Landslides PDF - IRDR0001-1.1

IRDR0001: NATURAL AND ANTHROPOGENIC HAZARDS AND VULNERABILITY Landslides Dr Ting Sun What are landslides? How do landslides occur? How to mitigate landslide-related hazard risks? IRDR0001: NATURAL AND ANTHROPOGENIC HAZARDS AND VULNERABILITY Landslides Dr Ting Sun What are landslides?...

IRDR0001: NATURAL AND ANTHROPOGENIC HAZARDS AND VULNERABILITY Landslides Dr Ting Sun What are landslides? How do landslides occur? How to mitigate landslide-related hazard risks? IRDR0001: NATURAL AND ANTHROPOGENIC HAZARDS AND VULNERABILITY Landslides Dr Ting Sun What are landslides? How do landslides occur? How to mitigate landslide-related hazard risks? Landslide Definitions “The movement of a mass of rock, debris or soil down a slope.” (Cruden, 1991) “The perceptible downward sliding or falling of a relatively dry mass of earth, rock or combination of the two under the influence of gravity. Also known as landslip” (McGraw-Hill, Dictionary of Earth Science, 2002) “A landslide is the movement of rock, debris or earth down a slope. They result from the failure of the materials which make up the hill slope and are driven by the force of gravity. Landslides are known also as landslips, slumps or slope failure.” (Geoscience Australia website, accessed 2016) “Any kind of moderate to rapid soil movement including lahar, mudslide, debris flow. A landslide is the movement of soil or rock controlled by gravity and the speed of the movement usually ranges between slow and rapid, but not very slow. It can be superficial or deep, but the materials have to make up a mass that is a portion of the slope or the slope itself. The movement has to be downward and outward with a free face.” (EM-DAT website, accessed 2016) Global landslide distribution Landslides: A Secondary Hazard Landslides usually occur in response to other hazards: Heavy precipitation Rapid snowmelt Earthquakes Volcanoes Wildfires kills vegetation, less integrity of soil, spraying water to end fire => cascading hazards Faure Walker et al., (2013) Landslides: A Secondary Hazard Landslides and flooding closely associated with: Precipitation Runoff Saturation by ground water Faure Walker et al., (2013) Landslides: A Secondary Hazard Earthquakes: Nepal, 25th April and 12th May, 2015 5,600 landslides identified as of July 2015 Generally areas affected by landslides in the region of overlap of the two events were affected by both earthquakes EWF Project (2015) Landslides: A Trigger Hazard Landslides can cause other hazards: Flooding Damming E.g. Slumgallion Landslide, Colorado, USA Tsunami when mass falls in sea or ocean E.g. 1929 Grand Banks (off coast of Newfoundland, Canada) E.g. Lityuva Bay, Alaska, USA, July 9th 1958. Earthquake triggered landslide, which triggered 30m wave – largest landslide generated tsunami recorded (USGS, 2008) Slumgallion Landslide, Colorado, USA Landslide dammed Lake Fork of Gunnison River, flooded valley forming Lake Cristobal Photograph by Jeff Coe, USGS USGS Circular 1325 (2008) Landslides: Timing During monsoon in eastern and southern Asia During hurricane and typhoon season in Central America and Caribbean. This is during the summer and autumn in the Northern Hemisphere Pan American Highway, El Salvador, near town of San Vicente, 2001, Photograph by Ed Harp, USGS USGS Circular 1325 (2008) Classification: Material Landslides can be grouped into “rock” or soil” with respect to material type Rock Soil Debris: coarser sediments Earth: sand-sized or finer particles Classification: Movement Increasing slope angle Shallow Steep Flow Spread Slide Topple Fall (translational / rotational) Falls Falls occur where there are very steep slopes. The material falls through the air under gravity The material does not stay in contact with the slope as it falls. Often associated with undercutting Occurs very quickly Indicators of imminent rockfall include terrain with overhang, fractured or jointed rock along steep slopes, frequent freeze- thaw, cut faces in gravel pits Common on steep and vertical slopes, also in coastal areas Cause economic losses due to interference with transport Cause death from falling rocks Falls - examples Bridport, Dorset, UK, 2013 Credits Glenwood Canyon, 2003 Credits Topples Forward rotation out of the slope of a mass of material (soil and rock) about a point or axis below the