Modify or Enhance Structure

Structures in place for managing flood risk can have significant impacts on the hydromorphological and ecological functioning of rivers and transitional waters. Depending on the type of structure, the impact can result in changes to flow velocity, sediment dynamics and resultant morphological forms e.g. change in size, structure, channel or foreshore gradient and morphological diversity. These hydromorphological impacts can, in turn, have a detrimental impact on ecology.

The removal of an obsolete structure should be investigated as the first possible option (see Remove Obsolete Structure). Where the asset is still required as part of flood risk management infrastructure and removal is not possible, modification of the structure should be considered. It should be noted that there are many options for modifying structures to change their function whilst still maintaining their overall performance in terms of flood risk management; for example, reducing the height of cross-shore structures to increase long shore sediment transport.

As structure types are extremely varied and typically designed to perform a specific function, modifications to improve working with natural processes should be considered on a case-by-case basis. In these cases, staged modifications to structures may be needed to assist a change in natural processes and allow gradual adaptation of the waterbody (for example Tidal exchange systems).

Specific information within this guidance is provided on the structural modification of culverts, and increasing the ecological value of structures.

Culverts can be modified to address a number of key issues:

  • The downstream end of the culvert may be 'perched' above the watercourse so that upstream movement of aquatic species is hindered or prevented. Regarding or modification of the culvert exit, or the installation of a downstream resting pool, may assist fish passage.
  • Flow velocities through the culvert may be too fast for fish migration, particularly where the culvert has a steep gradient. Baffles can be constructed within culverts of gradient <5% to slow water velocities.
  • Sharp differences in light at the culvert entrance may act as a behavioural barrier for some species.
  • Ponding of water around the entrance can lead to stagnant conditions and water quality problems.

The type of structural modification that can be achieved will be dependent on a number of different factors, including available space, access, bed slope, water depth, and flow velocities.

The loss of habitat from flood risk management infrastructure can be mitigated by either incorporating features that reduce the impact of the structure or that improve the ecological value of the structure itself. These might be bespoke alterations to the size and shape of an existing or new structure to change localised morphology or to provide niche habitats within the structure. In addition or as an alternative, different material types may be used to encourage new habitats to develop.