Traditionally, interventions in river channels have been carried out to reduce flood risk at a particular location. This approach has produced artificial river geometries which have often been found, for a variety of reasons, to be unsustainable. A core principle of modern river engineering is that, in general terms, rivers tend to return to their natural ‘regime’ state, in which the main channel has the capacity for a particular flow and no more.
Pepper, A.T. and Rickard, C.E. 2009 Fluvial Design Guide. Bristol: Environment Agency. Chapter 8. (Contains public sector information licensed under the Open Government Licence v3.0.)
While major rivers, especially in developing countries, must be treated as being unique, current thinking is that this flow- rate corresponds roughly to the mean annual flood (Pepper and Rickard, 2009). Greater flow-rates, therefore, are not necessarily contained in the main channel. Artificial deepening of a river channel increases cross-sectional area but may reduce slope; the result can be reduced velocity and increased deposition of silt, tending to a reversal of the initial deepening. Artificial widening may cause deposition close to the river banks. Cutting off a meander will shorten the channel between two points and therefore increase the bed slope; this will increase flow- carrying capacity but will also increase velocities. The result may be erosion of the banks or the bed (termed ‘scouring’) together with deposition further downstream; this will also tend to reverse the initial steepening.
In some circumstances, decreasing roughness or straightening the course may solve local flooding problems, by increasing the capacity of the channel, but a reduction in both storage and attenuation is inevitable. In contrast, where channel naturalization or river restoration includes returning a watercourse to a more natural condition (for example, by reinstating meanders) this can increase storage, enhance the amount of attenuation and thereby reduce flood risk downstream.
Making the channel bigger seems the obvious way to increase its capacity and thereby overcome a flooding problem, and many flood alleviation schemes have gone ahead on this basis. Though, as promoters of such schemes have found to their cost, there are drawbacks too. These include:
- environmental degradation as of the result of the loss of natural channel features and vegetation during the enlargement process (recovery may take a long time);
- channel instability due to the removal of vegetation that helps to prevent erosion and maintains the integrity of the riverbanks;
- sedimentation in the engineered channel leading to the formation of shoals and bars, and eventually to the reversion to a smaller, more natural channel (frequent intervention may be required in the form of desilting to keep the desired channel size, again with adverse environmental impacts);
- a stark and unnatural appearance, particularly in the usual low-flow conditions (which are much more common than flood flows).
Some of these problems can be overcome by creating a two-stage channel, in which the usual flows are contained within a smaller channel and flood flows occupy a much larger second stage channel. This design emulates the operation of a natural floodplain, but in a more contained and controlled manner.
As a variant of the two-stage channel, a low-flow section can be engineered by the provision of low groynes that confine these flows and result in enhanced flow velocities, but do not interfere significantly with flood flows. Low weirs can also be used to maintain the depth of water in the channel in low-flow conditions without impacting on the flood capacity.
All these options require careful consideration and the involvement of an experienced geomorphologist to ensure they are stable in the long term and do not result in adverse impacts.