Case Studies

1963 Vaiont Disaster

Case Studies

Ancient slide and recent small slide

gave reason to worry

Case Studies

Geology of the area led to problems

Rainfall

and water level

led to earth

movement

Slumgullion

Translational Slide: goes along existing weaknessThis one cost taxpayers $400 million

Thistle, Utah: 1983Snowmelt Town was floodedLake was drained slowlyRailroad/Highway movedSlide is still moving

Landslides Create Dams and Lakes

Failure is inevitable:

• Time before failure and size of flood depends on– Size, height and geometry of dam– Material making up dam– Rate of stream flow, how fast lake rises– Use of engineering controls (artificial breaches, spillways or tunnels)

• Dams from mudflows, debris flows and earth flows are noncohesive and erode quickly

2008 Earthquakein

China

Failure of Landslide Dams

• Most landslide dams fail when water overflows and erodes spillway that drains lake

• If dam-failure flood incorporates significant sediment, can turn into debris flow – much more dangerous

• Useful dams can be constructed on top of rock dams • China solution: Use explosives to remove the dams

Landslides are a major problem

• Not covered by insurance• In U.S., $2 billion and 25-50 deaths/year• Globally, $20 billion, 7500 deaths/year• Disasters increase with population

Mitigation of Damages from Landslides

Colorado Springs

Cities Can Map Ancient Slides

Colorado Springs 1995: Rainfall and Landslides abundantClay-rich rock and steep slopes of the Rockies1999 major slides and floodingCity purchased landslide-affected properties ($4.5M)GIS maps of old slides and potential slidesWill this be used wisely?

Oakland, CA

Oakland, CA

Oakland, CA

USGS Landslide Hazard Program

Research the causes of landslides

Suggest mitigation strategies

Wildfires increase susceptibility

San Diego: 2003 and 2007

• 2003 Fires led to 16 deaths from landslides• $10sM in damage• Moderate rainfall on burned areas• Runoff increases, infiltration decreases• San Diego: 2” rain in 3 hrs is 10 year storm

Fire and Rain Lead to Slides

• 30% slopes or steeper are vulnerable• % of area burned• Average storm intensity• Clay volume in soil• How much water can be absorbed by soil

Variables include:

Videos

Chemical Weathering-dissolving rocks

Chemical Weathering-dissolving rocks

Carbonic acid formswhen CO2 and H2O molecules combine.

Carbonic acid ionizes to form hydrogen andbicarbonate ions.

Feldspar

KAlSi3O8

Al2Si2O5(OH)4

Kaolinite clay+

Bicarbonate ions react with feldspar.

EROSION BY GROUNDWATER

Artesia: 7/19/08