centre of gravity of the displaced mass cascading way Can occur slowly or very fast A rock topple is similar to a pile of dominos falling. Usually occurs due to erosion at base of cliff e.g. by wave action Can be due to water or ice in cracks in the mass Common in columnar-jointed volcanic terrain, and along stream and river courses where the banks are steep Topples Topple in Portland Stone, south of Mutton Credits Cove, Portland Credits Slides Slides occur where there are moderately steep slopes of 20º to 40º The material maintains contact with the slope it moves down Slides can be rotational (slump) or translational Slides can occur at various speeds Slides Most movement occurs along base of slide and coherence of material maintained. Can move as largely intact or in broken pieces. Can have some internal deformation. Volume of displacing material enlarges from the area of local failure. One of most common types of mass wasting worldwide. Generally translational slides are shallower than rotational slides. Slides Surface of rupture has a depth-to-length ratio of < 0.1 in translational slides (Highland and Bobrowsky, 2008). Size – small or up to several kilometres. With increased velocity, the landslide mass of translational failures may disintegrate and develop into debris flow. Common causes are water saturation or earthquakes. Widening cracks at top or toe bulge may be an indicator of imminent failure. Anatomy of a landslide Credits Slides (Translational Landslides) Batton River Valley, British Credit: British Geological Society Columbia, Canada, 2001 Highland and Bobrowsky (2008) Slumps (Rotational Landslides) Slumps are a type of slide. In a slump, the material does not slide very far. Sometimes refereed to as interrupted landslides. At the top of a slump, a scoop-shaped scarp is characteristic. In a slump, the material rotates rather than flowing downslope parallel to the slope. Slumps (Rotational Landslides) The slump generally behaves as one large mass of material, but the material at the bottom of the slump may be poorly consolidated. Surface of rupture generally has a depth-to-length ratio between 0.3 and 0.1 (Highland and Bobrowsky, 2008). Secondary slumps may occur eroding the rear scarp. Slumps (Rotational Landslides) New Zealand, 2007 Holbeck Hall, North Yorkshire, 1993 Crozier, Encyclopedia of New Source - bgs Zealand, Sep 2007 Spreads Gradual lateral displacement of large volumes of material. Can be up to hundreds of metres. less harmful but damages infrastrucuture over time Extension of an upper layer over and subsidence into a softer underlying material. The under layer may cause the upper layer to break into blocks by squeezing upwards. If the weaker unit is relatively thin, the overriding fractured blocks may subside into it, translate, rotate, disintegrate, liquefy, or even flow. Spreads May result from liquefaction or flow of the underlying material possibly due to saturation. Can be block spreads, liquefaction spreads, and lateral spreads. Occur on very shallow or almost flat terrain (up to a couple of degrees inclination) where there are liquefiable soils. Spreads Following 2001 Nisqually Earthquake, Puget Sound Following 1989 Loma Prieta Earthquake, area in Washington, USA California, USA Photograph from Seattle Times Photograph by Ellen, U.S. Geological Survey, Highland and Bobrowsky (2008) Flows Internal deformation Flows occur when the material (rock, mud, or soil) mixes with water. If a slope becomes saturated with fluid, then the pore spaces become full leading to the slope to lose all its cohesion and can spontaneously flow. Flows The water causes the material to behave like a viscous fluid. A large amount of material can be deposited The material can travel a great distance Usually stop at base of slopes in debris fans or cones. Flows cannot be stopped but can be deflected. Flows: Mud Flows Mud flows and debris flows contain a lot of mud while debris flows also have a large amount of rock debris. Mudflows and debris flows travel with speeds of 0.1-35 [km hr-1]) Lahars = Mudflows associated with volcanoes Soliflucation = Mud glaciers In Arctic or Alpine conditions, soil flows can occur slowly during the summer when the frozen soil thaws and becomes saturated with water above the layer of permafrost. Flows: Debris Flows Debris avalanches are very large