Why rivers are channelised, and the methods used

Overview

Realignment of a river channel and environmental costs of re-sectioning.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is channelisation?

   Model answer

   Channelisation is the direct engineering modification of a river channel to change its course, its cross-section or its bed, in order to control the flow of water and sediment. It includes realigning the channel to make it straighter, re-sectioning it to make it wider or deeper, dredging accumulated sediment from the bed, and lining or reinforcing the banks. It is a form of hard engineering, working against the natural behaviour of the river rather than with it.

2. 2

   Why do some rivers need to be channelised?

   Model answer

   Chiefly to reduce flood risk where valuable property and people already occupy the floodplain: enlarging the channel and speeding the flow means a greater discharge can be contained before the banks are overtopped. Other reasons are to drain waterlogged farmland and bring it into more productive use, to keep the channel deep and straight enough for navigation, to stop bank erosion undermining bridges, roads, embankments and buildings, to remove sediment that has reduced the capacity of the channel, and to allow land beside the river to be developed.

3. 3

   Why is it usually cheaper to channelise a river than to move the people at risk?

   Model answer

   Because the settlement, its infrastructure, its services and its economic activity are already in place and represent a very large sunk investment, while the political and social costs of relocating a community are enormous. Engineering works are a one-off capital cost with predictable maintenance, and they can be funded and delivered within a single political term, whereas relocation requires land elsewhere, compensation, new infrastructure and public consent. This is why hard engineering has historically been chosen even where planners recognise that it is not the most sustainable answer.

4. 4

   Explain why channelisation is more likely to be used on an urban reach than a rural one.

   Model answer

   The value of what is protected is far higher per square kilometre in an urban reach - housing, commercial property, transport routes, utilities and industry - so the benefits of a scheme are large enough to justify a high capital cost in a cost-benefit assessment. Space in a town is limited, so soft options that need land, such as washlands or restored meanders, are often not available, whereas a narrow engineered channel fits the constraints. Urban rivers are also already heavily modified, culverted and constrained by existing development, so there is little natural channel left to conserve.

5. 5

   How does making a channel more efficient help to prevent flooding in that reach?

   Model answer

   Flooding in a reach occurs when discharge exceeds bankfull capacity. Deepening and widening the channel raises the discharge that can be contained, so a bigger flow is needed before the banks are overtopped. Straightening and smoothing the channel increases the gradient and reduces friction, which raises velocity, while the deepening and widening above raises the hydraulic radius, so the same volume of water is moved out of the reach in less time and the water level for a given discharge is lower. Together these mean the local flood peak passes through the reach inside the banks.

6. 6

   What is the fundamental drawback of solving a flood problem by moving water downstream faster?

   Model answer

   The water has not been removed from the system, only delivered somewhere else sooner. A straightened, deepened reach discharges its flood peak downstream more rapidly and in a shorter, sharper pulse, and if that pulse now coincides with the peak from a tributary the combined discharge downstream is greater than it would naturally have been. Communities below the scheme therefore face a higher and earlier peak, so the benefit to one settlement is bought at the expense of another, which raises real questions of fairness as well as of effectiveness.

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