عنوان مقاله [English]
Damage caused by the sand dunes movement is one of the most important environmental and socio-economic issues in desert region. erosion and wind processes study began with the work of Bagnold (1954). After significant advances in laboratory and physical approaches to the elements and forces involved in wind erosion and at the contemporary with the development of remote sensing tools and data and changes in methods and algorithms for interpreting aerial photographs and satellite images, the rapid emergence of planetary geomorphology and the search for analogies and similarities on other planets. Rapid developments in the geomorphology of wind processes took place. Using Landsat, ASTER and Quick bird images and LIDAR data, many studies have been done to classify sand dunes. After the launch of two ERS remote sensing radar satellites in 1991 and 1995, the value of CCD Was considered. But in Iran, most of the studies conducted in the desert region, such as the Damghan playa, have studied the changes in long-term periods, which are mainly of sediment origin and classified sand dunes using multispectral satellite data. The aim of this study was to use Sentinel-1 IW SAR time series data in arid regions to detect surface changes in the short term due to wind morph dynamic activity and on the other hand to evaluate the effectiveness of using both radar and optical data and Object-oriented classification model in the events and morphological changes detection of sand surfaces and forms. the results obtained from the processing of remote sensing data and classification and achieving the dimensions of sand dune mobility with the results of wind data analysis will be evaluated and verified.
Materials and methods
Damghan plain located in Damghan plain with longitude 54 10 to 54 40 east and latitude 36 36 to 36 10 north has a hot and dry desert climate and an average rainfall of 100 mm per year, which due to the desert nature, is prone to wind morph dynamic performance. Therefore, in the present study, we aimed to evaluate the mobility of sand dunes as part of the natural hazards active in the region. The research method is library, remote sensing and surveying. Data analysis is based on two main concepts; segmentation and classification. Initially, based on geological and topography maps and field survey, geomorphology maps were created. Then based on the prepared and adapted maps and field surveillance, sandy forms were limited. Then, in order to determine the working units, using the CCD technique with Sentinel-1 radar images, the active and inactive parts of the sand forms were detected. Two radar interfrograme (Master and slave) related to the two dates of 14/05/2017 and 22/03/2018 were used to extract the CCD (based on phase difference).
Finally, with the identification of work units, automatic detection and extraction of sand dunes was targeted, and for this purpose, the bottom-up hierarchical object-oriented method and top-down classification using the growing region technique was used. Also, by extracting sand dunes using object-oriented classification, the values and direction of moving the dune were extracted using Guy, 1995 optimized model and the corresponding sand rose were drawn. Wind rose analysis and drawing related to wind statistics of Damghan synoptic station (the closest meteorological station to the study area) in the statistical period of 1384-96 was also performed with the aim of verifying the findings of the previous step.
Result and discussion
A: Extract sandy forms
Image enhancement is the first step in preparing an image for the extraction of image elements (including sand dunes). Due to the importance of the dune slip face, in the process of identifying the displacement and sand dunes movement, and its lower compaction coefficient than other parts of a sand dune, in order to detection This enhance method, by using the most abundant discontinuity search, distinguishes brighter borders that forming sand dune steep slope from other parts of dune and around environments. The output of this filter is an image in which the sand dunes slip face, with different radiometric intense, is marked from the surrounding sand surfaces.
B: Detecting and extracting sand dunes
In order to evaluate the displacement amount and direction, the object-oriented classification paradigm was used to automatically detect the edges as dune front. Instead of just evaluating pixels, the spatial pattern of objects and forms is also considered. Therefore, the initial segmentation was performed using a scale factor "100" that determined the maximum heterogeneity in the diagnosis of the forms. in addition to using radiometric values, classes can be formed based on geometry and related elements. The rules used are Brightness and Compactness. First, by analyzing the values of average brightness with a threshold of 165, the overall sand dune pattern as the first layer was created. Then, using the Compactness rule, the pixels that were recognized as the dune slip face by the spectral feature in the previous step were eliminated from the classes.
C: calculation the sand dunes amount and direction displacement.
Sand dunes displacement calculated by considering the end edge, as the progressive edge at successive times and measuring the distance between two consecutive lines in two consecutive years. To evaluate the dunes movement direction, the axis of symmetry of each hill was selected as the main axis and the initial and final point of this line on the downwind front of sand dunes in both the first and last years were considered. The azimuth line or the direction of movement relative to the north was drawn and this angle was calculated and its sand rose with an angle of 135 degrees was drawn
This study, suggested a new approach to detect sand dynamics using radar InSAR techniques and object-oriented classification using high resolution optical images. The results of InSAR processing, and CCD technique, was able to recognize active and inactive sand dunes dynamic, and display them in continuous numerical values (fully active to fully stabilized hills).
The application of OBIA on Bird’s eye and Geo eye images (2003-2016) results, indicates that the 22.4 m movement of hills is mainly in the southwest direction in a period of 13 years and 1.7 m for each year. The result of comparing wind rose (wind data analysis) and sand rose (sand movement data analysis) shows a significant relationship between 80% of northwest-southeast wind frequency in relation to 135 ° azimuth for 75% of sand dunes movement and 15% of north-south wind frequency in relation to 180° azimuths of 25% of sand dunes movement.