Tidal exchange systems

Description of measure

Tidal exchange encompasses a range of techniques for intertidal habitat creation at sites where breaching or full removal of the existing line of defence is impractical.  It differs from other techniques for implementing Managed Realignment in that the sea wall remains intact but pipes, sluices, tide gates or culverts are inserted to allow and control regulated tidal flushing by seawater to create saline or brackish conditions behind the defence. This is a technique that slowly allows the land and local species to adjust gradually to the siltation and more saline conditions that develop.

Application

Tidal exchange is a potentially valuable tool in two particular scenarios: where coastal defences are likely to remain in place for the foreseeable future; and as the first phase of a longer term realignment strategy where it is necessary to build up site levels in advance of Breach or Bank Realignment in order to created the desired habitat. In some cases, where refurbishment of tidal flap valves offer the opportunity for saline intrusion over time.

A number of small projects have been developed in the UK (The Online Managed Realignment Guide (ABPmer, 2009 ) provides details on 12 schemes), but there are examples of larger projects overseas in the Netherlands, Germany and the US. The technique enables habitat creation on a wider range of sites than other managed realignment techniques due to the fact that the technique requires only a limited amount of land to function and it allows precise control of the flood and ebb regimes with environmental, social and economic benefits.

Most examples of the technique utilise a variety of pipes of varying diameter (e.g. Seal Sands, Tees Estuary; Horsey Island, Hamford Water), sluice (e.g. Ryan’s Field, Hale) and often a series of internally excavated creeks.

Benefits for the Water Framework Directive

Tidal Exchange can help deliver objectives of the WFD, by:

  • Helping trap sediment, providing suitable habitat for benthic/macro invertebrates and benefits to angiosperms (through saltmarsh colonisation);
  • Preparing sites for larger managed realignment schemes that will deliver a greater area of intertidal habitat;
  • Providing a valuable nutrient and pollution sink, improving water quality and reducing the undesirable effects associated with eutrophication.
  • Providing benefits to fish (through provision of suitable nursery habitat).

Tidal exchange also enables the existing flood defence to remain fully intact, preserving any access routes (i.e. footpaths) or heritage interest that may be associated with the wall or embankment.

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

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