Culvert Guidance

Summary of requirements

The following works require ordinary watercourse consent from the Lead Local Flood Authority (Devon County Council) under Section 23 of the Land Drainage Act 1991:

1. a) erect any mill dam, weir or other like obstruction to the flow of any ordinary watercourse or raise or otherwise alter any such obstruction; or
2. b) erect a culvert in an ordinary watercourse, or
3. c) alter a culvert in a manner that would be likely to affect the flow of an ordinary watercourse.

Devon’s Local Culverting Policy

Devon County Council considers it beneficial for watercourses to remain open wherever possible for both flood defence and environmental purposes. Culverting can exacerbate the risk of flooding, increase maintenance requirements and create difficulty in pollution detection. It also destroys wildlife habitats, damages a natural amenity and interrupts the continuity of a watercourse. The Council will consider each application to culvert a watercourse on its own merits and will only approve a culvert if there is no reasonably practicable alternative, or if is thought that the detrimental effects would be so minor that a more costly alternative would not be justified.

Nevertheless, it is understood that there may be cases where culverting is unavoidable. For example, short lengths for access purposes or where highways cross watercourses. In such cases the length involved should be restricted to a minimum, the hydraulic and environmental design assessed, and appropriate mitigating enhancements to the surrounding environment included.

However, culverting will not be considered until other options have been thoroughly explored, for example:

• Clear open span bridges with existing banks and bed retained;
• Revision of site layout to incorporate an open watercourse;
• Diversion of the watercourse in an environmentally sympathetic channel and corridor.

Design Summary

Detailed design plans will need to be submitted with your Land Drainage Consent application and also consider the following:

• An applicant should demonstrate that they have considered the environmental implications of all options, and preferably settle on the least environmentally damaging solution. If no other alternative is feasible, any proposed culvert length should be as short as possible and the diameter as large as possible.
• The culvert shall be aligned with the direction of flow and existing gradient of the watercourse. The culvert shall be on a straight reach of the watercourse.
• Depending on local circumstances Devon County Council will look for a minimum culvert diameter of 600mm. The culvert should be at least as wide as the natural channel.
• All culverts should be designed to safely convey the 1 in 100 year flood event, plus climate change flood discharge or greater where appropriate. Wherever possible flood estimation methods should be used.
• Culverts must be designed so they do not cause a restriction to flow. They must not increase the risk of flooding or prevent maintenance of the adjacent open watercourse. Where possible designs incorporate a specified amount of freeboard to allow for floating debris, minor blockage and variations on the ‘design’ water surface. Consideration must also be given to overland flow paths in the event of a culvert becoming obstructed. It should be ensured that flows will not affect property or cause unreasonable nuisance or harm.
• The responsibility for future maintenance and clearance of a culvert must be agreed and details of those responsible submitted with your application for consent. The responsibility for the maintenance of a culvert lies with the landowner or the person who owns the culvert unless otherwise arranged.
• Appropriate inlet and outlet structures should be provided in order to ensure smooth hydraulic transition and avoid erosion. Headwall arrangements at the upstream and downstream ends of a culvert should be suitably keyed into the bed and banks of the watercourse, and should be appropriate to the local environment.
• Suitable access arrangements for maintenance should be included in the design. Access chambers must be provided at each change of direction if the culverting is not straight. Other access/inspection chambers should be installed at suitable intervals.
• Inlet and outlet screens should not be used unless absolutely necessary. An appropriate risk assessment must be submitted with your application to demonstrate when a trash screen is necessary, and a formal maintenance regime must be agreed prior to approval. Where screens are to be adopted, these should be designed to the standards and principles adopted in the CIRIA Culvert, Screen and Outfall Manual (C786F).
• In most situations it is appropriate for the inverts of culverts to be set slightly below the existing bed level to allow for future maintenance or other works on the watercourse. It also aids the provision of a more “natural” bed to establish within the culvert.
• The provision of some transitional erosion protection between hard invert and natural riverbed should be provided to prevent the invert becoming perched.
• A Single large culvert is preferred to multiple small culverts, to maximise hydraulic performance and reduce the risk of debris accumulation.

Size of Culvert

• Design the culvert to flow freely (part-full rather than surcharged) unless there are overriding reasons to do otherwise. Devon County Council will look for a minimum culvert diameter of 600mm.
• Choose a size that safely conveys the design flow (the 1% annual flood – the 1 in 100 year flood event) with no appreciable increase in water level upstream. They must be designed so they do not cause a restriction to flow, increase the risk of flooding or prevent maintenance of the adjacent open watercourse. Allow for future development of the catchment upstream and for climate change effects (please see the latest guidelines and peak river flow map). Wherever possible flood estimation methods are required.
• Adopt a single culvert 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 larger 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).
• Adopt the shortest length possible. Fish will migrate through 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.

Where possible, designs should incorporate a specified amount of freeboard (F), to allow for floating debris, minor blockage and variations on the ‘design’ water surface. A degree of sedimentation (S) within the culvert is recommended.

Invert Levels and Slope

When installing a culvert, the invert should be buried below the existing natural bed level to ensure bed sediment transport continuity and minimise the likelihood of scour of the bed material, within and downstream of the culvert. Suggested sedimentation and freeboard depths are recommended below:

Maintaining the natural bed level and slope will ensure adequate water velocity (and depth) for fish passage. It will ensure the slope of the culvert is not too steep, increasing risk of fast flows, erosion and ‘perching’, or too shallow, increasing the risk of deposition which may reduce flow capacity, and increasing flood risk.

