Changing channel characteristics
How and why do river channel characteristics change with distance downstream?
A- The Bradshaw model
The Bradshaw model is a theoretical model which describes how a river’s characteristics vary between the upper course and lower course of a river. It shows that discharge, occupied channel width, channel depth and average load quantity increases downstream. Load particle size, channel bed roughness and gradient are all characteristics that decrease.
It is depicted below as a series of triangles. If the triangle gets wider towards the downstream section it means that on average that variable increases with increasing distance downstream. If the triangle decreases in size it means that on average the variable decreases with increasing distance towards the mouth, Figure 1.
B- Therefore, the key river channel variables, which should change with distance downstream, are:
1- velocity (the speed of flow of water in the channel)
Velocity is influenced by gradient, channel roughness and efficiency. It increases downstream despite the fact that gradient normally decreases downstream. Two factors, which are related to friction, are more important than gradient to explain the increase of velocity:
The channel shape becomes more efficient and there is less friction from the banks and bed of the channel.
Erosion also reduces the size of bedload material and decreases the roughness of the channel.
2- discharge (rate of transfer of water through the channel)
It increases downstream as more water is added to the channel by tributaries and groundwater flow and because the channel becomes more efficient with distance downstream. As velocity is also a component of discharge, the higher velocities downstream also affect it.
3- channel shape and 4- efficiency (hydraulic radius)
The shape of the channel affects the amount of water in contact with the bed and banks, where energy is lost through friction. Therefore it influences the velocity of the river. In the upper course, the river channel is narrow and there are large boulders as bedload, (Figure 2). The rougher the bed, the greater the wetted perimeter(the length of the bed and banks in contact with the water) and, therefore, a higher proportion of the total water volume is in contact with the bed. This explains why a river velocity is low in its upper course, in spite of the steepness of the gradient. In its lower course, a river is often in contact with a smoother channel, as mush of the sediment has been transported and, by attrition, is reduced in size to small particles such as sand, silts and clays. Even if the gradient is more gentle than in the upper course, the water is moving at a higher velocity as a whole.
A channel is described as more efficient if there is a large area of water in the cross-section (cross-sectional area = channel depth x channel width) compared with its wetted perimeter. This can be found using the hydraulic radius (cross sectional area ÷ wetted perimeter) (Figure 3).
For instance, if the hydraulic radius = 0.938 m, this value indicates that if the bed or wetted perimeter were stretched out flat and the water in the cross-section were heaped upon it, it would form a layer 0.938 m deep. In effect, each metre of the wetted perimeter affects 0.938 m² of the water in the cross-section. If the value of the hydraulic radius is large, a large area of water in the cross-section is affected by each meter of the bed. The frictional effect of the bed is therefore very limited. With a high hydraulic radius, the channel loses less energy through friction with its channel.
The more efficient the river is, the more energy the water will have to:
- move downstream (so as hydraulic radius increases, velocity increases)
- carry load (so as hydraulic radius increases, the river’s competence and capacity increases)
- increase the rate of erosion (in the upper course, as hydraulic radius increases, there is a higher rate of vertical erosion, so gradient increases; further downstream where the river is closer to base level, as hydraulic radius increases, there is a higher rate of lateral erosion).
1- Study Figure 4 which shows the dimensions of two river channels.
(i) Calculate the hydraulic radius of channel B. (2)
(ii) Which channel has the more efficient shape? (1)
(iii) Explain why this channel is more efficient. (3)
(iv) State TWO other factors that influence the velocity of water in a river channel. (2)
(v) Explain why channel efficiency usually increases with distance downstream. (3)
2- Study Figure 5 which shows variations in hydraulic radius (channel efficiency) with velocity in a river.
(i) State the range of values of hydraulic radius as shown on Figure 5. (1)
(ii) How is the hydraulic radius of a channel calculated? (2)
(iii) Describe the relationship between hydraulic radius and velocity shown in Figure 5. (3)
(iv) Explain the effect of hydraulic radius on velocity. (3)
(v) Outline the influence of factors other than hydraulic radius on velocity. (3)
Channel characteristics such as cross profile, wetted perimeter, hydraulic radius, roughness and efficiency change downstream. Describe and explain how channel characteristics change downstream. (15 marks)