The Role of Wind Direction and Unidirectional Index in the Time Development and Determining the Sand Dunes Morphology (Case Study: Sadegh Abad Erg-Bafq)

At present, about 36% of the land surface is covered by arid and semi-arid areas, with 19% of these levels completely dry and no plant life. In Iran, between one third and one-fourth of Iran's dry surfaces are covered with Sand and Sand Dunes. The arid and semi-arid conditions of Iran have caused about 80 million hectares of Iran to be covered by desert areas, sand dunes and insignificant vegetation areas. Therefore, the sand dunes are as one of the geomorphologic types dominate the, which is called Erg in a region of collection of sand dunes. From the shape, sand dunes can be divided and studied from different perspectives. For example, sand dune morphology may consider in transverse, linear, Compound and complex sand dunes or star, Barchan and crescent sand dune morphology stellate or pyramid sandy hills. In general, the first studies on sand dunes relate to the Bagnold study in 1941, which shows that influence of the wind regime on the sand particles diameter and sand dune morphology. He believed that the similarity between morphology and the behavior of sand dune morphology can be used different models to study the sand dunes and Erg morphology. In this research, by studying the wind regime of Sadegh Abad area and studying the effect of wind direction, Different conditions for the formation of various types of sand dunes morphology Influenced by wind, have been discovered. Daranjir Graben is located in the geographical coordinate’s 55 00 15 to 55 49 30 E, longitude, and 32 34 50 to 31 08 20 N, latitude along the southeast of Yazd, which extends from Kharandagh to Bafq. The Erg of Sadegh Abad is located in Daranjir Playa. Because of the role that wind characteristics in eroding area, Study of sand dunes requires to examine of wind characteristics of an area. Also, because of the dominant morphology of the sand dunes in ergs are dependent on wind direction variations, wind directional studies methods are necessary to investigate the morphology of the sand dunes. The easiest method to study wind regime in a region is to draw wind rose using wind speeds and wind direction data. Wind rose can be plotted monthly, seasonally, and yearly. Therefore, in this study, wind speed and wind direction data of Bafq weather station during a 20 year period (1997-2017) were used in Wrplot software to determine the wind direction in Sadegh Abad. In addition to the prevailing wind direction and drift potential sand, Sand rose was drawn up by Sand Rose Graph software. Considering the type of morphology of the sand dunes depends on the degree of stability and the wind direction, in order recognize wind regime at a meteorological station. In this condition may change the direction of the wind in a month or season and changing the morphology of the sand dunes over time. Therefore, in the first stage, seasonal and annual wind rose was used to determine the direction of the dominant wind in different seasons. In the next stage, with the fact that the unidirectional index (UDI) slightly changing, with the annual, seasonal and monthly calculated sand rose and another sand movement indexes in the synoptic station Bafq. The Bafq station annual wind rose shows that the direction of the wind dominates is northwest. However, winds could be seen in other directions. The annual sand rose at the summer and spring indicate that the direction of the predominant sand transport is some direction with the dominant wind direction. But, the autumn and winter sand rose show the opposite of this and can be seen the dominant winds of the sand carrier were in a direction perpendicular to the direction of the prevailing north-west winds. In addition to determining the direction of the wind, another factor that plays a more important role in morphological changes and the sand dune dominant shape in an Erg, is the UDI index, or the rate of wind stability at different time periods. In order to determine the UDI value, firstly, sand indicators such as DP and RDP should be calculated and, based on the RDP / DP ratio, calculate the UDI value. The UDI was calculated at an annual rate about 0.24 at the bafq weather station at the Bafq station. In the spring, the homogeneity index of winds has a condition that is approximately equal to a ratio of 0 and 1 (UDI = 0.51) so that in the spring, and to some extent in the winter, there is a prevailing condition that, like the summer season, one can see stability in the winds and the presence of relatively one-side winds, and not like the fall of the year saw different winds toward the Sadeq Abad. The morphology of the Sand dunes is dependent on the region's wind regime, especially wind direction and its sustainability. In the monthly review of the change in RDD, it is evident that the maximum slope of wind change is in the fall season and between two months of October and November, about 90 degrees. This sudden change of wind direction from the southwest in November has had an important role in changing the morphology of the barchan and Crescent sand dunes to star sand dunes. Therefore, by examining the Bafq's wind regime at different time periods, it can be stated that the best conditions for the development of the Star sand dunes are provided in the winter and mid-spring While summer with the highest unidirectional index (UDI = 0.74) and autumn with the lowest unidirectional index (UDI = 0.19) respectively, are suitable for the development of barchan and star sand dunes in the erg of sadegh abad.