Summary • Description • Constraints • Potential effects • Monitoring • Locations • Further information

Summary

Regulated tidal exchange is a form of saltmarsh creation that allows the controlled inundation of previously defended land with saline water, using a combination of pipes and sluices. It differs from realignment schemes in that the sea wall remains intact. It 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. A small number of projects have been developed in the UK but there are examples of larger projects overseas in the Netherlands, Germany and the United States .

Like managed realignment, this is not strictly a technique for the management of saltmarsh however, again, it is an option that should be considered as part of a broader strategy for managing marsh habitat (i.e. coastal management).

Description

Regulated Tidal Exchange (RTE) Systems enable an area behind a sea defence to be gradually converted to saltmarsh and/or mudflats. The process uses pipes, sluices or tide gates to allow regulated tidal flushing by seawater to create saline or brackish conditions behind the defence. This is a slow process that allows the land and local species to adjust their soil chemistry to the more saline conditions required by saltmarsh and siltation to proceed gradually.

There are several techniques used to control the flow of sea water and not all techniques are suitable at all sites; some are more suitable for creating mudflats than saltmarsh. Techniques include:

• An open culvert, with no tidal flap through the sea wall. Tidal water will flow in and out on every tide as long as the invert level is around the mean low water mark. A variation on this method is to have a drop board on the landward side to prevent water flowing out of the culvert, creating a permanently flooded area.
• Culverts with manually operated flaps that let water through into an impoundment at high tide over several high tides, until desired water level is reached.
• Self regulating tide gates (SRTs) have one moving part and an adjustable float system, allowing the SRT to stay open and float on flooding and ebbing tides until the specified desired water level has been reached, at which point the SRT will close and stay closed. When the tide recedes on the outside of the site, the SRT automatically reopens, allowing the impounded water to flow out.
• Electronically operated tide gates. Flow is regulated by a vertical lift, rectangular tide gate on the seaward side that opens and closes electronically at desired water levels, which are monitored by pressure sensors. The gate is normally open for a short period on each rising and falling tide.

There are several essential requirements of potential sites for RTE:

• An existing sea defence such as a seawall into which a pipe, sluice or tidegate could be integrated;
• An area that can be flooded without flooding adjacent farmland (may require a bund to be constructed behind the primary defence);
• A nearby source of sea water to permit saltwater flushing. Sea water should ideally have enough suspended sediment to enable accretion at a higher rate than sea level rise;
• The site must be no less than 0.1m lower than sea level at the highest part of the tidal cycle;
• The site must have a tidal range of at least 3m;
• Impermeable underlying geology, not prone to erosion (i.e. not peat or chalk);
• Gradients of at least 1-6%, this will determine the ratio of saltmarsh to mudflat.

When developing a RTE scheme it is essential to ensure adequate water exchange and a sufficiently high rate of accretion to keep pace with predicted sea level rise. Broadly speaking, sites with less than 450-500 inundations a year will tend to develop into saltmarsh, while those with 450-600 will tend towards mudflat.

RTE can be useful in large managed realignment schemes where it is impractical to allow all areas that may potentially flood to be inundated at once. It therefore provides the opportunity for a phased approach to managed realignment. Large sites may be compartmentalised and only small areas introduced to tidal inundation at any one a time, to potentially minimise impacts. RTE may also be a useful prior to breaching a managed realignment site. By exposing land behind the defence line to carefully controlled inundations, the land may be encouraged to warp up to a higher level (through sedimentation) in preparation for breaching or the removal of a defence.

However, RTE is a particularly useful technique where defences are likely to stay in place for some time. Furthermore, a new defence may not need to be constructed, potentially representing a cheaper option than managed realignment. RTE may also be appropriate at a number of sites in eastern England, where managed realignment may have negative impacts on wider estuary systems associated with increased tidal volume.

RTE has many benefits in common with both foreshore recharge and managed realignment, with the added benefit of a higher degree of control over hydrological processes. This means that a site could be set up to encourage a specific type of saltmarsh or mudflat to develop and any undesirable impacts associated with increasing tidal volume following a managed realignment could be controlled.

Constraints

The relatively small hydraulic capacity of spillways, culverts and pipes compared with defence removal or breach creation (realignment) usually tends to restrict their use to smaller sites of only a few hectares in size.

In addition, the existing defence line has to be maintained for as long as the tidal exchange system is to function, so potential defence cost savings associated with breach or bank retreat would not be realised or may be deferred.

Potential effects

The potential effects associated with RTE are similar to those associated with managed realignment.

Monitoring

Monitoring objectives will be similar to those for realignment schemes. In particular, baseline monitoring of ground levels, vegetation, invertebrates, current bird usage, water levels, salinity, changes in vegetation and invertebrates should be undertaken.

Locations

Regulated tidal exchange was first tried at Horsey Island, Essex and has subsequently been trialled in Chichester Harbour, Devon, at Goosemoor.