Mass Wasting January 30, 2021 Big Sur, CA the transfer of rock/soil downslope under gravity Gravity: how it works • If the shear component of gravity is stronger than the forces holding an object on the slope, then the object will move down-slope. • If the slope is made of/covered in soil, clay, sand, etc., and the shear stress becomes greater than the forces holding the particles together, the particles will move down-slope Angle of Repose Gravity, for unconsolidated material It is the maximum slope, measured from the horizontal, at which loose solid material will remain in place without sliding Examples of angle of repose • Each material has a specific angle of repose • for a given material, if the slope angle is higher than angle of repose, the slope is unstable and mass wasting will occur Mass wasting features Talus: rocks accumulating at the base of the cliffs. Slope at angle of repose No minimum angle is required for mass wasting to occur! Mass Wasting, Hawaii Mass Wasting on Mars Mass Wasting features can be seen anywhere there is unconsolidated or unstable material and gravity at work As a landscape ages, the evidence of mass wasting become less dramatic – surface processes and biosphere at work! Triggers of Mass Wasting • Loss of cohesion among particles Weathering , water, removal of vegetation • Geology how the local rock layers and structures relate to the slope • Ground shaking Earthquakes and volcanic eruptions • Anthropogenic: – Steepening or undercutting of the slope: changing the shape of the slope – Additional weight – Building on top of slopes, or water from increased precipitation The role of water in mass wasting • • Diminishes particle cohesion Water adds weight Water triggered mass wasting examples: 1 – mudflows lahars 2 – Submarine mass wasting • Called Turbidity Currents • The largest mass wasting on Earth happen underwater • Scar the continental slopes Mass wasting triggered by earthquakes • Earthquakes can trigger mass wasting The Madison river rockslide (Montana) August 17, 1959 was triggered by an earthquake west of Yellowstone. Triggers of Mass wasting: volcanic activity • Explosive eruptions • Lahars Mt. St. Helens Triggers of Mass wasting: changing/removal of vegetation • Roots keep slope material in place Triggers of Mass wasting: geologic structures and slope angle Triggers of mass wasting: Oversteepening Slopes can become instable when they are over-steepened Common when roads are carved at the base of slopes Triggers of mass wasting: Oversteepening A massive rock slide blocks Sky Highway near Porteau Cove, B.C. Triggers of mass wasting: undercutting Types of mass wasting Mass wasting classification Based on three elements: Type of material – Mud, Earth, Rock, (Water) Type of motion – Fall – Slide (along a flat surface) – Slump (along a curved surface) – Flow (chaotic mixture with water: e.g. mudlfow) Velocity of the movement – Fast – Slow by type of material Soil Mud Rock Avalanche Type of motion: Fall most common in weathered rocks Yosemite 1996 Type of motion: Slide • Movement along a flat surface rock layers that slide on top of each other • lf the layers of rock are inclined in the same direction of the slope, the slope is very unstable • → importance of orientation of geologic structures Which side is more stable? Gros Ventre, Wyoming 1925 Type of movement: Slump • Also known as “rotational slide”, it occurs along a curved surface – Tends to form irregular “steps” Examples of slumps Type of movement: Flow • Soil and rocks mixed with a large amount of water • Often confined to channels Mudflow or Earthflow • The ground seems to flow away debris flow • Large boulders, fragments from rock falls • Ladak debris flow Mt. St.Helens’ mud flow Velocity of the movement • Fast (example: rock falls, avalanches, landslides, debris flow • Slow (creep, solifluction) – Gradual movement of soil downhill Some visible effects of creep Very slow mass wasting: creep • A SLOW downhill movement of soil and regolith. Creep results in tree trunks that are curved at the base, tilted utility poles, fence posts, and tombstones, and causes retaining walls to be broken or overturned. Bent tree trunk illustrating creep. National Zoo, Rock Creek Park, Washington, D.C. solifluction • In dense clay hardpan or impermeable bedrock layer – Common in regions underlain by permafrost Permafrost regions in the Northern Hemisphere: solifluction can happen there! Track deformation by solifluction Damage to pipeline Mass Wasting HAZARD Mass wasting is a recurrent hazard • Mass wasting will happen again and again as long as the conditions for it are met. Blue line: 1995; Yellow line 2005; red line predevelopment Case study: mass wasting Hockings River, Athens Ohio 1968-1976 1968 1971 1975 1976 Case study: mudslide – Washington state, 2014 Before the 2014 mass wasting video How to study landslides hazards • Experiments: the USGS Flume laboratory • Monitoring Engineering for mass wasting containment: catchment/debris basins Debris basins are engineered for risk mitigation, they collect