and rapid Debris flow channels have levees look like stream channels but the material inside is different Flows: Earth flows Earth flows occur on gentle to moderate slopes deeper and slower (a few metres per day) Usually occur in fine-grained soil Flows Southern Leyte debris avalanche, Philippines, Caraballeda debris flow, north coast of Venezuela, February 2006 December 1999, killed 30,000 Photograph by University of Tokyo, Photograph by Smith L.M., Waterways Experiment Station Highland and Bobrowsky (2008) Creep: Slow Earthflow Soil creep occurs very slowly Usually less than 1mm/decade Individual soil particles will gradually be displaced downhill due to the expansion and contraction of individual particles in response to increases and decreases in the amount of moisture. Creep can also occur due to freeze-thaw. Note in cold climates rock creep can occur Creep: Slow Earthflow Creep may occur in response to ground shaking during an earthquake. Creep does not constitute a sudden hazard, but it does cause damage to structures. Creep can be observed indirectly by fences and trees leaning downhill. Creep: Slow Earthflow East Sussex, UK, Photograph by Ian Alexander, Highland and Bobrowsky, 2008 IRDR0001: NATURAL AND ANTHROPOGENIC HAZARDS AND VULNERABILITY Landslides Dr Ting Sun What are landslides? How do landslides occur? How to mitigate landslide-related hazard risks? why classification ? to understand the science behind the phenomenon which will benefit predictions and then our impatc analysis and preparations Slope Stability Forces acting on a block on a slope normal force friction weight θ Slope Stability Forces acting on a block on a slope normal force friction W = weight W sinθ W cosθ weight θ Slope Stability Forces acting on a block on a slope normal force = W cosθ friction W = weight W sinθ W cosθ θ Slope Stability Forces acting on a block on a slope normal force = W cosθ friction = μN W = weight μ = friction coefficient N = normal force W sinθ W cosθ θ Slope Stability Forces acting on a block on a slope normal force = W cosθ friction = μW cosθ W = weight μ = friction coefficient N = normal force W sinθ W cosθ θ Slope Stability Forces acting on a block on a slope normal force = W cosθ friction = μW cosθ W = weight μ = friction coefficient W sinθ The object will slip downslope when: W cosθ θ W sinθ > μW cosθ balance equation tanθ > μ Slope Stability normal force = W cosθ friction = μWcosθ Angle of Repose The maximum stable inclination that a loose pile of material can sustain W sinθ W cosθ θ Determinants: W = weight Coherence of the material μ = friction coefficient Size of particles Sorting of particles The object will slip downslope when: Angularity of particles W sinθ > μW cosθ tanθ > μ  θ > arctan(μ) Slope Stability normal force = W cosθ friction = μWcosθ Moisture content matters Small amounts of water strengthens soil by increasing the surface tension, however, once W sinθ the pore spaces between the grains are full of W cosθ θ water the cohesion between the grains is reduced Water adds weight to the slope W = weight Water acts as a lubricant on existing planes of μ = friction coefficient weakness, reducing the friction The object will slip downslope when: For a typical material friction coefficient ≈ 0.6 (typically 0.3 - 1) angle of repose ≈ 30° W sinθ > μW cosθ tanθ > μ  θ > arctan(μ) Size of Particles Sorting of Particles better cohesion btw particules Angularity of Particles better cohesion btw particules Pore Fluid - Unsaturated cohesion is unhanced Pore Fluid - Saturated less cohesion  Pore Fluid and Angle of Repose Effective normal stress = friction = μWcosθ? W cosθ – pore pressure Dry particles have no cohesion W sinθ Damp particles have increased cohesion W cosθ θ between the particles due to dipolar forces within the water creating surface tension, increasing the angle of repose W = weight μ = friction coefficient When the pore spaces are filled with water The pressure from the water reduces the effective normal stress The object will slip downslope when: (= normal stress – pore pressure) This reduces the friction and thus decreases the angle of repose W sinθ > μ(W cosθ – pore pressure) Pore Fluid – Sandcastles Dry sand Damp sand Wet sand Saturation A primary cause of landslides is slope saturation by water Triggers: Intense rainfall Snowmelt Changes in ground-water