Fluvial Design Guide - Chapter 8

Works in the river channel

8.1 Introduction

8.1.1 Striking the right balance

Successful – and indeed sustainable – engineering works are those that are planned, designed, operated and maintained with due regard for the environment in which they function. In this context, the term ‘environment’ is used in its broadest interpretation, including the physical forces that have to be resisted as well as the ecological, social and visual contexts. Works in a river or stream are no different in this regard; indeed it can be argued that the fluvial environment is one of the most challenging for engineers.

A common mistake in the past was to ‘over-engineer’ river works. Although such an approach may have achieved the hydraulic objectives (increased flood capacity, for example), this was often at the expense of other matters such as ecology and visual amenity. The quest to improve the hydraulic conveyance of river channels also created a huge maintenance obligation. This is because, left alone, a river tries to return to its ‘regime’ condition, in which flows significantly greater than the mean annual flood are not contained by the dominant channel. Today’s river managers must strive to achieve the right balance (Environment Agency, 2003).

This is not to say that under-engineering is the right approach. If the works cannot withstand the hydraulic forces imposed upon them, they will fail to achieve the desired objective and may damage the environment in the process. Nor is the complete abandonment of historic maintenance practices an appropriate response to environmental pressures, as this can also result in environmental degradation of a different kind (see Figure 8.1).

Figure 8.1 Inadequate maintenance?

In the past this urban stream has been concrete-lined to improve its hydraulic capacity. But it has not been maintained regularly and the sediment in the bed now supports a lush growth, which acts to obstruct flow.

The appropriate level of maintenance should be determined by the desired hydraulic capacity of the channel as well as by the ecological value of the stream. Complete clearance of the vegetation may not be appropriate. However, the removal of most of the vegetation, perhaps leaving a strip on in the inside of the bend, could improve flood capacity without unduly damaging the ecological status.

Getting the right balance ensures that both hydraulic and environmental objectives are achieved.

The stability of a natural channel (that is, its resistance to short-term change) is inextricably linked to its geomorphology and ecology. Aggressive channel ‘improvement’ works or vegetation clearance not only inflict severe damage on the ecology and visual amenity, but may also lead to erosion on the channel bed or banks, with consequential damage to adjacent infrastructure. It is therefore important that engineers and river managers:

  • seek expert advice on geomorphology and ecology when contemplating significant works in rivers (see Chapters 3 and 4);
  • consider the impact of any works on other users of the river or stream, from those who simply appreciate the fluvial environment to those who actively use it for sports and recreation (see Chapters 5 and 6).

Works in a river channel not only have the potential to alter the river environment permanently, but they can also have a significant impact during construction and when maintenance works are required. Consultation with all affected parties (stakeholders) in the early planning stages will therefore pay dividends later on. In particular, it may be possible to programme works both to minimise the environmental impact and to avoid conflict with river users (such as anglers, boaters, canoeists and ramblers).

When works in river channels are planned, opportunities for the enhancement of the fluvial environment should also be considered. Examples include:

  • improvements to riverside access;
  • the provision of fish ledges;
  • installation of nesting pipes in the riverbank for sand martins;
  • the removal of invasive plant species (see Chapter 4).

The overall aim should be to achieve the objectives in a manner that is sustainable, while adding to the environmental value whenever possible. There is useful guidance in an RSPB handbook (1994).

Since the starting point for channel improvement works, including vegetation removal and desilting, is usually the desire to increase (or maintain) flood conveyance, it is clearly important to define the channel capacity required. In its simplest form this means defining a flow (in m3/s – cubic metres per second) for given water levels along a reach of the channel. The recently developed ‘conveyance estimation system’ (CES) allows the user to estimate the flow capacity of any channel for a range of assumptions regarding channel maintenance; for details of the CES and its development, see the project website (http://www.river-conveyance.net).


8.1.2 Legal issues

The legal framework for works in rivers is outlined in Chapter 1. The fundamental legal issues are set out in Acts of Parliament which define the rights and responsibilities of all parties with respect to rivers and streams. More recently, the European Union has enacted the Water Framework Directive with the aim of securing and, where possible, improving the ecological status of watercourses throughout all Member States.

Anyone contemplating works in any watercourse must therefore consider carefully the impact that such works may have, either directly or indirectly. In particular, the works should not cause damage or loss to other users of the river – in the reaches upstream and downstream as well as in the reach in question (see Box 8.1) – and there should be no long-term reduction in the ecological status of the watercourse. It is recognised that a temporary, short-term reduction may be inevitable during and immediately following the works. An environmental impact assessment (EIA) considers both short-term and long-term impacts of proposed works.

