EESA05 LEC 6.docx

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University of Toronto Scarborough
Environmental Science
Mandy Meriano

EXAM MC: memorization is not good, test understanding of the concepts T/F Online practice question from textbook CAUSES FOR SLOPE FAILURE Numerous factos typically come together to produce a mass wasting event The most important ones are… 1. Water saturation is the single most important factor in mass wasting. It canresult from heavy rain, rapid snowmelt, seasonal freezing, leaking of water or sweage lines, poor drainage. Water saturation weakens rock by building up pore pressure and by lubricating contacts between particles 2. Reduction of slope strength by weathering, by burrowing animals, or by water infiltration 3. Increase in slope steepness 4. Increase in weight at the top of the slope 5. Removal of support at the base of a slope 6. Shaking by either natural processes (earthquakes) or human activities (explosions) 7. Presence of bands of weakness, such as beds of clay, pre existing failure surfaces or faults FACTORS AT LA CONCHITA The jan 2005 landslide at La Conchita occurred on the site of an earlier landslide Steep slopes, previous landslide activity, geology suitable for landslides, episodes of prolonged and intense rainfall Mitigation strategies: move, engineering, warning, nothing FUTURE TRENDS FOR LANDSLIDES It is going up because of…. 1. Increased development and urbanization 2. Increased cutting of forests 3. Changing global climate patterns increasing variability in precipitation HAZARDSASSOCIATED WITH LANDSLIDES Tsunami Infrastructural damage Flooding Spread of disease TSUNAMI FROM LANDSLIDES The tallest tsunami ever inferred occurred in Lituya Bay, Alaska 1958 The forest was destroyed up to 518m above sea level The tsunami was generated by a rockfall which was triggered by an earthquake The tsunami that struck southern newfoundland in Nov 1929 was generated by an undersea landslide which was triggered by an earthquake Happened at night and there was no warning Massive tsunami over 350 m high have struck parts of the hawaiian islands These were not caused by earthquakes, but by landslides The geological history of Hawaii records numerous large landslides that have generated tsunami which have left deposits over 800m above sea level Collapse of a fractured block on the island of La Palma may caus the most destructive tsunami in history Ablock material weighing 500 billion tonnes if poised to slide into the sea Models suggest waves will be 100 m high on the West Saharan coast HUMAN IMPACTS ON LANDSLIDES Deforestation and loss of soils are major contribution to landslides Soil losses in haiti were a major factor in landslides which killed over 1000 people in 2004 Development may play a role in landslides Human activities may change the water balance flowing through an area Human activities also change the angle of slopes (usually steepening) as well as increase the load on the load of slopes Urbanization may occur in inappropriate areas MITIGATION 1. Hazard zone identification and mapping to identify and avoid building on areas where movements have occurred in the past 2. Engineering works to prevent or reduce number of landslides 3. Engineering works that divert flows from some developed areas 4. Warning system MITIGATION THROUGH MAPPING Cracks and terraces on slopes Talus piles at the bottom of cliffs Exposed bedrock with layering parallel to slope Tongue-shaped masses of sediment at the base of a slope or the mouth of a valley For an area facing development, the hazards may be mapped, and used to establish rules for land use Many approaches to slope stabilization Improve drainage – either allow water to leave the grounf or divert surface water to prevent it from being infiltrated into soils The slope may be reduced by grading (reducing the slope), frequently by cutting benches (taking material from the top and putting it at the bottom to increase the resisting force) Buttress fills and retaining devices hold slopes in place Involves removing the entire failed mass, stepping the former failure surface, and filling the hole with compacted material, retaining materials and drainage Plant vegetation to stabilize slopes Using slope-stabilizing structures (wire mesh, shotcrete, etc) Deflection walls may be used to divert the flows around structure MASS WASTINGAT SCARBOROUGH BLUFFS Canadas most serious erosion problem is at the scarborough bluffs The problem is sever because of the intensity of development and the relatively high rate of retreat of the bluffs (~1m/year) In winter the surface of the bluffs is frozen, damming up water During the spring thaw, the pent-up water drains out along the bluff face, washing out sand (forming pipes) and undermining the more resistant layers Toronto sits on a thick package of alternating sand and slit layers that have been deposited in a large bedrock channel beneath a glacial lake Groundwater flow is concentrated in the sands, encouraging the washing out of pipes during the spring thaw MITIGATIONAT SCARBOROUGH BLUFFS Reduce erosion at the toe of the s
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