Structural modification of culverts

Design Guidance

The Culvert Design and Operation Guidance (CIRIA, 2010) covers the design and operation of culverts from conception to decommissioning, encompassing the whole-life cycle of an infrastructure asset. It is intended for use in the United Kingdom and reflects the particular requirements for England, Scotland, Wales and Northern Ireland whilst drawing on wider guidance. 

Options for upgrading or replacing the culvert must be considered first to ensure that better environmental options are not overlooked. Alternatives to a culvert may include one or more of the following:

  • Relocating the infrastructure elsewhere to avoid the need to cross the watercourse (perhaps by making use of an existing crossing point).
  • Using a bridge instead of a culvert (a bridge can be designed to have much less impact on the hydraulics and ecology of a watercourse).
  • Using a ford instead of a culvert for a seasonal watercourse crossing a minor road (not appropriate for flashy streams or high flow rates which could result in people or vehicles being swept downstream).
  • Diverting the watercourse (and taking the opportunity to enhance the ecology and amenity at the same time).
  • Combining channels so that only one crossing point is needed (for example, replacing two small culverts with one large one).
  • Removing the culvert and restoring the channel (sometimes referred to as ‘daylighting’ see Remove Obsolete Structure and Remeandering straightened rivers).

In many circumstances however, there will be no other practical option than to keep a culvert in situ and ensure that where possible the impacts of the culvert are mitigated.  There are a number of measures that can be adopted to improve the culvert as part of the river system. The following measures may be appropriate, subject to an assessment of the impacts on the hydraulic capacity of the culvert.

Improving fish passage

Historic culverts may well be causing either delay to or total exclusion of migrating fish. The common reasons for both of these problems include excessive water velocities, inadequate depth or culvert diameter, sudden change of invert level between the culvert and the watercourse, rapid change in stream hydraulics at the upstream inlet, lack of resting places, and debris accumulations causing physical blockage or combination of any of these factors. Mitigation should therefore be focused on retro-fitting to deal with the above issues, providing that the reduction in channel capacity does not pose an unacceptable increase in flood risk.

Useful design guidance on easing fish passage related to existing culverts can be found in Armstrong G.S et al., (2004). Environment Agency Fish Pass Manual: Guidance notes on the Legislation, Selection and Approval of Fish Passes in England and Wales. Version 1.1.

Problems related to fish passage within the culvert itself can generally be improved by using the combined effects of increased water depth and reduced water velocity. This can be achieved by increasing roughness, in the form of some type of baffle or other structure, or else by back-watering the culvert using a series of small low head notched weirs if this does not increase the risk of sedimentation in the culvert, or health and safety risks at the site, or by a combination of these. Some basic guidance on flow velocities, flow depths and water level drops at the outlet is shown below.


Coarse fish; Roach, Dace, Chub etc. (< 25cm)

Brown trout (150 mm) and coarse fish (25 to 50 cm)

Sea trout and Brown Trout (250 to 500 mm) Large Coarse Fish  (>25 cm)

Salmon (more than 500 mm)

Maximum acceptable mean flow velocity through culvert and any screen fitted


Culvert length < 20m





Culvert length 20 to 30m





Culvert length >30m





Minimum depth of water in culvert (mm)





Maximum water level drop at either inlet or outlet (mm)





Minimum gap between trash screen bars (mm)


100 trout

150 coarse fish



Adapted from River Crossings and Migratory Fish: Design Guidance - a consultation paper, April 2000, Scottish Executive. NB The velocities for the shorter culverts are approximate to the burst speed achievable by salmonids at 5�C, and the velocities for culverts >30m approximate to the cruising speed. For coarse fish they equate to mean burst speed and the median cruising speed achievable at 10�C.

The conditions in the culvert for successful fish passage must include adequate swimming space, adequate depth of water, appropriate water velocity, and no physical or behavioural barriers. Although these conditions need not exist at all times or all flows, they should prevail during the seasons and flow windows when fish are actively migrating. If the latter is not known it is suggested that these conditions are present for river flows between Q90 and Q10.

