تحليل فضايي ناپايداري دامنه اي در حوضه آبريز رودخانه مهاباد

Spatial Analysis of the slope Instability in Mahabad Dam Watershed

امروزه استفاده از مجموعه هاي فازي به دليل در نظر گرفتن محدوده اي از احتمالات به جاي اعداد، نداشتن محدوديت روش هاي كمّي و برخورداري از دقت به مراتب بيشتري در پيش بيني وقوع ناپايداري دامنه اي، مورد توجه ژئومورفولوگ ها واقع شده است. اين پژوهش با هدف مشخص كردن مناطق با پتانسيل بالاي وقوع مخاطرات دامنه اي در حوضه آبريز سد مهاباد به روش فازي انجام شده است. به اين منظور، ابتدا متغيرهاي مؤثر در وقوع حركات دامنه اي شناسايي، سپس با استفاده از تصوير ماهواره اي سنجنده + ETM ، نقشه هاي توپوگرافي 1/50000 ، نقشه زمين شناسي 1/100000 مهاباد و انجام عمليات ميداني، لايه هاي مورد نياز تهيه شد. در مرحله بعد لايه ها، ابتدا با استفاده از مدل فازي، استاندارد و فازي شده و در قالب رستر به صورت ارزشي از صفر تا يك درآمده اند. سپس تابع هاي جمع و ضرب جبري فازي روي لايه ها انجام گرفته و با هم همپوشاني شده اند. در نهايت با در نظر گرفتن بهترين توان گاما، نقشه نهايي پتانسيل وقوع حركات دامنه اي حوضه سد مهاباد با استفاده از تابع Gamma0.7 تهيه شده است. نتايج نشان داد كه در حدود 254/64 كيلومتر مربع معادل 33/8 درصد از كل مساحت حوضه سد مهاباد، داراي احتمال وقوع حركات دامنه اي "خيلي زياد"و "زياد" مي باشد. مقايسه نقشه نهايي با نقشه هاي متغيرهاي مؤثر در ناپايداري دامنه اي نشان مي دهد كه نواحي با احتمال وقوع "خيلي زياد و زياد" منطبق بر نواحي با شيب توپوگرافي بالاي 20 درصد، ارتفاعات بالاي 2300 متر، نواحي با تراكم بالاي گسل ها و همچنين تراس هاي پرشيب رودخانه اي مي باشد

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.