| Other defra.gov.uk sites:

Beneficial use

Over the last decade or so there has been a shift in thinking about how to handle the disposal of dredged material derived from port maintenance and similar activities.

The emphasis has turned from disposal to the beneficial use of such material for environmental gain. For example, beneficial use has been shown to successfully protect and/or create salt marshes and mud flats, which in turn may provide useful flood and coastal defence. These inter-tidal habitats are also important ecosystems in their own right, and relatively rare in Britain.

However, beneficial use must not be seen as the solution to all problems. For a start some material may be better suited for such a use than others.

"Capital" dredged material is typically coarse or bulky material which, once deposited at a marine disposal site, tends to remain in place. In contrast, "fine" material (e.g. muddy sand) obtained from maintenance dredging is less likely to remain at a disposal site. Sediment movement can be potentially detrimental and have greater indirect ecological effects.

Within the UK there have been a number of small-scale trials for beneficial use. This is because of our lack of understanding of the biological processes following sediment deposal. The re-colonisation of sediments by marine plants and animals may have profound effects on the stability and fate of deposited material, in both the short and long term.

We also don't know enough about how beneficial use affects invertebrates, which limits our ability to predict any indirect effects on birds and fish. This is particularly important as most beneficial-use schemes are situated on estuarine inter-tidal habitats, areas important for sustaining such populations.

Beneficial use and disposal options have their own logistical, legal, economic and environmental limitations. Each case should be assessed on its own merits. Nevertheless, under certain circumstances, the concept of the beneficial use of dredged material can result in solutions that satisfy the needs of industry, the regulator and society at large.

Advantages

There is a dynamic, self-regulating process found in many tidal estuaries: a net balance between the amount of material being deposited and eroded. Such a balance may be disturbed when an estuary is dredged. Continuous removal may eventually lead to erosion of inter-tidal banks and salt marshes.

The "in estuary" placement of dredged material during beneficial use schemes ensures that potential damage during essential dredging is minimised.

Nature conservation bodies in the UK have become increasingly concerned at the progressive loss of inter-tidal habitats, including salt marshes and mud flats, due to erosion and reclamation. Overall, there are only 44,370 ha of salt marsh in Great Britain. This compares with approximately 1,300,000 ha of peat land and 350,000 ha of ancient, semi-natural woodland (1996 figures), themselves rare in national terms.

Salt marshes are a rare and specialised habitat and are among the most natural ecosystems remaining in Britain. Many of their plants survive nowhere else and are specifically adapted to the high and often changing salinities of the soils and regular tidal immersion. These habitats support specifically adapted invertebrates, which include a number of rare species and are home for a wide variety of breeding birds.

Mud flats are productive systems, inhabited by huge numbers of surface-dwelling marine invertebrates. Consequently, mud flats are very important for supporting large bird and fish populations.

Case studies

The Wallasea Island re-alignment project was undertaken by Defra in 2006 to create a new wetland on the north shore of this island, situated in the Crouch Estuary in mid-Essex.

As part of the scheme 550,000m3 of maintenance material dredged from the approach channels to the Port of Felixstowe was supplied by Harwich Haven Authority with the aim of creating an area of salt marsh within the re-alignment site.

The dredged material was pumped ashore and deposited in a 45-metre-wide strip at the rear of the site prior to the breaching of the sea wall. It had to be contained by a new sea wall and a clay bund to its seaward side to allow for consolidation. The topography was raised to a level just below the Mean High Water Springs level using this sediment-recharge approach.

Experience at Wallasea Island suggests that imported sediment can colonise quickly: the signs of colonisation by salt-marsh plants were evident at this site within the same year.

Wallasea Island, sediment discharging

Dredged material being pumped ashore at Wallasea Island (© ABP mer)

 Changes in the River Orwell have resulted in a loss of inter-tidal mud levels adjacent to the Suffolk yacht harbour, which is on the east bank of the river. The harbour authority has always disposed excavated material from the site locally, and has been keen to place the regularly available maintenance material on the adjacent foreshore.

The fluid dredged material is pumped through pipes positioned within the marina to fixed pipes at the disposal area. The placement is within wattle hurdles or faggots (bundles of twigs stapled to the foreshore). Sufficient material has remained to raise the tidal height on the foreshore to allow salt marsh plants to colonise.

 Saltmarsh creation. Suffolk Yacht Harbour, Levington, Orwell Estuary, Suffolk, UK

Horsey Island is part of the Walton Backwaters. The island is strategically important because it provides protection against wave action for the Backwaters, thus mitigating erosion.

In 1998, the Environment Agency recharged 20,000m3 of mud from Harwich Haven Authority's port development, between a shingle berm and the sea wall. After nine months, considerable salt marsh growth had occurred over parts of the recharge area.

The area was again recharged during January 2001, the aim to raise the tidal height of the mud surface to facilitate the establishment of higher salt marsh plant species. 

 Saltmarsh creation, Horsey Island, Essex

© Crown Copyright 2014
Last Modified: 27 April 2014