Lecture 13 - Landslide classification and risk management Flashcards
What are mass movements are usually classified by?
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.
What can mass movements sometimes be classified by?
speed of the mass movement
its water content.
What are some examples of slow mass movements?
earth flows and soil creep
What are some examples of rapid mass movements?
debris flows, snow avalanches and rock falls
What are some examples of mass movements that can be slow or rapid?
rotational and translational landslides
What direction is the movement in a fall?
In falls, the movement is mostly downward.
What does a rock fall involve?
The free fall of separate blocks from a free face, following opening of a fracture.
What direction is the movement in a slide?
In slides, material slips along a basal failure plane, making the movement downward and outward.
What does a block slide involve?
A block slide involves a block of intact rock sliding along a basal plane as a semisolid mass.
What happens during a rock slide?
In rock slides the rock breaks up as it slides.
What are the different types of rock slides?
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.
When does a debris slide occur?
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.
When is a landslide called a flow?
If the material becomes fluidized, the landslide is called a flow.
What are different types of flows?
earth flows (soil) debris flows (fragmented rock) mud flows (wet mud) lahars (wet volcanic ash)
What were the 2013 debris flows at Kedarnath, India triggered by?
monsoon rains
How many people were killed by the 2013 debris flows at Kedarnath, India?
~6,000 people
What is the slowest but most common type of slope failure?
Soil creep
What is soil creep caused by?
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.
What are composite landslides?
They are landslides that start off as one type and evolve into another.
What do rock falls often morph into?
They often morph into rock avalanches when they hit slopes below and the intact rock disintegrates. Over time, these build scree slopes.
What can landslide hazard maps help to do?
limit development within the most prone areas
What is the most effective method of landslide mitigation on small slopes? And what are some examples of this?
To remove the hazard. This may involve scaling (removing loose rock) or benching (remodelling the slope as a series of steps).
How can a hazard be mechanically reduced?
unloading the head
reinforcing the body
supporting the toe
How can coastal rock falls can be mitigated?
Offshore breakwaters are built which reduce erosion. This is “supporting the toe”.
What can be built to provide protection for roads and railway tracks against rock falls and snow avalanches?
tunnels and snow sheds
How can satellites be used as a part of earthquake preparedness?
Satellites can monitor ground deformation associated with slow-moving landslides, but predicting if and when these fail catastrophically is difficult.
What are sturzstroms?
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.
What are some instruments that are relevant to early warning?
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.
Why is there an early warning system in Illgraben, Switzerland? What does it consist of?
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.
