Mass Movement

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Information about Mass Movement
Travel-Nature

Published on March 11, 2008

Author: AscotEdu

Source: authorstream.com

GLG110 Geologic Disasters and the Environment:  GLG110 Geologic Disasters and the Environment Instructor: Professor Stanley Williams Email: stan.williams@asu.edu Course Website: http://glg110.asu.edu TA: Carol Butler Email: clbutler@asu.edu Today: Chapter 8 Mass Movements Disaster of the Day – Pollution from Pipeline Burst:  Disaster of the Day – Pollution from Pipeline Burst 7/30/2003 - Significant environmental problem, 10,000 gallons of gasoline Underground pipeline When burst blew off soil and spread out through soil and across surface Disaster of the Day – Pollution from Pipeline Burst:  Burst in new residential development Saturated homes under construction, soil, got into dry wash that goes into Santa Cruz River Fortunate no one living in area at time of burst (and no one smoking) Clean-up Homes torn down Excavate soil Groundwater monitoring wells Disaster of the Day – Pollution from Pipeline Burst Disaster of the Day – Pollution from Pipeline Burst:  Disaster of the Day – Pollution from Pipeline Burst 10/21/2003 report extensive groundwater pollution found 150 ft below surface Pipeline may have been leaking before burst >10,000 gallons according to Arizona Dept. of Environmental Quality Could be other leaks as well due to problems getting pipeline running again Disaster of the Day – Pollution from Pipeline Burst:  Tests Look at type of gas found to determine how long it was leaking (at time of break was carrying premium) Look at downslope monitoring stations see how far plume traveled and how spread out to determine if was leaking earlier than thought Disaster of the Day – Pollution from Pipeline Burst Overview:  Overview Mass Wasting Definitions Slope Processes Slope Stability Types and Examples of Mass Movements Human Activity and Landslides Minimizing the Hazard Definitions:  Definitions Mass Wasting = comprehensive term for any type of downslope movement of Earth materials More precisely = downslope movement of rock or soil as a coherent mass Includes: Landslides, earthflows, mudflows, rockfalls, debris and snow avalanches, and subsidence Mass Wasting:  Mass Wasting Downslope movement of rock, regolith, soil Force of gravity dominates Often initiated by heavy rainfall or earthquake Distinct from erosional processes Follows weathering Mass-wasting + running water = stream valleys Mass wasting & erosion sculpt Earth’s surface Slope Processes:  Slope Processes Slopes are the most common landforms Most appear stable but really are dynamic and evolving Material on slopes is constantly moving Slope form depends on: Underlying rock type Climate Regional tectonics Regional drainage pattern Slide10:  Slope Processes Concave Slope Cliff Face Straight Slope Hard Granite Weak Rock Soil Talus Slope (rock fall debris) Convex Slope Note – the slopes on this figure in your text are mislabeled! Slope Stability:  Slope Stability Slope = how steep the inclination of the hillside is Slope stability depends on the driving and resisting forces that act on the slope As angle of slope increases, driving forces increase Slope Stability:  Slope Stability Gravity Weight of Material Friction Upward force of supporting materials Driving Forces and Resisting Forces Driving forces move material down the slope (Most common = weight of slope material, vegetation, and structures on slope) Resisting forces oppose downward movement (Most common = strength of slope material) Slope Stability:  Slope Stability Slope stability is evaluated by computing safety factor Safety Factor = ratio of resisting forces to driving forces If S.F. > 1 the resisting forces are stronger and the slope is stable If S.F. < 1 the driving forces are stronger and the slope is unstable Slope Stability:  Slope Stability Slope stability changes over time as conditions change causing ratio to change Driving and resisting forces are determined by interactions between: Type of earth materials Slope angle and topography Climate Vegetation Water Time Role of Slope Material:  Role of Slope Material Affects type of mass movement Shale or weak volcanic material slopes tend to creep, flow, or slump Affects falls when strong, resistant rock is underlain by weak, easily erodable rock, can result in undercutting and subsequent rock fall Role of Slope Material:  Role of Slope Material Affects type and frequency of slides Patterns of movement: Rotational = occurs along curved slip surface, produces terrace-like structures Translational = planar, along slip planes within the slope Slip planes include fractures, layers, bedding planes, foliation, etc. Role of Slope & Topography:  Role of Slope & Topography Angle of Repose = maximum angle at which unconsolidated material on slope is stable ~30-35° The larger and more uniform the size of fragments the steeper the angle of repose Controlled by: Coarseness, shape, size and uniformity of size of fragments Water content Steep slopes often associated with rockfalls and debris avalanches Role of Climate:  Role of Climate Climate = characteristic weather at specific place over time Includes: Average temperature Amount