Land subsidence

Land subsidence

Land subsidence is a gradual settling or sudden sinking of the Earth's surface owing to subsurface movement of earth materials.

Causes of  Subsidence 

Subsidence is caused by a diverse set of human activities and natural processes, including :-
1.mining of coal , metallic ores,
2.Limestone , salt, and sulfur; withdrawal of groundwater, petroleum, and geothermal fluids;
3.dewatering of organic soils;
4.pumping of groundwater from limestone;
5.wetting of dry, low-density deposits, which is known as hydro compaction; natural sediment compaction;
6.melting of permafrost; liquefaction; and crustal deformation

Catastrophic Subsidence as Result For Water Level Decline (Sinkholes).

•Water is stored in underlying carbonate rocks and moves through interconnected openings along bedding planes, joints, fractures, and faults, some of which are enlarged by solutioning.
•Recharge from precipitation, in response to gravity, moves downward into this system of openings or toward the stream channel, where it discharges and becomes streamflow

•Sinkholes can be separated into categories described as“induced” and “natural.” Induced sinkholes are those caused or accelerated by human activities, whereas natural ones occur in nature. Sinkholes resulting from water level declines

Induced sinkholes

•Induced sinkholes (catastrophic subsidence) are those caused, or accelerated, by human water development/management activities
•activities. These sinkholes commonly result from a water level decline due to pumpage.
•Are most predictable in a youthful karst area impacted by groundwater withdrawals.

Triggering mechanisms resulting from water level declines

(1)loss of buoyant support of the water
(2)increased gradient and water velocity
(3)water-level fluctuations