Subsidence

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Outline

• Deltas

• Groundwater pumping

• Sinkholes

Subsidence

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

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

Slow subsidence

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!

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

New OrleansEssentially town is a bowl, ringed by levees for protection.

Deepest point in the city is ~14 ft. below sea level.

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

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

Pumping Subsidence

Example: Las Vegas in 1990s

Several inches of subsidence resulted from over-pumping groundwater

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!

Limestone Caverns

• Today - groundwater flows through limestone, dissolves to make underground caverns

• If lose groundwater, support in caves disappears– Roof collapse - sinkhole

Common in Florida

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