Landslides

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Mass Wasting

• Downslope movement of earth materials

• Generally gravity driven• Generally result from

undercutting of a slope– Either natural or human

induced• Landslides– General term for all

types of mass wasting Not to be confused withmass wasted…

Landslide Factors

• Steepness of slope– Steep slopes are

generally unstable

• Vegetation– Roots hold soil together

and absorb water– Vegetated slopes

generally more unstable than non-vegetated

Landslide Factors Continued...

• Water– Sandcastle analogy

• Nature of unconsolidated stuff– Angle of repose—

maximum slope at which loose material remains stable

– Higher for angular rocks

Landslide Factors Continued...

• Type of rock and orientation of rock layers– Sedimentary rocks

dipping in same direction of slope = bad

– Sedimentary rocks dipping in the opposite direction of slope = good

Landslide Factors Continued...

• Earthquakes and volcanoes– Earthquakes can

destabilize slopes– A volcanic eruption can

melt glaciers at summit—creating a landslide

“You’re just my type.”

• Flow– Loose, unconsolidated sediment/soil moves in a

fluid-like way• Slide– Movement of a coherent block of material along a

fracture• Fall– Rapid, free-fall motion

Types of Mass Wasting

Flows

• Creep– Slow downhill flow of

rock or soil under the influence of gravity

– Very slow—1cm/yr– Shallow stuff moves

more quickly than deeper stuff

– Pistol butt trees and leaning fences

Natural creep

Human creep…

Flows: mudflows

• A flow composed entirely of fine-grained sediment

• Lobe shaped deposits• Can be wicked fast—60mph• Two types– Lahar—volcanic ash mobilized by water– Jokulhlaup—large release of water from a sub-

glacial lake

The Armero Tragedy

• Armero, Columbia: 1985• Small eruption from nearby volcano, Nevado del

Ruiz melted the glacier on top of the volcano• Water from the glacier mixed with volcanic ash

creating a lahar• “People can evacuate if they feel like it…”• ~29,000 people killed• 80 yrs previous, town buried by mud flows

Slides

• Slump– Occurs when blocks of

material slide downhill over a curved fracture

– Rotated trees– Jumbled, hummocky front

• Rockslide ( aka rock avalanche– Bedrock slides downslope

over a fracture plane– Breaks up—jumbled

chaotic mess of rock– Fast 500 km/hr

RockfallIndividual blocks

plummet in a free fall from a cliff or steep mountainside

Lituya Bay, Alaska:Rock Fall Example

• July 9, 1958– 8.0 earthquake on

Fairweather Fault– Rock fall (slide?) from

face of Gilbert Inlet slams into Lituya Glacier removing 1,300 ft (1km fall)

• Total volume = 30 million m3

– Wave was generated 1720’ tall (525m) which swashed back and forth due to the Bay’s shape

– 100 ft (30m) tall when reaches inlet

Out of 6 boaters in the bay, 2 killed, 2 tossed over the spit into the Pacific, two completely unharmed

8X greater than any tsunami

Speed of wave 97-130 mph

Lituya Bay Images

• Tsunami inundates 13 km2 of woodland– Sharp line of tree inundation measure of wave

height– Wave travels 1.1 km inland

Landslide Prevention: Intro

• Consequences of construction– Land more susceptible

to mass wasting• Undercutting—removing

support for upper part of slope

• Vegetation removal• Weight of buildings • Irrigation—adding water

Landslide Prevention 1

• Preventative measures– Retaining walls with

drain pipes– Terraces– Planting fast growing

grasses and shrubs– Sloping “sheds” or

tunnels– Building roads in low

landslide risk areas

Landslide Prevention 2• Radio-transmitted, real-

time monitoring of areas that are prone to landslides– Especially places where

roads might be affected• Bedrock “stitching”

– Basically drilling holes into bedrock and reinforcing with concrete and steel cable

• Education