landslide hazard zonation mapping

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  1. 1. Landslide Hazard Zonation Mapping BY Prof. A. Balasubramanian Centre for Advanced Studies in Earth Science, University of Mysore, Mysore-6
  2. 2. Landslide Is the down slope movement of loose soil, uncompact rock, and organic materials, under the effects of gravity along a sliding plane, Triggered by severe Rf, Sws, wt of overburden. Creates a new landform resulting from such movement. Hazardous/ disastrous.
  3. 3. Landslides associated with specific mechanics of slope failure and the properties and characteristics of failure types. Landslides are experienced in the hilly and mountainous areas, all over the world. May start from the top of a hill slope and move till the end of foot hill, or may start from the bottom of a hill slope, drawing all materials from the top to bottom.
  4. 4. Mass movement It is the movement of surface material caused by gravity. Landslides and rockfalls are examples of very sudden movements of this type. Of course geological agents such as water, wind and ice all work with gravity to cause a leveling of land.
  5. 5. Hill slopes Hills and mountains are typical geomorphic features. They are characterized by gentle or steep slopes. They are unique climatic zones. They are characterized by thick soil profiles, richness with soil moisture and plant growth.
  6. 6. Landslides as natural hazards Landslides normally occur on hill slopes. When the slopes are stable there will be no problem. When the slopes become unstable then they may lead to landslides.
  7. 7. Landslides occur when the stability of the slope changes from a stable to an unstable condition. The change in the stability of a slope may happen due to several factors. These factors may be acting alone or acting together.
  8. 8. The overburden may be mostly loose soil, may have a thick weathered zone, adequate natural or man-made vegetation, good root penetration, and all are under the direct influence of Climatic variations, especially rainfall.
  9. 9. Slope and bedrocks The Bedrock is related to the basement rocks of the hill. These may be massive or fresh, structurally weak or deformed. All of these may be under the influence of a tectonic force originating from the deep interior or a nearby huge structure like a dam.
  10. 10. The next factor is the degree of slope of the area. Slope is a common factor for a landslide. The categories of slope is to be understood first. It varies depending upon the geomorphic conditions, bedrock, nature of overburden, vegetation and drainage systems.
  11. 11. Slope A slope is the rise or fall of the land surface. It is important for the farmer or irrigator to identify the slopes on the land.
  12. 12. TYPES OF SLOPES Gentle Steep Concave Convex
  13. 13. The slope of line a is m=5/6 The slope of line b is m= -1/4 The slope of line c is m= 3/7 The slope of line d is m= -6/3 or m=-2 The slope of line e is m= 0 (but how do you use m= rise/run to get this number?)
  14. 14. Categories of slopes The following are the categories of slopes: Cliff > 80 degrees Precipitous 50-80 degrees very steep or steep 20-50 degrees moderate slope 6-20 degrees gentle slope 1-6 degrees flat terrain < 1 degree.
  15. 15. Slope (%) Approximate degrees Terminology 0 - 0.5 0 Level 0.5 - 2 0.3 - 1.1 Nearly level 2 - 5 1.1 - 3 Very gentle slope 5 - 9 3 - 5 Gentle slope 9 - 15 5 - 8.5 Moderate slope 15 - 30 8.5 - 16.5 Strong slope 30 - 45 16.5 - 24 Very strong slope 45 - 70 24 - 35 Extreme slope 70 - 100 35 - 45 Steep slope > 100 > 45 Very steep slope
  16. 16. Slope may be expressed in several ways but all depend upon the comparison of vertical distance (VD) to horizontal distance (HD). Before we can determine the percentage of a slope, we must know the VD of the slope. The VD is determined by subtracting the lowest point of the slope from the highest point.
  17. 17. Overburden soils and Bedrocks Rock type Structure Dip and strike Soil profile- properties of soils Rock material properties Rock mass properties & lastly Triggering mechanisms.
  18. 18. What do these pictures show?
  19. 19. Parts of a landslide zone
  20. 20. Landslide classification The landslide classification based on Varnes' (1978)system has two terms: -the first term describes the material type,- the second term describes the type of movement.
  21. 21. The material types- Rock, Earth, Soil, Mud and Debris Rock: is a hard or firm mass that was intact and in its natural place before the initiation of movement. Soil: is an aggregate of solid particles, generally of minerals and rocks, that either was transported or was formed by the weathering of rock in place. Gases or liquids filling the pores of the soil form part of the soil.
