Landslides and Mass Movements PDF

Mass Movements Dr. Neil J. Porter Introduction to Natural Hazards ESCI-1000 The lecture material is the intellectual property of Dr. Neil J. Porter....

Mass Movements Dr. Neil J. Porter Introduction to Natural Hazards ESCI-1000 The lecture material is the intellectual property of Dr. Neil J. Porter. Distribution or publication of this material in whole or in part is illegal, under the Canadian Copyright Act, RSC 1985.  Landslides and Mass Movements Overview 7.0 Introduction to Mass Movement 7.1 Types of Landslides and Mass Movements 7.2 Slope Processes 7.3 The Effects of Mass Movements 7.4 Human Interactions 7.5 Prevention of Mass Movements 7.6 Adjustment to Landslide Hazards  Landslides and Mass Movements 7.0 Introduction to Mass Movements Landslide and mass wasting Terms used to describe the downslope movement of rock or sediment due to gravity What is a snow avalanche? Masses of snow, generally more than a few cubic metres in volume, that separate from the intact snowpack and slide or flow downslope Types of landslides are determined by: mechanism of movement 1. 2. Rate of movement 3. ammount of water present 4. type of material  Landslides and Mass Movements 7.0 Introduction to Mass Movements: Geographic Locations Anywhere that have significant slopes and mountains  Landslides and Mass Movements 7.1 Types of Mass Movements One can distinguish between: 1. fall 2. slide 3. flow  Landslides and Mass Movements 7.1 Types of Mass Movements: fall Rock fall Bounding of rock or blocks of sediment from the face of a cliff Blocks pile up at the bottom to form talus fpund in temperate regions water gets into joints and cracks and when water freezes it expands and causes rocks to dislodge  Landslides and Mass Movements 7.1 Types of Mass Movements: slides Rock slides Downslope movement of a coherent block of rock or sediment along a discrete failure plane  Landslides and Mass Movements 7.1 Types of Mass Movements: The Frank Slide  Landslides and Mass Movements 7.1 Types of Mass Movements – slides ( soil slip) Slopes covered by mud are seen after landslides in Nova Friburgo, Brazil on Monday, Jan. 17, 2011  Landslides and Mass Movements 7.1 Types of Mass Movements: slides slump ( only occurs in unconsolidated materials) Failure plane is curved upward  Landslides and Mass Movements 7.1 Types of Mass Movements : Flows Flow Downslope movement of sediment in which particles move semi- independently of one another, often with the aid of water Debris flow Typically move rapidly; mixtures of mud, debris and water  Landslides and Mass Movements 7.1 Types of Mass Movements: Flows Creep Very slow flow of rock or sediment Sackung - slow movement of large masses of rock Topple – a rock mass pivots about a point  Landslides and Mass Movements 7.1 Types of Mass Movements: Earth flows : are the downslope, viscous flow of saturated fine grain materials. making tounge shape depression Resemble mudflows, however slower moving and covered with nonfluid material.  Landslides and Mass Movements 7.1 Types of Mass Movements: Flows Avalanches Rapid downslope movement of snow and ice (mixed with air). Depend on steepness of slope, stability of snowpack, weather. Avalanches move down chutes Avoiding these areas minimize hazard Landslides and Mass Movements 7.1 Types of Mass Movements: Flows point- release avalanches 1. Begin with failure of a small amount of loose fluffy snow Widens as it moves downslope Commonly happen after a heavy snowfall  Landslides and Mass Movements 7.1 Types of Mass Movements: Slab avalanches 2. Snowpack fractures along a weak layer at depth Moves as a cohesive block More dangerous than point-release avalanches  Landslides and Mass Movements Complex 7.1 Types of Mass Movements: Most large mass Fall movements consist of several elements. For Slide example, a rock fall can lead into a debris avalanche, which can in Flow turn, become a debris flow. Hope Slide, BC  Landslides and Mass Movements 7.2 Slope Processes Rain and snowmelt increases the driving force by increasing the weight on the Irrigation, septic tanks etc slope and by decreasing the resisting force by increasing pore water pressures The driving force is GRAVITY. Its effect increases with: a) steeper slopes Weight of new b) height of slope buildings c) weight on slope MATERIAL STRENGTH (COHESION) of the material (rock etc) resists movement. This decreases with weathering and increased pore water pressure Erosion by rivers, from rain and snowmelt waves, glaciers, humans etc  Landslides and Mass Movements 7.2 Slope Processes Driving and resisting forces are not static As local conditions change, these forces may change Factor of safety may increase or decrease Forces on slopes are determined by: Angle of the slope 1. 2. vegetation climate 3. 4. type of material water 5. 