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Using GIS & RS in Mass Movements Hazard Zonation - A case study in Germichay Watershed, Ardebil, Iran
Methodology
1. Studying of landslides triggering factors
The common method to study the triggering factors is to use from questionnaire and morphometring the landslides inside the watershed as field works, which this method has been done for Germichay watershed. In field works, some instances such as, the location of landslides, lithological structure, vegetation coverall, type of land use around the landslides, slope angle, road construction, river & water ways, altitude, slope aspect etc. must to be determined that expert could recognize the important factors in triggering the landslides. Then the important factors must to be determined with their priority arrangement.
2. Providing the landslides distribution map
With interpreting the aerial photographs, the sustainable areas for landslides were recognized, so as the areas that their morphology show as a landslide must to be marked on the photos in order that in field checks to be controlled. But the most important part of providing the landslides distribution maps, has been done in field controls and field checks. In this stage all of the marked areas on the aerial photographs must be controlled with field observations and questionnaire complementing. Then the marked areas that probably are not the slides, be removed. For determining the minute location of landslides and to provide the landslides distribution map, was used from GPS (Global Positioning System). In the end, the provided map was entered to Geographic Information System (GIS) with using GISs software, Arc/ Info, in order that to overlaying with the other maps to determine the landslides triggering factors, and the other objectives.
3. Providing the landslides triggering factors maps
GIS & RS techniques provided the maps of seven factors that had been recognized as important factors and had been arranged for their priority. Then for the reason of districting the maps, each map was classified into several classes. These are:
1. lithology agent map, 2. slope map, 3. land use map, 4. lineament factors map, 5. precipitation classes map, 6. altitude classes map, 7. slope aspect map
For providing the lithology agent, slope, lineament factors (river & road), precipitation classes, altitude classes and aspect maps, were used the GIS software's, Arc/Info, Idrisi and ArcView simultaneously, and for providing the land use map were used form satellite images and their digital data in Idrisi software circumference.
Zonation of Germichay watershed with using Analytical Hierarchy Process (AHP)
In this method, at first for the reason of determining different factors preference and conversion they into quantitative values, is used from oral judgements. In this case, the preference of a factor as compared with the other factor is taken into as a table 1, then these judgements are changed into quantitative values from 1 to 9 [1&5].
Table 1. The preference of factors and conversion they into quantitative values
| Preference of a factor as compared with the other |
Numerical value |
| Extremely preferred |
9 |
| Very strongly preferred |
7 |
| Strongly preferred |
5 |
| Moderately preferred |
3 |
| Equally preferred |
1 |
| Intervals between preferences |
2,4,6,8 |
After formation of table 1, in this time for the reason of zonation the area, under stages have been done orderly [5]:
1- Couple comparing of the factors and their priority based on their weights
For the reason of couple comparing of the factors and determining their preference as compared with each other, was formed table 2. At present for the reason of to calculate each alternative weight of couple comparing matrix, is used from arithmetic mean method. In this method, at first, the values of each column are summed together (that this stage has been done in table2). Then the values of each alternative (element) of the matrix is divided by the sum value of the same factor column, and in the end stage, the factors mean values is derived in each rows (table 3). These mean values of each row are the weight values for each map.
Therefore, the priority of each factors based on earned weights in connection with landslides hazard in the study area is as the under arrangement:
1. lithology, w1= 0.423, 2. slope, w2 = 0.216, 3.land use, w3=0.118, 4. lineament factors,
w4 = 0.118, 5. precipitation, w5= 0.058, 6. slope aspect, w6= 0.039, 7. altitude, w7= 0.028
Table 2. Couple comparing of the factors in landslide triggering of Germichay watershed
| |
Lith. |
Slo. |
Lan. U. |
Lin. F. |
Pre. |
Asp. |
Alt. |
| Lithology |
1 |
3 |
5 |
5 |
7 |
8 |
9 |
| Slope |
1/3 |
1 |
3 |
3 |
4 |
5 |
6 |
| Land use |
1/5 |
1/3 |
1 |
1 |
3 |
4 |
5 |
| Lineament F. |
1/5 |
1/3 |
1 |
1 |
3 |
4 |
5 |
| Precipitation |
1/7 |
¼ |
1/3 |
1/3 |
1 |
2 |
3 |
| Aspect |
1/8 |
1/5 |
¼ |
¼ |
½ |
1 |
2 |
| Altitude |
1/9 |
1/6 |
1/5 |
1/5 |
1/3 |
½ |
1 |
| Sum |
2.112 |
5.283 |
10.783 |
10.783 |
18.883 |
24.5 |
31 |
Table 3. Arithmetic mean method for calculating the factors weights (maps weights)
| |
Lith. |
Slo. |
Lan. U. |
Lin. F. |
Pre. |
Asp. |
Alt. |
Mean |
| Litho logy |
0.4735 |
0.5679 |
0.4637 |
0.4637 |
0.3717 |
0.3265 |
0.2903 |
0.423 |
| Slope |
0.1578 |
0.1893 |
0.2782 |
0.2782 |
0.2124 |
0.2041 |
0.1935 |
0.216 |
| Land use |
0.0947 |
0.0631 |
0.0927 |
0.0927 |
0.1593 |
0.1633 |
0.1613 |
0.118 |
| Lineament F. |
0.0947 |
0.0631 |
0.0927 |
0.0927 |
0.1593 |
0.1633 |
0.1613 |
0.118 |
| Precipitation |
0.0676 |
0.0473 |
0.0309 |
0.0309 |
0.0531 |
0.0816 |
0.0968 |
0.058 |
| Aspect |
0.0592 |
0.0379 |
0.0232 |
0.0232 |
0.0265 |
0.0408 |
0.0645 |
0.039 |
| Altitude |
0.0526 |
0.0315 |
0.0185 |
0.0185 |
0.0177 |
0.0204 |
0.0323 |
0.028 |
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