Access Points

• Culverts shall have frequent access points for rodding, or manholes at changes of direction and section. Manhole spacing shall be more frequent on culverts with slow bends and on culverts located in gardens.
• For small diameter culverts (man-entry not possible) it is suggested that manholes should be provided at spacing of about 50 times the culvert diameter (or height for non-circular culverts) to provide access for inspection and jetting. A maximum spacing of 30m is suggested for culverts that have to be cleaned by rodding or winching. For man-entry culverts it is suggested that manholes should be spaced at intervals of no more than 100m.
• Manholes lifting off in flood conditions present a severe safety hazard because anyone walking through the floodwaters may not be aware of the open manhole. It is important that manholes located in public spaces cannot be lifted and displaced in flood conditions.

Culvert Entry and Exit

• The exit velocity under the design flood conditions shall be limited to 1.5m/s.
• Protection against erosion shall be provided where necessary, for backfilling up to flood level at both the culvert entry and exit. Soft bank protection should be used in preference to hard engineering wherever possible and shall be designed in keeping with the local environment.
• Headwalls and wingwalls shall be provided, where necessary, at entry and exit, and be adequately keyed into the banks and bed to prevent erosion. Design and construction materials shall be in keeping with the local environment. Smooth concrete finish is not normally acceptable.
• Where culverts are steep or the head loss is high (for example, outlet from a flood storage reservoir) it may be necessary to provide an energy dissipating device in the outlet structure. This destroys the excess energy and creates relatively tranquil conditions as the flow joins the channel.
• Safety fencing shall be provided at entry and exit wherever there is a possibility of public access and shall be designed in keeping with the local environment.

The transition from open channel to closed conduit is an important part of the culvert structure, and is frequently a source of problems.

An inlet structure:

• provide a transition from the natural channel to the culvert barrel, so as to avoid excessive head loss or the creation of sediment or debris traps
• to support the earthworks at the entrance to the culvert
• to prevent local scour that might undermine the culvert entrance to help the installation of a trash screen or security screen

The form of the outlet structure can usually be similar to that of the inlet structure, providing a transition from the culvert barrel to the channel downstream. It is important to design a structure that does not amplify any tendency for erosion, and that eases access to the culvert exit for inspection purposes

(Picture on the left is a cast in situ headwall and wing walls for pre-cast box culvert units. Picture on the right shows a similar inlet but with the addition of concrete bag work to stabilise the steep watercourse bank)

Safety Screens, Trash Screens and Fencing

Devon County Council does not recommend the use of screens. Upstream screens shall only be used where there is a real danger to children, or where industrial, rubbish or tree branches are likely to block the culvert and cause major flooding. The screen shall be designed for easy raking and have a horizontal top section which allows additional flow capacity when the inclined section becomes blocked. Raking of the screen remains the owner’s responsibility.

• New culverts should avoid the need for a screen wherever feasible. If the applicant can justify that a screen on a new culvert is a necessity, then they will need to provide hydraulic calculations including in a blockage scenario.
• Outlet screens are only permissible alongside an inlet screen.

Screens should be designed to the standards and principles adopted in the CIRIA Culvert, Screen and Outfall Manual (C786F). It is recommended that centre-to-centre bar spacing for a screen is:

• Maximum of 150mm for a security screen
• Minimum of 300mm for debris screen on culverts with a height or width ≤ 1.2m
• Minimum of 450mm for debris screens with either height or width > 1.2m but < 3m

Environmental Considerations

Culverts must not present a barrier to fauna by:

• creating a step or drop in the bed at the culvert inlet or outlet which will hinder the passage of fish and other fauna
• creating any other restrictions to the free passage of migratory fish and other fauna.

Mammal Passage

• Culverts may affect the movement of mammals such as otters and badgers along a watercourse, particularly if the culvert has no safe route above water level, limited headroom or there is a risk of harm crossing a busy road or railway.
• Mammal passage can be encouraged by providing safe routes across infrastructure, for example a mammal ledge or additional pipes above the water level.

• The requirements for mammal passage depends on the species present, but typically:
  • Small mammals (e.g., voles) – mammal ledge with minimum width 150mm and 600mm headroom
  • Large mammals (e.g., badgers) – separate pipe with minimum 600m diameter, or mammal ledge with minimum width 600mm and 1050mm headroom.

Fish and eel passage

It is an offence, under the Salmon and Freshwater Fisheries Act (SAFFA) 1975, to reduce fish passage for migratory salmonids at a permanent structure (new or existing) without providing a fish passage (approved by the Environment Agency). Under The Eels Regulations 2009, it is also an offense to impede the passage of eels. The Environment Agency can serve notice to require that eel passage is maintained.

Any maintenance or repairs to existing culverts (or structures which cause an obstruction) must prevent deterioration of the status of the watercourse and should contribute to improving ecological status, fish and eel passage.

Hierarchy of preferred culvert shape for fish and eel passage:

Retrofit Solutions

In addition to new structures, environmental considerations are also required when maintaining an existing structure. Any new structure or maintenance of an existing structure which are likely to amount to an obstruction will require eel passage to be addressed.

Example of fish pass retrofit solution:

Examples of eel pass retrofit solutions:

For further details on fish and eel passes, please make use of the following guidance:

Environment Agency Fish Pass Manual

SEPA 2010, Engineering in the water environment: good practice guide – River crossings

CIRIA Culvert, Screen and Outfall Manual (C786F)

This is a supporting document to Devon’s Local Flood Risk Management Strategy and should be seen as a live document to be updated as new best practice, grants and schemes become available.

We welcome comments from land owners and farmers on best practice and any case studies you would like to share. Please contact Devon County Council Flood Risk Management at floodrisk@devon.gov.uk.