عنوان مقاله [English]
The phenomenon of land subsidence involves collapse or sinking of the earth's surface, which can also have a slight horizontal displacement vector. This movement is not limited in terms of intensity, extent, and measure of the involved areas (Ge et al., 2007).
Several factors may cause land subsidence. Earthquakes, dissolution, freezing of ice and compaction of deposits, the slow movement of land and lava outflow, or human operations such as mining or extraction of groundwater and oil are some factors that play a role in land subsidence (UNESCO, 2014). Land subsidence is an issue that is exacerbated by anthropogenic factors including excessive groundwater mining (Chaussard et al, 2014), it also affects a multitude of constructions and facilities, therefore, it causes problems for environmental industries (Schumann and Poland, 1969; Sun et al., 1999; Chen et al., 2003; Sneed et al., 2003; Ge et al., 2007, Teatini et al., 2006; Galloway and Burbey, 2011).
Numerous techniques and methods have been implemented to study the extent, action and land expansion and behavioral measurement in relation to land subsidence. One of the most recent and the most effective methods is the radar image interference technique. Interferometric Synthetic Aperture Radar (InSAR) is a powerful technique for measuring the topography of a surface and its changes over time . Due to its broad spatial coverage and high accuracy, InSAR has become a preferred geodetic method for the study of land deformation in developed groundwater basins and provides insight into the geological and hydrological parameters that characterize the underlying aquifer systems(Haghighi and Motagh,2019).
Varamin plain is one of the areas affected by subsidence in Iran, which is located in the alluvial fan of Jajrud river and its development and prosperity is due to the quality and fertility of the soil in this region.Irregular and over pumping of groundwater has caused many subsidence cases in the study area. This study investigates the subsidence of Jajrud alluvial fan with emphasis on Javadabad Varamin from 2016 to 2019, using permanent scatter radar interferometry method.
materials and methods:
In this study, 41 images of Sentinel (2016-2019) were used to determine the rate and amplitude of land subsidence using the method of permanent dispersants. Permanent scattering points (PS) were selected based on the domain scattering index.
The amplitude scattering index, according to the threshold limit is usually considered to be 0.4 or 0.42 in various studies, which means that all selected points are not located in the set of PS points and only those points to be selected that the amplitude scattering index value is exceeding the threshold. Then, at this stage, points network (PS) was prepared in order to evaluate the processing performed. The prepared spatial network in this section is derived from the Delaunay model. Due to this fact that the amount of displacement is relative in this method, therefore, one of the points in the area is selected where the amount of displacement is relatively zero in compare to other points and other PS points are applied to it. Because of seasonal temperature changes and its effect on buildings that cause elevation changes (the effect of temperature on the structure of buildings), selected point must be located on the ground. SNAP software was selected for radar interferometry processing. . In order to find the cause of subsidence, the information of piezometric wells (1996-2016) in the study area and its temporal changes were investigated.
Results and discussion:
In general, land subsidence has occurred in two large areas. The northern region is located in the Jajrud alluvial fan. The length of this zone is 44 km and its width is 7 km, which covers an area more than 300 square kilometers.
The highest amount of subsidence in this area is related to Golabbas area, which is about 120 mm per year. The presence of Jajrud alluvial fan and agricultural lands in this area is considerable.
The second region is related to the southern zone. The northern part of this zone is bounded by the city of Pishva, Qala-e-Sin and the east of Varamin city. In these areas, the annual subsidence rate reaches to maximum 6 cm. The southern part of this zone is surrounded by Hesargol and Jahanabad areas.
In these areas, the rate of land subsidence is lower compare to the central areas and it is about 20 to 40 millimeter per year. In the northern part of Jahanabad region, the maximum land subsidence is 60 mm per year. The southern zone in which the study area is located has circular shape and the amount of land subsidence increases from the periphery to the center of the region. The highest rate of land subsidence in the study area is occurred in Salman-Abad, Khaveh, Javadabad, Hesar-e-Sorkh and Zavarehvar areas, with the amount of between 160 to 200 mm per year. Areas such as Tajreh, Rostamabad and Hesar Kouchak also show significant land subsidence, that is about 120 to 160 mm per year.
The result of radar interferometry studies has demonstrated that subsidence has occurred in the main parts of the Jajrud alluvial fan's surface. Javadabad region which is located in Varamin is one of the areas with significant subsidence that subsidence occurs at a rate of 20 cm per year Studies conducted in the study area depicted that the decreases in groundwater level in the study area is linear and this is acceptable in justifying land subsidence. So that the patterns obtained using the radar interferometry method in order to find the pattern of land subsidence and groundwater were consistent and somewhat uniform. In this regard, one of the main causes of land subsidence in the region in Javadabad due to the increasing depth of piezometric wells in the study period can be attributed to the high pumping of groundwater resources.
According to the physiographic shape of the region and the pattern of land subsidence, the highest rate of land subsidence was observed in areas consist of low slope, including Javadabad and Varamin. In terms of risk, population, housing, roads and buildings, rail roads and important roads are involved in the phenomenon of land subsidence in this region. The railroads of Tehran, Mashhad, Garmsar and Qom are the connecting roads in this region.
The occurrence of land subsidence has caused change in the local surface slope and this has caused disturbance in the drainage network and the flow path of surface water, which has ultimately result the occurrence of various types of erosion.