Document Type : Original Articles
Assistant Professor, Department of Physical Geography, Faculty of Earth Science, Shahid Beheshti University
Associated Professor, Department of Range and Water Management, Faculty of Agriculture and Natural Resources, Hormozgan University
Extraction of water from underground sources is on the increase, especially from basins stored by non-strengthened alluvium, sediments and from shallow areas of the sea. This results in sinking on the surface and leads to ground subsidence. In recent years, the Minab Plain, like many areas of Iran has faced drought, which has led to more uptake of subterranean water; this lowers the underground water level and results in sinking which appears on the surface as a sinkhole following subsidence. A field study of this sinking phenomenon involves the following procedure: Firstly, the analysis of the drought occurrences in the area including features of water table, sediment and bedrock. Secondly, the consideration of exploitation of the subsurface water; and finally, identification of the different forms of subsidence in the area from the information acquired above. Statistics from the Sheikh Abad rain-gauge station (covering a period of 27 years) were used for this investigation. Data on precipitation are presented with the decimal method and the standard precipitation index, an index that shows any decrease of climatic rainfall in the area. This data shows that in recent decades there has been less than normal rainfall and that the area has undergone a period of intense drought. In most parts of the plain new alluvium was formed at a depth of 2-3 meters from the surface, the sediment of which often constitutes large grains, which gradually changed to form finer sediment in the form of sand, silt and clay containing salt-water. Therefore, it can be said that with increasing depth, the sediments become tinier (smaller) with an increase in salinity. All of the wells in this area have settled on washed silt sediment and have a high density around the rivers of the Minab area. Because of recent droughts and the drying up of the water supply canals of Minab dam, the digging of wells has increased from 164 in 2002 to a current figure of 607 dug wells. Critical factors related to occurrences of subsidence, are that the existing alluvium covering with the varying thickness up to 150 meters has formed small-grain, silt clay-silt loam soil, which has increased surface subsidence following extraction of underground water over the past two decades.