Topic 11 Earth Surface Process Mass Wasting l

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

Topic 11 - Earth Surface Process: Mass Wasting l 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

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

What is Mass Wasting? l One end-product of weathering is called regolith: l According to Ehlen (2005), regolith is the weathered loose material lying above the unaltered bedrock l 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

Mass Wasting: A Major Hillslope Process

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

What is Mass Wasting? l Some mass wasting processes (mud flow, lahar, landslides, rock avalanche, etc. ) are sources of geologic or natural hazards l When human settlements are located on or near steep hillslopes, they become exposed to this form of natural hazards l Over 20 lives are lost yearly in the 50 states of the U. S. due to mass wasting

United States Map of Landslide Potentials

What is Mass Wasting? l 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) l 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 l Least hazard areas occur on the plain & Florida

What is Mass Wasting? l 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. l The global Landslides map indicates that areas with high topographic slopes are also areas of high mass wasting risks

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

Causes of Mass Wasting – Force of Gravity l Fp is related to total force of gravity (Fg) as follows: Fp = Fg sinø where Fg = weight in pounds or kilograms ø = slope angle l 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

Causes of Mass Wasting – Force of Gravity l However, local conditions or factors of the hillslope may work in synergy to increase the shear stress due to gravity l 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

Other Factors Contributing to Increasing Shear Stress Due To Gravity Removal of Lateral Support Erosion by rivers & glaciers, Through Undercutting or previous rock falls or slides, Slope Steepening construction works Removal of Underlying Support Undercutting by rivers or waves, subsurface solution, loss of strength by extrusion of underlying sediments Loading of Slope Weight of rain water, vegetation, accumulated debris, weight of army tanks or construction equipment Lateral Pressure Water in cracks, freezing in cracks, hydration of swelling clay Transient Stresses Earthquakes, movement of trees in wind

Causes of Mass Wasting – Resisting Forces l 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 l Hence: s = c + Sn tanø Where: s = shearing stress c = cohesion Sn = normal stress ø = angle of internal friction or shearing resistance

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

Causes of Mass Wasting – Resisting Forces l Normal force (Fn ) is related to Fg as follows: Fn = Fg cosø where Fn is an indication of frictional resistance l 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)

Causes of Mass Wasting – Resisting Forces l 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

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

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

Factors Contributing to Reduce Resisting Forces of slope Material (Slope Strength) Weathering Effects Disintegration of granular rocks, hydration of clay minerals, dissolution of cementing minerals in rock or soil Changes in Pore-water Saturation, softening of material Pressure Changes in Structure Organic Effects Creation of fissures in shale and clays, remoulding of sand sensitive clays Burrowing of animals, decay of tree roots

Causes of Mass Wasting – Resisting Forces l In conclusion, slope failures resulting in mass wasting occur when the resisting forces (shear strength) of slope materials are less than its shear stress due to gravity l 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)

Causes of Mass Wasting – Resisting Forces - conditionally stable slope: failure occurs when shear strength changes temporarily (or safety factor 1 -1. 3) l Mass wasting tends to occur under the state of actively unstable or conditionally stable slopes or when safety factor is generally less than 1. 3

Causes of Mass Wasting – Trigger Factors l 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)

Causes of Mass Wasting – Trigger Factors l 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

Causes of Mass Wasting – Trigger Factors l 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

Mass Wasting: Angle of Repose of Slope Materials

Causes of Mass Wasting – Trigger Factors l 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 l Role of Vegetal Cover: - Plant roots hold the soil in place - They protect soil against erosion and contribute to slope stability

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 l 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

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

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: l Slow/Moderate Mass Wasting: - Creep - Solifluction - Rotational Slides or Slump (Moderate) l Rapid Mass Movement: - Falls or Avalanche - Slides - Flows l

Main Types of Mass Wasting l A three dimensional grouping, by adding type of material moved, yields a larger list of mass wasting l 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)

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

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

Slow-Moderate Mass Wasting l Slow - Moderate Mass Movement: - Creep - Solifluction - Rotational Slides or Slump (Moderate)

