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Slope Failures
Natural Disasters
| Question | Answer |
|---|---|
| Mass wasting (movement) | gravitational downslope transport of rock, regolith, snow or ice |
| What are some factors that affect slope shape? | Material strength: ability to avoid failure. Depends on if it is coherent or unconsolidated or has planes of weakness. Climate: affects which agents of erosion are at work (ice, water, wind) and how fast; also affects soil and vegetation formation on s |
| What are some factors that affect downslope? | Slope angle Critical angle of repose Cohesion and water saturation Material strength |
| Shear stress | stress resulting from application of force parallel to a surface (downhill) |
| Normal stress | component of stress perpendicular to the Earths planar surface |
| Critical Angle of Repose | Definition: maximum angle at which sediment particles can stand without falling Depends on: Grain size Grain angularity (rounded grains < angular grains) Moisture content |
| Cohesion means??? | Definition: the attraction between small soil particles that is provided by the surface tension of water between the particles. Friction + surface tension of water can provide resistant to sliding! |
| Types of slope failures: Creep | extremely slow downslope flow of sediment on the surface. Caused by cycles of freeze-thaw, wetting-drying and/or warming-cooling Occurs slowly Loss is mostly from damage to buildings rather than casualties |
| Types of slope failures: Solifluction | downslope creep driven by sequential freezing and thawing in tundra regions. Results in a soggy layer of ground above frozen permafrost that then moves downhill. |
| Translational slump | a landslide that moves along a regular sloping planar surface Typically occurs where a block of material is underlain by a weak surface that is parallel to the slope Block can travel down slope as a cohesive unit or lose internal cohesion and break up |
| Rotational Slump | a landslide in which the mass rotates on a concave failure surface |
| key term: flows | slope failures involving material that has lost internal cohesion. |
| Key term: mudflow | a flow of mud, rock, and water dominated by clay-sized particles (includes lahars) (ex. Afghanistan: heavy rain turned a slump into a mudflow and buried a village) |
| Debris flow: | a slurry of rock, sand, and water flowing downslope; water usually makes up less than half of the flow volume |
| Case study: Vargas Tragedy | Several debris and mudflows over a 2-day period caused by heavy rainfall Towns were built on debris from past flows 30,000 deaths and 75,000 people displaced |
| Quick clay: | water-saturated mud deposited in salty water composed of clay flakes with large pore spaces between the flakes; highly unstable |
| rockslide: | sudden movement of rock and debris down a non-vertical slope |
| rockfalls: | a slope failure that occurs when a mass free-falls down a cliff or steep slope; slope is often vertical |
| Frost wedging: | splitting of rock through pressure exerted when water freezes (freezing water expands it by ~10%). Coastal areas are more prone to frost wedging where temperature oscillates around the freezing point. |
| Sturzstroms (rock avalanches): | extremely rapid downslope movement of large volumes or rock and debris Largest and most destructive landslides Typically begin as rocks slides, but breaks up, entraining air and water Their mechanism is still under debate |
| Example: Frank, Alberta | Started as translational slide: gained speed and developed into a sturzstrom Event lasted less than 100 seconds Slope failure occurred along fractured limestone planes Based on the speed and distance, it is thought to have travelled on a cushion of com |
| How can we reduce and mitigate the risk of a slope failure? | Roots stabilize potential failure surfaces Potential failure surface dries and becomes stronger when water is removed Walls can be put in place to prevent damages from rock falls/avalanches Prohibit building at the base of mountains Zone hazardous are |
| Hazard Mapping and Mitigation | here are GIS softwares that can help identify areas of past and potential failures |
| Introduction to lost ground: Subsidence | relatively slow downward movement of land, typically at rates of cm/year |
| Introduction to lost ground: collapse | rapid movement of land, ranging from cm/hr to m/s of material disappearing almost instantaneously |
| What is the definition of karst? | Definition: topography resulting from dissolution of limestone rocks by water |
| Sinkholes: | a ground depression caused by collapse into an underground cavern |
| Collapse sinkholes: | formed when weak rock overlying a cavern collapses. These are the most dangerous kind of sinkhole. |
| Dissolution sinkhole: | depression formed from dissolution of rock at the ground surface |
| Cover-subsidence sinkhole: | depression formed when loose sediment at the surface washes into caverns or voids below. |
| Climate Factors: | Rainfall: water is the driving force behind dissolution Temperature: limestone and dolostone dissolve more readily in cold water Vegetation: plants can make water more acidic = rocks dissolve more readily Soil Type: influences the permeability of land |
| Land Subsidence | Subsidence is the settling of the ground in response to: extraction from ground water, extraction of oil, drying of peat When groundwater is extracted, pore spaces collapse, and the ground subsides |
| Permafrost | Ground that remains frozen (below 0 degrees) for as least a couple years |
| Interstitial ice | Ice that crystallizes in pores between grains of sediment |
| segregated ice | lenses of pure ice developed in permafrost sediment |
| Ice wedge: | wedge-shaped accumulation of ice in permafrost regions; formed from freeze-thaw cycles Melting of permafrost (interstitial and segregated ice) causes land subsidence, slumping and mudflows |
| thermokarst: | karst-like landscape in permafrost terrain caused by melting of permafrost under increasing temperature |
| Causes behind thermokarst: | Causes: Warming climate Erosion along coastlines and rivers where ice-rich sediment is exposed Heating the ground surface Vegetation removal |
| Thermokarst lakes: | circular oval or square lakes formed by thermokarst melting |
Created by:
SAdriana
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