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Subsidence

Subsidence. Outline. Deltas Groundwater pumping Sinkholes. Subsidence. Ground moves downward due to Slow compaction of water saturated sediment Rapid collapse of caves. Slow subsidence. Ground sinks when fluids are removed from depth in some way. Delta Subsidence.

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Subsidence

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  1. Subsidence

  2. Outline • Deltas • Groundwater pumping • Sinkholes

  3. Subsidence • Ground moves downward due to • Slow compaction of water saturated sediment • Rapid collapse of caves

  4. Slow subsidence • Ground sinks when fluids are removed from depth in some way

  5. Delta Subsidence • Delta: water saturated loose pile of sand and mud deposited by river • Compact and sink as new layers of sediment are added (forcing water out) • Problem: popular places to live!

  6. Example: New Orleans • On Mississippi Delta • Parts dropped ~10 ft in last 50 years • 45% of city is below sea-level • Many retaining walls keeping it dry • Significant danger from hurricanes

  7. New Orleans Essentially town is a bowl, ringed by levees for protection. Deepest point in the city is ~14 ft. below sea level.

  8. Possible Hurricane Danger - New Orleans • Poor evacuation routes, estimates of 1 in 10 killed during a very large storm • Huge economic impacts - • Produces 1/3 U.S. seafood, 1/4 natural gas, 1/5 oil • Significant housing/structural damage

  9. Subsidence from Groundwater Pumping • Water beneath surface used for drinking • Typically, amount pumped out can be replenished by rain, snow • Pump too much, ground subsides • Examples: Mexico City, Los Angeles

  10. Pumping Subsidence Example: Las Vegas in 1990s Several inches of subsidence resulted from over-pumping groundwater

  11. Pumping Subsidence

  12. Sinkholes • Catastrophic collapse • Typically find in areas where limestone is bedrock

  13. Limestone • Rock made of calcium, oxygen, carbon • Form in shallow, warm ocean waters • Common in south and central U.S. • Used to be shallow ocean!

  14. Limestone Caverns • Today - groundwater flows through limestone, dissolves to make underground caverns • If lose groundwater, support in caves disappears • Roof collapse - sinkhole

  15. Common in Florida

  16. Sinkholes

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