SAJC Cluster 2.2 Lecture 10 Notes (Geomorphic Processes Pt.1: Fluvial Processes and their factors)

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 10_2024/pg1 Lecture 10 Geomorphic Processes (I): Fluvial Processes and their Factors KEY QUESTIONS: ✓ What influence fluvial processes? With the completion of this lecture, attached readings and tutorial, you should be able to understand the: Fluvial processes: erosion, transportation and deposition of materials by rivers Sources of materials in rivers Influence of river’s energy on fluvial processes Natural and human factors influencing river’s energy Lecture Outline 10.1 Introduction: Rivers as energy systems Box 1: Some useful terms to do with channel morphology What a river does with the energy it has... 10.2 Fluvial Erosion 10.2.1 Corassion 10.2.2 Corrosion 10.2.3 Cavitation 10.3 Fluvial Transportation 10.3.1 Solution 10.3.2 Suspension 10.3.3 Saltation and Traction Box 2: Capacity and Competence 10.4 Fluvial Deposition 10.5 Tying fluvial processes together: The Hjulström Curve Box 3: Where do materials in a river come from? What factors will affect the energy a river has... 10.6 Factors Influencing a River’s Energy 10.6.1 Hydraulic Radius 10.6.2 Channel Slope 10.6.3 Channel Roughness

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 10_2024/pg2 10.1 Introduction: Rivers as energy systems Rivers are energy systems. They derive their energy from: • their volume of flow, or discharge (influenced by OVF, TF and BF; recall Lect 5) • flow velocity (which is dependent on a number of factors, see Section 10.6) This energy is expended (or used) through fluvial processes of erosion and transport. Deposition sets in when river energy is low. • At high flow, rivers have surplus energy which is expended on sediment erosion (“picking up” load; see Section 10.2) and transport (“moving” load; see Section 10.3). • At low flow, rivers may deposit (“drop”; see Section 10.4) its load to ensure it has enough energy to overcome the frictional resistance of the channel and keep the water flowing. At any one time, a river will be performing all these processes, but depending on the existent factors, there will be a dominance of one over the other. 10.2 Fluvial Erosion Broadly, erosion refers to the progressive removal of load (i.e. rocks, sediments, and mineral material) from the bed and banks of the channel. Erosion is associated with high energy levels in the river. By the process of erosion a river can deepen (by eroding the river bed), widen (by eroding the river bank) or even lengthen its channel. There are generally three ways through which fluvial erosion can take place (see Fig. 1). These are abrasion, attrition, solution and hydraulic action.

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 10_2024/pg3 10.2.1 Corassion Corrasion refers to the scraping away (like a sandpaper would) of the bed and banks by materials transported by the river. The main tools of corrasion are coarse bedload (like pebbles) and sand-sized particles. As these particles are transported (see Section 10.3), they scour the channel. This process is responsible for much of the downcutting or vertical erosion that creates and deepens channels, and is likely to be most effective in upland channels in times of flood when the large angular fragments of bedload can be activated upon the exposed rock of the bed. This process is also responsible for sideways cutting or lateral erosion that widens the channel of the river. If the base of the bank is eroded by abrasion it might result in collapse of the upper part of the bank of the river as well, thus also widening the river channel. Active corrasion along a streambed can produce potholes (see Fig. 2A), especially in fast- flowing rivers with strong eddying (i.e. water moving in a circular manner). • Potholes are cylindrical holes ‘drilled’ into the bedrock by turbulent high velocity flow. It occurs when fragments of load are whirled around in turbulent eddies to drill smooth depressions into the rock bed (see Fig. 2B). Corrasion is a major cause of the bedload particles themselves becoming smaller and rounder as they move downstream (see also later, attrition).

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 10_2024/pg4 10.2.2 Corrosion Corrosion refers to the chemical action of water, which dissolves soluble materials and carrying them away. It is most marked on carbonated rocks, such as limestone, where both river water and carbonic acid dissolves the lime in the rock and forms gaps, openings and hollow spaces on the bed and banks of the river. (We shall learn more about limestone in a subsequent lecture). However, many other chemical compounds are soluble, and thus a wide range of rocks on the bed and banks of the river may be vulnerable to the process of corrosion. Like corrasion, corrosion can serve to widen and/or deepen a channel. 10.2.3 Cavitation Fig. Cavitation Cavitation involves the formation and collapse of bubbles within the water. As the bubbles burst, very tiny jets of water (microjets) are produced that move at intense velocities and hammer the river bed and banks (Fig. 4). It involves shockwaves released by imploding bubbles, which are produced by pressure changes in fast-flowing streams, smashing into the channel walls, hammer-like, and causing rapid erosion. Cavitation occurs when flow velocities are high, as at the bottom of waterfalls (Fig. 5) or in rapids • Cavitation is an important process in rapids and waterfalls, and is generally accompanied by corrasion.

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 10_2024/pg5 Fig. Waterfall (plunge pool) Comparison of the Different Fluvial Erosional Processes (Table 3) Corrasion Corrosion Cavitation Definition The mechanical process of wearing away by friction. Chemical erosion of specific soluble material in rocks by river water. Cavitation involves the formation and collapse of bubbles within the river water. Tools Coarse river load or materials from channel Acidic water Air bubbles in high velocity river Actions Coarse and regular fragments or river load are rolled and dragged along the channel bed and banks. Minerals dissolved in acidulated water in the river and are removed leaving pits in the bed and banks of the river. It involves shockwaves released by imploding bubbles, which are produced by pressure changes in fast- flowing streams, smashing into the channel walls, hammer-like, and causing rapid erosion. Landform it affects Wears away rock outcrops, exposed rock of river bed. Scratches and smoothens channel Downcuts and deepens the bed of the channel Sideway cuts or widens the channel All rocks with minerals Which are susceptible to dissolving action. Forms depressions known as plunge pools, at the base of waterfalls or rapids. Remaining Material Angular fragments. Rock fragments Highly effective in Upper course of the river or High flood River beds and banks made of soluble minerals, Tropical rivers. High velocity river mainly at the base of waterfalls and rapids. Table 3 Plunge pool develops

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