On deltas and passive margins

Subsidence related to compaction and removal of water from sediments deposited on continental margin deltas, in lake beds, and in other wetlands poses a serious problem to residents trying to cope with the hazards of life at sea level in coastal environments. Deltas are especially prone to subsidence because the sediments that are deposited on deltas are very water-rich, and the weight of overlying new sediments compacts existing material, forcing the water out of pore spaces. Deltas are also constructed along continental shelves that are prone to regional-scale tectonic subsidence and are subject to additional subsidence forced by the weight of the sedimentary burden deposited on the entire margin. Continental margin deltas are rarely more than a few feet above sea level, so are prone to the effects of tides, storm surges, river floods, and other coastal disasters. Any decrease in the sediment supply to keep the land at sea level has serious ramifications, subjecting the area to subsidence below sea level.

Some of the world's thickest sedimentary deposits are formed in deltas on the continental shelves, and these are of considerable economic importance because they also host the world's largest petroleum reserves. The continental shelves are divided into many different sedimentary environments. Many of the sediments transported by rivers are deposited in estuaries, which are semi-enclosed bodies of water near the coast in which fresh water and seawater mix. Near-shore sediments deposited in estuaries include thick layers of mud, sand, and silt. Many estuaries are slowly subsiding, and they get filled with thick sedimentary deposits. Deltas are formed where streams and rivers meet the ocean and drop their loads because of the reduced flow velocity. Deltas are complex sedimentary systems, with coarse stream channels, fine-grained inter-channel sediments, and a gradation seaward to deepwater deposits of silt and mud.

All of the sediments deposited in the coastal environments tend to be water rich when deposited, and thus subject to water loss and compaction. Subsidence poses the greatest hazard on deltas, since these sediments tend to be thickest of all deposited on continental shelves. They are typically fine-grained mud and shale that suffer the greatest water loss and compaction. Unfortunately, deltas are also the sites of some of the world's largest cities, since they offer great river ports. New Orleans, Shanghai, and many other major cities have been built on delta deposits and have subsided 10 or more feet (several m) since they were first built. Many other cities built on these very compactable shelf sediments are also experiencing dangerous amounts of subsidence, as shown in the table "Subsidence Statistics for the 10 Worst-Case Coastal Cities" on page 726. The response to this subsidence will be costly. Some urban and government planners estimate that protecting the populace from sea level rise on subsiding coasts will be the costliest endeavor ever undertaken by humans.

The fate of these and other coastal cities that are plagued with natural and human-induced subsidence in a time of global sea level rise is subaqueous. The natural subsidence in these cities is accelerated by human activities. First of all, construction of tall heavy buildings on loose, compactable water-rich sediments forces water out of the pore spaces of the sediment underlying each building, causing that building to subside. The weight of cities has a cumulative effect, and big cities built on deltas and other compactable sediment cause a regional flow of water out of underlying sediments, leading to subsidence of the city as a whole.

New Orleans has one of the worst subsidence problems of coastal cities in the united States. Its rate and total amount of subsidence are not the highest,

SUBSIDENCE STATISTICS FOR THE 10 WORST-CASE COASTAL CITIES

City/State or Country

Maximum Subsidence

Area Affected

Tectonic Environment

Feet (m)

square miles (km2)

Los Angeles (Long Beach), California

29.5 (9.0)

20 (50)

Oil eld subsidence

Tokyo, Japan

14.8 (4.5)

1,170 (3,000)

Delta

San Jose, California

12.8 (3.9)

312 (800)

Delta

Osaka, Japan

9.8 (3.0)

195 (500)

Delta

Houston, Texas

9.0 (2.7)

4,720 (12,100)

Oil eld and coastal marsh

Shanghai, China

8.6 (2.63)

47 (121)

Delta

Niigata, Japan

8.2 (2.5)

3,237 (8,300)

Delta

Nagoya, Japan

7.8 (2.37)

507 (1,300)

Delta

New Orleans, Louisiana

6.6 (2.0)

68 (175)

Delta

Taipei, Taiwan

6.2 (1.9)

51 (130)

but since nearly half of the city is at or below sea level, any additional subsidence will put the city dangerously far below sea level. Already, the Mississippi River is higher than downtown streets, and ships float by at the second story level of buildings. Dikes keep the river at bay and usually keep storm surges from inundating the city. However, the catastrophes of Hurricanes Katrina and Rita in 2005, of Hurricane Camille in 1969, and many before this, show that the levees cannot be trusted to hold. Additional subsidence will make these measures unpractical, and lead to greater disasters than Hurricane Katrina. New Orleans, Houston, and other coastal cities have been accelerating their own sinking by withdrawing groundwater and oil from compactable sediments beneath the cities. They are literally pulling the ground out from under their own feet.

The combined effects of natural and human-induced subsidence with global sea level rise have resulted in increased urban flooding of many cities, and greater destruction during storms. Storm barriers have been built in some cases, but this is only the beginning. Thousands of miles of barriers will need to be built to protect these cities unless billions of people are willing to relocate to inland areas, an unlikely prospect.

something must be done to reduce the risks from coastal subsidence. First, a more intelligent regulation of groundwater extraction from coastal aquifers, and oil from coastal regions, must be enforced. If oil is pumped out of an oil reservoir then water should be pumped back in to prevent subsidence. sea level is rising, partly from natural astronomical effects and partly from human-induced changes to the atmosphere. It is not too early to start planning for sea level rises of a few feet (about 2 m). Sea walls should be designed and tested before given on massive scales. Consideration to moving many operations inland to higher ground should be given.

See also basin, sedimentary basin; deltas; groundwater; karst; passive margin; plate tectonics.

FURTHER READING

Beck, B. F. Engineering and Environmental Implications of

Sinkholes and Karst. Rotterdam: Balkema, 1989. Dolan, Robert, and H. Grant Goodell. "Sinking Cities."

American Scientist 74, no. 1 (1986): 38-47. Holzer, Thomas L., ed. Man-Induced Land Subsidence. Reviews in Engineering Geology VI. Boulder, Colo.: Geological Society of America, 1984. Whittaker, Barry N. Subsidence: Occurrence, Prediction, and Control. Amsterdam: Elsevier, 1989.

Sun The Sun is an average star that sits very close to Earth, a mere eight light minutes away, and 300,000 times closer than the next closest star, Alpha Centauri, which is located 4.3 light-years distant. It is a glowing ball of gas held together by its own gravity and powered by nuclear fusion in its core. Because of its proximity and fairly average characteristics, t

Solar flare image taken by NASA's SOHO satellite, July 1, 2002 (AP Images)

the Sun has been extensively studied and forms the basis for many of the concepts and models about other stars in the universe. The Sun is also the only source of light and the main source of heat for life on Earth.

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  • lena
    Why delta is related to the plate tectonics?
    2 years ago

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