Fluvial Design Guide - Chapter 8

Works in the river channel

8.6 Culverting of watercourses

8.6.1 General policy regarding culverts

Environment Agency policy is that no watercourse should be culverted unless there is an overriding need to do so. This is because:

  • the ecology of the watercourse is likely to be degraded by culverting;
  • culverting introduces an increased risk of blockage (with consequent increase in flood risk);
  • it can complicate maintenance because access into the culvert is restricted (in some cases being classified as a confined space and requiring trained operatives and specialist equipment).

A blockage in a culvert can be very difficult to remove and likely to result in a severe flood risk. For these reasons the provision of a screen at the entrance to the culvert is often considered. Such a screen eliminates the risk of a blockage inside a culvert, but introduces a significant maintenance obligation (to ensure that the screen is kept clean) which far exceeds the typical maintenance requirements of an open watercourse.

Alternatives to culverting include:

  • construction of a bridge – much lower impact on the watercourse hydraulics and ecology;
  • constructing the infrastructure elsewhere – often not a practical option;
  • diverting the watercourse – this has its own disadvantages but also some opportunities for environmental and hydraulic improvement;
  • for small streams, constructing a ford.

8.6.2 Impacts of culverting

The culverting of a watercourse can have a many different impacts on the water environment including ecology, channel form, flow regime and chemistry. Table 8.3 summarises the impacts that need to be considered in the design of any new culvert in addition to the risk of blockage and impact on flood regime described above.

Table 8.3 Impacts to be considered in the culvert design

Aspect affected

Description of impact

Ecology

Culverts can be impassable to riverine fauna and can create barriers to the movement of fish.

Culverting results in the loss of natural in-stream and bankside habitats through direct removal and loss of daylight.

Pollution

In urban areas, culverted watercourses are often highly polluted due to misconnected foul sewers, overflows from blocked sewers or discharges of contaminated surface water.

Morphology

Culverted sections may create or exacerbate downstream or upstream bank and bed erosion or promote sediment deposition, as a result of altered water velocities and disruption to the natural transport of sediment.

Restoration

Culverts can hinder future restoration options. This is particularly significant where urban development results in the burial of once open watercourses beneath housing or commercial centres, or where new development is placed on top of existing culverted watercourses which otherwise might be available for restoration.

Landscape and amenity

Culverting of urban waters leads to the loss and degradation of distinctive components of the local landscape.

Culverting leads to the loss of green amenity space along river banks and reduced access for recreational opportunities such as angling, walking or canoeing.


8.6.3 Culvert design

In cases where culverting is unavoidable it is necessary to seek land drainage consent from the Environment Agency to allow culverting to go ahead. The Environment Agency scrutinises design submissions carefully to ensure all steps have been taken to reduce environmental degradation (or mitigate it) and to reduce the risk of blockage. Box 8.3 sets out the ‘golden rules’ of culvert design.

Detailed guidance on the design of culverts can be found in Culvert design guide (CIRIA, 1997). An updated version of this guide is expected to be published in 2009 as Culvert design and operation guide.

Figure 8.24 shows an example of good practice in culvert design.

Box 8.3 ‘Golden rules’ of culvert design

Size

Design the culvert to flow freely (part-full rather than surcharged) unless there are overriding reasons to do otherwise.

Choose a size that readily accommodates the design flow (for example, the 1% annual flood) with no appreciable increase in water level upstream. Allow for future development of the catchment upstream and for climate change effects (it is suggested that 20% is added to the estimated 1% flood to allow for this).

Adopt a single barrel in preference to multiple barrels, so as to present the largest possible waterway through the culvert and hence reduce the risk of large debris getting trapped inside it. Multiple barrels are acceptable for large watercourses and they have environmental benefits if one or more of the barrels is above normal water level (reduced sedimentation risk and provision of passage for mammals).

Length

Adopt the shortest length possible. Fish will migrate though a short culvert, but will be discouraged by a long culvert. The shorter the culvert the less likely are problems associated with blockage, and the lower the hazard associated with unauthorised or accidental entry into the culvert.

Invert level

The invert level of the culvert should generally be set lower than the existing bed level of the channel. This allows for any future regrading of the watercourse and also promotes the formation of a more natural bed through the culvert (which helps to maintain ecological continuity).

Trash and security screens

Avoid – only install in situations where the benefits significantly outweigh the risks (see Section 8.6.3).

Bends, steps, changes of cross section

Avoid – because they reduce the hydraulic efficiency and increase the risk of debris getting trapped.

If a bend is unavoidable, adopt a long gradual bend.

If the bend has to be sharp, provide an access shaft at the bend to afford easy access in the event of a blockage.

Hydraulic performance

Should be investigated for very high flows and low flows, as well as for the design flow.

If very high flows have the potential to overwhelm the culvert and cause flood damage, then options such as a flood bypass route should be considered.

Low flows should be examined from the perspective of watercourse ecology.

 

Figure 8.24 Urban culvert

This culvert has a simple inlet structure formed from a stone revetment. The approach channel is well-maintained and free from debris.

Although the culvert is close to houses, the potential hazard associated with children entering or being swept into the culvert is not considered to be serious enough to require the provision of a security screen.

The culvert is short, straight and of reasonable size, and therefore presents a low hazard rating. These features also mean that the culvert is unlikely to trap debris internally, so there is no need for a trash screen.


8.6.3 Trash and security screens

Screens serve two main purposes:

Regardless of the primary purpose, all screens accumulate trash and debris over time (see Figure 8.25), reducing the hydraulic capacity and causing the water level upstream to rise. The build-up of trash can be rapid, and the consequences can be severe in terms of flood damage to local properties and infrastructure. Many more problems have been caused by blocked screens than have resulted from blocked culverts. The Environment Agency therefore strongly discourages the use of screens and designers are urged to investigate alternative measures such as addressing the trash problem at source.

Figure 8.25 Even coarse screens can become blocked

This screen has been designed to exclude large debris, though it is clear that it has the potential to be obstructed by accumulations of mat-forming vegetation, which could otherwise readily pass through the culvert without causing any problems.

No proposal to install a screen is acceptable unless and until the inspection and maintenance obligations (principally routine and emergency cleaning the screen) have been assessed and accepted by the responsible party.

Screens can also be used to trap coarse bed material (cobble and boulders) that might otherwise cause a problem further downstream. Such screens must be capable of being safely overtopped when blocked by debris. In Boscastle, for example, a coarse screen upstream of the critical areas traps large bed material before it can cause an obstruction at pinch points in the channel through the town. It is located at a point where any trapped material can easily be removed after the flood has passed.

Fully detailed guidance on screens can be found in Trash and security screen guide for flood risk management (Environment Agency, 2009). Figure 8.26 shows an example of a well-designed screen.

Figure 8.26 Well-designed screen

This screen prevents unauthorised access into a long urban culvert. It also has to cope with trash and debris in the flow, so it has been designed in accordance with accepted practice for trash screens.

Note the two-stage screen arrangement, with cleaning platforms for each stage. Note also the additional flow capacity provided by the horizontal section of screen to the rear of the lower platform.

Safe access has been provided and the whole site is fenced off to reduce the risk of vandalism. The site is provided with lighting to facilitate cleaning at night.

 

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