and timing of precipitation Which then infiltrates slope thus affecting stability Affects vegetation which influences slope stability Seasonal weather patterns Common mass movements in arid and semiarid regions = fall, debris flow, shallow slips Role of Vegetation:  Role of Vegetation Vegetation is a function of climate, soil type, topography, and fire history Can increase or decrease the chances of mass movement Shields soil thus mitigates runoff during heavy precipitation Roots help fix soil in place Adds weight to the slope In subhumid to humid areas vegetation is abundant, thick soil develops so mass movements = complex landslides, flows, and creep Role of Water:  Role of Water Almost always directly or indirectly involved with mass movements Water fills pore spaces between sediments, reduces internal resistance, adds weight Affects: Decreasing stability when slope becomes saturated Slumps or slides can occur years after deep water infiltration Can erode base or toe of slope decreasing stability Role of Water:  Role of Water Water & Erosion Stream or wave erosion may remove material creating steeper slope thus reducing safety factor May reactivate old landslides Liquefaction Some clays behave as liquid and flow when disturbed Does not always require earthquake, can occur through toe erosion Role of Time:  Role of Time Forces on slopes often change with time Driving and resisting forces can change seasonally as water content changes Chemical weathering introduces elements into soils which change properties A slope that is becoming less stable with time may exhibit increasing creep until failure occurs Types of Mass Movements:  Types of Mass Movements Important variables in classifying downslope movement of Earth materials are: Types of mass movements slide, slump, fall, flow, subsidence, or complex Slope material Amount of water Rate of movement Slide24:  Classification of Mass Movements based on water content and speed of movement Types of Mass Movements Slide25:  Types of Mass Movements Slide/Fall Types of Mass Movements:  Types of Mass Movements Slide = downslope movement of coherent block of Earth material Slide27:  Types of Mass Movements Blackhawk Landslide, California Started as slide Rode on blanket of air Was pulverized when hit the base of slope Created debris blanket 10-30 m thick Types of Mass Movements:  Types of Mass Movements Fall = free fall of Earth material Yosemite National Park, California Types of Mass Movements:  Types of Mass Movements Debris Avalanche = very rapid downslope movement of soil, rock, & organic debris In <2 minutes 40 million cubic yards of rock covered the town of Frank, Alberta, Canada, in 1903 Types of Mass Movements:  Types of Mass Movements Avalanche Rapid downslope movement of snow and ice sometimes with rock, soil, and trees Often begins with slab weighing millions of tons, falling from an overloaded slope Can travel as much as 62 mph Tend to travel down chutes where previous avalanches have flown Slide31:  Types of Mass Movements Flow Types of Mass Movements:  Flow = downslope movement of unconsolidated Earth material saturated with water particles within also move with respect to each other Types of Mass Movements Debris Flow = >50% of particles coarser than sand Mudflow = >50% of particles finer than sand Types of Mass Movement:  Types of Mass Movement Lahar = mudflow produced when large volume of volcanic ash and ejecta becomes saturated with water Armero, Nevado Del Ruiz, 1985 Types of Mass Movement:  Types of Mass Movement Rock Glacier = mass or rock and ice frozen together and flowing downslope Mt. Sopris, Colorado Slide35:  Types of Mass Movements Creep/Heave Slide36:  Types of Mass Movements Creep & Heave = slow, ~imperceptible downslope movement of unconsolidated Earth material Often related to seasonal effects and/or precipitation rates Slow Flow Slide37:  Types of Mass Movements Slump/ Subsidence Types of Mass Movements:  Types of Mass Movements Slump = rock or soil moving downslope along curved slip plane producing slump blocks Black Hills, N.D. Types of Mass Movements:  Types of Mass Movements Subsidence = sinking of mass of Earth material below level of surrounding material can occur on slopes or flat ground San Joaquin Valley, California Types of Mass Movement:  Types of Mass Movement Subsidence can occur as a result of: Withdrawal of groundwater Withdrawal of oil and gas Dissolution of limestone Mining (coal, ore, salt) See Lake Peigneur example in book Subsidence can cause earth fissures and sinkholes Slide41:  Types of Mass Movements Complex Landslide Types of Mass Movements:  Types of Mass Movements Landslide = complex combinations of sliding and flow La Conchita, California, 1995 Slide43:  La Conchita Complex Landslide Slide44:  Aerial photos of complex landslide at La Conchita, California Do you see the landslide scarp? La Conchita Complex Landslide 1927 1967 Slide45:  Aerial photos of complex landslide at La Conchita, California Do you see the landslide scarp? La Conchita Complex Landslide 1927 1967 Slide46:  Note how quickly the vegetation is returning and how it helps conceal the slide scar La Conchita Complex Landslide 1998 Image: Jeffrey J. Hemphill

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