TMJC 2025 JC1H2 C2 Lecture 11 Channel Patterns

Higher 2 (9173) Cluster 2: Tropical Environments TMJC Lecture 11 Channel Patterns and Deltas 2025 Page 1 of 10 LECTURE OBJECTIVES 1. Explain the meandering and braided channel patterns 2. Compare the characteristics and conditions that leads to the formation of meandering and braided channels 3. Explain how the characteristics of the drainage basin affect the drainage efficiency Introduction Channel Patterns: Channel Straight Meander / Sinuous Braided Channel Photo Plan View Cross- section view

Higher 2 (9173) Cluster 2: Tropical Environments TMJC Lecture 11 Channel Patterns and Deltas 2025 Page 2 of 10 (A) WHAT IS A MEANDER CHANNEL PATTERN? 1. A meandering channel is a sinuous channel with a sinuosity index of at least 1.5 with asymmetrical cross-sections, formed by erosion on the outer bank and deposition on the inner bank. 2. In terms of channel features present, meandering channels contain pool and riffle sequences as well as river cliffs and slip-off slopes (Fig. 1). Fig. 1: Meander Channel Pattern Sinuosity is a measure of how irregular a stream’s path is. Sinuosity is measured by the Sinuosity Index, which is obtained by dividing channel length between 2 points by the straight line distance between the two points (also known as the downvalley length). Sinuosity index = Channel length between A and B/Downvalley length Where sinuosity is 1.0, it shows that channel length is equal to the straight line distance, meaning it is a straight channel.

Higher 2 (9173) Cluster 2: Tropical Environments TMJC Lecture 11 Channel Patterns and Deltas 2025 Page 3 of 10 (B) HOW IS A MEANDER CHANNEL PATTERN FORMED? 1. The main condition required for the formation of meander channel patterns is the presence of an alternating pool and riffle sequence. a. Any form of turbulence will create decelerated flow of water resulting in deposition forming a riffle . This deposition result in release of energy allowing for accelerated flow which causes erosion, creating a pool. b. Alternating pool and riffle sequences refer to alternating occurrences of pools (areas of deep water) and riffles (areas of shallow water) along the river channel 1. Typically the distance between the pools are 5–7 times the bed width of the river. c. Pool and riffle sequences may affect channel form by increasing the sinuosity of the channel which will then eventually lead to the formation of meanders once the sinuosity index exceeds 1.5. 2. Thus, meanders form as a result of differences in channel efficiency and competence over space due to the pool and riffle sequence. 3. Along a straight section of a river, a pool and riffle sequence results in a diversion of the thalweg (the path of maximum velocity) from side to side of the river banks due to diffusion over riffles. a. This causes the thalweg to develop a sinuous course. b. As the thalweg swings, it results in erosion on the outer bank due to higher velocities of flow and deposition on the inner bank to form point bars due to reduced flow velocities and reduced channel competence (Fig. 2). Fig. 2: Diversion of thalweg due to pool-riffle sequence 4. When the river emerges from the bend, it encounters the riffles in the channel and due to the shallowness of the riffles, velocity falls and deposition of material takes place. 5. The river, having deposited sediment on the riffles, emerges with more energy. 6. As the thalweg swings to the next bend, this results in increased erosion of the outer bend again. This makes the channel more sinuous and also creates an asymmetrical channel at the meander bend (Fig. 3). 1 Riffles are formed in shallow areas by coarser materials such as gravel deposits over which water flows. Pools are deeper and calmer areas whose bed load (in general) is made up of finer material such as silt.

Higher 2 (9173) Cluster 2: Tropical Environments TMJC Lecture 11 Channel Patterns and Deltas 2025 Page 4 of 10 Fig. 3: Increasing sinuosity of the channel, with pools at the apex of the curvature and riffles at inflexion points 7. The swinging thalweg in a meander system also generates a secondary flow known as helicoidal flow, which is a corkscrew-like flow of water (Fig. 4). a. The high velocity flows hit the banks of the channel with force before dropping with gravity. b. There is a surface movement of water to the outside of a meander bend, and a return drift at the bed. c. This results in erosion and undercutting of the channel bank, accompanied by deposition on the convex bank opposite. 8. This accentuates the asymmetrical nature of the channel as erosion occurs on the outer bank to form river cliffs, while deposition occurs on the inner bank to form slip-off slopes. Fig. 4: Helicoidal Flow 9. In fully developed meanders, pools are located at the apex of curvature close to the cut bank where flow velocity is highest. 10. These pools are areas of scour, laminar flow, deep water and fine sediment. Riffles occur on crossover points of channel where the thalweg swings from one bank to the other. They are areas of shallow water, coarse sediments, steep gradient and turbulent flow.

Higher 2 (9173) Cluster 2: Tropical Environments TMJC Lecture 11 Channel Patterns and Deltas 2025 Page 5 of 10 11. Meandering rivers can migrate laterally due to erosion on the outer banks of the channel and transference of materials to the inner banks, increasing the sinuosity of the meander. When 2 meander bends intersect, they may form horse-shoe shaped lakes known as oxbow lakes. 12. The availability of higher continuous input of water is essential in the formation of meandering channels and hence meandering channels are found largely in the humid tropics. An example of a meandering channel in the humid tropics is the Mamore River in Bolivia and Brazil within the Amazon Basin.

Higher 2 (9173) Cluster 2: Tropical Environments TMJC Lecture 11 Channel Patterns and Deltas 2025 Page 6 of 10 (C) WHAT IS A BRAIDED CHANNEL PATTERN? 1. A braided channel is a channel in which there is not one single channel but a series of small interconnecting channels separated by mid-channel bars (eyots) (Fig. 5). 2. The whole body of bars are located within the banks of the channel and which is referred to as a containing channel. 3. The individual channels within the containing channel is known as anabranches/ anastomising streams. Fig. 5: Features of a braided channel pattern (D) HOW IS A BRAIDED CHANNEL PATTERN FORMED? 1. There are three main conditions leading to the formation of braided channel patterns. 2. Braided channel patterns tend to form in streams having (1) highly variable discharge, (2) high sediment load and calibre, as well as (3) easily erodible banks that form broad containing channels and supply abundant sediment load to the stream. 3. Braided channels therefore form as a result of the variation of channel hydraulic efficiency and channel competence over time. 4. During periods of high discharge, a stream with easily erodible banks may carve out broad channels, where the high discharge increases channel competence and provides energy for bank and bed erosion to take place. These broad channels are thus hydraulically efficient (possess high hydraulic radius) during periods of high discharge (Fig. 6).

Higher 2 (9173) Cluster 2: Tropical Environments TMJC Lecture 11 Channel Patterns and Deltas 2025 Page 7 of 10 Fig. 6: Broad containing channel carved out during a period with high discharge to produce a hydraulically efficient channel 5. However, during periods of low discharge (such as during the dry season), the broad bed of the containing channel makes it hydraulically less efficient (lower hydraulic radius) due to the relatively greater wetted perimeter compared to the cross -sectional area. The increased friction results in lower vel