Buckinghamshire Council Culvert Policy

Detailed design plans will need to be submitted with your land drainage consent and planning application. Applicants will be required to adhere to the following guidance:

Applicants should demonstrate that they have considered the environmental implications of all options, and settle on the least environmentally damaging solution.

If no other alternatives are feasible, any proposed culvert length should be as short as possible and the diameter as large as possible, the culvert should be designed to flow freely part-full rather than surcharged.

Applicants are required to:

• 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)

Depending on local circumstances the minimum acceptable culvert diameter would be 450mm. Any application for a proposed culvert with a smaller diameter than 450mm will need supporting drainage calculation for evidence (using CIRIA C689 Culvert Design 2010).

If any proposed culvert has an internal diameter of 900mm or larger then additional requirements may apply in relation to planning, design, construction, maintenance, adoption and/or demolition. Contact the council’s structures team at [email protected] for further information.

In exceptional circumstances when a longer length of culvert is required, innovative and new technologies should be utilised, for example artificial lighting or natural daylight through openings.

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 watercourses. Considerations 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.

If culverting is unavoidable, the preference is to adopt a single culvert rather than multiple culverts. This ensures the largest possible waterway through the culvert and hence reduces the risk of large debris getting trapped inside and causing a blockage. Where multiple culverts are unavoidable, a minimum number of culverts should be used and cutwaters should be provided between pipes at the culvert inlet, at least one of the culverts should be positioned above the normal water level to reduce sedimentation risk and provide passage for mammals.

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.

The responsibility for future maintenance and clearance of a culvert must be agreed and details of those responsible submitted with the application for land drainage consent. The responsibility for the maintenance of a culvert lies with the landowner or the person who owns the culvert unless otherwise arranged.

Bends, steps and changes of cross section within a culvert should be avoided 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 had to be sharp, provide an access shaft at the bend to afford easy access in the event of blockage.

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.

Inlet and outlet screens should not be used unless absolutely necessary. An appropriate risk assessment, such as a public safety 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.

Screens serve two main purposes:

• reducing the amount of debris entering a culvert
• preventing unauthorised access into the culvert

Regardless of the primary purpose, all screens accumulate trash and debris over time, 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. Buckinghamshire County Council therefore strongly discourages the use of screens and designers are urged to investigate alternative measures such as addressing the trash problem at source.