The Influence of Hydrogeology Characteristics on Sand dune movement (Case study: Daranjir playa)

Introduction About 1/4th of the world deserts is covered with Aeolian sand (McKee, 1979; Ghadiry et al., 2012). Sand dunes are considered as one of the most important geomorphic features of such environments the formation of which shows the abundant resources of sand, the sand-moving powerful wind and the suitable conditions for sediment deposit (Tsoar, 2001; Michel et al, 2018). The most significant feature of these landforms is their dynamics (Maghsoudi et al, 2017) and The dimensions of these landforms is a function of parameters like sand origin, wind power, wind aspect, the amount of particles carried and time (Ahmadi, 2008). Considering the controlling elements of sand dunes, the depth of groundwater and surface moisture are determining elements in specifying the amount of portable deposits and the formation of sand dunes (Hesp, 2000; Bauer et al, 2009; Poortinga et al, 2015; Silva et al, 2018). As a matter of fact, the high water tables limit the movement of sands (Arens, 1996; Oblinger and Anthony, 2008).   Materials and methods In the first stage, two neighboring sites covered by sand dunes were selected based on Google Earth satellite images and field observations. In the second stage, after georeferencing aerial photos taken in 1967 and the satellite images taken in 2017, the rate of sand dune movements was determined and using Transec tool their movement rate with a distance of 1 meter was calculated by GIS software. In each site, regarding the location of the sand dunes and transects, four borings were located for drilling in a field operation and the status and height of the borings was determined by GPS. After the collection of sediment samples, each sample was completely mixed and 100 grams of each sample was taken for chemical analyses. After moving from the depth of groundwater, the depth of the water was measured by a tape measure in each bore. Then, 100 millimeters of water was collected by a water sample collector device for doing experiment. In the geomorphology laboratory, initially the amounts of TDS, EC, and pH in the sample waters were measured by a multi parameter device, version HI9811-5. To measure the amounts of TDS, EC and pH in sediment samples, saturated paste method was utilized. Here, the samples were initially dried in a drying device, and then 50 grams of each sample was measured with an accurate scale and mixed with 50 millimeters of distilled water. In the following step, the distilled water was mixed with sediment samples and the amounts of EC and TDS was measured using the multi parameter device and other devices.       Results and discussion The analyses results of sand dune movement in case study indicate average, minimum and maximum of sand dunes movement are respectively 22.91 m, 0.11 m, 51.24 m in the first site and 11.74 m, 0.01 m, 41.6 m in the second site within 50 years. The result also indicate average of sand dune movement rate in site 1 (0.45 m in year) with Haloxylon vegetation is nearly double of site 2 (0.23 m in year) which haven’t any vegetation. The results of examinations on the qualitative nad quantitative features of the groundwater indicated that the average of ground water depth, TDS, and EC are respectively 1.25 m, 8595 ppm, 17280 µS/cm in the first site and 0.71m, 12990 ppm, and 26160 µS/cmin the second site. The finding of sample sediment analyses shows that the lowest degree of salinity belongs to the sediments of 80-100 depth in the first bore Of the first site with an amount of 840 ppm and 1820 µS/cm and the highest 11900 ppm and 23870 µS/cm belongs to the depth of 0-20 in the bore 2 of the second site.   Conclusion The obtained results indicate that the relative difference in sand dune movements of the two neighboring sites is caused by the hydrologic conditions of the area. In other words, the quantity and quality of groundwater have influenced the rate of sand dune movements in two different ways. In the first site, the existence of groundwater with a lower salinity has resulted in the formation of vegetation and reduced the speed of sand dune movements. However, in the second site high water tables, high Capillary action power,  and high salinity have led to a lower rate of sand dune movements compared to that of the first site.