Mass movements Flashcards
What are the two types of avalanches?
Slab avalanches:
- involves an entire slab of ice and snow shearing away from the hillside and moving downhill rapidly, carrying rocks and trees as it goes, and therefore cause the most amount of damage
Loose snow avalanches:
- forms with powdery snow and start at one point
- don’t cause as much destruction
What are the causes of avalanches?
- Heavy snowfall
- steep slopes
- rising temperature
- human activity
- aspect
Iceland preparation for avalanches
- IMO observe probability of avalanches
- SMS warning messages to those in affected areas from civil protection
- recommend locals take precautionary measures:
- spending as little time in danger zones
- staying in part of house furthest away from hillside
- don’t stay in basement unless fully underground and has a concrete roof
- establish communications regularly with someone outside of danger zone
Flateyri avalanche case study
- built an A shaped earthen dam in response to an avalanche which occurred in 1995
Successes: - a large avalanche a year later and multiple small ones in the years after and the village has been protected
Failures:
- only keeps Flateyri safe - other places in danger
- too late as people are suffering from the effects of the previous avalanche - 48% of the survivors of Flateyri developed PTSD four months later
- still 230 homes in danger zone C to avalanches – people living in these areas are 10 times more likely to die from an avalanche than a car crash
- 19 billion ISK estimated to be needed to protect the rest of the village
Vargas state debris flow
- debris flows can be about 80% water and travel at over 50kph and be up to to several metres thick
- houses built on alluvial fans (remnants of former flows)
- foundations were not strong
- lots of regolith which flows when saturated
- rainfall has increased continually over the decades
- human development on alluvial fans and upwards on the canyon slope
- 30,000 people died but actual figure could be much more upwards of 100,000
Austrian Alps landslides
- residents detected earth movements prior
- authorities used a probe hanging from a helicopter which emits electro-magnetic waves which can penetrate 100m deep in order to find danger zones and create a hazard map
- the hazard map shows areas of vulnerability and makes landslide prediction more accurate
Oregon USGS debris flow studies
- created more than 100 artificial debris flows
- researchers looked at what slows or speeds up landslides
- they looked at the velocity, gradient, water content and how debris flows pick up material en route down-hill
- this research allows for prediction of the destructive potential of a debris flow and could inform authorities of locations of potential hazards and allow the preparation for their mitigation
- slow debris flows may seem harmless but can cause as much damage as fast
ones
San Francisco landslides
- popular residential areas
- knowledge of residents is limited
- there are previous scars on hillsides which are obvious
- in 1982, 15-25 inches of rainfall fell in 24hrs which led to 18,000 debris flows in one night
- big houses are built at the base of potential landslides
- the bedrock on either side of the fault plane is very weak so when sediment becomes wet it over-comes friction on the slope very easily
- the rock is weak and has little strength and becomes heavy very quickly - gravity takes over
- a protective wall was constructed over a 10 month period to hold the earth in place where slow creep was apparent along with a drainage pipe and any loose earth above the foundations was removed
- however, the wall is not holding the hillside up - friction does this - it is only a matter of time before the earth will become saturated again
what exacerbated the debris flows in Sarno 1998
- mass deforestation and unregulated building
- two days of excessive rain had saturated the surface layers right down to an underlying limestone layer
- 31,000 debris flows occurred killing 137 people
Gondo, Swiss Alps
- added an 8m high protection rock fall wall
- sensors placed in 3 drill holes 50-120m deep
- monitored malformation of the drill holes, water pressure and tiny tremors inside the rock
- why was the response difficult at first?: swollen rivers around Gondo due to unprecedented levels of rainfall - streets were destroyed by landslides and fog made visibility difficult
- retaining wall didn’t protect the town and made it worse as 200 tonnes of debris per metre pressed against it
