| چکیده انگلیسی مقاله |
1- Introduction Iran is one of the countries that is witnessing an increase in dust events and wind erosion recently. Although wind erosion is a global phenomenon, its severity depends on the environmental circumstances (Chorley et al., 2000). In the arid and semi-arid regions, wind is the main factor in desertification process (Yan, 2004). Coastal zones are dynamic parts of the Earth. They and arid lands are the most favorable areas for wind processes (Mahmoodi, 2002). Experiments have shown that in dry areas, wind velocities of up to 4.5 meters per second can be considered as an erosive factor. From this point on, changes in wind erosion rates will be followed by changes in velocities (Mahmoudi, 2002). Of course, the transport of sediment by wind is the result of the interaction between the wind and the earth's surface, which depends on the size of the sediment grains (Bagnold, 1941). Highly fine grains (< 60 – 70 µm) are transported in suspension, or they are transported over long distances via the turbulent airflow (Lancaster, 1995). In addition to the characteristics of surface soil, the sources of soil moisture supply from the atmosphere are the main factors affecting wind erosion (Shayan et al., 2014). There is also an undeniable relationship between dust emission and geomorphological features and land cover. Studies in various parts of the world, including West Texas in the United States (Leea et al., 2012), Hamoon in Iran (Dahmardeh et al., 2019) confirm this. In arid regions, dust events are one of the most important threats to the human environment (Kermani et al., 2016). According to the studies by the Iranian National Plan for the Identification of Crisis Centers of Wind Erosion, over 20 million hectares of Iran are in a critical wind erosion area. About 13 million hectares are in the origin zone, about 2 million hectares are transport or transit areas and the remaining 5 million hectares are sedimentation zones or ergs (Desert Affairs Office, 2002). The western part of Mackoran coastal plains exposed to wind erosion, as a part of the wind erosion critical area, due to its topographic and soil characteristics (Asadpour & Akbarian, 2009; Akbarian et al.,2008). The present study attempted to identify the role of geomorphological factors that affected the dust events in the western part of Makran coastal plain as a dry coastal region. The study area is located in the western coastal plain of Mackoran, southern Iran at 25°31´N - 27°09´N, 56°54´ - 59°19´E, which is north of the Gulf of Oman and east of the Strait of Hormuz. Generally, the region consists of dry land with very little rain, windy with sandstorms, torrents, thunderstorms, high humidity, and fog. Geologically, the area is affected by the general structure of the Makoran Mountains, and is mainly composed of shale, marl, and sandstone layers. The Mackoran Plain is divided into six parts by five relatively high hills from the remnants of the Mackoran Foothills, which reach the coast in the form of capes. Based on the position of the first cape in Bunji (west of Kuh Mobarak), the region of the research is divided into two main sections that have completely different orientations (Figure 1). The west of Kuh Mobarak includes the shores of the Strait of Hormuz and has a north-south trend; The eastern part also includes the coasts of Jask county with the western-eastern trend. 2- Methodology The data collected and processed in this study include Modis satellite images, the wind velocities, the percentage of relative humidity and precipitation in the period from 1/1/1993 to 12/31/2018, as well as the granulometric information about the plain surface sediments, including wind depositions, bed of dry rivers and coastal plain landforms. The research tools included topographic and geomorphological maps of the area, GPS, laboratory instruments of granulometry, and computer software such as Envi. After referring to the research records, 31 samples were taken. The samples were granulometric and the diameter characteristics of the particles were determined. From the relationship between particles' diameter and threshold velocity, wind erosion threshold velocity was determined and with experimental formulas, threshold velocity was obtained in humid air conditions. Also, the probability of the occurrence of wind erosion in different months during the selected time period was calculated and analyzed. Finally, by processing the optical depth of terra satellite images from the MODIS sensor, the possible dust days were investigated and compared with a map of geomorphology in two parts of the research area. 3- Results The study of the geomorphological map indicates the existence of two different trends north-south and west-east in the stretch of plains and wind-sensitive landforms, respectively in the eastern part of the Strait of Hormuz and the coast of the Sea of Oman, east of Hormozgan province. The diameter of sediment particles in the research area varies between 94 and 375 micrometers. Most of the lowland landscapes' sediments have a diameter of 187.57 μm, so according to the Zakhar table (Table 1), the threshold of wind erosion at a rate of 7.21 ms-1 will be result in dry conditions. The wind erosion threshold in terms of humidity changes, varies between 7.21 and 12.31 meters per second during the research period. The months of February, March, and April respectively have the highest probability of erosion; October, November, and September respectively have the lowest probability of wind erosion. Image processing shows different concentrations of dust in the western and southern parts of the region. On dusty days, the concentration of dust in the western plain is far more than of the southern plain. 4- Discussion & Conclusions According to the results, it seems that the spatial difference in dust concentration in the western plain of Mackoran is affected by the geomorphological arrangement of landforms against the prevailing wind and their erodibility to wind is not a determining factor. In the western plains, where sensitive zones extend perpendicular to the wind direction, the wind is able to remove large volumes of suspended particles without being saturated. This process is not applicable to the southern plains, where sensitive forms have a narrow width and long elongation on wind direction. |