Slope processes Flashcards
(26 cards)
Mass movement
Movement downslope of rock fragments and soil under the influence of gravity
Falls, slumps, slides and flows
Water and speed are the main influences on the type of movement, with water influencing in soil creep, slumping and flows, not so much in rockfall
Between 2004 and 2010, 2620 fatal earthquakes killed 32,322 people (not even including earthquake-triggered landslides
What causes mass movement
Rockfalls often occur due to weathering
Poor road construction decreases shear strength of slope as gradient increases
Basal undercutting reduces stability of slope
Earthquakes trigger mass movement
Physical geography can influence the level of mass movement as some areas are naturally mountainous and steep
Loose soil composition, lack of vegetation and the number of fractures in rocks are also minor reasons
Excess water / rainfall and basal undercutting are largest reasons
Rock geology, structure and lithology
Porous or impermeable - if a rock can absorb water it becomes heavier so mass movement is more likely
Weak or resistant - weaker rocks are broken down quicker so lose shear strength and will collapse under gravity
Angle of orientation and dip - some cliffs have bedding planes facing the sea so are more likely to be basally undercut
Extent of faults and folds
Arrangement of rocks / combination of different rock types
Less resistant sandstone overlying more resistant and impermeable granite
Heavily jointed rocks more susceptible as water and chemicals can weaken deep within the rock structure making it liable to failure
Soil and aspect
Soil is the organic overlying layer of lithosphere
Determined by the underlying parent rock material and climate
Clay soils coagulate to form large impermeable layers prone to waterlogging, increasing weight to increasing risk of creeps
Granite soil is more coarse and has little water content so falls occur
Aspect refers to the compass direction in which a slope faces, affecting levels of insolation, affecting weathering processes
Solifluction
Slow downhill flow of saturated soil
Common in tundra environments where the seasonal thawing of the uppermost layer provides sufficient water to enable flow to occur
Water reduces cohesion and friction, promoting movement
Gelifluction is the same but on frozen ground
Vegetation
Improves slope stability due to:
Greater root density binds soil better, increasing cohesivity, and removes more moisture from the soil, reducing instability due to increased shear strength and decreased soil permeability
Different types affect slope stability e.g. trees offer protection in terms of interception, shrubs less so
Climate and weather
Heavy rain and melt water both add volume and weight to soil
Peltier Diagram - high temperatures and rain increase weathering
More moisture decreases shear strength of slope
Most mass movement occurs in tropical areas because of this
High precip means rounded slopes and debris due to fluvial action whereas low precip means angular, steeper slopes
Rockfalls occur on hard, shear rock faces undergoing physical weathering whereas slumps occur on sloping hills undergoing chemical weathering
Rills and gullies
As runoff travels across land it creates small grooves in the soil called rills
Rills join together to make larger rills which become gullies when big enough
They only contain water in them after it rains
As they carry water, it picks up soil and rock, eroding the sides, making the gully larger
Rainsplash
Raindrops can erode hillslopes
On a 5° slope about 60% of the movement is downslope but increases to 95% on a 25° slope
Mostly effective on slope angles between 33%-45% and at start of rainfall event when soil is still loose
On even slopes raindrops hit the soil, compacting it but also dislodging particles equally in all directions
Soil Creep
During heavy rainfall, volume and weight of soil increases, causing it to move downhill, bunching up in ridges called terracettes
During cold periods, moisture freezes and soil expands by 9% and during melting falls vertically down (heave)
Repetition of expansions and contractions (freezing/thawing or wetting/drying) that causes soil creep
Very slow - 1 inch per year
Can damage infrastructure and nature
Earthflows
Broken down fragments of rock on slopes of more than 5-15° that are saturated with water are prone to flows of earth in lobes or tongues
Around 1-15km of movement per year
Mudflows
More rapid form of movement than earth flows and exceed 15 km/h so break vegetation
In extreme cases they can move at 40 km/h but these are normally caused by volcanic eruptions
Slides / slumps
Slides are sudden movements of landmass with no internal derangement (unlike earthflows and mudflows) under the force of gravity
The break is clean and straight leaving an exposed scarp
Rocks that are jointed or have bedding planes are particularly susceptible
Where rotational movement occurs a slump is formed
Common feature on coasts with clay cliffs where slope becomes saturated leading to vertical movement along a slip plane
Turbulent flows
Internal derangement
Turbulence in material acting like a fluid
E.g. earthflow / mudflow which causes it to move more slowly
Laminar flows
No internal derangement
In a slide, material moves more efficiently as a whole
Leads to faster speed of mass movement
Rockfalls
Occurs on steepest slopes over 40° and often results from extreme physical or chemical weathering
Material bounces or falls vertically forming scree slopes or talus at the foot of the slope
When falling from cliff faces, rock can exceed speeds of 50 km/h
Sheer cliff faces and scree slopes at the bottom are left behind
Debris avalanche
A sudden catastrophic collapse from an unstable steep-sided mountain, most frequently on volcanoes
Speeds of over 50 km/h, very large scale, extreme internal deformation
Soil creep example: Trinidad and Tobago 2012
Tropical storm Helen caused prolonged heavy rainfall
Infrastructure caused less infiltration
Saturation increased and pore pressure was higher
This and steep sided terrain made mass movement more likely
Flooding and falling trees damaged houses, roads and communications
$61mn investment into new roads as a result
Mudflow example: Nevada del Ruiz 1985
Volcano eruption triggered catastrophic mudflows/lahars
Relatively small eruption with devastating consequences (VEI 3)
Mudflows travelled up to 100 km at speeds of 60 km/h
23,000 people lost their lives, which was 75% of Armero’s population
Over 5,000 injured and thousands left homeless
The town of Armero itself was completely destroyed and is now uninhabited
$1 billion in damages
Lahars
Flows of volcanic material (ash) which is very viscous caused by the melting of snow on volcanoes
Lahars flow down volcano and into rivers through gullies
Mudflow and Lahar example: Mount Pinatubo 1991
2nd largest volcanic eruption of the 20th century (VEI 6)
Mudflows travelled over 50 km from eruption
Lahars continued for over 5 years
more than 350 people died
over 100,000 people displaced
73,000 homes destroyed
$700+ million in damages
Landslide example: Lituya Bay Earthquake 1958
The earthquake of magnitude 8.3 caused a large landslide.
The landslide slid into water which caused a tsunami of height 524 meters
Landslide example: Oso Landslide, Washington 2014
Portion of unstable hill collapsed, sending around 11,000 tonnes of mud and debris around 4 miles around, engulfing neighbourhoods and area of around 1 square mile
43 deaths and 49 homes destroyed
Most deadly landslide in US history not caused by earthquake or eruption
Rockfall example: El Capitan, Yosemite 2017
Freeze thaw weathering and exfoliation weakened the rock
An unusually wet winter increased water content
1300 tonnes of rock fell, leaving a visible scar on the cliff face
1 person died and 1 was injured
Only one small point of contact with the rock, showing that this was a process that had been occurring over millions of years
