Assessment and analysis of subsidence risk in Hashtgerd Plain

Land subsidence is a geological hazard that occurs in connection with human activities and environmental changes. This hazard has occurred in 60 countries and more than 150 cities of the world. Similar to many regions of Iran, the southern plains of the Alborz mountain range are facing rapid land subsidence in connection with excessive extraction of groundwater. Considering the average water consumption and the scattered data related to the drop in the underground water level in the country, it is not difficult to recognize that subsidence and its consequences have become a problematic phenomenon in the country. In recent years, many cracks have appeared in agricultural lands and residential buildings, streets and infrastructures of the study area; In such a way that in the agricultural sector of the region, it has caused the destruction and destruction of part of the lands, and in the residential sector, it has caused successive settlements in the buildings and caused irreparable damages.

In this research, data analysis was done in several stages, which are described below. In the first stage, using the information related to the water level drop of the wells in the studied basin, the amount of changes in the water level of the wells was calculated and then a map of the groundwater level drop in the studied basin was prepared. In the second step, using Sentinel 1 images for a period of 5 years (January 2017 to January 2022) and using the SBAS time series method, the amount of subsidence of the studied basin has been calculated. At this stage, the desired images with 24-day intervals have been prepared. After preparing the images, Linux operating system and SNAP software have been used to process the images. In the next step, the correlation between the amount of subsidence with the state of groundwater level drop in the region and other geomorphological parameters has been evaluated, for this purpose, the interpolation map related to the drop in the groundwater level of the region was entered into SPSS software in pixel format and then the subsidence map was also created. In the same way, it was entered into the SPSS software, and in this way, the relationship between the amount of subsidence and the groundwater loss of each pixel was evaluated together. At this stage, using the integrated model of fuzzy logic and AHP, areas prone to subsidence have been identified. To do this, first, the desired parameters including lithology, height, slope, the amount of groundwater loss, proximity to the river and topographic units have been selected. After preparing the desired information layers, the layers are scaled based on the subsidence potential. After fuzzy membership of the layers, the layers are weighted based on the opinions of experts (3 Geomorphologists) and the Analytical Hierarchy Model (AHP) and finally, the layers of information are combined using the fuzzy gamma operator and the final map of the areas prone to subsidence is prepared.

The area of Hashtgerd Plain lacks the variety of geomorphological landforms and is mainly located on the plain and old conifers. The absence of geomorphological barriers has made the residential and agricultural development in this area not to face much restrictions. Therefore, in the period of 30 years, there have been major changes in the land use of the region. The land use map of the studied area during the years 1990 to 2020 shows the increase of residential and agricultural uses. According to the results of calculating the water level drop in the studied wells, in this research, using the spline interpolation method, an annual groundwater drop map has been prepared in the study area, based on this, the middle and southern areas of the range Studies have the highest average annual groundwater level drop.
The correlation between the annual average drop of the underground water level and the amount of subsidence has been evaluated, and the results show that the correlation coefficient between them is about 7%. According to the results, it can be concluded that the Hashtgerd range has experienced subsidence between 18 and 320 mm in our five-year period.

Land subsidence will ultimately be the "death of the aquifer" and the destruction of agriculture and horticulture in the Hashtgerd Plain. With the continuation of this process, the fertile lands of Hashtgerd region will face a dark prospect and irreparable social and economic effects will affect the region. The results of radar interferometry indicate that the amount of subsidence in the study area during a 5-year period is between 18 and 32 cm. The final subsidence map shows that the highest amount of subsidence is in the southern and western areas and the lowest amount is related to the northern areas of the study area. Based on the assessment of areas prone to subsidence risk, it can be seen that the results of radar interferometry are highly reliable to a significant extent.