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Understanding Mass Wasting Processes: Causes, Hazards, and Landforms

This topic explores the concept of mass wasting, including its causes and triggers such as water, slope angle, vegetation cover, and clay type. It also delves into the classification of mass wasting processes, including rapid forms like slump, rockslide, and debris flow, as well as slow movements like creep and solifluction. The hazards and landforms associated with mass wasting are also discussed.

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Understanding Mass Wasting Processes: Causes, Hazards, and Landforms

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  1. Topic 11 - Earth Surface Process: Mass Wasting • What is Mass Wasting? • Causes and Triggers of Mass Wasting- Role of Water- Degree of Slope- Vegetation Cover- Type of Clay- Earthquakes

  2. Topic 11 - Earth Surface Process: Mass Wasting • Classification of Mass Wasting Processes:- Types of Materials- Types of Motions- Rate of Motion: => Rapid Forms of Mass Wasting: - Slump - Rockslide - Rockfall - Debris Flow/Mudflow/Lahar - Earthflow

  3. Topic 11 - Earth Surface Process: Mass Wasting • => Slow Movements: - Creep (soil Creep) - Solifluction - Permafrost • Mass Wasting and Landforms

  4. What is Mass Wasting? • One end-product of weathering is called regolith: • According to Ehlen (2005), regolith is the weathered loose material lying above the unaltered bedrock • These loose materials are easier to move by geomorphic processes or denudation agents (i.e., mass wasting, running water, wave action, glacier, wind and others) to produce landforms=>

  5. Mass Wasting: A Major Hillslope Process

  6. What is Mass Wasting? • Mass wasting, also called Mass Movement, involves the downslope movement of loose weathered materials (regolith) in bulk or individually under the influence of gravity • Though regolith wetness may increase the rate of movement, it does not require a physical transportation medium ( like water, glacier, wind) to move materials • The rate of movement may be very slow and imperceptible to very fast moving, like during a rock avalanche

  7. What is Mass Wasting? • Some mass wasting processes (mud flow, lahar, landslides, rock avalanche, etc.) are sources of geologic or natural hazards • When human settlementsare located on or near steephillslopes, they become exposed to this form ofnatural hazards • Over 20 lives are lost yearlyin the 50 states of the U.S. due to mass wasting 1995 Earthflow Hazard at La Conchita, CA

  8. United States Map of Landslide Potentials

  9. What is Mass Wasting? • In the United States of America, over 60% of all mass wasting hazards occur along: - mountainous hillsides in the Pacific Northwest triggered by heavy rainfall or earthquakes- over steepened coastal California cliffs triggered by strong coastal waves- Seattle area volcanic mudflows (Lahars) • Fewer mass wasting hazards occur: - in the East on Appalachian hillslopes and- in the Central Plains on steeper river bluffs on the Mississippi and other major river systems • Least hazard areas occur on the plain & Florida

  10. What is Mass Wasting? • The least mass wasting hazard zones in the country occur on the low hillslopes in the:- Central Plains - Florida and adjacent Atlantic and Gulf Coastal Plains. • The global Landslides map indicates that areas with high topographic slopes are also areas of high mass wasting risks

  11. Causes of Mass Wasting – Force of Gravity • The most important driving force of mass wasting is the force of gravity acting to pull hillslope regolith downslope • Total force of gravity (Fg) resolves into 2 forces:- normal force (Fn) (resisting force or stress)- parallel force (Fp) (shear force or stress)

  12. Causes of Mass Wasting – Force of Gravity • Fp is related to total force of gravity (Fg) as follows: Fp = Fgsinø whereFg = weight in pounds or kilograms ø = slope angle • As slope angle changes, Fp will vary from zero (stable slope) on a horizontal surface to a maximum Fp (unstable slope) on a vertical slope

