Structural modification of culverts

Description of technique

This measure is concerned with the structural modification of existing culverts. Improvements to culverts have the benefits of:

  • Creating a more natural bed profile within the channel.
  • Providing points within the culvert, or at the entrance/exist to a culvert, in which velocity changes occur locally to provide shelter, access and resting points for fish.
  • Maintaining flow depths.
  • Removing/reducing drops at the inlet and/or outlet of the culvert.

Good practice guidance exists on design of new culverts in the Culvert Design and Operation Manual (2010) which should be referred to where new culverts are required as part of flood risk management projects. Tide gates are typically attached to culverts that are placed in watercourses where there is tidal influence. These barriers to fish passage and their modification are dealt with in Fish passes at tidal outfalls.

Implementation of the measure

The potential to remove a culvert should be considered as the first option. Removing the culvert and restoring open channel has a number of significant benefits:

  • Improving flow regime and variability in flow conditions.
  • Restoring potential for bankside and in-channel vegetation.
  • Reinstating natural substrate (where bed of culvert was previously artificial) and continuity of the sediment movement.
  • Enhancing physico-chemical quality elements (e.g. dissolved oxygen and thermal conditions).

However, in many instances, culvert removal may not be practicable (e.g. where a culvert flows beneath existing infrastructure), and modification of the culvert may be the only form of possible mitigation.  Changes to the conveyance and risk of flooding will determine the extent to which measures can be implemented within a culvert.

Benefits for the Water Framework Directive

Implementation of this measure can help deliver the requirements of the Water Framework Directive, by:

  • Re-creating a more natural, self-sustaining river system, within which hydromorphological and ecological processes can be re-established.
  • Assisting with improving or restoring fish passage; offering benefits to other mobile species (e.g. invertebrates).
  • Providing access to valuable habitat that may be otherwise isolated from migratory species.

Culverts can also be a barrier to the longitudinal movement of other species, such as mammals (e.g. water voles). Specific guidance for modification for benefit of mammals is given in the Culvert Design and Operation Manual (see Case Study A3.5, pp 306).

To read more about the effectiveness of the measure within academic literature please click here: Effectiveness for Biological Quality Elements

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