SAJC Cluster 2.2 Lecture 12 Notes (Geomorphic Processes Pt.3: Movement of materials on slopes)

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2 Cluster2_Lecture 12_2025/pg1 Lecture 12 Geomorphic Processes (III): Movement of Materials on Slopes KEY QUESTION: ✓ What influences the movement of materials on slope? With the completion of this lecture, attached readings and tutorial, you should be able to discuss: • how materials move on slopes through mass movement and water erosion • the different forms of mass movements (fall, slide, flow, creep) and water erosion (splash erosion, rainwash and rillwash) • the human and natural factors influencing movement of materials on slopes Lecture Outline 12.1 Introduction: Movement of Materials on Slopes 12.2 Slope Stability 12.3 Classification of Mass Movements 12.4 Fall 12.5 Slide 12.6 Flow 12.7 Creep 12.8 Factors Affecting Shear Stress and Shear Strength 12.8.1 Natural Factors 12.8.1.1 Gravity 12.8.1.2 Climate – The Availability of Water 12.8.1.3 Vegetation 12.8.1.4 Slope Characteristics (a) Steepness of Slope (b) Geology (c) Underlying Rock Type (d) Character of Regolith Box 1: Factors Affecting Mass Movements in Singapore 12.8.1.5 Tectonic Forces – Seismic and Volcanic Activities 12.8.2 Human Factors Box 2: Recovery, construction efforts after Clementi landslide 'may take a few months': Desmond Lee 12.9 Water Erosion 12.9.1 Splash Erosion 12.9.2 Rainwash 12.9.3 Rillwash 12.10 Factors Affecting Water Erosion 12.10.1 Natural Factors 12.10.2 Human Factors 12.11 Conclusion

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2 Cluster2_Lecture 12_2025/pg2 12.1 Introduction: Movement of Materials on Slopes • As we learnt in Lect. 11, different weathering processes break down rocks into regolith and soil. All these broken materials lie on slopes of the earth’s surface. Since these broken materials are weak and loose, these are susceptible to the pull of gravity, running water, wind, and the movement of glaciers to move them downslope from one place to another. In this lecture, we will focus on the processes involved in the movement of materials down the slope due to the pull of gravity (mass movement) and due to water (water erosion). • Mass movements and water erosion are considered geomorphological processes as their actions alter slope surfaces. • In geography, a ‘slope’ refers to a small strip or patch of the land surface that is inclined from the horizontal. Most slopes are covered by a layer of regolith, which transitions downward into solid, unaltered rock, known simply as bedrock (see Fig. 1). Fig. 1 Profile of a Slope 12.2 Slope Stability • We use the term mass movement to refer to the downslope movement of materials under the influence of gravity (see Fig. 2). The rock materials move as a single unit. Mass movements take place when the driving force of gravity, which creates shear stress, exceeds the resisting force created by the shear strength of the slope materials. o Mass movement is different from erosion in that mass movement is not accompanied by a moving erosion agent (such as wind, water or ice) but just the influence of gravity. o The word “mass” in mass movement refers to the accumulation of materials of varied nature – from gigantic boulders, fragmented rock, regolith, to fine soil particles. The volume of these material that is moved can be very large, making mass movements potentially hazardous. Fig. 2 Some key terms

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2 Cluster2_Lecture 12_2025/pg3 • From Fig. 3, we can see that: o When shear stress < shear strength → the slope is stable. All weathered and loose rock materials lie at rest (i.e. no downslope movement) on a stable slope. o When shear stress > shear strength → the slope is unstable. Mass movement will result to bring the slope towards attaining slope stability by lowering the slope angle. o When shear stress = shear strength → slope is at critical threshold. • Shear Stress: The force that acts parallel to the slope and attempts to pull materials downslope. (aka downslope force) o The primary driver is the pull of gravity, which the weight of the weathered material is a function of. o The magnitude of shear stress varies with the gradient or steepness () of the slope. The steeper the slope, the greater the tendency for gravity to pull materials down the slope. o The steepest angle at which materials on a slope remain stable is called the angle of repose (see Fig. 4 and Section 12.8.1.4(a)). Beyond this angle, gravitational force will move materials downslope. o Shear stress can be increased by a range of factors (see Section 12.8.1.5) including violent shaking caused by earthquakes, volcanic eruptions and manmade explosions. Fig. 4 Angle of repose • Shear strength: The forces that cause soil material to stay in place on a slope contribute to shear strength. It is the underlying resisting force that acts against shear stress. (aka upslope force) o Shear strength is dependent on the friction and resistance forces between particles such sands and gravels, and cohesive forces (cementing) between silt and clay particles. These Fig. 3 Slope stability is achieved when shear stress is less than shear strength. When shear stress exceeds shear strength, slope instability results, giving different forms of mass movement.

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2 Cluster2_Lecture 12_2025/pg4 particles are all found within the “mass” that is potentially or eventually “moved” (hence the term, “mass movement”). o Shear strength is also dependent on root cohesion (capacity of plant roots to bind soil together). 12.3 Classification of Mass Movements • Mass movement show great variety of scale, rate, material and resulting landforms. Many different classifications have been proposed using these criteria, but one of the more frequently cited ones is by Carson and Kirby (1972) as shown below. • Carson and Kirby acknowledged the presence or absence of water as an important role in triggering and determining the type of mass movement. Hence, they used: o the amount of moisture present, and o the speed of movement as characteristics to distinguish between the various types (see Fig. 5). Fig. 5 Carson and Kirby’s classification of mass movement processes • Carson and Kirkby recognised three main types of mass movement; fall (see Section 12.4), slide (see Section 12.5), flow (see Section 12.6) and creep (see Section 12.7). • As Fig. 5 shows, there are many different (sub-)types of mass movements. In our syllabus, we will focus on the main types, i.e., fall, slide, flow and creep. • The notes will outline the characteristics of each mass movement as if they were discrete movements, although in nature the various types often overlap. Creep Slide Flow

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2 Cluster2_Lecture 12_2025/pg5 12.4 Fall • Fig. 5 does not include fall, but if it were to, the placement of “fall” would be somewhere close to slide (see Section 12.5) because it has the fastest rate of movement and is also the driest. • Falls are generated on the steepest slopes (angles greater than 40°), allowing a