Lecture 13 - Landslide classification and risk management

Question 1

What are mass movements are usually classified by?

Answer 1

the material being moved: e.g. rock, mud, earth, debris

the type of movement: e.g. fall, topple, avalanche, slide (block, rotational, or translational), flow, or creep.

Question 2

What can mass movements sometimes be classified by?

Answer 2

speed of the mass movement

its water content.

Question 3

What are some examples of slow mass movements?

Answer 3

earth flows and soil creep

Question 4

What are some examples of rapid mass movements?

Answer 4

debris flows, snow avalanches and rock falls

Question 5

What are some examples of mass movements that can be slow or rapid?

Answer 5

rotational and translational landslides

Question 6

What direction is the movement in a fall?

Answer 6

In falls, the movement is mostly downward.

Question 7

What does a rock fall involve?

Answer 7

The free fall of separate blocks from a free face, following opening of a fracture.

Question 8

What direction is the movement in a slide?

Answer 8

In slides, material slips along a basal failure plane, making the movement downward and outward.

Question 9

What does a block slide involve?

Answer 9

A block slide involves a block of intact rock sliding along a basal plane as a semisolid mass.

Question 10

What happens during a rock slide?

Answer 10

In rock slides the rock breaks up as it slides.

Question 11

What are the different types of rock slides?

Answer 11

They can be rotational slides above curved (concave upward) basal surface, or translational slides above planar basal surfaces. Many have a distinctive geomorphology with a head scarp and toe.

Question 12

When does a debris slide occur?

Answer 12

If the material fragments as it slides, it becomes a debris slide. The debris commonly comprises a mix of broken up rock, soil, and vegetation.

Question 13

When is a landslide called a flow?

Answer 13

If the material becomes fluidized, the landslide is called a flow.

Question 14

What are different types of flows?

Answer 14

earth flows (soil)
debris flows (fragmented rock)
mud flows (wet mud)
lahars (wet volcanic ash)

Question 15

What were the 2013 debris flows at Kedarnath, India triggered by?

Answer 15

monsoon rains

Question 16

How many people were killed by the 2013 debris flows at Kedarnath, India?

Answer 16

~6,000 people

Question 17

What is the slowest but most common type of slope failure?

Answer 17

Soil creep

Question 18

What is soil creep caused by?

Answer 18

It is caused by cyclic expansion and contraction of the soil due to freezing/ thawing, wetting/ drying of clay minerals, or heating/ cooling. Expansion acts in the direction perpendicular to the hill slope, whereas contraction under gravity is vertical. This results in a slow, downward creep.

Question 19

What are composite landslides?

Answer 19

They are landslides that start off as one type and evolve into another.

Question 20

What do rock falls often morph into?

Answer 20

They often morph into rock avalanches when they hit slopes below and the intact rock disintegrates. Over time, these build scree slopes.

Question 21

What can landslide hazard maps help to do?

Answer 21

limit development within the most prone areas

Question 22

What is the most effective method of landslide mitigation on small slopes? And what are some examples of this?

Answer 22

To remove the hazard. This may involve scaling (removing loose rock) or benching (remodelling the slope as a series of steps).

Question 23

How can a hazard be mechanically reduced?

Answer 23

unloading the head
reinforcing the body
supporting the toe

Question 24

How can coastal rock falls can be mitigated?

Answer 24

Offshore breakwaters are built which reduce erosion. This is “supporting the toe”.

Question 25

What can be built to provide protection for roads and railway tracks against rock falls and snow avalanches?

Answer 25

tunnels and snow sheds

Question 26

How can satellites be used as a part of earthquake preparedness?

Answer 26

Satellites can monitor ground deformation associated with slow-moving landslides, but predicting if and when these fail catastrophically is difficult.

Question 27

What are sturzstroms?

Answer 27

They are a type of rock avalanche that have large horizontal distances compared to their initial vertical drop — the ratio can be 20 or 30 times. How these long runout distances are facilitated remains a matter of debate. The ∼17,000 year-old Blackhawk slide in the Californian Mojave desert is one of North America’s most spectacular examples of a sturzstrom.

Question 28

What are some instruments that are relevant to early warning?

Answer 28

Rain gauges to detect extreme rainfall events that precede many mud and debris flows and slides.
Acoustic flow monitors to detect ground vibrations or infrasound at frequencies associated with large debris flows or lahars.
Laser ranging instruments to measure channelized flow levels or velocities in real time.

Question 29

Why is there an early warning system in Illgraben, Switzerland? What does it consist of?

Answer 29

A system of acoustic and laser flow monitors is in place at Illgraben, Switzerland — one of the most active debris flow catchments in the Alps — with data telemetered to a control centre where an alarm can be raised.