Subsidence QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.
Jan 02, 2016
Subsidence
• Ground moves downward due to – Slow compaction of water saturated sediment– Rapid collapse of caves
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
Sinkholes
• Catastrophic collapse
• Typically find in areas where limestone is bedrock
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
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