Improving culvert inlet or outlet

The treatment of the inlet and outlet structures needs consideration to ensure that fish are not discouraged or prevented from entering or exiting the culvert. As with other obstructions a simple solution is the installation of a weir or prebarrage downstream of the outlet. This can be used to mitigate perching and to improve the water depth in the culvert. Low weirs can be constructed of stone or other materials. They should have head drops of not more than 0.3m (and ideally 0.1-0.2m for coarse fish passage), and be not less than 6m apart if possible. Where necessary, notches should be provided to ensure a sufficient width and depth in which fish can traverse the weir. Depth below the traverse should be at least twice the head drop. The most downstream structure should be flush with the river-bed to help prevent erosion. A series of such weirs can be installed over a length of the downstream channel depending on the overall head difference that is to be overcome.

When considering the modification of long stretches of culvert, consideration should also be given to appropriate resting pools immediately downstream of the culvert entrance. These should have a minimum depth of 30cm for trout and coarse fish and 45cm for salmon.

Improving flow conditions within the culvert

In some cases the installation of baffles in the invert of a culvert can improve conditions for fish by slowing the flow and locally increasing depth. However, this approach is not yet universally accepted because the baffles may have an adverse effect on culvert hydraulics in flood flows possibly trapping debris, increasing the risk of blockage. These issues should be carefully considered through feasibility, taking account of the location of the asset within the wider river system.

Baffles are relatively low structures placed on the culvert bed and spaced closely together, to allow flow to stream over them. Various simple baffle designs may be applied to any of the typical culvert cross sections (most usually round or square).

Weir baffles usually between 0.15-0.45m in height, are used to create roughness. They are not recommended for slopes over 1%.

Corner baffles are used typically in culverts with slopes between 1-2.5%.

Notch baffles are particularly useful in wide (e.g. plate-arch) culverts where the slope is between 2.5- 5%. The central length of notch can be zero, thus giving two corner baffles.

Bates (1997) gives a succinct description of culvert baffles, and further sources are listed within the reference list.

Where low water depth cannot be ameliorated through the use of baffles or back-watering it may be possible to cut low-flow channels in the base of the culvert to ensure there is sufficient water depth for fish passage.

Baffle in the base of culvert to improve fish passage, including low flow channel (Source: Transport for Scotland).

Improving substrate conditions

Knowledge of the channel bed material upstream and downstream and replicating the size and grade of material is key to successful substrate placement within the culvert and upstream and downstream where bed levels need altering.  The culvert should be filled with material to the natural channel bed level, ideally using materials that were excavated from the bed during installation. If this is not possible, or additional material is required, then uncontaminated material should be used which is sufficiently graded.

In gravel bed rivers, it is important that a coarse layer is reinstated, acting as a bed armour layer and reducing access of high flows to the finer substrate and reduce the potential for elevated fine sediment loads until the armour layer naturally re-establishes itself.  The design team should consider the depth of water and velocities through the culvert after placement of material to ensure fish passage remains feasible.

Mitigating poor light conditions

Culverts may also form a barrier to non-migratory fish because such species are reluctant to swim through culverts. It has been suggested that fish find the interior of a culvert less discouraging if natural light is present in the interior, but there is little evidence to support this in the United Kingdom (CIRIA, 2010). For culverts longer than 50m it may be worth considering the provision natural light “chimneys” at intervals along the culvert, but it is recommended that guidance is sought from the local fisheries officer who will be familiar with the species of fish present in the watercourse. Fish can also be reluctant to pass sudden light/dark interfaces at culvert entrances and exits. This can be ameliorated through judicious planting of vegetation

Design guidance for screening culverts   

Screens should be constructed from square, oblong, or wedge-wire section materials, not round ones (which more easily lead to gilled and trapped fish).

Where security is an issue the guidelines recommended free gap in the screens is ≥75mm≤150mm. Screens should be a minimum of 75mm because of the increased risk of blockage with smaller gaps. Generally a gap of 150mm is regarded as sufficient to exclude children, although in areas of very high risk it might be considered appropriate to reduce this (Environment Agency / Defra, 2002). Where occasional horizontal bars are used on vertical screens they should be spaced at least 400mm apart.

Where security is not an issue and it is simply a case of protection from water-borne debris, then the free gap used for screens should err on the generous size and not be limited to the minimum gap specified in the table. Thus, the size of free gap employed in the screen should be 250-300mm.

The Environment Agency approach to best practice for the design and installation of screens is detailed in the Trash and Security Screen Guide (Environment Agency, 2010). 

prev   1  2  3   next