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Mass Wasting

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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

Tectonic Activity

Land either uplifts or subsides due to tectonic activity.

Sedimentary basins are going down: San Fernando Valley -0.045 feet/yr

Hills are going up: Santa Monica Mountains +0.013 feet/yr San Gabriel Mountains +0.02 feet/yr

Data and image from: https://pubs.usgs.gov/of/1996/0517/fema/html/FEMA-1.2.html

 

De-watering

Subsidence due to de-watering sediments occurs where water table is lowered due to drainage or pumping. Solution: reduce pumping rate to equal recharge rate. This is done via injection wells. Reclaimed water is pumped back into the ground in areas where groundwater extraction is occuring.

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

 

From the USGS: subsidence caused by groundwater extraction, Santa Rita Bridge, San Joaquin Valley

Image showing land elevation change in the northern San Joaquin Valley from 2008-2010, based on InSAR data


 

Oil Extraction

Oil extraction is similar to water extraction, and has the same problems associated with it. Petroleum commonly occurs in partially consolidated sedimentary deposits. Like water, petroleum fills up the pore spaces within sediments, providing stability. When the liquid is extracted, the sediments have less support and the weight of the overlying sediments is not supported, causing subsidence.

Solutions: Reduction of pumping rate to equal recharge rate and / or injection of water in oil and gas fields or even artesian aquifers.

 

Mining

Extraction of resources can cause the ground beneath to resemble an ant farm. The formation of tunnels removes material from the ground, creating voids where solid rock used to exist. This decreases the stability of the area, leading to tunnel collapse, collapse of overlying sediments / bedrock, and the lowering of water tables (kind of hard to mine in an aquifer).

Solution Relocate or diversify mining locations

 

Cave Collapse / Sinkholes

Cave collapse and sinkholes are caused by lowering of the water table and reduction in buoyant support of the rocks above the cave, and typically occurs in regions underlain by carbonate rocks such as limestone. Here is an animation showing how karst topography forms. Nova also has a good animation of how caves form (choose "rainwater").

Yucatan Peninsula - The Cenotes of Chichén Itzá

 

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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CSULA Department of Geosciences and the Environment
Pasadena City College Department of Geology    
    © Sonjia Leyva 2018