عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Sand dunes as one of the most dynamic geomorphic features of the Earth's surface are directionally and spatially influenced by wind. Sand dunes also have a dramatic effect on the economies, communities and the fragile ecosystems of dry and desert areas. Therefore, the formation, transformation and displacement of these hills as well as the way they change, plays a vital role in understanding the environmental changes in these areas.The purpose of this study was to investigate the altitude fluctuations of the sand dune of Rige-Zarrin, one of the most significant sand dunes of Central Iran, during the period of 40 years by designing two questions as: whether the height of sand dune was changed and if so, where did these altitudinal changes take place? Rige-Zarrin is extended from 54 ˚ 38' to 55 ˚ 04' E and 32 ˚ 54' to 33 ˚ 36' N and located at 40 km of Bafq Desert (Dre - Anjir) to the Northwest, Saghand village to the Northwest and Bayazeh village to the South. This area is lies inside a plat surrounded by four hills with the highest ones in the southeast (3000 meters). The area of the Rige-Zarrin is about 460 Km2 and its length is about 60 km.
To determine the height and area of the Rige-Zarrin, the topography map with 1: 50000 scale, the DEM of study area, the Landsat TM satellite image, and the Google Earth software were used. The time series satellite imageries from 1977 to 2017 obtained from Google Earth (stored as KMZ files) were transferred to the Global Mapper software. The files were converted into DEM maps with a spatial resolution of 30 m. Z profile (height changes during time for each pixel) was determined by stacking all output images (1977, 1987, 1993, 1998, 2002, 2008, 2013, and 2017 years) in ENVI software. To do so, 9 points were randomly selected along with a cross-sectional profile from the North-West to the South-East and in the middle parts of study area with relatively regular space and specific geographic coordinates. Then, the height of each point were calculated during all the mentioned years using ENVI software. In the next step, the variations of height versus time (during years) for those points were drowned. Then, using the Excel software, the linear gradient graph for the elevation profiles was plotted for the points in the cross-sectional profile as well as for each point in different time.
Results and discussion
The results of this study showed that, in average, the height of the Rige-Zarrin has increased in the Western and Northwest parts and decreased in the East and Southeast parts. The lowest and highest altitude were seen in the Northwest and the Western part of study area with about 900 m and 1150 m height, respectively. Thus, the difference in elevation along its longitudinal profile is more than 250 meters. Based on the plotted height profiles, point 1 and the point 9 have the lowest and highest elevation, respectively. Nevertheless, in the middle parts of all height profiles, a natural drop occurs, which amounts to more than 30 m. The reason is the topographical planar fracture of the beneath infrastructure, where the lowest section of the Rige-Zarrin is located. But the highest peak is located in the southeast, where high sandy hermits have been created at the intersection and convergence of winds. The final results showed that the elevation has increased in all part of study area. While the westernmost and easternmost part of the Rige-Zarrin has experienced an increase of less than one meter over a period of 40 years; the middle sections have risen above 4 meters. Thus, it was observed that sand sedimentary materials move eastward under the influence of regional dominant winds and accumulate in the middle sections. The presence of sedimentary materials in the flange cones of the amplitude Mountains and the existence of regional and local winds are the main cause of the height increment of the Rige-Zarrin.
The main results indicate elevation changes in all parts of the study area. But these changes have varied widely over the years and in different parts of the Rige-Zarrin. In the middle part of the study area, the hills have risen above 8 m in the range of less than 10 years. Accordingly, the maximum altitudes and elevation change of the hills occurred between 1977-1987, 1987-1993, 1998-2002 and 2008-2013. Observations showed that the altitudinal fluctuations of the hills were affected by soil humidity, as the higher sand migration has accrued during dry years and vis versa. The final results showed that the Rige-Zarrin is influenced by the dominant regional winds (Northwest-Southeast) moving toward East and South-East, but converging winds cause local reciprocal formation in sandy hermits and, due to the continuity of these winds, the size and height of sand dunes of the central parts of the Rige-Zarrin have been increased. However, the height of the middle sections of the Rige-Zarrin increased with a steeper gradient and less fluctuation. The reason is also the local convergent winds that crossed this section