Dredging and disposal


Dredging and disposal in marine SACs


Why dredge?

Dredging is a fundamental activity for most, but not all, ports and harbours. The Central Dredging Association states that "in its simplest form dredging consists of the excavation of material from the sea, river or lakebed, and the relocation of the excavated material elsewhere for disposal" (IADC/CEDA 1997). In ports and harbours dredging can be undertaken to meet a number of different objectives, which include the following:

  • Navigation: to maintain or improve/extend navigable depths in ports, harbours, marinas and shipping channels which is usually a statutory requirement for port and harbour authorities.
  • Flood control: to improve drainage or sea defence.
  • Construction and reclamation: in support of coastal development or for the provision of foundations for civil engineering works, for example barrages, bridge piers and pipelines.
  • Mining/Aggregate: to win minerals and aggregate materials from underwater locations (Aggregate extraction is the subject of a further report of the UK Marine SACs Project).
  • Beach nourishment: to supply material to reinstate or improve the performance of a beach as a sea defence or an amenity.
  • Environmental: to improve and clean up the environment, generally for the removal of contaminated sediments which is commonly called remedial dredging.

Dredging for navigation purposes encompasses two main types, maintenance and capital dredging.

The main difference associated with these definitions relates to the renewal of existing consents and the application for new consents for the disposal of dredged material. The main reason a distinction is applied by the consenting authorities concerns the physical characteristics of the dredged material and how, or if, it will disperse during its disposal or beneficial placement and subsequently move away from the disposal site. Generally for capital dredging, there is unlikely to be as much information available about the characteristics of the material and an additional assessment is made to determine the potential environmental impacts and the predicted pattern of dispersal away from the disposal site.

In their guidance on European marine sites in England and Wales, DETR/Welsh Office describe dredging and disposal as an example of an activity which might occur in European sites (DETR & WO 1998). However disposal of dredged material is subject to consent and licensing, and therefore also falls into the definition of a project and plan which is "in general, any operation which requires an application to made for specific statutory consent, authorisation, licence or other permission".

This section of the guidelines focuses on routine maintenance dredging in ports and harbours that will be managed under the marine SAC management scheme. Ports generally have a statutory responsibility to maintain navigation for port users. This remit includes dredging to keep the navigational channels open and may include commercial agreements to maintain the channel at a certain depth for a specific customer. It is therefore imperative for ports, harbours and marinas to carry out dredging when necessary.

Capital dredging for new port, harbour and marina developments, includes the construction, extension or deepening of berths and navigation channels for access by larger vessels. These operations will, in general, require consent under the Harbours Act 1964 (or equivalent local Act) and will therefore be subject to the Assessment of Environmental Effects Regulations. Capital dredging will generally not be considered within the SAC management plan and is therefore not considered in these guidelines. However, the impacts of different types of navigation dredging can be generic to some extent, and case studies of capital dredging will be discussed where considered relevant to the development of SAC management plans.

Dredging and disposal in marine SACs

Between 20 and 40 million tonnes of material is dredged from English and Welsh ports, harbours and their approach channels every year, and in 1994 the amounts dredged were estimated at some 40 million tonnes (Lee et al 1995). However, the levels of dredging that take place varies greatly from port to port and from year to year. For example, whilst Milford Haven may only have a minimum requirement to undertake maintenance dredging in some years, in others the oil companies in the Haven have dredged substantial amounts. The variation in dredging effort in ports and harbours across the UK is indicated in the table below, which provides estimates of the amounts of material dredged per year in selected SACs.

For example, the ports of the Severn Estuary were reported to dredge around 4.5 million tonnes of sediments in a typical year (Severn Estuary Strategy 1997), whereas in Strangford Lough only 2,000 tonnes were dredged a year (Portaferry Harbour personal communication 1998). In contrast, no dredging is undertaken at Millbay Docks and Sutton Harbour in Plymouth Sound, although small amounts of maintenance dredging activity is undertaken within the Dockyard Port of Plymouth.

In addition to undertaking maintenance dredging to improve or extend navigable depths in ports, harbours, marinas and shipping channels, it is also an important activity in the vicinity of lock and dock gates to ensure there is efficient operation and continued access to dry docks and basins.

Indication of the variation in dredging effort in or near selected UK marine SACs


Marine SAC


Total estimated amounts of material dredged

Fal and Helford





3,000-4,000 m3 per year

Morecambe Bay







1,270,000 – 2,670,000 m3 per year 1

Pembrokeshire Islands


Milford Haven

Pembroke Dock

50,000 m3 per year

Plymouth Sound and Estuaries


Cattewater Harbour

Millbay Docks

Sutton Harbour



Dockyard Port of Plymouth

Very small amounts of maintenance dredging per year

Severn Estuary












3,460,000 m3 per year 1

Strangford Lough

(Northern Ireland)



1,500 m3 per year 1

1 converted from tonnes assuming that the material has a density of 1,300kg/m3

In the UK, the majority of material from maintenance dredging is disposed of at sea at about 150 licensed disposal sites (MAFF 1996; IMO 1997). Quantities of maintenance dredgings disposed to sea in England and Wales varied from 17.6 – 34.1 million wet tonnes between 1985 and 1993 (Murray 1994a). Gradual reductions in the amounts of material disposed of at sea have resulted from changes in port operations and dredging practices and the increased use of beneficial options for the disposal of sediments. Less than a quarter of UK marine SACs have disposal sites located in or near the site. These include four UK Marine SAC Project sites, namely Morecambe Bay, Fal and Helford, Solway Firth and The Wash, and a further seven sites, Moray Firth, Flamborough Head, Essex Estuaries, Thanet Coast, Solent Maritime, Severn Estuary, and the Pembrokeshire Islands. The amount of maintenance dredgings disposed within or near these sites varies greatly, as does the nature of the various disposal sites.

In recent years, all applicants for sea disposal licences for dredged material in the UK have been required to consider whether the material can be managed in such a way to derive environmental or other benefits or the potential for beneficial use of the material. Dredged material from ports and harbours have been put to a range of beneficial uses, including construction, agricultural and environmental uses. A number of ports and harbours within or near marine SACs are considering the feasibility of using dredged material for intertidal recharge schemes and saltmarsh restoration schemes. Small-scale schemes of this kind have been undertaken on an experimental basis at over 15 locations along the south east coast of England, including a number within the Essex Estuaries SAC. The Port of Truro has also been investigating the feasibility of mixing dredge spoil with china clay waste to produce a soil substitute for use in land reclamation on contaminated sites.

Whilst intertidal recharge schemes can provide long-term benefits of environmental enhancement and protection, the act of placing material over existing intertidal habitats has the potential to cause the same short-term impacts of any disposal operation, generally associated with smothering and increased suspended solids. In recognition of the dual nature of intertidal recharge schemes they are discussed both as an impact of the disposal of maintenance dredgings and as a means of addressing potential impacts arising from dredging operations.

Existing regulations for dredging and disposal

Environmental impacts of maintenance dredging and disposal

Means of avoiding, minimising and addressing the potential impacts of maintenance dredging and promoting benefits


Good practice