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Subsidence

Subsidence. Patrick Kahn GEOG 366. Subsidence. The downward (vertical) sinking of earth materials. It is caused by:. Compaction of recently deposited sediment Shrinking of expansive soils Deflation of magma chambers Earthquakes Thawing of frozen ground.

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Subsidence

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  1. Subsidence Patrick Kahn GEOG 366

  2. Subsidence • The downward (vertical) sinking of earth materials.

  3. It is caused by: • Compaction of recently deposited sediment • Shrinking of expansive soils • Deflation of magma chambers • Earthquakes • Thawing of frozen ground

  4. Human-induced subsidence is caused by: • Withdrawal of fluids from subsurface reservoirs (water, oil) • Collapse of soil and rock over subsurface holes (underground mines) • Draining of wetlands causing the oxidation of organic soils

  5. Sinkholes and Subsidence • Sinkholes • Solutional • Collapse • Subsidence • Sediment and Soil Compaction • Collapse soils • Fine Sediment • Expansive Soils • Tectonism • Permafrost • Lava Tubes

  6. Karst • Certain rocks are water-soluble, particularly those with gypsum and salt. • Limestone and other carbonate rocks need acidic water for sinkholes to develop (karst terrain) • More than 40% of the humid USA east of Oklahoma is underlain by such rock

  7. Sinkhole, Jan 2005, Colorado. On the northern limit of a swarm of sinkholes that extends to Carbondale. The underlying bedrock is evaporite (e.g., gypsum, anhydrite, siltstone and halite). Two electric golfcarts, stored under the structure, disappeared into the sinkhole.

  8. Karst terrain • Areas underlain by limestone or dolostone. • Susceptible to dissolution by acidic rainwater (carbonic acid) • H2O + CO2 = H2CO3 (carbonic acid in water) • Areas underlain by jointed rock are especially vulnerable.

  9. Sinkholes • Come in a variety of scales. • Some may be individuals. • Others occur in large numbers forming a pockmarked plain known as a karst plain. Minnesota

  10. Karst plain, Arizona

  11. Many karst areas underlain by extensive cave systems. • In humid regions, less soluble material is left behind, creating tower karst.

  12. Solutional and collapse sinkholes • Solutional sinkholes: • Form by dissolution as acidic groundwater is concentrated in holes associated with joints. • Collapse sinkholes • Most common type: develop by collapse of near surface material into part of an underground cavern systems. Can be spectacular

  13. Hazards? • Easily polluted water supplies • Rapid water flow and little filtration. Point source pollution drains rapidly to drinking water source. • Three municipal sewage lagoons have collapsed in southeastern Minnesota, sending millions of gallons of sewage to the aquifer! • And what about…manure storage areas; feedlot runoff… • Loss of buildings and infrastructure

  14. Collapse of dissolving bedrock Florida

  15. Sierra Karst

  16. Sierra Karst

  17. Sierra Karst

  18. Sinkholes • The sudden and sometimes catastrophic collapse may be triggered by groundwater declines caused by pumping • The high solubility of salt and gypsum permit cavities to form in days to years • Carbonate bedrock: slow process- centuries to millennia • Human activities expedite cavity formation

  19. Sinkholes in urban areas and transportation corridors, N. Carolina

  20. Many sinkholes are interspersed between homes

  21. Going, going….

  22. Sinkhole at Winter park, Florida, 1981. Occurred in a single day. It was sealed and converted to an urban lake.

  23. Other Karst • Disappearing Streams • Karst Springs • Tower Karst

  24. Disappearing Streams

  25. Karst Springs

  26. Tower Karst

  27. Subsidence • Less obvious than catastrophic sinkhole formation • Develops gradually and is typically widespread • You can’t tell by looking that the San Joaquin Valley floor was once 35 ft higher in some locations • Mapping is critical: InSAR (inteferometric synthetic aperture radar) uses repeat-pass radar images from Earth-orbiting satellites to measure subsidence at sub-centimeter resolution

  28. Ground subsidence • Can occur owing to the compaction of material as • Water is drained from the soil, often caused by a reliance on groundwater supplies. • Organic material decays • Oil is removed

  29. In the arid southwest, a visible sign may be fissuring of the earth.

  30. Groundwater pumping and land subsidence

  31. Amount of subsidence • West of Phoenix: 18 feet • Las Vegas: 6 feet • Albuquerque: 1 foot • California • Lancaster: 6 feet • Davis: 4 feet • Santa Clara Valley: 12 feet • Houston, TX: 9 feet

  32. San Joaquin Valley

  33. The future? • Increasing population in the southwest means more groundwater pumping, and continued subsidence, possibly spreading to metropolitan areas where damage will be great.

  34. Well head protrusion in Las Vegas.

  35. Subsidence…free home remodeling. Las Vegas

  36. Some of the most spectacular examples of subsidence-related earth fissures occur in south-central Arizona.

  37. Subsidence in the LA Basin? Causes • Tectonic activity • Water and oil withdrawal • Examples include • Pomona (water); Beverly Hills (oil), San Pedro and Long Beach (probably oil)

  38. Groundwater withdrawal, Lancaster

  39. Texas: oil related subsidence. Bay waters intruded the oil field.

  40. Historic Houston subsidence. Over 3,200 mi2 of Houston has sunk at least one foot. The Houston ship canal has sunk 10 ft. Cause: groundwater withdrawal.

  41. Drainage of organic soils • Occurs when soils rich in organic carbon are drained for agriculture or other purposes. • The most important cause is microbial decomposition, which, under drained conditions, readily converts organic carbon to carbon-dioxide gas and water. • Compaction, desiccation, erosion by wind and water, and prescribed or accidental burning can also be significant factors.

  42. Two important areas of subsidence: • Sacramento-SanJoaquin Delta • Everglades • Continuing organic-soil subsidence threatens agriculture • Affects engineering infrastructure and water transfers • Complicates ecosystem restoration • Threatens the integrity of levees

  43. Santa Clara Valley

  44. Expansive soils • Soils which shrink in dry periods and swell during wet periods. • Causes differential movement which damages foundations. • Clay-rich

  45. Expansive Soils

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