عنوان مقاله [English]
The displacement and instability of sand dunes are the most important environmental and socio-economic issues of the central regions of Iran. The field scale studies consist of varieties of landforms, such as; mapping and classification, pattern analysis, spatial analysis, topographic changes and morphology studies of landforms and the estimation of mineralogical and sedimentary characteristics. These studies include Al-Masrahy and Mountney (2013), Bazzichetto et al (2016), Al-Mutiry et al (2016) and Metelka et al. (2018). Particularly, in Iran Dafar (1392), Weso et al. (1393), Aharari Roody (1396) have some investigations in this field.
Reviewing of above studies indicates the vertical displacement of landforms has been neglected, although the information of vertical displacement of landforms may not have same importance as horizontal displacements, however, accessing to vertical displacement for 3D modeling and accurate estimation of displaced sediment volumes can be determinative and it is absolutely vital.
The present literature is based on the use of radar Interferometry techniques for estimating vertical displacement and spectral indices derived from satellite bundle combinations in order to estimate the horizontal displacement of sand dunes and wind landforms.
This study used Sentinel 1 satellite radar imageries and Sentinel 2 multispectral images have taken in the spring of 2018, which covered the rugged area of Ardestan. In addition to visual data and in order to validate the results of the GPS receiver, a wide range of studies was taken at the time interval between the images have being taken at 110 points in which included the coordinate information and the class of land use study.
The plane displacement rugged area of Ardestan is located in the central area of Ardestan province in north of Isfahan state, with an area of over 2000 square kilometers, which encompasses longitudinal to east and latitudinal to north.
Generaly, the implementation of the research was carried out in two stages of modeling and estimation of the vertical displacement and plane displacement of sand dunes.
Vertical displacement: for this purpose, both Interferometric wide of Sentinel 1 image have been used. Through interferometric processing, you can eliminate the sources of error and just continue with the so-called desired sources, like; height or displacement. In the construction phase of the interferometer, you can also estimate correlation.
Horizontal displacement: for estimating horizontal displacement just used two Sentinel 2 images. Spectral indices were used to estimate the range and extent of sand dunes. In this study, three spectral indices were used, such as; Normalized Difference Vegetation Index (NDVI), Normalized Difference sand Index (NDSI) and Bare soil index.
The range of altitudinal displacement values in comparison with plane displacement indicates lower values, the factor of the displacement of wind landforms; whether vertical or plane is wind force and Vertical displacement evolves by the sequence of plane motion, which results to aggregating or slipping of the sand particles, Therefore, the domain of its values at any specific time interval is less than the range of displacement values of the planes.
To assess the accuracy of reviewing vertical and horizontal displacement values of sand dunes, they used three parameters, such as; average square error (RMSE), average mean error (MAE) and coefficient of detection or determination ( )(Powell et al., 2007) (Table 2). Although the accuracy obtained in digital elevation model and both vertical and horizontal displacements seem to be acceptable, however, precision parameters in DEM and vertical displacement show higher accuracy.
MAE RMSE outcome
0.73 2.06 2.42 Digital elevation model
0.86 0.037 0.052 Altitudinal displacement
0.67 3.74 4.68 Horizontal displacement
Table 2: Accuracy of elevation and plane displacement maps is based on precision parameters
The general direction of the movement of sand dunes is from the northeast to the southwest which is in compliance with the recorded position of prevailing winds of Ardestan Meteorological Station (weather station).
The displacement of sand dunes can be attributed to their initial volume and height. In general, you can mention that the height and volume of the hill become higher; there is a greater desire for changing in its original location, this change can result in displacement of any flat or vertical dimensions. To evaluate this assumption, points are taken out in each of the three outputs of the digital elevation, vertical displacement, and horizontal displacement and their corresponding values are compared in pair, in both cases, altitudinal values on the X axis are considered as independent variables in relation to the vertical displacement, on the other side, the displacement of the planes on the Y axis as a dependent variable. From linear fit between the two independent and dependent variables in both cases and also forming of the regression equation between the two variables, it can be concluded that the higher altitudinal values in sand dunes area increase unrest and desire to change, also coefficient of determination in case of changing altitude function has increased. This difference indicates that there is a more meaningful relationship between the two variables, In other words, by accelerating the height of the wind landforms, Vertical displacement of landforms is more likely happened, and its values are higher than those dunes with lower altitude.
These findings of this study are: The direction of the overall movement of Ardestan sand dunes from northeast to southwest in April and May, The size and extent of displacement of sand dunes depends on dimensions of the planes and the altitudes of dunes, basically the instability of elevated and more massive landforms is the effect of their stimulus against movement of prevailing wind.
This study has used Radar Interferometry for vertical displacement modeling, emphasized on the importance and potential of radar images in monitoring wind landforms which have mobile and fluid nature that are threatening factors life of the inhabitants of central zone and desert margin in Iran. The process and results of this study could be used in future studies and local geomorphological planners have exposed to wind landforms that have executive value.