As a general rule, the first point of contact when considering river works should be the Environment Agency, which will be able to provide advice on current use of the river and any restrictions or legal requirements. This should then lead the way to establishing contact with all persons with rights or responsibilities for the reach of river in question, not least of all the riparian owners (the owners of land that abuts a watercourse, with ownership usually extending to its centreline).

Box 8.1 Consider and consult before starting work

In a recent legal case, one landowner (the plaintiff) sought compensation from another (the defendant) when works carried out on behalf of the defendant fundamentally changed the flow regime through the plaintiff’s land. The watercourse in question comprised two roughly parallel channels, one being the original course of the stream and the other a man-made channel constructed generations ago to feed water meadows. The crude structure that regulated the division of flow was on the defendant’s land and he owned the land through which the man-made channel flowed. The plaintiff’s land was on the original channel downstream of the division structure.

The defendant asked the Environment Agency to carry out maintenance works on his channel because it was overgrown. The work was carried out with the result that the majority of the stream flow was thereafter channelled through the defendant’s land, leaving the plaintiff with a mere trickle. One would have thought, perhaps, that this would be an easy problem to resolve, but the reality was different. The case lasted for several years and the Environment Agency (as executor of the maintenance works) was inevitably drawn in to the legal battle.

Two very important lessons come out of this example:

  • Always consider fully the range of impacts that any proposed works might have before undertaking them.
  • Always consult all stakeholders in advance of doing the work.

Anyone who intends to construct works in a watercourse must seek land drainage consent from the Environment Agency. This applies to weirs, culverts, sluices and any works that could have an impact on the flow or water level in the watercourse.

Anyone wishing to take water from a watercourse (for irrigation, for example) must first obtain an abstraction licence from the Environment Agency or from the Internal Drainage Board (IDB) as appropriate for the specific area. The Environment Agency has particular responsibility for rivers designated as a ‘main river’. Other watercourses are generally the responsibility of the local council (or the IDB in an IDB area).

In the specific case of culverting a watercourse, Environment Agency policy is that watercourses should not be culverted except where there is no other viable option because of the environmental degradation that would result. This policy is strongly reinforced by the Water Framework Directive. Similarly, the construction of weirs is generally discouraged because of the potential impact on fish migration.

Riparian owners have certain rights and responsibilities regarding use of the watercourse (Environment Agency, 2007). There may also be byelaws that define rights and responsibilities. There is no duty in common law for a landowner to improve the drainage capacity of a watercourse, but there is a responsibility to maintain the bed and banks and any trees and shrubs growing on the banks. The riparian landowners must also keep the channel clear of debris, including the removal of material that does not originate from their land. Figure 8.2 shows an example of neglect.

Figure 8.2 Fluvial neglect by riparian owners

Allowing a watercourse to deteriorate to this degree is courting disaster. There are potential hazards relating to pollution, obstruction of the flow, and health and safety.

The tree on the right has been allowed to grow too large. The sheetpile wall is collapsing into the channel. The demolition debris invites vandalism.

Although this is clearly a temporary state, the risks of flood damage, pollution or injury will persist until the channel has been restored to a more natural state with adequate capacity and stable banks.

A riparian landowner has the right to receive water in its natural quantity and quality, although it is often difficult to define what is meant by ‘natural’ in this context. By the same token, a riparian landowner has the responsibility to pass on flow without obstruction, pollution or diversion affecting the rights of others.

Although many of the fundamentals of the law affecting watercourses are straightforward and sensible, their interpretation can be legally complex. If there is any doubt about the legality of any proposed works – whether new works or the maintenance of existing works – the promoters of such works should seek legal advice at an early stage of their proposal.


8.1.3 Rivers as dynamic systems

Natural channels are subject to continuous change in response to a wide range of influences. Not least of these is the ever-changing flow pattern which reflects day-to-day weather conditions, seasonal changes, and longer term changes to the catchment and in the global climate. Figure 8.3 illustrates this hydrological variability with flow data from the River Trent. Fluvial designers ignore this dynamic nature of rivers at their peril.

 

Figure 8.3 Hydrological variability

This graph shows the changing flow conditions in the River Trent. The three bars for each month represent (from left to right) the long-term average flows, flows in 1998 (a wet year) and flows in 1996 (a dry year).

The variability is pronounced, even more so when it is appreciated that these are monthly averages. Although the monthly average varies from a low of 25 m3/s to over 200 m3/s, mean daily flows (not shown here) fell as low as 15 m3/s in August 1976 and reached a peak of 1019 m3/s in November 2000.

This natural variability has implications for construction, operation and maintenance activities in the river.

