CHAPTER 4 RESULTS AND DISCUSSION
4.1 Weighting System and Construction of Thematic Maps
4.1.2 Weight Values and Thematic Map of Lithology
proportion of landslide areas, and 2) LSI considers the size of landsides while LFI disregards it.
The assignment of weight values for each class of lands use land cover is described as follows. The land use type that has the highest value of LSI is assigned the maximum weight value of 100. The other land use types are assigned weight values proportional to their LSI values divided by the largest LSI of a class within land use land cover category. For example, open land has the highest LSI value of 32.57 so it is assigned the maximum rating weight value of 100. Crop land is assigned a rating weight value by means of dividing its LSI value, 3.69, by 32.57, the highest LSI value. Hence, the rating weight value of crop land is 3.69/32.57*100=11.
The rating weight values for the remaining classes were derived using the same manner.
Thematic map of land use land cover was constructed based on weight values derived from LSI. Fig. 4.3 shows thematic map of land use land cover that has a similar appearance as the original land use land cover map except that the map replaces the attribute of land use land cover types with LSI values. The LSI values for open land-cut slope, urban-built up, cropland-bushes, forest and river-lake are 100, 60, 11, 6, and 0 respectively. Areas with the highest weight values are mainly located along roadsides. Few of them are located in the middle of forest and the surrounding of urban/built up areas.
Fig. 4.2 Landslide Susceptibility and Landslide Frequency Indices of LULC
Fig. 4.3 Thematic map of land use
3.69
1.87
19.70
32.57
0.00 4.10
2.08
21.89
36.19
0.00 0.00
10.00 20.00 30.00 40.00
Crop land, bushes, thin vegetated area
Forest Urban, Built up Open land, Cut slope
River & Lake
LSI / LFI
Landslide Susceptibility Index Landslide Frequency Index
Weights 11
6 100
0 60
The LSI and LFI values indicate the same trend as shown in Fig. 4.4 (a). Fig. 4.4 (b) shows a different trend between the number of landslides and LSI of lithology type. Granite-covered areas had the lowest LSI value, 2.78, even though this area had the highest number of landslide occurrences, 231. The wide range of granite coverage was responsible in determining this small value of LSI. Almost similar condition went to sedimentary rock type. Schist-covered area was found to be the most prone slopes to fail referring to its highest LSI value, 18.29. As mentioned previously, this is the consequence of applying bivariate statistical approach to produce an objective measure of landslide occurrence relative to different types of lithology. From high to low, LSI values of 18.29, 7.0, 5.67, and 2.78 belongs to schist, alluvium, others/undifferentiated, and granite types respectively.
The existence of a large number of landslides on granite-covered areas has been a questionable phenomenon. According to Mohr‘s scale of hardness, the hardness of granite is quite high, about 7–8. LHEF of Anbalagan [2] also put granite rock type as having a small rating value meaning that granite rock type was quite hard. Hence, slopes covered with this rock type are least likely to fail. It seemed to be a contradiction having these facts. However, previous investigations along Pos Selim- Cameron Highlands highway, part of the study area, carried out by Jamaluddin [219]
showed that the discontinuities existed in the bedding planes of rock mass, both granite and metamorphic bedrock. The area experienced weathering both chemical and physical weathering. There were two weathering profiles of granite that tended to cause rock falls and landslides. The thickness of overburden soil was about 5-20m.
Overlay of maps of lithology, landslide sites and river including tributary rivers shows that most of post landslide events occurred near or alongside the rivers. High rate of erosion along the riverside accelerates the weathering process of respective rock mass (Fig. 4.5). Maps overlay of land use land cover, lithology, and landslide site shows that the land use types on top of granite covered area are dominated by urban or built up and crop land types (Figure 4.5b), as the result of image classification. This means that anthropogenic activities might have changed the land cover from the original type into both types. During which, the land cover might appear as landslide scars and then recognized as landslide sites during construction of landslide inventory map by ARSM.
(a)
(b)
Fig. 4.4 (a) LSI and LFI, (b) Landslide occurrences number and Landslide Susceptibility Index of lithology
The rating weight values for lithology were computed the same manner as land use land cover causative factor. Schist was assigned the maximum weight value of 100 due to its maximum LSI value. It was followed by alluvium, sedimentary rock, and granite with their corresponding weight values are 38, 31, and 15. These weight values were computed proportional to their susceptibility indexes (see Table 4.1).
Using all these weight values, thematic map of lithology was constructed (Fig. 4.5).
Schist area with the highest weight value is located at the top center of the map where Pos Selim big landslide and other slope failures occurred.
2.78
5.67 7.00
18.29
3.09
6.30
7.78
20.32
0.00 5.00 10.00 15.00 20.00 25.00
Granite Others/undiff. Alluvium Schist
LSI / LFI
LSI LFI
231
82
1
44 2.78
5.67
7.00
18.29
0 4 8 12 16 20
0 50 100 150 200 250
Granite Others/undiff. Alluvium Schist
LSI
Landslide occurrences number
Landslide Occurrences Number Landslide Susceptibility Index