levels Surface-water level changes along coastlines, earth dams, and in the bank of lakes, reservoirs, canals, and rivers Highland and Bobrowsky, (2008) IRDR0001: NATURAL AND ANTHROPOGENIC HAZARDS AND VULNERABILITY Landslides Dr Ting Sun What are landslides? How do landslides occur? How to mitigate landslide-related hazard risks? Ground Indicators of A Landslide Springs and wet or saturated ground in previously dry areas on or below slopes Ground cracks: cracks in snow, ice, soil, or rock on or at the head of slopes Sidewalks or slabs pulling away from structures if near a slope Soil pulling away from foundations Offset fence lines, which were once straight or configured differently Unusual bulges or elevation changes in the ground e.g.: pavements, paths, or sidewalks Tilting telephone poles, trees, retaining walls, fences Highland and Bobrowsky (2008) Ground Indicators of A Landslide Excessive tilting or cracking of concrete floors and foundations Broken water lines and other underground utilities Rapid increase or decrease in stream-water levels possibly accompanied by increased turbidity (soil content clouding the water) Sticking doors and windows and visible open spaces walls and frames are shifting and deforming Creaking, snapping, or popping noises a house, building, or grove of trees (for example, roots snapping or breaking) Highland and Bobrowsky (2008) Active and Inactive Indicators Source - geocases1 Input Maps for Hazard Analysis Topographic Map Indicates slope gradient, terrain configuration, drainage pattern. Terrain Map Identifies material, depth, geological processes, terrain configuration, surface and subsurface drainage, slope gradient (also called surficial geology or Quaternary geology maps). Bedrock Map Identifies bedrock type, surface and subsurface structure, surficial cover (overburden), and age of rock over a topographic map base. Engineering Soil Map Identifies surficial material type, drainage, limited engineering characteristics, soils characteristics, vegetation cover. Forest Cover Map Identifies surface vegetation, topographic features, surface drainage pattern, and in some cases, soil drainage character. Highland and Bobrowsky (2008) Sources of Input Maps Aerial Photographs Identification can be made of vegetation cover topography drainage pattern soil drainage character bedrock geology surficial geology landslide Example of aerial photograph of La Conchita landslide in California, USA, taken in 2005 AirPhoto USA, Randy Jibson USGS USGS Circular 1325 (2008) Sources of Input Maps InSAR (Interferometric Synthetic Aperture Radar) Merging of two images taken in same place at different times can show any ground displacement that has occurred between the timing of the two images SAR (Synthetic Aperture Radar) can produce much higher resolution results than ordinary radar on a typical Earth-orbiting satellite Sketch of two SAR acquisitions occurring at successive satellite passes. Wasowski and Bovenga (2022) Sources of Input Maps InSAR (Interferometric Synthetic Aperture Radar) Merging of two images taken in same place at different times can show any ground displacement that has occurred between the timing of the two images SAR (Synthetic Aperture Radar) can produce much higher resolution results than ordinary radar on a typical Earth-orbiting satellite Interferogram from InSAR, Cascade Range central Oregon 1997-2001 USGS Circular 1325 (2008) Sources of Input Maps LiDAR (Light Detection and Ranging) Can produce high resolution and high precision DEMs (digital elevation models) to help identify sites of past landslides, which may not be possible by other means due to things like Oblique LiDAR image of La Conchita, California, USA, taken in 2005. vegetation cover Outline of 1995 landslide (blue),2005 landslide (yellow) and ancient landslide scarp (red) la are shown. Airborne 1 and Randy Jibson, USGS USGS Circular 1325 (2008) Sources of Input Maps LiDAR (Light Detection and Ranging) historical landslide Left: lidar point clouds Right: bare earth DEMs the vegetation is stripped away to reveal past landslides steep slopes at risk of failure masked by forested canopies In dense forests, landslides - especially old landslides - might be invisible on aerial images and hard to detect from the ground. Credit: USGS Landslide Susceptibility Maps The relative