Subsidence

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 Orleans Essentially 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

Sinkholes Catastrophic collapse Typically find in areas where limestone is bedrock

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

Sinkholes