PowerPoint Presentation - Geophysics 189 Natural Hazards

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Subsidence
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Outline

• Deltas
• Groundwater pumping
• Sinkholes

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Subsidence

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

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Slow subsidence

• Ground sinks when fluids are removed from depth
  in some way

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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!

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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

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

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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

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

• Catastrophic collapse
• Typically find in areas where limestone is bedrock

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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!

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(No Transcript)
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Limestone Caverns

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

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Common in Florida
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Sinkholes