Preliminary investigation of sinkholes occurrence and their hazards in Kabudar Ahang, Famenin and Hamekasi plains(Hamedan province)

Description

Base on the reports about occurrence of sinkholes in Kabudar Ahang area, in Hamedan province, and following the request of local authority a team of experts of geological survey of Iran, Engineering geology group, made a visit of affected area. In the area two major types of sinkholes were observed. First type occurred on Hamekesi area on the bare karst. A group of more than 11 sinkholes were recognized. The mechanisms of these sinkholes were not so complex; they have happen to be developed on Oligomiocen limestone of Qom formation. They belong to solution and collapsing karstic sinkhole classes and some of them may come under Paleokarst category, with natural origin. The rest sinkholes of the area (in Kerd Abad, Nou Abad, Famenin, etc.) appeared to have a more complex origin. In these locations, depth and distribution of limestone layer and the existence of cavities in it have not been known clearly. The evidence about underground geology has been scare and insufficient. But two points were clear, the extent of abuse of groundwater resources and relatively lower depth of limestone layer in affected area. These conclusions were obtained from the limited drilling charts and water table data available in the area. A preliminary examination of data were pointed out that these sinkholes belong to subsidence sinkholes category that were provoked by underground erosion due to change in water table gradient. A set of preliminary suggestions and advises have been made to the local authority. But it should be emphasized that any long-term plan to control and minimize the risk of occurrence of sinkholes and probable subsidence should be based on underground geology and geophysical studies in detail.