  22. 22. Earth: describes material in which 80% or more of the particles are smaller than 2mm, the upper limit of sand sized particles. Mud: describes material in which 80% or more of the particles are smaller than 0.06mm, the upper limit of silt sized particles. Debris: contains a significant proportion of coarse material; 20% to 80% of the particles are larger than 2mm, and the remainder are less than 2mm.
  23. 23. VARNES CLASSIFICATION OF SLOPE MOVEMENTS The five kinematically distinct types of movement are described in the sequence: - fall, - topple, - slide, - spread, - flow. Combining the two terms gives classifications such as: Rock fall, Rock topple, Debris slide, Debris flow, Earth slide, Earth spread, etc.
  24. 24. Landslides: Rock Fall Rock Falls are the detachment of rock from a steep slope along a surface on which little or no shear displacement takes place. The material then descends largely by falling, bouncing or rolling.
  25. 25. Rock Fall -Diagnostic Characteristics A fall starts with the detachment of soil or rock from a steep slope along a surface on which little or no shear displacement takes place. The material then descends largely by falling, bouncing or rolling. Free fall movement of material from a steep slope or cliff, EPOCH (1993).
  26. 26. Toppling failures are distinguished by the forward rotation of a unit about a pivot point. Typically involving tall columns of rock Vertical or steeply dipping discontinuities behind the block allow the rock mass to topple out of the face and second set of orthogonal joints, which defines the column height.
  27. 27. Landslides: Rock Topple A topple is the forward rotation, out of the slope, of a mass of soil and rock about a point or axis below the centre of gravity of the displaced mass.
  28. 28. Landslides: Rock Topple Diagnostic Characteristics Toppling failures are distinguished by the forward rotation of a unit about a pivot point. Typically involving tall columns of rock. Vertical or steeply dipping discontinuities behind the block allow the rock mass to topple out of the face and second set of orthogonal joints, which defines the column height. Similar to a fall but involves a pivoting action rather than a complex separation at the base of the failure, EPOCH (1993).
  29. 29. Rock Topple Diagnostic Characteristics
  30. 30. Landslides: Rock & Block Slide A slide is the downslope movement of a soil or rock mass occurring dominantly on the surface of rupture or relatively thin zones of intense shear strain.
  31. 31. Types of landslides
  32. 32. Translational slides move along a flat slippery zone.
  33. 33. Rotational slides move along a concave (bowl shaped) slippery zone.
  34. 34. Common types of slope failure
  35. 35. What Causes Landslides? There are two primary categories of causes of landslides: natural and human-caused. Sometimes, landslides are caused, or made worse, by a combination of the two factors. Natural Occurrences This category has three major triggering mechanisms that can occur either singly or in combination (1) water, (2) seismic activity, and (3) volcanic activity.
  36. 36. Effects of all of these causes vary widely and depend on factors such as steepness of slope, morphology or shape of terrain, soil type, underlying geology, and whether there are people or structures on the affected areas.
  37. 37. Landslide Causes and Triggering Mechanisms Physical CausesTriggers Intense rainfall Rapid snowmelt Prolonged intense precipitation Rapid drawdown (of floods and tides) or filling Earthquake Volcanic eruption Thawing Freeze-and-thaw weathering Shrink-and-swell weathering Flooding.
  38. 38. Natural Causes: Geological causes Weak materials, such as some volcanic slopes or unconsolidated marine sediments, for example Susceptible materials Weathered materials Sheared materials Jointed or fissured materials Adversely oriented mass disconti nuity (bedding, schistosity, and so forth) Adversely oriented structural discontinuity (fault, unconformity, contact, and so forth) Contrast in permeability Contrast in stiffness (stiff, dense material over plastic materials)
  39. 39. Natural Causes: Morphological causes Tectonic or volcanic uplift Glacial rebound Glacial meltwater outburst Fluvial erosion of slope toe Wave erosion of slope toe Glacial erosion of slope toe Erosion of lateral margins Subterranean erosion (solution, piping) Deposition loading slope or its crest Vegetation removal (by forest fire, drought)
  40. 40. Human Causes Excavation of slope or its toe Use of unstable earth fills, for construction Loading of slope or its crest, such as placing earth fill at the top of a slope Drawdown and filling (of reservoirs) Deforestationcutting down trees/logging and (or) clearing land for crops; unstable logging roads Irrigation and (or) lawn watering Mining/mine waste containment Artificial vibration su

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