Application of Geoinformatics on Mountain Land Hazard Mapping:
A Case of Annapurna Himalayas, Central Nepal
Table 1 Morphometric Characteristics of the Watersheds
|
CHARACTERISTICS |
WATERSHEDS* |
|
MODI |
SETI |
MADI |
MARDI |
Area (km2)
Perimeter (km)
Number of stream segments
Order of trunk river
Length of the trunk river (km)
Total length of stream segments (km)
Mean length of 1st order stream (meter)
Average drainage density (m/km2)
Mean bifurcation ratio ( Rb = NU/NU+1)
Mean length ratio (RL =
LU/LU-1) |
476.92
125.4
1236
5TH
31.62
1057.76
733.98
2217.91
5.7
2.911 |
342.63
87.01
1090
6TH
27.06
896.55
701.48
2616.69
4.071
4.5 |
502.31
118.16
1543
6TH
28.70
1239.23
741.42
2467.07
4.282
3.053 |
140.69
58.17
377
5TH
12.03
373.81
888.25
2656.99
4.183
2.10 |
* Only the upper parts of the basins have been delineated
Source: Computed from the base-map
3. Methodology
For the assessment of the terrain two different methods (method - 1 and method - 2) are applied. The method - 1 is designed to give a general spatial association of different factors, which are perceived as major determinants of the surface fragility as well as database available in the case of Himalayan watersheds. Eleven different map layers including geological lineament, lithology, altitude, relief ratio, slope inclination, soil, land use, stream frequency, drainage density, aspect and rainfall zone, and seventy-five classes of these factors have been considered (Table 2).
All these classes were scored in a sequential order of possibility of failure according to the procedure in general scale of regional mapping (Marsh 1978). Certain weight has been assigned to them accordingly (Table 3). Annapurna Region has been classified into five respective classes of potential geomorphic hazard units.
Table 3 Terrain Factors and Score Assigned (Method – 1)
| Map Layers |
CLASS CODE |
Weight |
Total Score |
|
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
|
|
1. Aspect
2. Relief Ratio
3. Drainage
Density
4. Elevation
5.
Stream Frequency
6. Geological Structure
7. Distance from lineament
8.
Land use
9. Soil
10.
Slope
11. Rainfall
Zone |
0.1
0.3
0.3
0.5
0.3
2
4
2.2
0.2
0.2
0.1 |
0.1
0.7
0.7
1
0.7
0.5
3
0.1
0.3
0.8
0.2 |
0.5
2
2
1.5
2
4
2
1.5
3
1
0.5 |
1
3
3
1.8
3
1.4
1
1
3.5
1.5
0.7 |
2.5
4
4
2.2
4
1
0
3.5
2
1.8
0.8 |
2.5
0
0
3
0
0
0
0.5
0.5
2.2
1 |
2
0
0
3
0
0
0
1
0.3
2.5
1.2 |
0.8
0
0
3
0
0
0
0.2
0.2
0
2 |
0.5
0
0
0
0
0
0
0
0
0
3 |
0
0
0
0
0
0
0
0
0
0
3.5 |
7
10
12
2
12
5
10
10
8
12
12 |
70
100
120
20
120
50
100
100
80
120
120 |
The method - 2 is applied based on the previous studies made on this field. In this method, Landslide Susceptibility Score (LSS) has been calculated as suggested in the studies (Aniya, 1985: 102-114; Sarkar, Kanungo and Mehrotra, 1995: 301-309; Dhakal, Amada, and Aniya, 1999: 3-16). Accordingly:
Landslide Susceptibility Score (LSS) = R (L)/R (A)
Where, R (L) = a ratio of the landslide area in a particular class of a factor to the total landslide area
R (A) = a ratio of the area belonging to that particular class to the total study area
A score greater than 1 suggests that the class contributes to landslide, whereas a score smaller than 1 implies the class inhibits landsliding. In this procedures seventy-five different classes of eleven factors in total were considered. However, there were fifteen classes having null score (Table 4). The LSS of each class of the factors were linked with the vector coverage. That coverage was later on overlaid with each other to obtain the cumulative score. The score of overlaid layers were added linearly.
Table 4 Landslide Susceptibility
Score (Method - 2)
| Class code |
Aspect |
Relief ratio |
Elevation |
Slope |
Geologic structure |
Geologic lineament |
Land use |
Soil |
Rainfall zone |
Drainage density |
Stream frequency |
1
2
3
4
5
6
7
8
9
10
11 |
0.49
0.958
1.477
1.964
0.806
0.696
0.843
0.769
0.492 |
0
1.773
1.049
0.771
1.24 |
1.875
1.236
2.881
0.129
0
0
0
0 |
0.1
0.526
0.781
1.053
1.644
0.834
0.987 |
1.75
0.21
1.212
4.074
0.719 |
2.514
1.103
1.175
0.579 |
0.187
0.026
0.774
0.107
3.218
0.252
0
0 |
0
0
1.405
1.421
1.469
1.592
0.897
0 |
0
0
0
0.204
1.992
0.629
1.895
0.177
0.023
0
0 |
0.808
0.565
0.715
1.562
1.2 |
0.733
1.433
0.622
2.25
1.056 |
What is generally considered is the high susceptible parcel that accounts for higher score and the lesser susceptible parcel is the vice versa. The range of LSS obtained is classified into five zones of relative instability. Theoretical potentiality of the terrain instability is further verified by overlaying the existing landslide maps on the result of both method-1 and method-2 and calculated the LSS. Finally overlaying the potential hazard map with settlement and cultivation land has given the coincidence of possible hazard and vulnerability of human life and property.
4. Result and Discussion
Based on the analysis of method -1, about five percent of the total area of the region is under the very high potential hazard zone, whereas only 0.2 percent area of the total is in the very low potential hazard zone. Distribution of the proportion of total area of the region is skewed towards the potentially high hazard prone classes (Table 5). The analysis reveals that the Madi watershed has the maximum share of its area under the very high potential hazard zone. This follows the Mardi watershed. Though the Modi and Seti have smallest share on very high potential hazard zones, the share of high zone has considerably large proportion. The extension of very low potential hazard area is less than one percent to all watersheds. The pattern of distribution is normal to some extent, but it is skewed towards the very high potential hazard class in all four watersheds under studied. According to method -2, the highest susceptible score is delimited to the lithological structure of the Ghan-Pokhara Formation. This group of lithology is consisting the black carbonaceous slates, green shales and grayish shales, white dolomitic limestone and limestone. This group is closely associated at the MCT zone of the region. Therefore, the geological structure seems very fragile. Again the second highest LSS is concentrated in close proximity to the geological fault line, therefore, the landsliding phenomena in the region are closely associated with weak geological structure.
Table 5 Distribution of Potential
Hazard Area in Different Watersheds (Method –1)
(Area in percentage of the
total)
| Class |
Hazard Class |
Modi |
Seti |
Madi |
Mardi |
Total of the Region |
1
2
3
4
5 |
Very Low
Low
Medium
High
Very
High |
0.08
29.42
43.46
25.72
1.31 |
0.11
19.40
48.15
29.85
2.50 |
0.25
22.66
35.74
32.26
9.09 |
0.06
5.44
59.98
30.49
4.03 |
0.2
22.5
43.4
29.4
4.5 |
| Total |
|
100 |
100 |
100 |
100 |
100 |
Source: Computed from the Analyzed Map
