Subsidence Outline Deltas Groundwater pumping Sinkholes Subsidence Ground

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 • Delta: water saturated loose pile of sand 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 1990 s 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