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 c