  13. Causes of Mass Wasting – Force of Gravity • However, local conditions or factors of the hillslope may work in synergy to increase the shear stress due to gravity • They include the:- removal of lateral support by human actions- removal of underlying support by rivers undercutting hillslope base- loading of slope with rain water or equipment- lateral pressure and transient stresses

  14. Causes of Mass Wasting – Resisting Forces • According to Coulomb (1773), slope failure (like mass wasting) occurs when shear stress (s) (due to gravity) is large enough to overcome the resisting forces of cohesion and frictional resistance of slope materials • Hence: s= c + Sn tanø Where:s = shearing stress c = cohesion Sn = normal stress ø = angle of internal friction or shearing resistance

  15. Causes of Mass Wasting – Resisting Forces • Thus, the second group of causes of mass wasting is the resisting forces aimed at preventing slope materials to be pulled downslope by gravity • The resisting forces include:=> Normal force or stress (Fn or Sn ) => Shear strength of slope materials

  16. Causes of Mass Wasting – Resisting Forces • Normal force (Fn ) is related to Fg as follows: Fn = Fg cosø where Fn is an indication of frictional resistance • Hence, Fn (frictional resistance of slope materials) is at a maximum on a horizontal surface (stable slope) and zero on a vertical slope (unstable slope)

  17. Causes of Mass Wasting – Resisting Forces • On the other hand, the shear strength of slope materials are enhanced by the: => cohesion of slope materials by the:- chemical bonding of rock & soil particles - presence of cementing materials of regolith- capillary cohesion in partially moist soils (e.g. silt and clay soils) causes film of water to be drawn over particles with the resulting negative pore-water pressure producing more adhesion

  18. Causes of Mass Wasting – Resisting Forces => inherent frictional properties of slope materials such as: - particle size distributions, shape and arrangement - number of contact points - crushing resistance

  19. Causes of Mass Wasting – Resisting Forces • However, local conditions and factors on hillslopes work to reduce the resisting forces (i.e., shear strength) of slope materials • They include:- weathering effects- changes in pore-water pressure- changes of structure- organic effects, among others

  20. Causes of Mass Wasting – Resisting Forces • In conclusion, slope failures resulting in mass wasting occur when the resisting forces (shear strength) of slope materials are less than its shear stressdue to gravity • Based on slope stability, hillslopes often exist in one of three states: - stable slope: shear strength > shear stress (or safety factor > 1.3) ( i.e. shear strength divided by shear stress) - actively unstable slope: shear strength < shear stress (or safety factor < 1)

  21. Causes of Mass Wasting – Resisting Forces - conditionally stable slope: failure occurs when shear strength changes temporarily (or safety factor 1-1.3) • Mass wasting tends to occur under the state of actively unstable or conditionally stableslopes or when safety factor is generally less than 1.3

  22. Causes of Mass Wasting – Trigger Factors • Based on field data, the four most important triggers of mass wasting are: - Role of water - Degree of slope - Type of clay - Vegetation cover and - Earthquakes (play different roles in creating downslope movements)

  23. Causes of Mass Wasting – Trigger Factors • Role of Water in Mass Wasting:- Heavy rains/rapid snow melt saturate weathered regolith and help to trigger mass wasting- Saturation reduces internal resistance of materials and making the materials to move easily- Water adds weight to the materials causing it to slide or flow downslope

  24. Causes of Mass Wasting – Trigger Factors • Role of Degree of Slope:- Over steepened slopes tend to collapse or move easily by gravity- Unconsolidated particles of sand or coarser materials tend to assume a stable slope called the angle of repose - The angle of repose is the steepest angle at which material remains stable

  25. Mass Wasting: Angle of Repose of Slope Materials

  26. Causes of Mass Wasting – Trigger Factors • Role of Degree of Slope:- If the degree of slope is increased, the slope materials come under increasing force of gravity and causing slope instability and verse versa • Role of Vegetal Cover:- Plant roots hold the soil in place- They protect soil against erosion and contribute to slope stability

