Manage and Restore the Intertidal Zone

This measure includes management practices which seek to protect or restore the intertidal zone, defined as the area of the foreshore between the average high and low water levels. This area can include many different types of habitats, including beaches, rocky cliffs, saltmarsh and mudflats which support key WFD biological elements indicating overall ecosystem health. The intertidal zone is a highly dynamic region which plays an important role in the structure and functioning of estuarine and coastal ecosystems. The itnertidal zone can be susceptible to a number of natural and anthropogenic pressures, including coastal squeeze of foreshore in front of sea defences (erosion exacerbated by sea level rise), pollution, dredging and sedimentation resulting from changes in sediment regime. Intervention is often required to mitigate or support the natural recovery of the intertidal zone associated to a disturbance.

There are a range of different solutions that can be applied to deal with hydromorphological changes resulting from different and / or combined FRM and land drainage activities.

The management of intertidal and/or foreshore erosion can encompass a wide variety of soft- and hard-engineered approaches. A number of techniques may be used in isolation or combination to manage intertidal erosion. There are essentially two main approaches to managing intertidal and/or foreshore erosion: (i) retaining existing sediment on the foreshore/intertidal and (ii) restoring sediment to the foreshore/intertidal.

Intertidal recharge may be used to either manage erosion or promote accretion. It is appropriate to both open coast and estuarine environments but application of this technique requires consideration of the availability of a suitable source of recharge material.

Tidal exchange has many benefits in common with both foreshore recharge and managed realignment in general, with the added benefit of a higher degree of control over hydrological processes. This control allows potential for greater application in a range of sites and the flexibility to design a system to encourage a specific type of saltmarsh or mudflat to develop and any undesirable impacts associated with increasing tidal volume following a Breach or Bank realignment could be controlled.

However it is important to bear in mind when applying this method as opposed to other managed realignment methods:

  • Increased costs due to insertion of hard engineering works (pipes, sluices, etc) at required ingress / egress points.
  • More limited habitat creation due to the relatively small hydraulic capacity of spillways, culverts and pipes compared with defence removal or breach creation.
  • There may be requirements to maintain the existing defence line for as long as the tidal exchange system is to function.

The relatively small hydraulic capacity of spillways, culverts and pipes compared with defence removal or breach creation has tended to restrict their use to the smaller Managed Realignment sites in the UK (of only a few hectares in size) although they have been used in the Netherlands and Germany to control tidal flooding and encourage sedimentation over extensive areas. The control systems may comprise lowering part of the crest of the existing tidal defence to create a spillway or the installation of culverts or pipes through the defence through which the flow is regulated by penstocks or sluice (flap) valves.

Breach Managed Realignment comprises the removal of certain lengths within an existing flood defence. A new defence alignment may be created some distance landward of the existing defence by the construction of a new structure or by naturally occurring high ground or a combination of the two may also be used.

This technique is well suited to schemes where the primary purpose for implementing the works is to rapidly create a definable new intertidal habitat for environmental purposes. It is appropriate where a measure of wave protection is required for the site and / or realigned defence and may reduce the extent of any scour protection that may be necessary. Over time, development of vegetation and accretion of sediments within the new intertidal area may reduce the need for the physical protection of the realigned defence.

Breach Realignment is suitable where sensitive local issues or potentially adverse effects of scheme implementation have been identified, for example erosion of downstream tidal defences within an estuary or detrimental effects on faunal communities due to increased turbidity. Carefully evaluating the number of breaches, and their width(s), and implemention of an appropriate monitoring programme can manage these effects.

Breach Managed Realignment may be more cost effective than Bank Managed Realignment as part of the existing tidal defence is left and allowed to deteriorate naturally over time, rather than having to physically remove it. However, the debris from such deterioration may cause safety problems as well as being visually obtrusive, particularly if the embankment is armoured.