عنوان مقاله [English]
Wind is one of the important causes of erosion in arid and semi-arid regions, which due to the limited vegetation in these areas, is able to move portable particles and results in wind erosion. Erosion defines as the transfer of discontinuous sediment from the earth's surface by water or wind. Wind erosion is a process that occurs because of increased speed and due to wind turbulence on a surface free of cover and as one of the main factors of land degradation and limiting soil fertility has created serious problems in many parts of the world, including Iran. Therefore, there is a serious problem against sustainable production and management of agricultural lands. The process of wind erosion destroys the soil and makes it inaccessible.
In arid and desert areas, soil erosion and particle transport are more affected by wind than other factors. Wind erosion and deposition of materials by wind or wind erosion occurs in more than one third of the land surface. Soil displacement and its destruction in such areas is very important; Due to the climatic conditions of these areas, soil is formed late. Wind erosion first picks up loose soil without vegetation and during the course of the attack, the impact of moving particles by the wind intensifies the erosion and increases the damage, and finally leaves the transported material in the form of sand dunes.
In this study, the physical-laboratory method of wind tunnel was used to estimate the amount of erosion and sedimentation of geomorphological facies of Gonabad city compared to wind erosion. Firstly, the landform units and their types were distinguished and after identifying the facies, one square meter was taken from the surface soil and the samples were transferred to the wind tunnel aerodynamics laboratory of Hakim Sabzevari University for analysis. Then, in order to measure the amount of sedimentation of lands through wind erosion measuring device, first the erosion threshold for each sample was determined and the sediments were exposed to wind erosion for ten minutes and its erodibility was determined. To measure the wind erosion threshold, initially the tray was filled with sediment and weighed, and the sediment sample was placed inside the wind tunnel and the wind tunnel was lit at a base speed of 4 meters per second. Then the speed was increased to reach the wind erosion threshold. Wind erosion threshold is the speed at which the first sediments begin to move. At the wind tunnel threshold speed remained on for ten minutes and the sediments were exposed to wind. After this time, re-sedimentation of weight and after calculating the percentage of pebbles in each facies, the amount of winding was estimated. Three samples of each facies were placed in the wind tunnel and the average velocity of the winding threshold was considered as the wind erosion threshold of the facies.
Results and discussion
The results of measuring the amount of eroded soil of geomorphological facies were estimated for the erosion threshold speed of each facies. Wind erosion threshold varies from 6 m / s in sand dunes to 15 m / s in low and high plains and relatively high water erosion and new deposits. Wind gauge was measured between 20 g / m2 in earthen hills with low and high erosion and moderate erosion up to 350 g / min in 10 minutes in sand dunes.
Examination of wind erosion threshold map in different facies and its adaptation to sand transport flow direction show that in sand dunes facies with threshold velocity of 6 m / s dominant direction of northwest flow was consequently threatening in north part It is in the way of communication. However, part of this facies is located in the north of Gonabad, which can be hazardous due to the location of industrial settlements and the establishment of industries for moving sand.
Lowland plains with relatively high water erosion and new deposits with a threshold speed of 15 meters per second are resistant to wind erosion and sand dunes with a threshold speed of 6 meters per second (12 knots) have the most sensitive facies and the highest amount erosion (350 g / m2 in ten minutes). After that, plains with gentle slopes without elevations and facies of earthen hills with elevations and thresholds with a threshold speed of 10 meters per second were estimated from areas sensitive to wind erosion in the region. The adaptation of the sand dunes to the speed of different thresholds showed that the city of Gonabad is prone to wind erosion by being located in the sensitive facies of a plain with a gentle slope and without elevation and the direction of the northern flow of sand in this facade. The results of calculating the wind erosion threshold in the study area showed a threshold speed of 6-15 m / s in different facies. The most sensitive facies are sand dunes (6 m / s) followed by plains facies with gentle slopes without elevation and elevation of earth hills with elevation and wind erosion (threshold speed 10 m / s). Considering that the city of Gonabad and Beidokht, as the two most important population centers, are located in the facies of plains with gentle slopes and without slopes and heights, special attention should be paid to this facies for stabilization. Also, sand dunes cover a large part of the city and in terms of location, are located on the main road north to south of the country (Mashhad-Zahedan) and also has the lowest erosion threshold and the highest wind volume. Management of these lands and planning to stabilize this facies seems to be necessary. An overview of the wind erosion susceptibility map showed that the northern areas of the city have more potential than the southern areas. As moving toward south, wind erosion decreases, and the topographic factor plays an important role. Due to the fact that the study area is one of the arid regions in terms of climate, so the unfavorable climate has prevented the establishment of vegetation and has intensified wind erosion in the region. It is suggested that in areas where the soil surface has hardened (floodplain facies and clay soils) and gravelly lands (sandy plains) the most important action to prevent wind erosion is to maintain this cover and prevent from damage cover.