EENS 3050 | Natural Disasters |
Tulane University | Prof. Stephen A. Nelson |
Subsidence: Dissolution & Human Related Causes |
Surface Subsidence and Collapse Subsidence hazards involve either the sudden collapse of the ground to
form a depression or the slow subsidence or compaction of the sediments near the Earth's
surface. Sudden collapse events are rarely major disasters, certainly not anywhere
near the scale of the earthquake, volcanic, tsunami, or landslide disasters, but the slow
subsidence of areas can cause as much economic damage, although spread out over a longer
period of time. The second most common type of sudden collapse involves dissolution of carbonate rocks (limestones) beneath the surface.This is the topic we will cover first.
Carbonate rocks such as limestone, composed mostly of the mineral calcite (CaCO3) are very susceptible to dissolution by groundwater during the process of chemical weathering. Such dissolution can result in systems of caves and sinkholes.
Water in the atmosphere can dissolve small amounts of carbon dioxide (CO2 ). This results in rain water having a small amount of carbonic acid (H2CO3) when it falls on the Earth's surface. As the water infiltrates into the groundwater system and encounters carbonate rocks like limestone, it may start to dissolve the calcite in the limestone by the following chemical reaction: CaCO3 + H2CO3 = Ca+2 + 2HCO3-2,
This reaction takes place as the water moves along fractures and other partings or openings in the rock. This results in dissolution of much of the limestone if the reaction continues to take place over a long period of time. |
Caves & Cave Formation |
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Sinkholes
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Removal of Solids and Mine Related Collapse Humans can play a large role causing collapse of the surface. Mining
activities that remove material from below the surface can result in collapse if
precautions are not taken to ensure that the there is adequate support for the overlying
rocks.
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Coal Mining Since mining often removes material from below the surface without dissolution, mining can create voids that may become unstable and collapse.
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Subsidence Caused by Fluid Withdrawal We have seen how fluids (particularly water) in the subsurface can dissolve rock to undermine support and cause collapse of the surface. Here we look at another role that fluids may play in causing subsidence. Any fluid that exists in the pore spaces or fractures of rock is under pressure due to the weight of the overlying rock. So long as the pressure of the fluid is enough to support the overlying rock, no subsidence at the surface will occur. But, if fluids are withdrawn from below the surface, a decrease in fluid pressure may occur resulting in the removal of support and possible collapse. The two most important fluids that occur beneath the surface are water (in the form of groundwater) and petroleum (in the form of oil and natural gas). Both of these fluids are often withdrawn for human use, and thus humans are often responsible for fluid withdrawal related subsidence. But, such withdrawal can also occur by natural processes. |
Groundwater
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Oil & Gas Oil and Natural gas are both fluids that can exist in the pore spaces and fractures of
rock, just like water. When oil and natural gas are withdrawn from regions in the
Earth near the surface, fluid pressure provided by these fluids is reduced, |
With a reduction in fluid pressure, the pore spaces begin to close and the sediment may start to compact resulting in subsidence of the surface. |
This has occurred recently in the oil fields of southern California. For example, in the Wilmington oil field of Long Beach, California, subsidence was first recognized in 1940 due to withdrawal of oil from the subsurface. The area affected was about 50 km2. Near the center of this area, the surface subsided by up to 9 meters. In 1958 repressurization of the area was attempted by pumping fluids back into the rocks below. By 1962 further subsidence had been greatly reduced, and the area continuing to subside had been reduced to 8 km2. Still, up to this point, very little uplift had occurred to restore the area to its original elevation. This subsidence event has cost over $100 million. |
Cities built on unconsolidated sediments consisting of clays, silt, peat, and sand are particularly susceptible to subsidence. Such areas are common in delta areas, where rivers empty into the oceans, along floodplains adjacent to rivers, and in coastal marsh lands. In such settings, subsidence is a natural process Sediments deposited by the rivers and oceans get buried, and the weight of the overlying, newly deposited sediment, compacts the sediment and the material subsides. Building cities in such areas aggravates the problem for several reasons.
The table below shows a list of cities throughout the
world that have been experiencing subsidence problems. Note that most of these
cities are coastal cities like London, Houston, and Venice, or are built on river flood
plains and deltas, like New Orleans, Baton Rouge, and the San Joaquin Valley of central
California. Mexico City is somewhat different in that it was built in a former lake.
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City |
Maximum Subsidence (m) |
Area (km2) | Cause |
LongBeach/Los Angeles | 9.00 | 50 | Petroleum withdrawl |
San Joaquin Valley, CA | 8.80 | 13,500 | Groundwater withdrawal |
Mexico City | 8.50 | 225 | filled lake |
Tokyo, Japan | 4.50 | 3,000 | coastal sediments |
San Jose, CA | 3.90 | 800 | bay sediments |
Osaka, Japan | 3.00 | 500 | coastal sediments |
Houston, TX | 2.70 | 12,100 | coastal sediments |
Shanghai, China | 2.63 | 121 | coastal sediments |
Niigata, Japan | 2.50 | 8,300 | coastal sediments |
Nagoya, Japan | 2.37 | 1,300 | coastal sediments |
New Orleans, LA | 2.00 | 175 | river sediments |
Taipei, China | 1.90 | 130 | coastal sediments |
Bankok, Thailand | 1.00 | 800 | river sediments |
Venice, Italy | 0.22 | 150 | coastal sediments |
London, England | 0.30 | 295 | river sediments |
The exact place and time of a disaster related to subsidence cannot usually be predicted with any degree of certainty. This is true of both slow subsidence related to fluid withdrawal and sudden subsidence related to sinkhole formation or mine collapse. Mitigation is the best approach to these hazards. In an ideal world, all areas susceptible to such hazards would be well known and actions would be taken to either avoid causing the problem if it is human related, or avoid inhabitance of such areas if they are prone to natural subsidence.
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Examples of questions on this material that could be asked on an exam
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