TMJC 2025 JC1H2 C2 Lecture 10 Channel Morphology and Fluvial Processes

1 Higher 2 (9173) Tropical Environments TMJC C2 Lecture 10 Channel Morphology and Fluvial Processes 2025

2 WHAT IS CHANNEL MORPHOLOGY? 1. Channel morphology refers to the form (size and shape) of the river channel. 2. There are various terms used to describe the morphology of a river channel: a. The width of the river channel is the horizontal distance across the river from the water's edge of one bank to the other, perpendicular to the general direction of flow of water (Fig. 1). b. The depth of the river channel refers to the vertical distance from the water surface to the channel bed measured at a particular point of the river (Fig. 1). c. The river slope (gradient) is the inclination of channel bed with reference to horizontal. Fig. 1: Width, cross-sectional area and depth d. The width-depth ratio refers to the ratio of the bankfull width by the average depth of the river channel. The higher the ratio, the more inefficient the river channel, as a lot of energy is lost through friction with the channel bed and bed load transport. e. The wetted perimeter refers to the length of the channel cross-section that is in contact with the water in the channel. f. The cross-sectional area of a river channel refers to the area in square metres of a vertical slice across the river channel at any point (Fig. 1). g. The hydraulic radius of a river channel is the proportion of water that comes in contact with the river channel. It is a measure of a channel’s efficiency (i.e. how well a channel is able to allow water to flow within it). The larger the hydraulic radius, the more efficient the channel (Fig. 2).

3 Calculating the hydraulic radius: Hydraulic radius (Rh) = cross-sectional area (m2) / wetted perimeter (m) Fig. 2: Contrasting the hydraulic radius of two streams with similar cross -sectional areas but different wetted perimeters. (The larger the hydraulic radius, the more efficient the channel i.e. Stream A is thus more efficient than Stream B) (B) HOW DOES THE CHANNEL MORPHOLOGY ALONG A RIVER VARY? 1. In general, the morphology of a river channel changes along a river. a. The long profile of a river (Fig. 3) shows how the gradient (slope) of a river channel varies from the source to the mouth of the river. i. In the upper stage of a river’s course, the river’s gradient is steep but it gradually flattens out as the river erodes towards its base level. ii. One thing to note is that the long profile of a river shows many irregularities called knickpoints. These are points where the gradient of the river changes suddenly and can be caused by features like waterfalls or lakes, where the geology of the river changes and differential erosion takes place. iii. Knickpoints can also be the result of rejuvenation, where the base level of the river falls, giving it some extra gravitational potential energy to erode vertically. iv. Theoretically, rivers should achieve a condition of equilibrium, or grade, and erode the irregularities. Throughout the long profile of a river, deposition and erosion are balanced , meaning that, given enough time, the river’s long profile would become a smooth, concave, graded profile and all the knickpoints would be eliminated as they are either eroded or filled in by deposition.

4 v. However, it would take a long time for a river’s long profile to become a graded profile though so the idea of a graded profile is, essen tially, theoretical as it does not really occur in nature. Fig. 3: Long profile of a river b. The cross profile of a river (Fig. 4) near the source at the upper valley is generally V - shaped, and the channel becomes deeper and wider with increasing distance downstream, with the channel being widest and deepest near the mouth. Fig. 4: Changes in cross profiles along a river c. The change in channel morphology along a river can be further illustrated using Schumm’s model (Fig. 5). i. According to the model, from the source to the mouth of the river, the gradient (slope) of the channel will decrease. On the other hand, channel width and channel depth will increase. ii. This is caused by the greater amount of energy available by the river downstream, as discharge and velocity increases. Fig. 5: Schumm’s Model (non-arid environment, no human intervention)

5 d. The changes in channel morphology along a river can also be illustrated using the Bradshaw model (Fig. 6). i. The model shows how characteristics of the river channel such as the depth, width increases downstream, while the gradient decreases downstream. ii. Similar to Schumm’s model, this is caused by the greater amount of energy available by the river downstream, as discharge and velocity increases. Fig. 6: Bradshaw Model

6 (C) WHAT ARE THE VARIOUS TYPES OF CHANNEL PROCESSES IN THE RIVER CHANNEL? 1. FLUVIAL EROSION a) Fluvial erosion is the wearing away and removal of material from the wetted perimeter of the river channel. The eroded material is then transported along the channel. All the material transported by the river is called load. b) There are 4 ways in which erosion may occur . These are the processes of Corrasion, Corrosion, Hydraulic Action (including Cavitation) and Attrition i. Corrasion (also known as abrasion) occurs when rock particles already being moved by the river strike or are dragged along the bed. Their action is like a hammer chipping the rock or like a file smoothing down the surface and producing small particles which are easily transported. If particles in the load are large, erosion is more rapid. Small particles tend to smooth or polish surfaces. Abrasion may also be known as corrasion. - Abrasion is possibly the most effective method of erosion and is normally responsible for most of the vertical erosion (‘downcutting’) in a river channel. If the river carries very little load, abrasion is ineffective. Potholes in the beds of rivers are a common feature of rapid abrasion. They are cylindrical holes drilled into the rock by turbulent high velocity flow. ii. Corrosion (also known as solution) is the process where soluble minerals in the rocks lining the channel are dissolved and carried away as solute load . It is most marked on carbonated rocks, such as limestone, where carbonic acid dissolves the rock. iii. Hydraulic action refers to the wearing away and removal of material from the force of air and water on the sides of the channel and in cracks. - It results in evorsion which is the direct force of flowing water on the river bed. Evorsion is generally responsible for removing only unconsolidated sedimenst such as sand or fine gravel. The velocities attained in a normal river flow will have little effect on hard, resistant rocks. - Besides evorsion, hydraulic action can also result in Cavitation, which is the force of air exploding. Cavitation occurs as bubbles implode and evict tiny jets of water at high velocities which can damage solid rock. iv. Attrition is the reduction in the size of the particles in transport as they strike one another or the bed of the channel. - As any particle moves downstream, there is a progressive reduction in size. In addition, sharp edges and angles of the particle become rounded since they are more exposed. The upper stretches of the river therefore tend to contain larger, angular sediment while the downstream parts of have fine, rounded particles. Attrition is a way of eroding the river’s load, not the bed and banks. Attrition is where pieces of rock in the river’s load knock together, breaking chunks of rock off of one another and gradually rounding and shrinking the load.

7 c) Erosion can deepen the river via vertical erosion, widen it via lateral erosion and lengthen it via headward erosion. - As a river moves downstream, both vertical and lateral erosion occurs, widening and deepening the ri