  27. Causes of Mass Wasting – Trigger Factors - When anchoring vegetation is removed by forest fire or farming or construction work, surface materials frequently move downslope, especially in semi-arid areas • Role of Earthquakes:- earthquake and its after-shocks can dislodge enormous volume of rock and debris and important in sudden release of rocks to cause rock avalanche

  28. Causes of Mass Wasting – Trigger Factors • - Massive slide can be triggered by earthquakes- Earthquake could cause liquefaction of sand with some water

  29. Main Types of Mass Wasting • Several methods of mass wasting classification. The simplest is based on rate of movement that groups mass wasting into two, Slow - Moderate and Rapid Mass Wasting and further subdivided based of type of movement: • Slow/Moderate Mass Wasting:- Creep - Solifluction- Rotational Slides or Slump (Moderate) • Rapid Mass Movement:- Falls or Avalanche- Slides- Flows

  30. Main Types of Mass Wasting • A three dimensional grouping, by adding type of material moved, yields a larger list of mass wasting • Thus, a list including three factors, (i) rate of movement, (ii) type of movement and (iii) type of materials moved, looks like the following:=> Slow/Moderate Mass Wasting: - Creeps: • Soil creep • Earth Creep • Talus Creep - Solufluction- Rotational Slide (Slump) (moderate rate)

  31. Main Types of Mass Wasting => Rapid Mass Wasting: - Falls: • Rockfalls • Debris fall • Rock avalanche and • Debris Avalanche: - Slides: • Debris Slides • Rockslides • Rotational Slides or Slump - Flows: • Earthflows • Debris Flow or Mudflows or Lahar

  32. Types of Mass Wasting Based on Rate of Movement and Amount of Water

  33. Slow-Moderate Mass Wasting • Slow - Moderate Mass Movement: - Creep - Solifluction- Rotational Slides or Slump (Moderate)

  34. SLOW/MODERATEMASS WASTING PROCESSES - Creep - Solifluction - Slump

  35. Slow Mass Movement: Creep • It is a slow imperceptible downhill movement that includes: - soil creep (Heave)(finer materials) - rock creep - talus creep (coarser materials) • movement consists of numerous minute discrete movements of slope materials under the influence of gravity

  36. Slow Mass Movement: Creep • rate of movement is slow (0.1 mm to 10 m/yr) depending on:- slope angle- susceptibility of the materials- intensity of the processes- water content • movement may extend up to about 20 cm below the surface but rate of movement decreases with depth

  37. Slow Mass Movement: Creep • movement is aided by heaving of the ground (expansion and contraction) caused by:- freezing and thawing- wetting and drying- or, other volumetric changes • mechanisms of soil creep include:- differential expansion-contraction- displacement of particles by organisms- downhill release of particles by weathering

  38. Mass Wasting in Cold Climate: Soil Creep

  39. Slow Mass Movement: Creep • Evidences of creep include: - tilting of surface objects like fence posts, tombstones, retaining walls- curvature of trees and - bending of rock strata downslope - formation of terracettes (i.e., step-like ridges along the hillside

  40. Effects of Soil Creep

  41. Soil Creep Effect: Curvature of Tree Trunk

  42. Soil Creep Effect: Curvature of Tree Trunk Base

  43. Soil Creep Effect: Curvature of Tree Trunk

  44. Soil Creep Effect: Tilting Fencepost

  45. Nearly Vertical Sedimentary Strata Bent in Downslope Direction

  46. Slow Mass Movement: Solifluction • Solifluction is a type of earth flow found in periglacial regions underlain by permafrostDuring the summer the surface layer of permafrost melts creating a water-saturated layer that becomes mobile • The underlying frozen ground acts as a sliding plane along which the mass of soil can slowly move down slope over

  47. Slow Mass Movement: Solifluction • It moves as imbricate tongues, lobes or sheets • Movement is most rapid in the center and slower near the lateral margins • It moves at a rate of about 1-10 cm/week • In the Yukon Plateau, it is as a major gradation process called equiplanation

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