Bridges 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 Policy

A bridge which crosses an ordinary watercourse in a free span technically doesn’t require Land Drainage Consent. If it’s clearly evident to you prior to a developer making an application that a proposed bridge won’t cause an obstruction to the flow, you will most likely be advised that you do not require consent. However, it’s good practice to confirm your proposal in writing and request a written response back. You may be requested to submit detailed drawings to confirm that no obstruction will be caused.

Some ‘bridge’ proposals, particularly for highways may actually be large section culverts with the invert placed below the bed level of watercourse. Developers often favour culverts rather than true bridges as they are often cheaper than bespoke designed bridge foundations. Proposals such as this will be treated as culverts and in line with Devon County Council’s guidelines set out in our culverting policy.

Most new bridge proposals will require planning permission and therefore a flood risk or consequence assessment (FRA or FCA respectively). Proposals to alter a bridge may require planning permission depending on the extent of the changes and the listed status of the structure.

The FRA or FCA must show that there’ll be no worsening of flooding as a result of the changes, and ideally an overall benefit. You must consider the effects on the surrounding land, especially properties, as well as the immediate effect on the river and its flow. If planning permission isn’t required, you may still be asked for the kind of information an FRA or FCA would contain.

Design Summary

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

  • Bridges shall be located over straight sections of channel and shall avoid:
    • riffles;
    • eroded sections of bank;
    • existing trees, scrub and adjacent wetlands;
    • protected species.
  • Clear span bridges which retain the natural banks and bed of the watercourse shall be provided where possible.
  • Open-type parapets are preferred to allow some over-deck flow in case of the bridge opening becoming partially blocked in an extreme flood event.
  • Where flood arches are proposed, a mathematical or physical model of the river flow would normally be expected.
  • To avoid problems of excessive siltation through single span structures, consideration shall be given to providing a two-stage channel beneath the bridge. With multi-span structures, consideration shall be given to staged bed levels.
  • Designs must include a clear indication of the consideration given to scour at all levels of flood risk.
  • Velocities shall be limited to 1.5m/s under flood conditions, (but with upstream and downstream training walls a velocity of 3.5m/s may be permitted).
  • Bank protection beneath the bridge deck shall extend both upstream and downstream where appropriate. Soft bank protection shall be used in preference to hard engineering wherever possible. (See DCC Bank Protection Guidance).
  • Afflux calculations shall be provided by the promoter.
  • Bridge decks shall be adequately restrained laterally.
Simple schematic cross section indicating the parapet, soffit and invert of a bridge or box culvert.
Simple schematic cross section indicating the parapet, soffit and invert of a bridge or box culvert.

Soffit

  • Road or rail bridge soffit levels and flood spans must normally be 600mm or more above the design flood level (or the maximum known flood level on minor watercourses) in order to allow floating debris to pass freely through the structure. The soffit level may be further influenced by what is in the vicinity, particularly upstream of the proposed bridge. This requirement may be relaxed on very small watercourses where we don’t know the maximum flood level so long as the soffit is above bank level and there is a low risk of damage nearby.
  • The soffit must be no lower than 300mm above either of the upstream bank tops. If a lower soffit is required on technical grounds, we may require a wider span to compensate.
  • Footpath and bridleway bridges in large flood plains can be treated differently. It may be more practical to construct a bridge with the soffit 300mm above bank top level and with open handrails, which will be submerged in a flood. A large structure raised above flood level with approach ramps could not be reached during a flood in any case.
  • Soffit levels on navigable rivers will need to take account of the clearance level required for boats legally using the river. You must consults the relevant navigation authority (British Waterways, ourselves as navigation authority, the county council, or a private company, as appropriate) if bridge works are proposed where they operate.
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 barrel is recommended. This protects the barrel invert and maintains low regimes (see Invert Levels).

Design Flows

  • Design flows must be based on a 1% annual event with an additional allowance for climate change in accordance with the latest UK guidelines [1]. Most structures will remain in place for a long time and we must ensure (as far as possible) their future suitability. Major new bridges will have a design life specified, and climate change allowances must be incorporated for the duration of the design life. If an application does not specify a lifetime, discussions between us and the developers should result in an agreement of what would be realistic. This will vary with the kind of bridge. Developers should justify why they have adopted a given lifetime when they are formulating their proposals. The impacts of climate change need to be taken account of in a realistic way.
  • We must consider the effect on overland flow routes of bridges and the transport routes leading to them. Long embankments across flood plains may cause more problems than the bridge itself. Where additional flood openings are proposed, we expect a model study to identify the optimum number, size and location. We’re likely to require a physical or detailed computer flow model in complex situations or if there is a wide floodplain requiring a number of openings.
  • The developer must adequately consider local scour to piers and abutments. A study of bridge failures has indicated that between 60 and 70% are caused by hydraulic action. This is the bridge owner’s responsibility.
  • Flood velocities should ideally be limited to between 1.5 and 2.0m/s. If this cannot be achieved developers will need to use training walls to design for velocities of up to 3.5m/s. The developer must include the design for any training walls in their consent application.
  • We must make developers aware that they need to take account of foreseeable increases in flow within a catchment due to further development and climate change. There is legal precedent for this.

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.

Bridge Span

Developers must provide clear span bridges as a general rule, though in larger rivers this may not be possible. In such cases, design calculations must confirm the capacity of the bridge is maintained. On rivers where a navigation right exists, there may be issues with proposals for piers in the channel. The developer must consult us and the navigation authority, in such cases.

If a multiple span design is needed, a single span covering the normal wetted channel with an approach span on each side (a three span bridge) can sometimes be more appropriate than a two span design which requires a central pier located in the channel.