6. time  Landslides and Mass Movements 7.2 Slope Processes: Role of Earth Material 1. Can affect both the type and frequency of movement Important characteristics: Mineral composition Degree of cementation or consolidation Presence of zones of weakness Ability of material to transmit water  Landslides and Mass Movements 7.2 Slope Processes: 2. Role of slope Slope steepness Steeper the slope, the greater the driving force Steep slopes are associated with falls Moderate slopes are associated with flows Gentle slopes are associated with creep Topographic relief Height of hill above land Mass wasting occurs more in high relief areas  Landslides and Mass Movements 7.2 Slope Processes: Avalanche Movements 2. Slope Angle The most important terrain factor for avalanche formation Avalanches tend to occur on slopes with angles between 25 and 60 degrees Most avalanches occur between 30-45 degrees Slope Orientation Leeward slopes are more likely to have avalanches Sun-facing slopes are more prone to avalanches during sunny, warm weather; shaded slopes are more prone to avalanches in cold weather  Landslides and Mass Movements 7.2 Slope Processes: Role of Climate 3. Influences Amount and timing of water that infiltrates or erodes slope Type and abundance of hillside vegetation Arid regions: rock falls, debris flows and soil slips Humid regions: complex landslides, earth flows, and creep, and debris flows  Landslides and Mass Movements 7.2 Slope Processes: 4. Role of vegetation ( influenced by climate) Function of climate, soil type, topography, and fire history Significant factor in slope stability 1. Vegetation provides a protective cover that slows surface erosion 2. Roots add strength and cohesion to the slope 3. Vegetation adds weight to the slope  Landslides and Mass Movements 7.2 Slope Processes: 5. Role of water Almost always directly or indirectly involved Saturated slopes: shallow soil slips and debris flows Deep infiltration: slumps develop months or even years after Water erosion: erosion of base of slope decreases stability Water can cause spontaneous liquefaction or quick clay Fine grained material that loses strength when disturbed and flows like a liquid  Landslides and Mass Movements 7.2 Slope Processes: 6. Role of time Forces change with time Driving and resisting forces change with season due to changes in moisture content or water table Chemical erosion occurs slowly over time  Landslides and Mass Movements 7.3 Effects of Mass Movements: Landslides 30 people are killed each year on average in North America; damage exceeds $1 billion People hit with or buried in falling debris Slides may damage homes, roads, and utilities constructed at the top, base, or side of a hill Slides may block roads and railways impeding travel, or may block streams causing flooding  Landslides and Mass Movements 7.3 Effects of Mass Movements: Avalanche Human deaths (600 in Canada since mid 1800s) Economic losses (destruction and blockage of roads, property damage) Damage to forests; soil removal  Landslides and Mass Movements 7.3 Effects of Mass Movements: Avalanche Safety Survival depends on the length of time the person is buried and the burial depth 92% survive if rescued within 15 minutes, only 30% survive after 35 minutes, almost 0% after 2 hours Only 5 to 10% survive burial in more than 1.5 m of snow Rescue Avalanche cords Avalanche transceivers Probes Shovels Avalanche dogs  Landslides and Mass Movements 7.4 Human Interactions Landslides are a natural phenomenon Humans increase through: Expansion of urban areas, transportation networks, and natural resource use Grading of land surfaces that increase instability of surfaces Humans can decrease incidence through: Building stabilizing structures Improving drainage  Landslides and Mass Movements 7.4 Human Interactions Major environmental and economic issue in some regions Two controversial practices Clear-cutting Removing all of the trees from a defined area Road building Increases erosion (soil slips) on geologically unstable land Interrupts surface drainage, alters subsurface movement of water, and changes distribution of earth materials  Landslides and Mass Movements 7.5 Prevention of Mass Movements Drainage Control ( get rid of water) 1. Keeps water from infiltrating a slope Drains can divert water Impermeable layers can keep water from infiltrating  Landslides and Mass Movements 7.5 Prevention of Mass Movements Slope support 2. rock bolts and steel mesh  Landslides and Mass Movements 7.5 Prevention of Mass Movements 3. Avalanche barriers and tunnels: Avalanche shed Wedge barrier  Landslides and Mass Movements 7.5 Prevention of Mass Movements: Triggering Mechanisms  Landslides and Mass Movements 7.5 Prevention of Mass Movements 4. Retaining walls  Landslides and Mass Movements 7.5 Prevention of Mass Movements: Terracing 5. Terracing  Landslides and Mass Movements 7.6 Perception of and Adjustment to Landslide Hazards People continue to build in landslide-prone areas Need to find adjustments Movement of critical facilities Hospitals, schools and police stations should be placed in safe locations Landslide correction Landslide may be stopped after it starts using drains  Landslides and Mass Movements 7.6 Perception of and Adjustment to Mass Movement Hazards Be wary of leaks in swimming pools or septic tanks, trees or fences that tilt, or sagging or taut utility wires Be wary of small springs Look for linear or curved cracks Look for surface features Don’t buy a home that has a landslide hazard  Landslides and Mass Movements 7.6 Perception of and Adjustment to Mass Movement Hazards Location of Infrastructure Risk is estimated by determining avalanche frequency, distribution, size Structures Fences, nets, berms, and avalanche sheds are used for protection Triggering Explosive charges are projected from cannons, fired by artillery, or dropped from helicopters