Announcements
• Kayaking Field Trip
– Show up at 12:30 pm
– More can come if paid by 5 pm today
• Defenses
– Due by 5 pm today
Plan for Today
• Define Mass Wasting
• Understand why slopes don’t
slide/fall/flow
• Types of mass wasting
• Causes of mass wasting
Mass Wasting
• Definition: Erosion without (much, if
any) transporting agent i.e.,
(wind/water/ice)
Why don’t slopes fail
• What keeps the top of Sehome Hill up
there, instead of down here?
– Rock Integrity
– Friction
Important types of mass
wasting
• Slide: Rock stays coherent, more or less
• Fall: Rocks fall
• Flow: Rocks behave like a fluid
Slump (a type of slide)
• Indicators:
– Scarp
– “Hummocky”
terrain on and
below
(earthflow)
Slump
• Earth moves on shallow, curved “fault”
scarp
Debris Flow
• Also called “mudflow”, “mudslide”
• Soil & regolith get saturated with water
and flow like liquid
– cement to soup consistency
– moderate to very fast movement
Debris Flow
Rockslide
• Rock moves because there’s nothing holding it back!
• Generally requires a pre-existing low-friction
surface...
Rockslide
• like a clay layer, once it’s wet...
“Earthquake Lake”, MT
• 28 deaths in 1959, triggered by
earthquake
Earthflow
• basically a very viscous (thick) debris
flow
• slow-moving
– faster in wetter weather
Earthflow
Creep
• very slow
• result of freezing and thawing
Creep
Creep
from D. Schwert, NDSU
Factors in mass wasting
• steepness
• wetness
– decreases friction by increasing pore
pressure
• soil weight
– water increases soil weight
• vegetation
– deep roots anchor soil
Triggers for rapid Mass
Wasting
• Rain
• Oversteepening
– cutting at foot of slope
– piling on head of slope
• Deforesting / Devegetating
• Earthquakes
a-debris flow b-rock fall c-earthflow d-slump
a-debris flow b-rock fall c-earthflow d-slump
a-debris flow b-rock fall c-earthflow d-slump
a-debris flow b-rock fall c-earthflow d-slump
Check out the mass wasting
links
• Links to Washington landslide stories
– Bainbridge Island story in the Atlantic
Monthly
– Kelso and the mobile subdivision
Mass Wasting WarmUp:
your dream house
• I would avoid buying a house on a cliff which
obviously is in the process of being eroded by the
water beneath it. This happens in California all the
time...then people complain when their million
dollar house falls into the ocean! Anybody can see
the bloody hill is being washed away!
• To ensure that I wouldn't buy property with a slope
stability problem I would look for an area that had
trees to solidify the ground with roots, and some
sort of solid material underneath the soil like
granite, instead of material that could slide like
clay.
Mass Wasting WarmUp:
your dream house
• I would look at the ground to try and find
evidence of a geologically recent landslide.