TMJC 2025 JC1H2 C2 Lecture 5 and 5.5 Mass Movement and Erosion by water

Higher 2 (9173) Theme 1: Tropical Environments - Physical Processes in the Tropics TMJC Lecture 5 Mass Movement 2025 Page 1 of 20 LECTURE OBJECTIVES 1. Discuss the mechanisms of mass movement. 2. Compare the different types of mass movement. 3. Discuss the factors affecting mass movement. Introduction Most of Earth’s surface is not perfectly horizontal; sloping surfaces are everywhere. On Earth, most slopes are formed by stream erosion and are related to the walls of stream valleys. Some, however, are the result of tectonic activity such as faulting; others result from wave erosion, extrusion of lava, glaciation, and even impact of meteorites. Regardless of a slope’s origin, gravity has a universal tendency to pull materials on it to a lower gravitational potential. Consequently, the downslope transfer of material through the direct action of gravity - mass movement - is extremely common. Left: Landslide and Right: Mudflow Mass movement processes can be classified into 4 categories: 1. Fall 2. Slide 3. Flow 4. Creep (move by heave mechanism) They can be compared based on: 1. Mechanism of movement 2. Conditions required for occurrence 3. Trigger 4. Coherence of material 5. Speed 6. Level of water content 7. Effects on slope

Page 2 of 20 (A) WHAT IS MASS MOVEMENT? Mass movement is the downslope movement of slope material such as soil and rock by direct action of gravity and does not involve a flowing transporting medium such as water, wind and ice. (B) WHAT ARE THE MECHANISMS OF MASS MOVEMENT? Mass movement occurs when shear stress exceeds shear strength (Fig. 1). Shear stress is the force which causes downward movement of the body parallel to the slope. a. Shear stress has a positive relationship to the (a) weight of the body and the (b) slope angle. b. The greater the weight of the body and the steeper the slope, the greater the shear stress and hence the greater the potential for shear stress to exceed shear strength and for mass movement to occur. Shear strength is the internal resistance of the body to movement. a. Shear strength has a positive relationship to frictional resistance, cohesion between particles and the binding action of plant roots. b. The greater the frictional resistance between the mass and the slip plane, the cohesion between particles and the binding action of plant roots, the greater the shear strength and hence the lesser the potential for mass movement to occur. Fig. 1: Shear stress and shear strength (represented by friction between the body and the slope surface). [Need to know how to draw and label]

Page 3 of 20 FALL Fig. 2: Diagram of a fall [Need to know how to draw and label] Mechanism of Movement Falls are the free-falling of detached bodies of bedrock from a cliff or steep slope (Fig. 2). Conditions for Occurrence Falls generally occur on steep slopes (greater than 70o) which have bare rock faces where joints are exposed. Trigger Falls are triggered when erosion or weathering processes such as freeze -thaw action prise open lines of weakness and reduce the shear strength of the slope. When shear stress exceeds shear strength, the rocks become detached and fall under the influence of gravity. Falls are also triggered by earthquakes which generate tremors that prise open lines of weakness and reduce shear strength of the slope. When shear stress exceeds shear strength, the rocks become detached and fall under the influence of gravity. Characteristics Falls display coherence of material, are very fast in speed, and have negligible water content.

Page 4 of 20 Effects on Slope Falls result in the retreat of steep rock faces and also form scree (also known as talus)1 slopes at the base of the original slope. Type of Fall - Rockfall A type of fall is rockfall (Fig. 3). A rockfall occurred in Cairo, Egypt in 2008 when 8 boulders weighing more than 50 tons each split from the edge of the Muqattam mountain and destroyed 40 houses and killed more than 100 people. Fig. 3: Photograph of rockfall with talus slope at the base of the slope 1 Scree is a collection of broken rock fragments at the base of crags, mountain cliffs, volcanoes or valley shoulders that has accumulated through periodic rockfall from adjacent cliff faces.

Page 5 of 20 SLIDE Fig. 4: Diagram of a slide [Need to know how to draw and label] Mechanism of Movement Slides are the downslope movement of detached masses of material which moves en masse (as a coherent mass without any internal shearing or deformation of the material) along a clearly defined inclined surface known as a slip plane (Fig. 4). Conditions for Occurrence Slides generally occur in areas where there has been undercutting of the base of a slope by building a road or coastal erosion, causing the slope to steepen and increasing shear stress. Trigger Slides may be triggered after periods of heavy rain when the topmost layer of surface material becomes saturated and extremely heavy which increases shear stress, and where water has lubricated the slip plane and reducing friction between the slope and rock / soil, decreasing shear strength. Slides may then occur when shear stress exceeds the shear strength. Slides may also be triggered by earthquakes and volcanic eruptions , as the shaking of the ground can reduce friction between the slope and soil / rock , thus decreasing shear strength. Slides may then occur when shear stress exceeds the shear strength. Characteristics Slides display coherence of material as it moves en masse, are fast in speed and the velocity is similar at all depths of the slide (Fig. 5) as the slide moves en masse, and have relatively high water content as water acts as a lubricating agent between slip plane and rocks.

Page 6 of 20 Fig. 5: Velocity Profile of a Slide [Need to know how to draw and label] Effects on Slope Slides result in the formation of a scar2 on the slope and a toe bulge3 at the base of the slope. Type of Slide - Rotational Slide (Slump) A type of slide is a rotational slide (also known as a slump) (Figs 6-7). Fig. 6: Diagram of a slump [Need to know how to draw and label] 2 Barren surface that forms after a slide occurs 3 Rock fragments that accumulate at the base of the steep slope.

Page 7 of 20 Fig. 7: Photograph of a slump (rotational slide) Mechanism of Movement A slump is a downward and outward rotational movement of rock or regolith which occurs along a curved concave slip plane. Effects on Slope Similar to slides, slumps result in the formation of a scar on the slope and a toe bulge at the base of the slope. However, slumps will also produce reverse slopes, as the top of the displaced block is usually tilted backward as slumps occur along a concave sliding surface causing some backward rotation. Type of Slide - Landslide and Rockslide Other types of slide include landslide (Fig. 8) and rockslide. For example, the Cherry Hills Landslide occurred in Antipolo City (Philippines) in 1999. Heavy rainfall due to Tropical Cyclone Ising intensified the normal monsoon rainfall, triggering a disastrous landslide. Many houses were destroyed and some 58 people lost their lives. Fig. 8: A landslide?? in Hong Kong in 1972

Page 8 of 20 FLOW Fig. 9: Diagram of a flow [Need to know how to draw and label] Mechanism of Movement Flows are the downward movement of regolith whereby the moving regolith mass undergoes internal deformation due to liquefaction of slope material. Conditions for Occurrence Flows generally occur in places where there is abundant loose and unconsolidated material such as ash and debris, and where torrential rainfall occurs. Trigger Flows are triggered usually as a result of heavy rainfall when the ground