SLOW/MODERATE MASS WASTING PROCESSES - Creep - Solifluction - Slump

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

Slow Mass Movement: Creep l 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 l movement may extend up to about 20 cm below the surface but rate of movement decreases with depth

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

Mass Wasting in Cold Climate: Soil Creep

Slow Mass Movement: Creep l 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

Effects of Soil Creep

Soil Creep Effect: Curvature of Tree Trunk

Soil Creep Effect: Curvature of Tree Trunk Base

Soil Creep Effect: Curvature of Tree Trunk

Soil Creep Effect: Tilting Fencepost

Nearly Vertical Sedimentary Strata Bent in Downslope Direction

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

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

Slow Mass Movement: Solifluction l According to Eakin(1916), Russell (1933), Peltier (1950), solifluction could result in the high-altitude planation of mountain ranges into flattened summit areas in a process called altiplanation

Solifluction Lobes and Terraces, Lewis Hill, Newfoundland

Solifluction Lobes and Terraces, Colorado Rocky Mountain National Park

Solifluction Lobes and Terraces

Patterned Ground: Stone Rings

Solifluction: The Permafrost Problem l In the polar regions of the world, the ground remains frozen throughout the winter season and the upper top layers thaws in summer l This poses a serious challenge to construction engineers l For example: Heating melts the permafrost causing land subsidence/ building collapse

Above-Ground Alaska Petroleum Oil Pipeline Why is the pipeline above ground when transporting warm petroleum oil product?

Moderate Mass Wasting - Rotational Slide or Slump l Slump or rotational slide involves a downward sliding of a mass of regolith moving as a single unit along a curved surface of rupture

Moderate Mass Wasting: Rotational Slide or Slump

Moderate Mass Wasting: Rotational Slide or Slump

Moderate Mass Wasting: Rotational Slide or Slump curves Rotational slide surface

Moderate Mass Wasting: Rotational Slide or Slump l Slumping leaves behind a crescent-shaped or cliff scarp created at the head and the block’s upper surface becomes tilted backward l The slump block moves downslope l Slump occurs when slope is over steepened l This may happen when anchoring materials at the base is removed making materials above to become unstable and reacts to the pull of gravity

Moderate Mass Wasting: Rotational Slide or Slump l Earthflows frequently form at the base of the slump

RAPID MASS WASTING PROCESSES - Rock Falls - Slides - Flows

Types of Mass Movements l Rapid Mass Wasting: - - Rockfalls and Debris Avalanche Slides: l Debris Slides l Rockslides l Rotational Slides or Slump Flows: l Earthflows l Debris Flow or Mudflows l Lahars (volcanic mudflows)

Rockfall

Rockfall

Mass Wasting: Landslide

Earthquake Triggered Landslide (2001), Santa Tecla, El Salvador

Landslides

Mass Wasting: Earthflow

Earthflow Caused by Infiltrating Septic Tank and Lawn Irrigation Water, Palos Verdes Hills, CA Sedimentary Earth Materials with buildings slipped downslope slowly (70 feet in 3 years) as infiltrating waste water lubricated the slippery clay layer underneath – A Major Human Factor

Mass Wasting: Rapid Rock Falls

Earthflow that Destroyed Some Houses at La Conchita, CA in 1995

Mass Wasting: Mudflows

Rapid Mass Movement: Lahars (Hot Volcanic Mudflows) l Lahars are volcanic mudflows formed by: -debris avalanches mixing with snow and ice melt water -pyroclastic materials mixing with rainwater or with surges produced by dam failures or with natural river water l Lahars with 20 -60% sediment are turbulent or smooth flowing if sediment content rises to over 80%

Mudflow and Lahar on Mount St. Helen

Number of Lahars Caused By Different Volcanic Events

Lahars (Volcanic Mudflows) l. Flow velocity may range from 1 meter per second to over 40 meters per second l. Rate of down valley movement of lahars depends on: -valley width and slope -flow volume -grain size composition l. Lahars cause people, cities & structures to be buried

