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