The high mountain cryosphere: processes and risks

GEO 415 / 856

Jérome Faillettaz & Christian Huggel

Slope stability & mass flows II: rock slides & landslides, debris flows

Andina

Landslides in Lurigancho / Chosica (Lima), Peru, March 2015

El Comercio

Dave Petley, landslide blog

INDECI

Existing hazard map…

C. Larsen

Mt. Steller region, Alaska, 2008

Huascarán, Peru

Flow transformations

J. Clague

Mt Meager (B.C.) Aug 2010

Flow transformations

Debris flow

depositionVal Varuna, Poschiavo (CH),

18.7. + 24.8.1987•Large event (200,000 m3) consisting of 3 – 4 surges (up to 50,000 m3) and many smaller surges

•Multiple lobes of deposits (stopped at railway tracks)

•Runout reached main river in the valley

•Similar “valley-blocking” cases may create a flooding hazard situation upstream and a dambreak flooding situation downstream

Debris flow processes and dynamics

Learning objectives

• Distinguish between trigger, flow/transport and deposition regimes of mass movements (in terms of processes, geomorphology, hazards implications)

• Recognize some basics of mass movement failure and flow classification

• Understand basic concepts of slope (in-)stability in terms of (dynamic) forces applied

• Understand how cascading processes related to mass movements can be produced and what effects can result thereof

Student presentation

Basic concept of mass movements

U.S. Geological Survey

Types of mass movements

After Coussot and Meunier, 1996, and Schatzmann, 2005

Classification of mass movements

Smithson et al., 2002

Classification of mass movements

Basic principles

Sliding block model

Coulomb friction (dry friction)

ϕ = angle of friction (also used: θ), max. angle before which block starts to slideμ = coefficient of friction

FN = normal force; FR = friction force; FG = gravitational force

Factor of Safety (FS) concept

resisting force (shear strength)

(downslope) driving force (shear stress)FS =

FS > 1 : slope considered ~ stable

FS < 1 : slope considered ~ unstable

(Infinite slope)

Shear strength

Infinite slope concept

Shear strength is the load / pressure per unit area a material can withstand before undergoing shearing failure

Shear strength is the normal stress plus cohesion of the material

Shear strength

Cohesion

Angle of friction

Normal stress on potential failure surface

Infinite slope concept

Shear strength

R

R

Shear stress

Angle of slope

Normal stress on potential failure surface

Infinite slope concept

Shear stress

D

D

Huggel et al., 2012

Slope instability and driving forces (incl. climatic change)

Huggel et al., 2010

Slope instability and time scales