RI 2.2 Fluvial Landforms and Ecosystems

CLUSTER 2 | TROPICAL ENVIRONMENTS TOPIC 2.2 LANDFORMS IN THE TROPICS Raffles Institution Year 5-6 Geography. For internal use only. 1 Q = A V 2.2.3 Fluvial Landforms in the Humid Tropics What fluvial landforms can be found in the humid tropics? • Fluvial landforms: meanders, braided channels and deltas • Features of meanders: sinuous channel, river cliff, point bar • Features of braided channels: mid-channel bars • Features of deltas: delta plain, delta front, prodelta Fluvial landforms are landforms produced by the erosion and deposition of streams or rivers, in another words, fluvial processes. Examples of fluvial landforms include bars, levees, braided channels, meanders and deltas. As you have learnt, fluvial processes are affected by both river energy, discharge and sediment characteristics. 1. River Energy Once water reaches the channel, it moves downstream towards the mouth of the river. This moving water possesses energy, which varies along its course. This energy is required for the three main fluvial processes—erosion, transportation and deposition. An increase in river energy usually results in an increase in erosion and transport, whereas a decrease results in deposition. A still body of water at any point above sea level has a certain amount of stored energy as a result of its position. This supply of potential energy, when converted to kinetic energy when the river flows downstream, enables it to erode the river channels and transport the eroded material. The amount of kinetic energy is determined by: 1. The volume of the flowing water. 2. Its mean velocity. The energy possessed by the river is therefore determined by its discharge (defined as a measure of the water flow at a particular point). An increase in any one of these two factors will thus mean an increase in the amount of kinetic energy. 1.1 Measuring River Discharge River discharge varies greatly over time and space. It is calculated as follows:

CLUSTER 2 | TROPICAL ENVIRONMENTS TOPIC 2.2 LANDFORMS IN THE TROPICS Raffles Institution Year 5-6 Geography. For internal use only. 2 V = 1.49 𝑅 2 3 𝑆 1 2 𝑛 𝑆 = ℎ𝑒𝑖𝑔ℎ𝑡 𝑎𝑏𝑜𝑣𝑒 𝑠𝑒𝑎 𝑙𝑒𝑣𝑒𝑙 𝑐ℎ𝑎𝑛𝑛𝑒𝑙 𝑙𝑒𝑛𝑔𝑡ℎ Q = discharge (usually expressed in m3/s) A = cross-sectional area of the river (in m2) V = mean river velocity The volume of water is important since an increase in the amount of water will mean a higher discharge and a more efficient river. This explains why floods can unleash so much energy and cause massive destruction to the environment. 1.2 Measuring river velocity Velocity is one very important determinant of river discharge and energy. Factors affecting the velocity of rivers are spelled out by the Manning’s equation. V = velocity S = channel slope R = hydraulic radius n = coefficient of roughness (1.49 is English units conversion factor, set to 1 for metric. However, you do not need to remember this formula for A Levels) 1.2.1 Channel Slope Since stream flow is caused by the force of gravity, a change in the channel’s gradient will affect the amount of energy the stream possesses. The gradient of a channel is calculated as such: If channel gradient is steep, the change from potential to kinetic energy is rapid and the velocity of the river is high. Conversely, on gentle gradients the velocity is low. 1.2.2 Coefficient of Roughness Channel roughness is another factor affecting velocity. Some of Manning’s coefficient of bed roughness is given in table 1 below. Notice that the higher the value the rougher the bed and the lower the velocity.

CLUSTER 2 | TROPICAL ENVIRONMENTS TOPIC 2.2 LANDFORMS IN THE TROPICS Raffles Institution Year 5-6 Geography. For internal use only. 3 Hydraulic radius = 𝑐𝑟𝑜𝑠𝑠 𝑠𝑒𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑟𝑒𝑎 𝑤𝑒𝑡𝑡𝑒𝑑 𝑝𝑒𝑟𝑖𝑚𝑒𝑡𝑒𝑟 Table 1: Manning’s coefficient of bed roughness. Surface Manning’s n Very smooth e.g. glass Concrete Unlined earth drainage channels Winding natural channels Mountain streams with rocky beds Alluvial channels with small ripples Alluvial channels with large dunes 0.010 0.013 0.017 0.025 0.040 - 0.050 0.014 – 0.024 0.020 – 0.035 In a downstream direction the river channel tends to become smoother because it is more likely that the banks and beds of the river will be made up of clay/silt/sand instead of boulders and pebbles. As the n value reduces the V value increases. Irregular beds in the upper course contain large protruding grains or boulder which increases turbulence, reducing velocity upstream. 1.2.3 The Hydraulic Radius A close relationship exists between the velocity of water in a river and the characteristics of the channel in which the water is flowing. These characteristics (which include depth, width and channel roughness) are collectively referred to as the hydraulic geometry of the channel. The width-to-depth ratio, for instance, is often used as a means of comparing different channel shapes and its influence on river velocity is illustrated below. The hydraulic radius is the ratio between the area of the cross-section of a river channel and the length of its wetted perimeter (the total length of the bed and bank sides in contact with the water in the channel), i.e.:

CLUSTER 2 | TROPICAL ENVIRONMENTS TOPIC 2.2 LANDFORMS IN THE TROPICS Raffles Institution Year 5-6 Geography. For internal use only. 4 Figure 1 shows how the hydraulic radius is calculated for two channels with the same cross- sectional area (i.e. with the same size) but different shapes. Stream A with a more balanced width depth ratio has a larger hydraulic radius because of the smaller wetted perimeter. This means that a smaller amount of water is in contact with the bed and banks of the channel which in turn creates less friction and energy loss. The river velocity is subsequently higher than that of stream B. In essence, the shape of a channel is determined by the materials forming the channel sides and the river forces working on them. Normally, channels in silt and clay tend to be deeper and narrower than those in sand and gravel, because the finer materials are cohesive and promote bank stability. Besides channel shape, channel size also affects the hydraulic radius. This can be easily illustrated by calculating the R value of two different sized rivers with the same shape. 2. Channel Morphology To study the features and formation of fluvial landforms, it is important to under the concept of channel morphology. Channel morphology is the study of the shape, size, and characteristics of a river or stream channel. It primarily includes the analysis of the channel's width, depth, gradient, cross-sectional area and the channel patterns they form. For this topic, we will need to understand channel morphology in terms of a river’s long profile (gradient), cross-sectional profile and plan form (patterns). Channel patterns such as meanders and braided channels are derived from looking at the plan forms of the rivers. As sediment transported by any river must eventually be deposited at or near the river mouth, the important landform produced will be a delta. Figure 1: Calculation of the hydraulic radius

CLUSTER 2 | TROPICAL ENVIRONMENTS TOPIC 2.2 LANDFORMS IN THE TROPICS Raffles Institution Year 5-6 Geography. For internal use only. 5 2.1 Long / Longitudinal Profile You have learnt this earlier on. The long profile refers to the gradient of a river from its source to mouth. For rivers in the humid tropics, it’s typical to see a concave profile where the slope becomes gentler downstream. Figure 1a: Long Profile and Cross Sections of a River 2.2 Cross / Cross-sectional Profile The cross-section or the cross profile of a river channel allows for the analysis of width and depth. Generally, cross sectional area increases downstream due to the increased volume of water added by the tributaries of the river (fig