likelihood of future landslide based solely on the intrinsic properties of a locale or site Classification, volume and spatial distribution of landslides which exist or potentially may occur in an area Include slope properties affecting landslide likeliness Include past landslide information Tend to show relative susceptibility rather than absolute susceptibility needed for probabilistic assessments Example of landslide susceptibility map from the Mackenzi River Valley, Northwest Territories, Canada, Graphic by Rejean Couture, Geological Survey of Canada USGS Circular 1325 (2008) Landslide Hazard Maps the possibility of landslides occurring throughout a given area. An ideal landslide hazard map shows not only the chances that a landslide might form at a particular place, but also the chance that it might travel Relative hazard downslope a given distance. High (more than 75 shallow landslides/km 2) Medium (20−75 shallow landslides/km 2) Low (fewer than 20 shallow landslides/km 2) Failure locations from landslide database Relative Hazard Map of Magnolia area in city of Seattle, Washington, USA, USGS Circular 1325 (2008) Landslide Risk Maps Landslide potential along with the expected losses to life and property, should a landslide occur. Risk maps = a landslide hazard map x all possible consequences Property damage Casualties loss of service risk = hazard x vulnerability x exposure Landslide Risk Index score of USA, https://hazards.fema.gov/nri/ Influence from Anthropogenic Activity How can anthropogenic activity increase landslide hazard? Undercutting for buildings, agriculture causing an over-steepening of the slope Building adding weight to slopes Adding impermeable structures such as roads Removal of plant roots Explosions such as mine collapse Manipulating drainage routes friction is not only influenced by normal stress, there are more developped equations likfe friction : c + mu x normal stress the c represents the cohesion of the material which depends on its properties Influence from Anthropogenic Activity How can anthropogenic activity decrease landslide hazard? changes pore pressure, which changes Drainage control normal force Slope supports and retaining structures Internal slope reinforcement Modification of slope geometry changes theta (see balance equation) Land use regulations Landslide Hazard Mitigation Drainage control Surface channels reducing water Site leveling infiltration Ditches and drains Subsurface drainage to reduce pore Drainpipes fluid pressure by removing Vertical or diagonal planting to direct groundwater water down the slope Impermeable barrier reducing water Root systems carry water down into infiltration (note the water will drain soil elsewhere) Straw wattles Debris flow basins Landslide Hazard Mitigation Drainage control - Drainpipes Widely used for landslide prevention in highway construction USGS Circular 1325 (2008) Landslide Hazard Mitigation Drainage control - Straw wattles Capture sediment Hold sediment onsite Enables seeds to settle and germinate Aids revegetation Photograph by Highland, USGS USGS Circular 1325 (2008) Landslide Hazard Mitigation Slope stability Slope supports and retaining structures Catch material moving over surface e.g. catch ditches, cable and mesh Support slope from below Gabions (wire cage or box filled with rocks, cement etc) Piles (concrete, wood or steel beams driven into the ground) Concrete retaining walls (at bottom of slope) Stream channel linings Bio-engineering: Vegetation such as trees, stout grass, broad-based shrubs, bamboos Internal slope reinforcement Rock anchors Anchors slope by pinning to layer below Landslide Hazard Mitigation Slope stability - Gabions Usually inexpensive Simple and quick to construct Can withstand foundation movement because flexible Do not require elaborate foundation preparation Very permeable providing drainage Gabion wall along highway, Pennsylvania Photograph by Highland, USGS USGS Circular 1325 (2008) Landslide Hazard Mitigation Slope stability - Vetiver grass local vegetation Vetiver grass system in the Democratic Republic of Congo for gully control in urban areas and highway stabilization. Final picture is after about 3 months. USGS Circular 1325 (2008) Landslide Hazard Mitigation Slope stability - Vetiver grass Worldwide distribution of active Vetiver grass programmes (http://www.vetiver.org) Landslide Hazard Mitigation Slope stability - retaining walls Retaining walls can fall down Mexico City, 15m high debris-flow rataining wall destroyed by landslide following heavy rain. Photograph by Chinagate/Xinhua USGS Circular 1325 (2008) Landslide Hazard Mitigation Slope Geometry Controlled removal of upper slope material Reduces stresses May use blasting Reduce slope angle Reduces stresses Increases the likeliness of remaining below the angle of response Constructing benches on steep slopes Scaling or Trimming Statistics of Landslide Disasters Comparing landslides with other hazards in fatalities Statistics of Landslide Disasters Comparing landslides with other hazards in affected people Statistics of Landslide Disasters Proportion of landslide and avalanche fatalities (1975 – 2000) www.worldmapper.org © Copyright Sasi Group (University of Sheffield) and Mark Newman (University of Michigan). Accessed March 2016. Data from the United Nations Environment Programme (2005) Statistics of Landslide Disasters Deaths in landslide and avalanche disasters per million people per year (1975 – 2000) Territory Deaths per million people per year Iceland 4.9 Ecuador 2.4 Papua New Guinea 2.2 Tajikistan 2.2 Peru 1.6 Colombia 1.3 Puerto Rico 1.2 Kyrgyzstan 1.2 Afghanistan 1.2 Nepal 0.9 www.worldmapper.org © Copyright Sasi Group (University of Sheffield) and Mark Newman (University of Michigan). Accessed March 2016. Data from the United Nations Environment Programme (2005) Statistics of Landslide Disasters Landslide fatalities – top ten events Country Date Fatalities Soviet Union 1949 12,000 Peru December 1941 5,000 Honduras 20th September 1973 2,800 Peru 10th January 1962 2,000 Italy 9th October 1963 1,917 China 7th August 2010 1,765 Philippines 17th February 2006 1,126 India 1st October 1968 1,000 Colombia 27th September 1987 640 Guatemala 1st October 2015 627 Statistics of Landslide Disasters Landslide losses – top ten events Country Date Losses (USD ‘000) Peru January 1983 988,800 China 1st May 1998 890,000 China 7th August 2010 759,000 Italy 14th December 1982 700,000 Switzerland 21st February 1999 685,000 Italy 28th July 1987 625,000 Guatemala 4th September 2010 500,000 Ecuador 28th March 1993 500,000 Soviet Union 10th March 1989 423,000 Bolivia 8th December 1992 400,000 (Plurinational State of) Statistics of Landslide Disasters Landslide Human Exposure Credits Statistics of Landslide Disasters Landslide GDP Exposure Credits Statistics of Landslide Disasters Landslide Fatalities Credits Statistics of Landslide Disasters Comparing hazard impacts (1900 – 2015) Total damage Continent Occurrence Total deaths Total affected (USD’000) Storm 3,853 1,390,345 1,008,573,839 1,047,616,229 Flood 4,565 6,949,389 3,626,235,717 700,667,499 Earthquake shaking 1,240 2,307,493 187,962,942 552,747,707 (Shaking and (1,301) (2,575,415) (109,918,875) (775,749,147) tsunami) Mass movement 720 67,817 13,797,835 9,079,598 (all) Volcanic activity 241 3,665 261,280 110,000 Statistics of Landslide Disasters Mass movement impacts by continent (1900 – 2015) Total Total Total Continent Occurrence Affected Injured Homeless deaths affected damage Africa 40 1,457 33,369 285 29,159 62,813 n/a Americas 189 23,200 5,281,857 5,093 245,876 5,532,826 3,086,727 Asia 390 25,597 4,172,289 5,158 3,954,628 8,132,075 2,878,916 Europe 81 16,931 39,657 524 8,625 48,806 3,111,489 Oceania 20 632 3,263 52 18,000 21,315 2,466 Total 720 67,817 9,530,435 11,112 4,256,288 13,797,835 9,079,598 Suggested Reading Farahmand A. and AghaKouchak A. (2013) A satellite-based global landslide model, Natural Hazards Earth Systems, 13, 1259-1267, http://www.nat-hazards-earth-syst- sci.net/13/1259/2013/nhess-13-1259-2013.pdf Faure Walker J.P., Landslides, Chapter 7.4 in Pompella M., Scordis N., Nicos A. (Eds) 2017, The Palgrave Handbook of Unconventional Risk Transfer, Palgrave ISBN: 978-3-319-59296-1 Perkins S. (2012) Death toll from landslides vastly underestimated http://www.nature.com/news/death-toll-from-landslides-vastly-underestimated-1.11140 Petley D. (2012) Geology http://dx.doi.org/10.1130/G33217.1 References Highland L.M. and Bobrowsky P. (2008), The landslide handbook – A guide to understanding landslides: Reston, Virginia, USGS Geological Survey Circular, 1325, 129p UNISDR (2009) Global assessment report on disaster risk reduction