Mass Wasting (Landslides)

Everything eventually gives in to gravity

What is Mass Wasting?

The process that

transports materials (e.g., bedrock, loose sediment,

and soil) down slopes by

the pull of gravity.

Laguna Hills, CA, 2005

Not always fast

Chuckanut Drive, Nov. 23, 2009

Naches Slide, October 2009

China March 2010 & June 2010

Landslides occur when:

Driving forces > Resisting forces

So what controls these two forces?

(Gravity) (Friction)

Factors that Affect

Slope Stability (Increase driving force or decrease resisting force)

1. Steepness of Slope

2. Composition of Slope

3. Presence of Water

4. Presence of

Vegetation

El Salvador, 2001

1. Steepness

Steeper Slopes = Less Stability

Force of Gravity

Flat Surface

Vertical Cliff

What Makes Steep Slopes?

Volcanoes - Mt. Rainier

Faults - Mercury

What

Makes

Steep

Slopes?

Erosion:

Rivers and Waves

2a. Composition: Weight

Heavier Weight = Less Stability

N. of Seattle (1997)

2b. Composition: Cohesion

Less Cohesive = Less Stability

2c. Composition: Layering

Layering Parallel to Slope = Less Stability

Slope and Composition: Angle of Repose

Angle of Repose: the maximum angle at which a pile of unconsolidated

particles can rest

Talus

Banff National Park, Alberta

3. Water

More Water = Less Stability

3. Water

More Water = Less Stability

4. Vegetation

Plants will:

• Add cohesion (roots)

• Removed water

4. Vegetation

Less Vegetation = Less Stability

Summary of Slope Stability

Factor Less Stability If:

1. Steepness Steeper

2a. Comp.-Weight Heavier

2b. Comp.-

Cohesion

Lower Cohesion

(Sediments)

2c. Comp. - Layers Parallel to Slope

3. Water More Water

4. Vegetation Less Vegetation

Human Impacts on Slope Stability

How do we impact?

1. Steepness

2. Composition

3. Water

4. Vegetation

Human Impacts Factor What humans do?

1. Steepness Cut off toes of slope

Human Impacts Factor What humans do?

2. Composition Build on slopes / add

weight

Human Impacts Factor What humans do?

3. Water Change slope drainage

Human Impacts Factor What humans do?

4. Vegetation Remove Vegetation

Lewis County,

WA

2007

Human Impacts on Slope Stability

Factor What humans do?

1. Steepness Cut off toes of slope

2. Composition Build on slopes / add

weight

3. Water Change slope

drainage

4. Vegetation Remove Vegetation

Landslide “Triggers”

Now that you have an “unstable slope” where:

Driving forces = Resisting forces

What does it take to upset the balance

and “trigger” an event?

Landslide “Triggers”

Natural triggers • Rainfall - Storms,

Hurricanes

• Floods

• Earthquakes

• Volcanic eruptions

Landslide “Triggers”

Human triggers • Poor land management

• Steepening &

overloading slopes

• Loud sounds and

construction vibrations

Landslide Triggers in the News

“…it is clear that just

above the top of the

slide, on the right side, the slope has

been regraded, and

indeed there is clearly

some fill in the

exposed slope” 23 May 2010

Landslide Triggers in the News

“On Tuesday evening

at 11:45 pm Peru

suffered a Mw=6.0 earthquake …”

20 May 2010

Landslide Triggers in the News

“Landslide at factory

as region is

by storms” Manchester Evening News -

18 Nov 2009

Landslide Triggers in the News

Norway:

Report release 19 Nov 2008

• Blasting of the cliff during the

construction of the apartment block was responsible for the

failure • This blasting, which was intended

to create the platform for the

building, weakened the hillslope

Landslide Triggers in the News “Southern California's fire-ravaged

areas prepare for possible mudslides” LA Times - 25 Nov 2008

Washington Landslides The Perfect Storm

1996

1. Steep slopes cut by waves and rivers

Washington Landslides The Perfect Storm

2. Wet winters

Washington

Landslides The Perfect Storm

3. “Right” composition

of sediments

Landslide Anatomy

Scarp

Toe

Rupture

Surface or

Plane

Head

Types of Mass Wasting Range of Speeds

Slow

(Creep)

Very Fast

(Rock Fall)

mm-cm’s

per year

10-100’s m

per second

China, 24 Nov 2008

Types of Mass Wasting

What is Moving?

Rock

Debris

Earth (Land)

Mud

How is it Moving?

Fall

Slide

Flow

Slump - Type of slide where material

moves along a curved plane

Avalanche -

A type of rock/debris flow

Creep - A very slow form of flow

Creep

Creep

Types of Mass Wasting

Rockfall (Zion National Park, Utah)

• Falls

• Slides

• Slumps • Flows

Rockfall • Falls

• Slides

• Slumps • Flows

Rt 20, Washington, 2003

Yosemite NP

Talus (Nevada)

• Falls

• Slides

• Slumps • Flows

Rockslide

• Falls

• Slides

• Slumps • Flows

Rockslide

• Falls

• Slides

• Slumps • Flows

Oregon, 2006

Chuckanut Rockslide, 2008

Slump

• Falls

• Slides

• Slumps • Flows

Slump E. of Lake Washington (January, 1997)

Earth

Flow

• Falls

• Slides

• Slumps • Flows

Debris Flow

• Falls

• Slides

• Slumps • Flows

Debris Flow

Debris Flow (Avalanche) (2002 Alaska Earthquake)

Mud Flow (Lahar)

• Falls

• Slides

• Slumps • Flows

Snow Avalanches

1. Steep Slopes (between 25° - 50°)

2. Start in areas with

no vegetation

3. Non-cohesive snow 4. Layers parallel to

slope (Slab avalanches)

5. Wet snow or thick

new snow - heavy

6. Ice layers - slippery