Afflux

A new bridge must produce no difference in water levels between the upstream and downstream sides (afflux) since this would increase flood risk upstream of the bridge.

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 should:

  • 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

Inverts

We must encourage the use of soft inverts to allow the natural river bed to be retained. The top of abutment footings of bridges with no solid invert must be set at least 600mm below the existing bed level.

Solid inverts must be set at least 600mm below the existing hard bed level to allow for future re-grading. The void up to the existing bed level must be filled a suitable inert material, commonly a clean local gravel. Developers can consider shaping the materials to allow dry weather flow to be concentrated rather than spread over the entire base, to prevent silting.

Parapets

 Open parapets/handrails may be appropriate to allow some flow over the deck in case the bridge opening becomes blocked or in an extreme flood event. Developers must consider the safety of bridge users in these circumstances. Although this is the developer’s responsibility, we must also make them aware of it.

Environmental Considerations

  • Developers must consider the choice of materials and their environmental implications for all works affecting a watercourse. They may need surveys to ascertain if any protected species are present, particularly water voles. The results of the surveys must accompany consent applications.
  • Bridges can provide potential nesting sites for a number of bird and bat species. Nest boxes can be incorporated into the design of new bridges. Our Biodiversity staff will be pleased to advise developers.
  • Developers must take account of fish, otters and other fauna. They can include flood arches, marginal shelves or wildlife underpasses (commonly a 600–1000mm pipe) in the structure to allow wildlife to move up and downstream. Developers may need to install wildlife proof fencing alongside roads in particular, to encourage animals to use these features.
  • We must consider and propose any suitable ecological features during pre-application discussion, as it’s not suitable to impose these features on already approved designs.

Temporary Works

You will need to pay special attention to proposals for temporary works around bridges as these can create an increased flood risk.

  •  All works should maintain the flow of water downstream through either sandbagging off one half of the watercourse whilst work is carried out on the dry half, or the use of a pipe or flume system. Any sandbag walls should be low lying so that in the case of a flood event, they can easily be overtopped and not create a blockage or flood hazard.
  • When using any scaffolding in the watercourse, this needs to be arranged so that there is minimal risk of blockage from debris build up in the channel.

Pollution causes and prevention

The following list is not exhaustive but covers the most likely areas encountered when working near water:

  • Discolouration of watercourse – effects fish movements and can reduce oxygen levels and increase Biochemical Oxygen Demand (BOD)
  • Rubble from reconstruction works falling into or being deposited in the channel
  • Disturbance to the bed from operations in the channel (i.e machines/operatives working in the channel and/or structures being founded on the bed)
  • Pollutants entering the watercourse, such as:
    • Cement from pointing and concreting operations
    • Oil from plant and formwork releasing agents
    • Sand and Hessian bags from damming operations

Sand Bag Cofferdams

  • The usual method of preventing pollutants entering the watercourse in shallow rivers and streams is to enclose the work area in a sand bag cofferdam. The dam should be constructed using new standard Hessian bags partly filled and laid in alternating directions to reduce water permeability. Plastic sheeting either under the dam or up the sides is not normally required if the dam is constructed properly. The dams normally have the width to height ratio of between 1:1 and 2:1 depending on water velocity and site conditions.
  • Sand bags and sand used in cofferdams shall be removed from site. In no circumstances should the bags be split and the contents emptied into the watercourse.
  • Cofferdams have a limited life as bags will rot in water; this should be taken into account on long duration projects when it may be necessary to use ‘rot proof’ bags manufactured to the technical specification BS1214:1977, this bag is constructed from a heavier weight of hessian cloth and includes a copper based rot proof treatment.
  • Dumpy bags are not as water tight as a conventional sand bag dam and should be avoided if possible; however if they are to be used it is advisable to lay a ‘carpet’ of conventional sand bags under the dumpy bags to provide a bed seal where much of the ingress occurs.

Filtering polluted water

  • A sump should be provided at the low point within the works to allow any seepage to be pumped away for filtering and return to the watercourse.
  • A suitably sized pump should be used with a suction filter fitted to prevent clogging. The pump should be sited so as to minimise pollution from any diesel or oil spills.
  • If the pump is close to the watercourse it should be sat in a sand tray or similar to catch engine pollutants.
  • The effluent from the pump should be filtered before returning to the watercourse by a means suitable to the location, ie:
    • Effluent can be filtered through grass or vegetation if the adjacent field has sufficient material and the outlet pipe can provide sufficient distance from the watercourse for this to be effective. The land owner should be consulted prior to this activity.
    • An alternative to grass filtration is to pump the effluent into a skip or similar large vessel where solids can settle out. There are proprietary products for this purpose such as Siltbuster Mobile Settlement Units
  • It should be noted that when using any pumps, the presence of fish must be considered and necessary precautions taken, such as use of screens or fish rescue. DCC or the EA should be consulted when working in the water where fish could be present.

Cement Anti Washout admixture

  • It is usual practise to incorporate an anti-wash out admixture to concrete that will be placed in watercourses; this applies even if a sand bag cofferdam is used. There have been instances where rivers have risen unexpectedly and the dam swamped creating a potential for pollution.
  • The use of Conplast UW is recommended as this was tested in trials at Rydon Depot by DCC and the Environment Agency. The specification should be adhered to and it should be noted that concrete mixes should have a minimum cement content of 400kg/m3. The admixture can delay the set. Other admixtures can be used provide they receive the approval of DCC and the Environment Agency

 

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.

[1] Flood Risk Assessments: climate change allowances https://www.gov.uk/guidance/flood-risk-assessments-climate-change-allowances