CHAPTER 4 RESULTS AND DISCUSSION
4.1 Weighting System and Construction of Thematic Maps
4.1.1 Weight Values and Thematic Map of Land Use Land Cover
The study area that covering 900,000,000 square meter was divided into five categories of LULC. They are 1) cropland, bushes, thin vegetated area in a group; 2) forest; 3) urban and built up; 4) open land and cut slope; and 5) river and lake.
The distributions of landslide occurrences within LULC types as well as the LSI values are shown in Fig. 4.1. The Figure shows that the high number of landslide occurrences in a particular class does not necessarily produce high LSI values. As the example of this case is the forest class. The number of landslide occurrence at this class is the highest one, 138. However, the LSI value is the lowest one, 1.87. Crop land-bushes-thin vegetated area class has a similar condition with forest. This is the consequence of using bivariate statistical method. This method offers an objective measure of landslide occurrence relative to different types of land use land cover by means of comparing the area of landslides within a class, i.e. forest, and the area of the respective class, i.e. forest. The size of the study area and the distribution of landslide sites affect the values of LSI. The remaining classes show inconsistencies between the number of landslide occurrences and LSI values. LSI value of open land- cut slope is the highest one, 32.57, although the corresponding number of landslide occurrences is the lowest one, 57. The second highest LSI values, 19.70, belongs to urban-built up class even though the corresponding number of landslide occurrences, 90, is higher than that of open land-cut slope class. The LSI values of these last two classes can be a preliminary indicator of intervention of human in contributing instability of slopes by changing natural slopes into for examples, cut slopes for road development, urban expansion, open land for constructions, etc. Since there is no landslide within lake or river, the respective LSI value is zero.
Fig. 4.1 Landslide occurrences number and Landslide Susceptibility Index of land use land cover
The identified landslide locations on the landslide map were mostly located in places that were formerly covered by forest. It seemed to be a contradiction to the fact that forested areas generally tend to increase the stability of the slopes. A reasonable explanation regarding this contradictory is that the sources of spatial data used for constructing the landslide map of Cameron Highlands are mainly from interpretation of aerial photographs acquired during the period of 1981-2002 by Malaysian Center for Remote Sensing as reported by Pradhan and Lee [229]. These authors added that landslides were recognized through image interpretation process as breaks in the forest canopy, bare soil, or geomorphological features such as head and side scarps, flow tracks, soil and debris deposits below a landslide scar. Fresh landslide scars were also found during field investigations conducted in 6 December 2006 and 4 May 2008.
The next step is to derive LSI values from which weight values of classes of a particular causative factor can be computed. The theoretical background can be found in Liao [47] and is explained in section 2.4.2.1. LSI is derived using Equation 2.4.
Adjustment of multiplication factor was applied to avoid a number of decimals and
73
138
90
57
0
3.69 1.87
19.70
32.57
0.00
0 5 10 15 20 25 30 35
0 20 40 60 80 100 120 140 160
Crop land, bushes, thin vegetated area Forest Urban, Built up Open land, Cut slope River/lake bank LSI
Landslide occurences number
Land use land cover
Landslide Occurrences Number
Landslide Susceptibility Index
facilitate computation of weight values. The detail explanation of obtaining LSI values is given as follows.
In Table 4.1, LSI values presented in column 8 were obtained by dividing area of landslides occurring within each class (column 5) with the total area of that class (column 7). This process produced very small values. As an example, landslide area of cropland is 65,700m2 while cropland area is 178,158,600m2. The division of the first value over the second one produced a very small value of 0.000368773. To facilitate calculation, this number was multiplied by 10,000 so that it resulted in a value of 3.69. This procedure was applied to compute all LSI values. The LSI values were used to derive weight values.
Another way to develop weight values was by using LFI values. Equation 2.5 was used to derive LFI. Adjustment of multiplication factor was also applied to avoid a number of decimals and facilitate computation of weight values. Practically, LFI values were derived from the ratio between the number of landslide occurrences within a class (column 3) and the total area of that class (column 7). As an example, the number of landslide within cropland area is 73. Division of this number with cropland area results in a value of 0.00000040974727. Again, to facilitate calculation, this value was multiplied by 107 so that it equaled to 4.10. Computation of LSI and LFI values for all classes of each causative factor was done using this manner. This method has been widely used such as by Liao [47], Lee, et al. [230] and Lee, et al.
[231].
According to Liao [47], both LSI and LFI values can be used as an objective measure for landslide occurrences that respect to different types of land use land cover. The higher the LSI or LFI values the more prone to landslides. Fig. 4.2 shows that LSI and LFI values indicate the same trend. In this research, LSI was used for generating rating weight values rather than LFI for reasons: 1) LSI measures the
Table 4.1 Landslide attributes from land use, geology, elevation, slope maps, and weighting systems
Landslide
factors Sub-Categories
Landslide number,
(%)
Landslide areas (m2)
Number of Pixel of each
category
Area of each sub category
(m2)
Landslide Susceptibility
Index (LSI)
Landslide Frequency Index (LFI)
LSI-based Weight Values
(1) (2) (3) (5) (6) (7) (8) (9) (10)
Land use land cover
Crop land, bushes, thin
vegetated area 73, (20) 65700 197954 178158600 3.69 4.10 11
Forest 138, (39) 124200 738129 664316100 1.87 2.08 6
Urban, Built up 90, (25) 81000 45685 41116500 19.70 21.89 60
Open land, Cut slope 57, (16) 51300 17502 15751800 32.57 36.19 100
River and lake 0, (0) 0 713 641700 0.00 0.00 0
Lithology (Rock types)
Granite 231, (65) 207900 830016 747014400 2.78 3.09 15
Sedimentary rock 82, (23) 73800 144535 130081500 5.67 6.30 31
Alluvium 1, (0) 900 1429 1286100 7.00 7.78 38
Schist 44, (12) 39600 24062 21655800 18.29 20.32 100
Elevation 80 - 540.96 0, (0) 0 80293 72263700 0.00 0.00 0
540.96 - 1130.56 54, (15) 48600 405843 365258700 1.33 1.48 16
1130.56 - 1249.51 92, (26) 82800 113435 102091500 8.11 9.01 100
1249.51 - 1394.33 75, (21) 67500 142847 128562300 5.25 26.30 65
1394.33 - 1585.69 100, (28) 90000 184277 165849300 5.43 6.03 67
1585.69 - 1864.97 29, (8) 26100 69831 62847900 4.15 4.61 51
1864.97 - 2100 (max) 0, (8) 0 3466 3119400 0.00 0.00 0
Slope 0 - 7.8 84, (23) 75600 149100 134190000 5.63 6.26 100
7.8 - 19.3 76, (21) 68400 235235 211711500 3.23 3.59 57
19.3 - 27.3 90, (25) 81000 310770 279693000 2.90 3.22 51
27.3 - 36.3 63, (18) 56700 215813 194231700 2.92 3.24 52
36.3 - 51 37, (10) 33300 83446 75101400 4.43 4.93 79
51- 89.6 (max. slope) 0, (0) 0 5628 5065200 0.00 0.00 0
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.