Subsidence

Subsidence is human caused in 37 states and affects 15,000 sq. miles. The cost nation wide is $100 million/year; The world wide economic impact is astounding.

Causes

• Tectonic Activity
• De-watering
• Oil Extraction
• Mining
• Cave Collapse / Sinkholes
• Unconsolidated sediments

De-watering

Subsidence due to de-watering sediments occurs where water table is lowered due to drainage or pumping.

Sacramento - San Joaquin Delta

When: 1850s - present
Type: subsidence
Cause: Draining of peat beds has lowered water table (de-watering of sediments)
Result: 12 to 14' subsidence
Solution: reduce pumping rate to equal recharge rate

Winter Park, Florida

When: 1981
Type: sinkhole
Cause: karst topography coupled with groundwater removal due to urbanization and farming.
Result: 100 m depression 13 m deep and $2 million in damage.
Solution: This is hard, pitting farmers again city dwellers. The obvious answer is to stop over-pumping and urbanization. However, it is not that simple. Recharging the groundwater with reclaimed water is one possible solution.

Hershey, Pennsylvania

When: 1949
Type: Sinkhole
Cause: De-watering at a limestone quarry created a cone of depression in water table
Result: 100 sinkholes appeared in an area over 26 km², each about 2-7 m in diameter, and 8 m deep. No new sinkholes after pumping stopped

near Johannesburg, S. Africa

When: mid 1960s
Type: Sinkhole
Cause: De-watering of a gold mine
Result: From 1962 to 1966 the water table lowered 450 m. Eight sinkholes > 50 m in diameter x 30 m deep formed in cone of depression.

Yucatan Peninsula - The Cenotes of Chichén Itzá

When: last several hundred years or so
Type: Cenote (say-NO-tay)
Cause:Karst topography
Result: Sacred Cenote, used for ritual offerings

Stages in the Formation of a Cenote (say-NO-tay) from the Science Museum of Minnesota
1. SOLUTION CAVERN - Naturally acidic groundwater seeping through cracks in the limestone bedrock dissolves areas of softer rock lying beneath the hard surface crust. Over time, this process creates large underground caverns roofed with only a thin layer of surface limestone.
2. YOUNG CENOTE As erosion continues, this thin roof eventually collapses, leaving an open, water-filled hole.
3. MATURE CENOTE Over thousands of years, erosion gradually fills the cenote with organic and mineral debris, reducing its depth. The Cenote of Sacrifice is currently in this stage.
4. DRY CENOTE As erosion continues, the cenote may completely fill, becoming a dry, shallow basin supporting trees and other vegetation.

Application of Water to Soils of Low Water Content

Irrigation water applied to low density alluvial sediments may cause subsidence This is most spectacular on the west side of the San Joaquin Valley. Test plots subsided 10.5 feet in 27 months. Flat surfaces become very irregular, and extensive damage to structures occurs to:

1) P. Gas & Electric transmission lines
2) Gas and oil pipelines
3) Drainage ditches
4) Freeway requires special protection

Solution? Use sprinklers to get more even irrigation.

New Orleans and region

The Gulf Coast region around New Orleans is sinking due to:

• sediment compaction
• de-watering
• isostatic adjustment

About 45% of city below sea level, prone to high-water surges in hurricanes - remember Hurricane Katrina in 2005?

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