Document Type : Full length article
Associate Professor, University of Yazd, Yazd, Iran
Msc of desert region management
Wind is considered as one the most important causes for erosion and land degradation in arid and semi arid regions. Thus, it is necessary to study temporal pattern of wind properties to prevent the huge damages. (Refahi, 1999: 22).
There are a variety of different methods, parameters, indicators and diagrams for analysis of windy status and wind dynamic conditions. These are wind rose, storm rose and sand rose diagrams, drift potential, dustiness indices and wind threshold velocity maps.
The main objective of this research is the investigation about the effects of wind direction on the orientation of coastal sand dunes in the southern coast of Iran. The results of this research can play a significant role in designing wind breaks and sand dune stabilizing projects.
Materials and methods
This study is done on coastal sand dunes, in south of Iran, in Khuzestan, Bushehr, Hormozgan, Sistan and Baluchestan Provinces. Commonly, western and south western winds influence the sand dunes located in southern Iran (Zareian Jahromi, 2008). These winds with high sand drifting potential cause strong sand storms in this area, from end of winter to begining of fall. In addition, Monsoon hurricanes blow in eastern and south eastern coasts of Iran, during summer.
The application of WR PLOT View 5.2.1. is employed for anemometer data analysis and drawing storm rose and wind rose. Data from five synoptic stations in the southern coast of Iran including Ahvaz, Kangan Jam, Bandar Lenge, Bandar Jask, and Chabahar with 25 years period (1982 - 2006) were selected. WD Convert 2.0. is used for converting data format (IRIMO to SCARM format) for wind data analysis and preparing data for drawing Wind Rose. Sand Rose software designed by Ekhtesasi and Saremi Naeeni (2003) based on Fryberger & Lettau (1975) equation is used for drawing Sand rose. Sand Rose was able to show different factors of wind data characteristics.
Results and discussion
Result of wind rose showed that dominant wind direction in surveyed stations is from west. Analysis of dust storm rose based on wind erosion threshold, at minimum speed (6.5 m/s), showed that hurricane and erosive winds in the study area is blowing from the west and southwest direction. Sand Rose Graph 3.0. software was used for sand rose drawing, sand drifting potential and sand dunes morphology analysis. The results showed that the direction of sand transition in study area is dominantly from south and west south to north and north east. Thus, high conformity can be observed between wind direction and sand dunes orientations (RDD index: Resultant Drift Direction), with 90.5 percent conformity by Boolean logic approaches. It is reported that Ahwaz station in the northeast sector of Iran has highest accuracy.
The most of the sand transport report (Total Drift Potential: DPt equal to 336.8) is belonging to Bandar e Jask station, that lead to the formation of sand dunes in Surgalm, Sedij and Bayahi Region. UDI index (Uni- Directional Index) value derived from sand rose is less than 0.59 for all stations. Therefore, this represents a bi- directional wind in study region.
Different landforms such as sand dunes, longitudinal dunes, zigzag silk and Sif are observed in study area by interpretation of aerial photos and satellite imagery on Google Earth. These formations are evidence for existing bi - directional winds that blows from the southwest (erosive winds) and southeast (formed winds).
Analyzing the results by sand rose and also sand dunes in a vast area of southern Iranian coasts revealed that southern (southwest to southeast) winds with local origin, reinforced by crossing the sea and warm coast (anabatic conditions), cause wind erosion and sand drifting from beach into plains and Pedi- plains.
Southeast and southwest winds and different sand drift potentials is resulted in reduction of unidirectional index (UDI) in all over the southern coasts of study area. Qua, the UDI varies from 0.2 to 0.5 in most cases, indicates the potential formation of longitudinal dunes in the region.
Bidirectional winds influence most of the southern Iranian coast dunes as longitudinal dunes such as sif, Linear Silk and zigzag Silk. The role sea shore wind play in abrasion, transition and formation of sand dunes in southern Iran is determined by matching results obtained from analysis of erosive wind the morphology of hill sand.