Fluvial Design Guide - Chapter 8

Works in the river channel

8.4 River restoration and improvement

People always have modified rivers for their own purposes. Many rivers and streams that appear, at first glance, to be natural, may in fact have been changed hundreds of years ago to feed mills or to permit navigation.

In the last century, many more rivers were modified to reduce flood risk, often by straightening and canalising them – frequently with concrete beds and sides, and sometimes with a concrete lid as well, forming a culvert.

More recently, the considerable disadvantages of such techniques have become apparent. Not only is there a realisation that loss of habitat represents a significant environmental disbenefit, but it has become clear that, in many cases, the flood risk has actually been increased by such interventions. For example, development constructed over a culverted watercourse lies in the floodpath when the flood exceeds the capacity of the culvert and the floodwater finds an overland route down the valley. Compare that to an open channel, where – even if the water is above the channel banks – the bulk of the flow is still in (or directly above) the actual channel. De-culverting (‘daylighting’) a watercourse is therefore beneficial from both an environmental and a flood risk management perspective (see Case study 8.4), even if the planform cannot be restored to a less artificial one.

Rivers that have in the past been straightened and constrained can be restored to their original length, complete with meanders and floodplains where space permits (see Figure 8.14). The drainage system is then able to store more water during a flood, and the attenuation provided by this storage reduces peak flows downstream compared with a straight, lined channel (where the aim is to move the water downstream as quickly as possible).

Figure 8.14 Restored River Cole

This reach of the River Cole has been restored to a meandering and more natural course. Active bank erosion is evident, but this is part of the natural process of channel adjustment and can be left to continue.

The natural features created by erosion and deposition create habitats for a wide range of aquatic flora and fauna. Many of these may be damaged or lost when a stream is over-engineered.

It is unfortunate that in many places where rivers have been straightened, lined or culverted, it is not possible to regain a natural (or naturalised) channel due to man-made development and infrastructure –probably the reason the river was altered in the first place. Where such a possibility does exist, the aim should be to restore the river to its natural course, as this reduces the need for bed or bank protection.

If old maps showing the natural course are not available, look for any natural reaches upstream and downstream, and aim to mimic these. Nearby rivers of similar size and slope may also give clues as to what a ‘natural’ river in that area should look like.

If the strata in which the restored channel is to run are erodible, it is advisable to create a new channel that is only approximately as the finished one should be. The flows will then fine tune it to a natural regime with, for example, pools and riffles forming naturally. But if the strata are likely to erode only slowly, then some extra work at the construction stage enables a more natural river to be achieved earlier. Features such as pools, riffles, varying bank slopes and berms can all be included with the expert advice of fluvial geomorphologists, ecologists and fisheries advisors.

A restored river invariably provides a wider range of habitats than its predecessor, not only within the river but also on the banks and in the floodplain (if included in the restoration). This will go some way to meeting local biodiversity action plan targets. A natural river enhances the amenity of an area and provides riverside walks, as well as providing an opportunity for formal and informal education.
 


8.4.1 Restoration techniques

The River Restoration Centre (RRC) collates information on river restoration schemes, and disseminates this information in a number of ways, one of which is the Manual of river restoration techniques (RRC, 2002). This should be among the first references to be consulted when considering a river restoration project.

Careful consideration is needed when proposing the removal of permanent artificial obstructions from a watercourse (such as a redundant weir) in order to restore a river to a previous state. Although there may be an immediate gain in flood conveyance and the removal of an obstruction to fish migration, there are also potential negative impacts. These include the release into the flow of accumulated sediments, which may be contaminated if the site has an industrial history. Removal of a weir may also be seen as a loss of amenity. It can also temporarily destabilise the river by causing bed erosion upstream, and this could undermine infrastructure such as bridges and riverside walls. But in many circumstances, a weir can be removed with no adverse – and many beneficial – impacts (see Case study 8.5).

If a weir is to be removed, consideration must be given to the changes in the pattern of energy dissipation that may be caused. A weir pool may have formed over many years, with a shape and size that allow it to act as an effective energy dissipator. If removal of a weir takes away that function, that energy must be dissipated elsewhere – which is along the bed and banks of the river upstream. Careful assessment is required to determine the mean and maximum velocities likely to be encountered and the shear forces generated at the bed and banks. If these are likely to cause unacceptable erosion, then either the areas at risk should be protected or perhaps the weir should be only partially removed.

An alternative is to replace one large weir with a series of lower weirs or possibly a cascade, which would permit fish passage and improve flood capacity while restricting velocities and potential bed erosion. In less steep rivers, a pool and riffles sequence can be established, often by the introduction of imported gravel to form the riffles. Guidance on the establishment of pools and riffles can be found in the Environment Agency’s Guidebook of applied fluvial geomorphology (Sear et al, 2003).

In some situations, a meandering channel with pools and riffles can replace a shorter straight channel containing a weir, to the benefit of fish passage, an increase in habitat potential and additional flood attenuation. Where restoration involves reintroducing meanders to a long straight channel, it is likely that the new channel will cut across the existing channel in a number of places. If possible, parts of the straight channel that are intersected should be left open rather than being backfilled to provide backwaters (see Figure 8.15). These will provide a still water habitat, as well as offer refuges for fish during periods of high flow. If land constraints permit, purpose-made backwaters or perhaps shallow bays can be excavated.

Figure 8.15 Backwater

When a floodbank along the River Roding was reconstructed further from the edge of the river, it gave the opportunity to construct a small backwater in the floodplain between the river and the floodbank.

The photographs show the backwater just after excavation and a year later. Some marginal planting had been carried out, but all terrestrial growth is from natural re-vegetation.

The entrance to the backwater can be seen at the far end of both photographs. In major floods the entire area is submerged by flowing water.

 

Not all river works are carried out in natural earth channels. In Boscastle, for example, the existing channel has been deepened through bedrock. Rather than just slicing a vertical wall through the rock, it was carefully broken out along its bedding planes, leaving the bank with the same appearance as the more natural banks elsewhere on the river. Such attention to detail and consideration of how to work with nature are the keys to successful river restoration.
 


8.4.2 River diversions

Sometimes it is necessary to divert a river or stream course to make way for major infrastructure such as a motorway. This should be seen as the opportunity to re-create the river corridor, incorporating existing and new environmental features to ensure the new reach of channel adds value to the environmental and ecological status of the stream in question. The Water Framework Directive requires this type of approach. The river restoration techniques described above can be adopted for river diversion schemes (Fisher and Ramsbottom, 2001).

In other cases it may be that the existing course of the river is too constrained to allow enlargement to carry flood flows but is adequate for low flows. In such cases an alternative route for excess flows must be found. This may be a new channel, although this can involve significant land acquisition for something that is rarely used, or it can be a floodway where land is used for its original purpose (except when a major flood occurs) provided that such use is compatible with occasional flooding (see Figure 8.16).

Figure 8.16 Floodway

This floodway is normally used for cattle and horse grazing but, when a major flood occurs in the adjacent river, floodwater is diverted across the fields by the operation of an automatic gate.

The scheme has been designed so that floodwater rises slowly, from the middle of the floodway, leaving no islands, and thus any stock in the fields are able to make their way to safety on the banks along each edge. There are large areas of field on each side which do not flood.

 

 

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