Introduction
The Mahabad Dam basin as entirely mountainous is faced with land use change from
rangeland cover and groveland to dry land farms. In recent years, field studies and
surveys indicate that the extent of instability slope has increased in the basin of
Mahabad Dam (Talebpour Asl and Khazari, 2010: 353).
Recently, fuzzy sets theory has been more remarkable by geomorphologists due to
consider a range of possibilities instead of numbers, no limits on quantitative
methods and a more accurate prediction of the instability slopes. The aims of this
study are identifying areas with high susceptibility of instability slope in Mahabad
watershed dam using fuzzy logic model. To achieve this aim, based on the literature
reviews, 9 factors affecting the mass movement were selected such as slope angle,
aspect, elevation, lithology, distance from the road, land use, distance from the fault,
distance from the river, and distance from the residential area.
Materials and Methods
At first, the factors affecting on the occurrence mass movement were determined.
These effective factors were mapped with satellite image (ETM+), topographic map
(1/50000), geological map (1/100000), and field surveys. In this paper, a linear
membership function was used. This function has 4 parameters that determine the
shape of the function (Figure 1). By choosing the appropriate mode values including
A, B, C and D, the trapezoid, triangular, S-shaped or l-type, membership functions
can be defined (Kainz, 2008: 5).
Membership Value b c
1 0.9 .. i d 0
d
0 20 40 60 80 100
Fig. llinear membership function
In this model, firstly, to determine the membership of each factor, the fuzzy
membership function based on the type of relationship was determined using the
following relation:
0
x- Min
IJ.(x) = Max
1
x Max
Then, the additive and multiplicative algebraic functions fuzzy were carried out on
factors and overlaps were done. Finally, based on the best possible gamma (Gamma
0.7), the final mass movement susceptibility map was obtained.
Discussion
The results showed that about 33.8 % (equal to 254.64 km2) of the total area of
Mahabad watershed dam have "high" and "very high" the susceptibility zones. Most
of these areas are located in the south and southwest of the basin. Comparison of the
final maps with the effective factors indicated that the areas which located in very
high zone are according to areas with slope degree over than 20 percent, elevation
over than 2300 meters, regions with a high density of faults and the steep terraces
river.
The final mass movement's susceptibility map divided into sub-basins showed that
the sub-basins of Dehbokr, Bagrdan, Jandaran and Baytas are more sensitive to mass
movements than other sub-basins. Also, comparing the spatial distribution of the
susceptibility map with the results of the research of Talebpour and Khezri in 2010
indicated that areas with high susceptibility for mass movements are completely in
corresponds to sub-basins which covers the most land use changes.
Conclusion
The main objective of this research is to prepare a mass movement's susceptibility
map in the Mahabad dam basin to manage sensitive areas, land use planning and
rural development. The model used in this research is fuzzy logic. Therefore, using
the Or, Sum, Product and Gamma functions, the instability slope in the Mahabad dam
basin was investigated. The results using the Gamma of 0.7 showed that about 33.8%
of the total area of the Mahabad Dam basin has a high potential for mass
movements, which field observations are confirmed their results.
At the end, according to the results of the research, it is suggested that a map with
high prediction accuracy can be prepared more precisely by new methods and
techniques. This map can help to managers, planners in land use allocation plans and
also urban and rural land development, especially in mountainous areas.