Pinatubo Mt. , Philippines Volcanic Lahars (1991)

Mudflow Covering the Garage Door

MASS WASTING AND LANDFORMS

Mass Wasting and Landforms l Weathering weakens and breaks massive country rocks into smaller fragments (clay, silt, sand, pebbles to large rock boulders) l Mass wasting moves these hillslope fragments (regolith) downslope by gravity alone l But when mass wasting combines with actions of running water (rivers), wind, glacier, ocean wave and groundwater, different landforms are produced

Mass Wasting and Landforms l For example, when a stream cuts down its channel floor alone without the help of mass wasting, it produces narrow channels with vertical walls in sandstone formation in semi-arid to arid environments l (Example: Zion Narrows of the Virgin River, Southern Utah)

Mass Wasting and Landforms l Expanded Grand Canyon Valley Walls: weathering and mass wasting working together has greatly expand the Colorado River channel width even in an Arid/Semi-Arid Region where mass wasting is highly limited l Humid areas with more mass wasting activities tend to have wider or more open river valleys

Channel Walls of Grand Canyon Expanded By Mass Wasting

Mass Wasting and Landforms l Typical hillslope elements are defined and dominated by different types of mass wasting and slope wash processes Source: Richard J. Huggett (2011)

Mass Wasting and Landforms l According to Gilbert (1909), convex slope segment of the hillslope is the result of soil creep l The cliff section underneath it called the fall face segment is dominated by rapid mass wasting called rock falls l The straight mid-slope below the cliff section is called the talus slope built with rock falls or screes often with high repose slope

Landform of Mass Wasting: Talus Slope

Landforms of Mass Wasting: Talus Slope of Screes

Talus Slope of Scree

Examples of Mass Wasting Hazards

Mass Wasting and Landforms l Over a long period of time, steep mountain slopes are gradually reduced to a more gentler and subdued slopes.

Mass Wasting and Landforms l Weathering weakens and breaks massive country rocks into smaller fragments (clay, silt, sand, pebbles to large rock boulders) l Mass wasting moves these hillslope fragments (regolith) downslope by gravity alone l But when mass wasting combines with actions of running water (rivers), wind, glacier, ocean wave and groundwater, different landforms are produced

Mass Wasting and Landforms l Mass wasting is an important earth surface geomorphic process responsible for the long term evolution of hillslopes Source: Richard J. Huggett (2011)

REVIEW QUESTIONS

Review Questions for Mass Wasting 1. In the evolution of many landforms mass-wasting is the step that precedes weathering. A. True B. False 2. Saturating the pore spaces of weathered debris with water will usually decrease the likelihood of downslope movement. A. True B. False 3. Which statement regarding debris flows is NOT true? A. debris flows may be caused by heavy rains B. In hilly areas debris flows follow canyons and stream valleya C. debris flows create talus slopes D. debris flow can move huge boulders and trucks

Review Questions for Mass Wasting 4. When and where is solifluction common? A. rainy season in the tropics B. dry season in subtropical deserts like the Sahara C. summer monsoon season in India D. summer in northern Alaska 5. Slump describes the very slow, downhill movement of soil. A. True B. False 6. This mass-wasting process is most frequently associated with hillsides in humid regions during times of heavy rains or snowmelt. A. earthflow B. rockslide C. unloading D. solifluction

Review Questions for Mass Wasting 7. This term is used to describe material that slides downslope as a unit along a curved surface. A. debris flow B. slump C. lahar D. solifluction 8. One of the primary causes of this mass wasting process is the alternate expansion and contraction of surface material caused by freezing and thawing or wetting and drying. A. rockslide B. solifluction C. creep D. debris flow 9. Which of these mass-wasting processes is slowest? A. slump B. rockslide C. debris flow D. solifluction

