Land subsidence
Land subsidence is the lowering of the land surface due to changes that take place underground. The common causes of land subsidence from human activity are:
- pumping water, oil, and gas from underground reservoirs;
- dissolution of limestone aquifers causing sinkholes;
- collapse of underground mines;
- drainage of organic soils; and
- hydrocompaction caused by initial wetting of dry soils.
Subsidence hazards involve either the sudden collapse of the ground to form a depression, or alternatively the slow subsidence or compaction of the near-surface sediments and soils. Sudden collapse events are rarely major regional disasters, certainly not anywhere near the scale of earthquake, volcanic, or landslide events, but slow subsidence of areas can cause significant economic damage, although spread over a longer period of time.
Overextraction of groundwater from aquifers is the major cause of subsidence (Bell 1997). In some systems, when large amounts of water are pumped, the aquifer compacts, reducing in size and number the open pore spaces that previously held water. This can result in a permanent reduction in the total storage capacity of the aquifer. For example if an aquifer has beds of clay or silt within or next to it, the lowered water pressure in the sand and gravel causes slow drainage of water from the clay and silt beds. The reduced water pressure results in a loss of support for the clay and silt beds. Because these beds are compressible they compact, so become thinner and the effects are seen as a lowering of the land surface. Recharging the aquifer until groundwater returns to original levels would not result in an appreciable recovery of the land-surface elevation.
Land subsidence causes many problems including:
- changes in elevation and slope of streams, canals, and drains;
- damage to bridges, roads, railroads, electric power lines, storm drains, sanitary sewers, canals, and levees;
- damage to private and public buildings;
- failure of well casings from forces generated by compaction of fine-grained materials in aquifer systems; and
- in some coastal areas, subsidence may result in tides moving into low-lying areas that were previously above high-tide levels.
Most of the major subsidence areas around the world have developed in the past half-century at accelerated rates due to the rapidly increasing use of groundwater, oil and gas. Even if the type of hazards associated with subsidence are different from that caused by sudden and catastrophic natural events like floods and earthquakes, because surface sinking is a slow event extensive damage can occur. In particular, most areas of known subsidence are along coasts where the phenomenon becomes quite obvious when ocean or lake waters start coming further up on the shore.
Accurate monitoring of land subsidence is required in order to give best recommendations for sustainable use of the underground resources. Possible measures include reducing groundwater use and switching to surface waters where possible, and careful determination of locations for pumping and artificial recharge that will minimise subsidence. Although the primary objective of artificial recharge is to preserve or enhance groundwater resources, it has been used for many other beneficial purposes, prevention of land subsidence being one of them.
Relevant Links
USGS Land subsidence from groundwater pumping
Tulane University Subsidence: Dissolution and human-related causes
UNEP Artificial recharge of groundwater
References
Bell JW, 1997. Las Vegas Valley: Land subsidence and fissuring due to groundwater withdrawal. Nevada Bureau of Mines and Geology. Impact of Climate Change and Land Use in the Southwestern United States