30%.
2. Type of Soil
Land is the result of rock weathering, with large sand or clay content, have weak
resistance which has low value of cohesion (c) and inside angle of shear (����. In the rainy season, the soil resistance decrease with increasing water content in the soil. The decrease is due to the soil volume expansion due to water content. Qualitatively, the resilience of the soil can be analyzed through the variable soil texture. Qualitative value of the land as a factor contributing to the landslide occurence that is measured based on the texture.
3. Landuse
Landuse can increase the burden that is borned by the lithology. If the burden borned is greater than the strength of lithology, there will occur landslide. Vegetation also affects the level of slope stability. Some vegetation can increase the stability of the slope because the roots can bind the rock mass making it more compact. But some kind of vegetations that have weak roots can actually reduce the level of slope stability which may impact on the landslides occurrence. Too tightly crops planting can result in heavy on the slopes so increasing the driving force of land on the slopes.
4. Rainfall Intensity
Rainfall is one of the determinants of the level of potential landslide hazard in the study area. If value of rainfall is high, it can be ascertained that the region is a region which has the highest potential for landslides. Basically, to find out the potential danger of landslides used daily rainfall data, but data is hard to find then it is used the average annual rainfall calculated based on its intensity. Because the average annual rainfall effect is not too significant to the occurrence of landslides, it is given a value of 10%.
5. Geology
Geology related to type of rock. The weather rock material is not massive and not stable, so with little influence of the movement will cause shift in position or landslides occurence.The more massive the rock without any cracks or fractures, the more resistant to weathering processes. It means that the rate of weathering of rocks getting smaller so the landslide susceptibility levels also getting smaller. Thus, the physical condition of rocks (the massive and the cracks or fractures presence) is one of the factors causing the occurrence of landslides.
6. Geological Structure (Fault)
Geological structure is a weak zone on a rock or lithology, especially fault zone. Fault that is occurred reduce holding capacity of rocks, thereby reducing the level of resistance of rock. In addition, fault form also became the place of entry of water so weathering and erosion run with more intensive. Rocks that have sufficiently intense structures have greater potential for the occurrence of landslides. The more getting close to the fault zone, the greater of this influence.
Landslide Hazard Map Acquisition àWeighted Overlay Operation Weighted Overlay is a technique for applying a common measurement scale of values to diverse and dissimilar inputs to create an integrated analysis. Geographic problems often
require the analysis of many different factors. In this case, determining the landslide hazard areas means assessing such things as slope, soil type, landuse, rainfall, and geological condition. Additionally, the factors in analysis may not be equally important. It may be that the slope is more important in choosing a site than geological structure. How much more important to decide. Weighted Overlay only accepts integer rasters as input, such as a raster of land use or soil types. Continuous (floating point) rasters must be reclassified as integer before they can be used. Generally, the values of continuous rasters are grouped into ranges, such as for slope, or Euclidean distance outputs. Each range must be assigned a single value before it can be used in the Weighted Overlay tool. The Reclassify tool allows for such rasters to be reclassified. You can either leave the value assigned to each range (but note the range of values to which the new value corresponds) and assign weights to the cell values in the Weighted Overlay dialog box later, or you can assign weights at the time of reclassifying. With the correct evaluation scale chosen, simply add the raster to the Weighted Overlay dialog box.The cells in the raster will already be set according to suitability or preference, risk, or some similarly unifying scale. The output rasters can be weighted by importance and added to produce an output raster. The steps for running weighted overlay are select an evaluation scale, add rasters, set scale values, assign weights to input rasters, assign restricted rule(optional), and run the Weighted Overlay tool. If the tool was used for suitability modeling (to locate suitable areas), higher values generally indicate that a location is more suitable. If the tool was used to generate a cost surface (to find out how much it will cost to travel through the landscape for instance), high values will generally indicate higher travel costs. You must understand the scale values you apply to input rasters so you know what the values in the output raster mean. In the illustration, the two input rasters have been reclassified to a common measurement scale of 1 to 3. Each raster is assigned a percentage influence. The cell values are multiplied by their percentage influence, and the results are added together to create the output raster. For example, consider the top left cell. The values for the two inputs become (2 * 0.75) = 1.5 and (3 * 0.25) = 0.75. The sum of 1.5 and 0.75 is 2.25. Because the output raster from Weighted Overlay is integer, the final value is rounded to 2.
b. Landslide Vulnerability Determination
Determination of landslide vulnerability considers parameter that has potential vulnerability to landslide hazard. The potential vulnerability is determined based on possible locations generate victims when landslide occured. The locations like that are lands that are used as settlement, because settlement areas represent that there are human populations in the area. The human populations can become to be victims if landslide occured. So all of settlement landuse are considered to be the vulnerable area, while all of landuse except settlement are considered to be the no vulnerable area. To operate this mapping using data query operation.
c. Landslide Risk Determination
To produce landslide risk map considers landslide hazard map and landslide vulnerability map. The area has landslide risk when the area is a vulnerable area to landslide hazard and located in landslide hazard area. But if the area is a no vulnerable area to landslide hazard, although located in landslide hazard area, so the area has not landslide risk. And if the area located in no hazard area, either vulnerable area or no vulnerable area to landslide hazard, so the area also has not landslide risk. Risk area has the levels of risk based on the levels of hazard. If the area is a vulnerable area and including in high landslide hazard area, so the
area also including high landslide risk area. To operate this mapping using vector overlaying operation.
E. Conclusions
1. Determination of landslide hazard areas is done by considering the parameters that
influence the landslide occurrence, each of those parameters has a weight factor that
describes the level of influence on the the landslide occurrence.
2. Hazard area mapping using weighted overlay analysis due to the weighted factors and
restricted rules consideration.
3. Result of landslide hazard area mapping shows that the potentially landslide hazard in
study area have percentage of 47.12% of total area.
4. The most extensive high level of landslide hazard is located in the Girimulyo district,
Kulonprogo (with percentage 36.66% of the entire area has high hazard).
5. To determine the risk map, it is needed information about the vulnerability map, the
vulnerability map is a map of the location of settlements which represent exciting
conditions of vulnerable populations to disasters occured.
6. Based on the hazard map and vulnerability map, risk map can be compiled using vector
overlay analysis due to it will be analyzed geometric size, related area measurement.
7. Result of landslide risk area mapping shows that the potentially landslide risk in study area
have percentage of 39.84% of total area.
8. The most extensive high level of landslide risk is also located in the Girimulyo district,
Kulonprogo (with percentage 52.46% of the entire area has high risk).
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