Review Questions for Mass Wasting 10. How do freezing, thawing, wetting, and drying contribute to creep? A. soil becomes much weaker when dry and frozen B. gravity exerts a much stronger force ehen soil is wet and thawed C. eventually these processes trigger sudden slides D. the soil expands and contracts, lifting and dropping particles a slight distance downslope 11. The steepest angle that a pile of dry unconsolidated particles can sustain before moving downslope is its angle of _____. A. repose B. talus C. stability D. retention

Review Questions for Mass Wasting 12. The transfer of rock material downslope under the direct influence of gravity is referred to as _____. A. weathering B. mass wasting C erosion D. deformation 13. A debris flow composed mostly of volcanic materials is called a (an) _____. A. earthflow B. solifluction lobe C. lahar D. slump 14. When _____ occurs, a crescent-shaped scarp (cliff) is created at its head. A. rockfall B. slump C. creep D. debris flow

Review Questions for Mass Wasting 15. This diagram illustrates which mass-wasting process? A. C slump debris flow B. D. rockslide solifluction

Review Questions for Mass Wasting 16. Curvature of tree trunk close to the base, as shown in this photo, is a strong evidence of this type of mass wasting: _____. A. C. solifluction soil creep B. D. lahar debris flow

Review Questions for Mass Wasting 17. Cliff profiles contain the following distinctive segments: A. a fall face segment B. a talus straight slope segment C. a concave slope D. A and B 18. The inherent frictional properties of slope materials are related to: A. Particle size of slope materials B. Particle shape C. Crushing resistance D. Particle arrangement E. All of the above answers

Review Questions for Mass Wasting 19. A slope in which the shear strength of slope materials is greater than shear stress or with a safety factor greater than 1. 3 is said to be: A. A stable slope B. Actively unstable slope C. Conditionally stable slope D. A fall face slope E. None

Review Questions for Mass Wasting 20. Which of the following statements is not correct about mass wasting? A. It is a downslope movement of slope materials in response to gravitational stress B. It does not require any physical medium such as water, glacier or wind to accomplish downslope movement of materials C. Occurs when shear strength of slope materials is less than the shear stress

Review Questions for Mass Wasting 21. D. Soil creep, solifluction and rock falls are good examples of mass wasting processes E. Actions of running water, wind, and glacier are required to accomplish mass wasting processes High-altitude planation of mountain ranges into flattened summits by solifluction processes is called: A. Peneplanation B. Etcplanation C. Pediplanation D. Altiplanation

Review Questions for Mass Wasting 22. The rate of movement of soil creep is a function of: A. Slope angle B. Susceptibility of slope materials C. Water content D. All of the above answers 23. A landform resulting from the free fall of rock materials is the: A. rock glacier B. peneplain C. talus slope D. alluvial fan E. Mudflow

Review Questions for Mass Wasting 24. Soil creep results from: A. the slow movement of soil organisms B. chemical reactions between the regolith and bedrock materials C. changes in the soil volume D. Disturbance of soil on a slope E. the drying out of fine particles on a slope 25. Slumping is different from other forms of earthflow or mass wasting in that:

Review Questions for Mass Wasting A. only gentle slopes are required B. it involves some backward rotation C. more water is needed than in other forms of flow D. lubricating water is unnecessary E. it occurs only on very steep slopes 26. Which of the following is the most fluid type of mass movement? A. earthflow B. mudflow C. Solifluction D. slump

Review Questions for Mass Wasting 27. 28. 29 30. The normal force (Fn) is an indication of frictional resistance A. True B. False The following equation Fn = Fg. CosØ shows that Fn is at a zero on a horizontal surface and at a maximum on a vertical surface or slope A. True B. False In dry slopes, pore-water pressure is zero or negative causing adhesion of slope materials A. True B. False The above-ground Alaskan pipeline transporting warm petroleum oil product

Review Questions for Mass Wasting cannot be transported through underground pipeline in this region because: A. It is a forbidden practice in the region by the local people B. the ground is permanently frozen C. the warm oil will melt the permafrost and induce land subsidence and the collapse of the pipeline D. the petroleum oil product must be kept at the same temperature throughout the transporting period E. A and D.