the body of the mass wasting from debris flow Slope stabilization measures • A variety of strategies are available for stlope stabilization, nets, anchors, landscaping, shotcrete, retaining walls, etc. Mass Wasting HAZARD Mass wasting is a recurrent hazard • Mass wasting will happen again and again as long as the conditions for it are met. Blue line: 1995; Yellow line 2005; red line predevelopment Case study: mass wasting Hockings River, Athens Ohio 1968-1976 1968 1971 1975 1976 Case study: mudslide – Washington state, 2014 Before the 2014 mass wasting video How to study landslides hazards • Experiments: the USGS Flume laboratory • Monitoring Slope stabilization measures • A variety of strategies are available for stlope stabilization, nets, anchors, landscaping, shotcrete, retaining walls, etc. Engineering for mass wasting containment: catchment/debris basins Debris basins are engineered for risk mitigation, they collect the body of the mass wasting from debris flow Hydrologic Cycle and Reservoirs • The hydrologic cycle consists of processes (evaporation, precipitation, runoff, infiltration) and reservoirs (oceans, lakes, clouds, etc…) Surface Water: how the stream forms • Runoff is water flowing on the surface, its duration and intensity depending on the precipitation. • Rills are formed by runoff mechanically eroding the surface. Rills are shallow channels eroded by running water • Rills can erode a significant amount of soil! Gullies Over time, when ruoff water flows with the same pattern, it forms a permanent incision on the ground called a gully Gullies of the badlands River • It is a stable stream of running water • Connected to the hydrologic cycle Drainage Basin • Land area that contributes water to the stream • Divide = imaginary line separating one basin from another Nesting drainage basins Drainage pattern • Pattern of the interconnected network of streams in an area • It depends on – the type of rocks eroded – the orientation of the rock structures – the geologic processes acting in the same drainage area Examples of drainage patterns Gotel Mountains along the border between Nigeria and Cameroon Drainage density A measure of how many branches of stream are in a given area – Rocks easy to erode have many stream branches → high drainage density. – Porous rocks have few branches (water is absorbed) Discharge Q The discharge is the volume of water moving past a given point in a certain amount of time. River Country Amazon Congo Yangtze Ganges Mississippi Brazil Congo China India USA Average Discharge at Mouth (cubic feet per second) 7,500,000 1,400,000 770,000 660,000 611,000 Stage • Discharge is related to the level/height of the stream = STAGE – Discharge may change, depending on the amount of water delivered to the stream – For any given stream, the higher the discharge, the higher the level of the water! • A gaging station collects data to monitor the level of the water in the river Movement of water in the stream: slope and ruggedness Smooth channel Turbulent flow Rugged channel Steep slope Water flows more slowly Laminar flow Smooth channel Water flows faster Evolution of a River along its longitudinal profile • Along its course, the river evolve through stages Headwaters the river starts here! Erosion dominant Mouth or base level, the river ends here Deposition dominant Headwaters • where the river “begins” • It can be one or more springs, a lake, a glacier, anywhere water is steadily present a flowing downhill Headwaters of the Shenandoah River South Fork Headwaters of the Mississippi Youthful/degradational stage – Near headwaters→ high energy erosion directed downward forming a steep V-shaped valley – Erosion is dominant – Turbulent flow with rapids and waterfalls (white water) Yellowstone River upper course Balanced/mature stage: Wide Stream Valleys – – – – Gradient of slope decreases Downward erosion is less dominant Stream channel form prominent meanders Erosion is directed side-to-side → the valley (lowland) becomes wider to side forming a floodplain. Yellowstone National Park, Wyoming, USA by Q T Luong terragalleria.com Meanders A meander forms because of turbulence, all stream channels form meanders In the balanced/mature stage the river forms pronounced meanders that erode the outer banks and widen the valley POINT BAR: water moves slowly →less energy → deposition CUT BANK fast moving water → more energy → erosion Meander evolution and oxbow lakes When the two eroding section of the meander face each others, the meander loop is cut out and forms an oxbow lake Meander cut-offs: Oxbow Lakes Meander cut-offs and international borders Meandering through the ages, the rivers design their own floodplain The Mississippi from satellite and its representation in a geologic map The Mississippi, many meanders ago… Aggradational stage: very low gradient • Stream at lowest gradient • Low energy, the channel breaks down in many smaller channels, taking up a typical braided appearance – A braided stream can also form 

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