Assessment and analysis of subsidence risk in Kaboudar Ahang-Famenin plain

Subsidence risk, unlike many risks such as floods and earthquakes, is insignificant and in the long run causes a lot of damage such as cracking of buildings, sloping of high facilities, destruction of agricultural lands, subsidence, etc. So the areas at risk are facing a lot of challenges. Among the areas that are at risk of subsidence are the plains of arid and semi-arid regions, including the plains of Iran. In fact, the location of a large part of Iran in the arid and semi-arid region has caused these areas to face a shortage of surface water resources, and this has led to overuse of groundwater resources in recent years and the occurrence of the risk has subsided. One of the areas that is at risk of subsidence is Kaboudar Ahang-Famenin plain in Hamadan province. Due to the lack of limiting geomorphological barriers, this plain has been associated with the development of many agricultural lands and due to the lack of sufficient surface water resources, the utilization of groundwater resources in this plain has been more than allowed and this has caused a decline. Extreme groundwater resources in this area and eventually the risk of subsidence. Due to the importance of the issue, in this study, the subsidence of Kaboudar Ahang-Famenin plain has been evaluated and the effective factors in its occurrence have been analyzed.

In this study, in accordance with the subject and objectives, statistical information (information about 13 piezometric wells in the study area), library and video (radar images related to Sentinel 1 satellite, Landsat satellite images and also 30 m high digital model SRTM) has been used as research data. The tools used in the research include GMT software (to prepare subsidence mapping using radar interference and Russian SBAS time series method), Google Earth (to monitor area and identify subsidence) and ArcGIS (to prepare final maps). According to the objectives, this research has been done in three stages. In the first stage, using the digital model of 30 m altitude SRTM and Landsat satellite images, the geomorphological status and land use of the region have been studied. In the second stage, in the second stage, using information related to 13 piezometric wells, the groundwater depletion situation of the region was investigated and in the third stage, using Sentinel 1 radar images and SBAS time series method, the amount was evaluated. Subsidence of Kaboudar Ahang-Famenin plain has been studied.

The study of the altitude situation of the region shows that there is a very small difference in height between the cities of Kaboudar Ahang and Famenin and also there are no significant obstacles and landforms in this distance. Also, the study of the slope classes of the region shows that the area of Kaboudar Ahang-Famenin plain is less than 10% in the slope class and the region does not have steep and restrictive areas. According to the prepared maps, Kaboudar Ahang-Famenin plain, in terms of geomorphology, has no limiting obstacles for the development of agricultural lands as well as residential areas. In fact, the lack of restrictive barriers has led to the development of agricultural lands in this region, especially irrigated agricultural lands in recent years, and this has led to excessive pressure on groundwater resources in recent years. The results of the study of the decline in groundwater resources in the region indicate that the rate of decline in water levels during a period of 24 years has been between 14.7 (Hemehkasi well) to 78.1 (Einabad well) meters. Also, according to the calculations, most of the studied wells have faced an average of more than 2 meters of water level drop annually. Also, the results of the assessment of subsidence in the region indicate that the study area has had a subsidence of 29 to 216 mm during a period of 5 years (from 16/01/2015 to 14/01/2020).

The results of studying the natural state of Kaboudar Ahang-Famenin plain have shown that this plain has a high subsidence potential due to its geomorphological and hydro-climatic conditions. In fact, in terms of geomorphological status, this plain is without limiting obstacles for the development of agricultural lands, and this issue has led to the development of irrigated agricultural lands regardless of environmental capabilities, including the hydro-climatic situation of the region. According to the above cases, the development of agricultural lands, regardless of the capacity of water resources in the region, has led to over-harvesting of groundwater resources and as a result, a sharp drop in groundwater levels Based on the results of the evaluation of 13 wells studied, most of the studied wells, with an average annual water level drop of more than 2 meters and this issue has caused the Kaboudar Ahang-Famenin plain during The 5-year period (from 16/01/2015 to 14/01/2020) should have a subsidence of 29 to 216 mm. Also, the results of the study of the spatial distribution of subsidence have shown that the highest amount of subsidence is related to the middle areas of the region and the distance between Kaboudar Ahang and Famenin cities, and considering that in these areas there was the highest level of groundwater loss, Therefore, it can be said that the main cause of subsidence in the region has been a sharp decline in groundwater resources. The sum of the results of this study has shown that Kaboudar Ahang-Famenin plain is in danger of subsidence and this issue has led to the emergence of numerous depressions in this plain which is a serious threat to human facilities and habitat.