Mass Movements: Surface Processes PDF

Mass Movements Surface Processes Mass Wasting Definition: mass wasting refers to a downslope movement of rock or soil as a more or less coherent mass. Comprehensive term – all inclusive term for any downslope movement of earth materials The downslope movement of rock, regolith and soi...

Mass Movements Surface Processes Mass Wasting Definition: mass wasting refers to a downslope movement of rock or soil as a more or less coherent mass. Comprehensive term – all inclusive term for any downslope movement of earth materials The downslope movement of rock, regolith and soil under the influence of gravity. : a. Erosion by rivers, glaciers, or ocean waves. b. Earthquakes. c. Volcanic eruptions (lose ash deposits, heavy rain and debris flow). e. Removal of deep-rooted vegetation. d. Excess weight of man-made structures. 1. Gravity. 2. Slope. 3. Water saturation. 4. And others. Forces on Slopes The stability of a slope expresses the relationship between resisting forces and driving forces Driving forces – forces which move earth materials downslope – Downslope component of weight of material including vegetation, fill material, or buildings Resisting forces – forces which oppose movement – Resisting forces include strength of material Potential Slip Planes Geologic surfaces of weakness in the slope material – bedding, foliation, fractures Angle of Repose The maximum slope angle at which a given unconsolidated material is stable. Safety Factor The ratio of resisting forces to the driving forces – RF/DF SF > 1 Slope is stable SF < 1 Slope is unstable  : is the sole force of all mass wasting but there are other forces that triggers landslides and mass movement including: : i. Stream undercutting a valley wall; waves pounding against base of cliff.  : i. Deep roots protects and stabilize ground. ii. Plants shield soil surface from erosional effects of rain drop. iii. Mass wasting is enhanced where plants are lacking (steep slopes, forest fires, human removal of plants).  : geological phenomena which include wide range of ground movement, such as rock falls, deep failure of slopes and shallow debris flows. Fall A type of movement common to mass- wasting processes that refers to the freefalling of detached individual pieces of any size Rockslide The rapid slide of a mass of rock downslope along planes of weakness.  Slump The downward slipping of a mass of rock or unconsolidated material moving as a unit along a curved surface. Creep The slow downhill movement of soil and regolith. Rock avalanche The very rapid downslope movement of rock and debris. May reach speeds in excess of 200 kilometers per hour. Debris Flow Debris flows are the downslope flow of relatively coarse material > 50% of particles in a debris flow are coarser than sand Movement may be very slow or very fast, depending on topographic conditions Mudflows, debris avalanches, and debris flows Small to moderate magnitude events, occasional large magnitude events slow-moving – faster in wetter weather Debris Flow  Rockslide Rock moves because there’s nothing holding it back! Generally requires a pre-existing low-friction surface... Role of Slope and Topography Hillslope angle is a measure of the steepness of a slope = slope gradient Steeper slope = increased driving forces Steep slopes associated with rockfalls Landslide activity includes deep complex landslides, earthflows, and soil creep. Role of Vegetation In subhumid to humid environments, vegetation is thick and abundant Vegetation influences slope stability by: – Providing a cover that cushions the impact of rain falling on slopes and retards erosion on surface – Vegetation has root systems that tend to provide an apparent cohesion which increases resistance to landsliding – Vegetation adds weight to the slope increasing the driving forces Role of Water Water can affect slope stability by: – Shallow soil slips can develop during rainstorms when slopes become saturated – Slumps or translational slides can develop months or years after slope is saturated – Water can erode the base or toe of a slope decreasing slope stability  basically a very viscous (thick) debris flow Earthflow slow-moving – faster in wetter weather Creep very slow result of freezing and thawing Creep Creep from D. Schwert, NDSU Triggers for rapid Mass Wasting Rain Oversteepening – cutting at foot of slope – piling on head of slope Deforesting / Devegetating Earthquakes Possible Preventive Measures Slope Reduction Reduce the slope angle Place additional supporting material at the foot of the slope to prevent sliding or flow at the bade of slope Reduce the load Retention Structure Use ground covers or vegetation Fluid Removal Decrease the water content or pore pressure of the rock or soil by improving the drainage on the slope surface.