Not only does a natural watercourse exhibit variability in its flow regime, but the boundaries of the channel are also subject to change. The rate of change depends on a wide range of factors including:

  • the nature of the soils through which the channel flows;
  • climatic conditions;
  • human activity.

In any given reach of channel, there tends to be a natural regime which defines the channel cross section (that is, the width, the depth and the slopes of the banks). Artificial changes to the channel cross section are generally temporary as the channel naturally reverts to its regime condition. Gravel-bed rivers can be particularly problematic, because large quantities of gravel can be deposited in a single flood event. This can result in any benefits achieved by dredging works being wiped out in a matter of hours.

Considerable research has been carried out into the regime state of natural channels (Nixon, 1959) and it is generally accepted that the regime cross section is one that is capable of conveying the mean annual flood. This makes it clear that engineering a channel to carry much bigger flows (for example, the flood with a 1% annual exceedance probability – see Section 2.4.1) represents a major change from the natural state.

An artificially deepened channel tends to silt up so that the bed level returns to its pre-dredged level (hence the cyclical nature of maintenance dredging in navigable rivers). Similarly, widening a river to achieve greater flow capacity often achieves only a temporary outcome, as shoaling tends to restore the natural width. Steepening a reach of channel (for example, by cutting off a meander loop) can introduce instability, leading to erosion of the bed or banks as the stream attempts to revert to its former regime state.

An obstruction in a river channel also changes the natural regime, though over time a new regime may establish itself. Thus the construction of a weir in a river causes a backwater effect, and the resulting slower flow velocities encourage sedimentation upstream of the weir. A new balance will be achieved after some time (possibly many years). Weirs and similar structures also create an obstruction for wildlife – especially fish – unless special measures are incorporated (such as a fishpass). Figure 8.4 shows an example of fluvial adaptation.

Figure 8.4 Fluvial adaptation

This large capacity culvert has been engineered to ensure that it does not cause a restriction to flood flows. But for most of the time flows in the stream are relatively small, with low velocities that encourage sedimentation. The islands of sediment become colonised with vegetation, making them erosion-resistant and prone to attracting more sedimentation.

Regular maintenance is needed to clear out the sediment and vegetation to avoid loss of flood capacity. Provision of an access ramp may be required to facilitate sediment removal. A two-stage culvert (with the outer boxes at a higher level), with similar approach channel geometry, could help to reduce maintenance obligations.

With construction works in river channels, there is a wide range of potential adverse impacts that must be addressed in advance in order to avoid or mitigate the impacts. Of course, there are often opportunities for positive impacts associated with works in river channels. It is up to the promoters of the works to liaise with river users and local interest groups (anglers, conservationists, fisheries, navigation interests, etc) to explore the possibilities for mutual benefit. Table 8.1 indicates potential negative impacts but also includes references to potential benefits.

Table 8.1 Works in river channels – potential negative impacts

Nature of work

Potential negative impacts

Notes

Construction of a structure in the watercourse (a weir, for example)

  • Rise in flood level upstream.
  • Obstruction to the passage of fish.
  • Sedimentation upstream.
  • Restriction to navigation.
  • These are all potentially permanent negative impacts which may require mitigating action (such as raising flood defences or creating a fishpass).
  • Potential benefits include improved amenity, aeration of the water, navigation and micro-power generation.

Diversion of the watercourse (resulting in shorter stream length)

  • Increase in stream slope, leading to erosion of bed and banks.
  • Possible interference with agricultural drainage systems.
  • Can be addressed by erosion protection measures.
  • Potential benefits include reduced maintenance requirement and freeing up land for other uses.

Widening of the watercourse

  • Temporary loss of marginal and bankside vegetation.
  • Damage to habitats (for example, water voles).
  • Reduction in flow depth.
  • Natural streams tend to revert to regime width over time.
  • Benefits include the potential for a wider range of habitats.

Deepening of the watercourse (dredging)

  • Promotion of sedimentation (also depriving downstream reaches of sediment).
  • Temporary damage to the ecology of the stream bed and hence the natural fauna.
  • There are also negative impacts while the work is being carried out (increased sediment entrainment in the flow).
  • In any sediment-transporting stream, a deepened reach of channel tends to silt up. In gravel-transporting streams, reversion to the former bed level may be rapid (perhaps in the course of one flood).
  • Dredged material has to be safely disposed of, and this may be expensive, particularly as the material may be contaminated.
  • Benefits are improved flood capacity and increased depth for navigation.

Construction of erosion protection works on the bed or banks

  • Damage to natural vegetation and habitats.
  • Water pollution (avoid by using appropriate materials).
  • Mature trees and shrubs should be maintained where practicable.
  • Benefits may include a reduction in regular maintenance liability and the security of adjacent land, property and infrastructure.

 

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