What is possible?

What is practical?

Possible constraints to the use of maintenance dredgings in beneficial use schemes

Case studies

Contaminated dredge material

Between 1989 and 1994 the amounts of maintenance dredged materials disposed of at sea under license in England and Wales almost halved due to improved port operations and dredging practices and the increased use of beneficial options for the disposal of sediments (Murray 1994a). The gradual reduction in the amounts of material being deposited at sea provides a means of minimising the overall potential effects from the disposal of sediments on the marine environment. In addition there are powerful economic arguments for ports and harbours to minimise amounts of material dredged and disposed at sea.

There has been over a decade of beneficial use schemes undertaken and planned by UK ports and harbours, mostly providing uses for coarse dredged materials such as gravels and sands for construction or coastal defence purposes, such as beach replenishment schemes. Beneficial use schemes using fine dredged silts are becoming more common. The use of maintenance dredged materials for environmental enhancement, such as habitat creation and restoration, has increased considerably in recent years, particularly intertidal sediment recharge (foreshore nourishment) schemes which provide a means of combating the erosion of intertidal flats and saltmarsh (ICES 1992).

Intertidal recharge schemes have been applied on a largely small-scale experimental basis in over 20 locations in Essex and Suffolk using dredged material from the Blackwater Estuary and Harwich Harbour (Carpenter & Brampton 1996). These schemes have the potential to be applied to address erosion problems in a number of marine SACs. A selection of beneficial use projects using dredged material from UK ports and harbours are summarised in the table below (Murray 1994a).

(Based on information in Legget & Dixon 1994, Murray 1994a, Dearnaley et al 1995; Carpenter & Brampton 1996; HR Wallingford & Posford Duvivier Environment 1998)

Although habitat creation and restoration using dredged material is still relatively rare in the UK, during the past decade over 40,000 hectares of wetland, both coastal and inland, have been restored, created or protected using dredged material in the USA (Landin 1998). Thousands of schemes have been undertaken, primarily by the US Army Corps. of Engineers, and to a lesser extent by other public agencies, such as the US Fish and Wildlife Service, conservation groups and by developers for federal and state permit applications under the Clean Water Act. An overview of the range of types of restoration and creation schemes undertaken through the USA has been described in numerous US Army Corps of Engineers guides and reviews (Landin et al 1995; Landin 1998; US Army Corps of Engineers 1987).

Considering the sheer volumes of material dredged in the UK every year, about 40 million tonnes, it is impossible to conceive sufficient beneficial use schemes to use such large amounts. However, this highlights the potential for the use of dredgings for beneficial uses and the creation and restoration of habitats. Possible constraints to the use of maintenance dredging in beneficial use schemes are summarised below.

Possible constraints to the use of maintenance dredgings in beneficial use schemes

• The perception that dredged material is a ‘dirty’ waste that must be disposed of at sea. This perception is gradually being changed and dredged material accepted as a useful material.
• The options for beneficial uses of fine materials are limited and silts are generally unsuitable for engineering or aggregate purposes. Cohesive muds require time to de-water and consolidate before becoming stable enough to support engineering structures or mature plant and animal communities. The time required for stabilisation in many cases may be outside the timescale for reclamation schemes and needs to be considered in planning habitat creation and restoration schemes using dredged material. However, fine material is vital for habitat creation purposes.
• Finding suitable locations within an estuary for such schemes may not be easy. Appropriate disposal sites may be highly restricted by coastal development, the location of intake and outfall pipes, navigation channels, land ownership, and fisheries, in addition to the presence of sensitive animal and plant communities.
• The high rate of production of dredging plant compared to the rate of use needed for most beneficial uses.
• Beneficial use schemes generally take longer to plan, find resources, obtain permits and undertake, than disposal at sea. Once dredging is underway the material needs immediate disposal (Burt & Paipai 1996). This highlights the need for strategic planning.
• Difficulties have been encountered in some beneficial use schemes on land, which often involve dealing with many different regulatory bodies, including the EA under the Waste Management Licensing Regulations 1994. If such beneficial use schemes are to be encouraged in the future there is a need for all relevant regulatory bodies to work together and reach consensus over ways that current regulatory disincentives may be removed, wherever possible.

These possible constraints to the promotion of beneficial use of dredged material need to be considered by the management scheme, however many of them can be addressed. If beneficial uses are adopted in the UK more often and a greater understanding of the issues involved is developed, the significance of these constraints is likely to be reduced. In many cases, the economic benefits of reducing the amounts of materials disposed of at sea, in terms of savings in steaming time to offshore dump sites, provides incentive and motivation enough to encourage beneficial use schemes in ports and harbours.

It is important to note that although the methods and techniques used to recharge and restore intertidal habitats in the UK are novel here, the methodologies used in these schemes have, in the vast majority of cases, been tried and tested elsewhere, particularly in the USA. Over the past 25 years the US Army Corps of Engineers have developed and improved techniques to place dredged material, whilst meeting environmental standards which has resulted in the completion of several thousand wetland restoration schemes. There are useful lessons to be learned from these schemes, the consideration, planning, design and construction of which are described in the US Army Corps of Engineers’ Engineering and design manual for beneficial uses of dredged material (1987).

There is increasing guidance available on the beneficial use of dredged material and procedures for developing economic and effective ways to use dredged material, including construction, agricultural and environmental uses. The practical guides prepared by HR Wallingford and PIANC provide a useful basis for assessing what beneficial use options are realistic for different types of sediments, including maintenance dredging material (PIANC 1992; Burt 1996).

The following case studies of beneficial use schemes are discussed further in Appendix M as an illustration of what is currently being achieved in the UK:

• Port of Truro, beneficial use of silts as capping material for the restoration of contaminated derelict land,
• Harwich Harbour, intertidal recharge using dredged sands and silts for coastal defence and habitat creation, and
• Medway Port, intertidal recharge (trickle charge) using silts to retain sediments in the estuary system.

In the USA the use of contaminated dredged material for habitat creation has been studied and undertaken for the past ten years and is considered to be the major innovation in beneficial uses (Brandon, Lee & Simmers 1992). In several other European countries including Denmark and Holland contaminated dredged material is treated so that it can be used beneficially. It may be possible to find out more information on the practicalities of the treatment of contaminated dredged material. However, the costs associated with these civil engineering treatment schemes are up to £80 per m³, whereas normal costs of disposal are of the order of £3 per m³. Therefore, with contaminated dredged material beneficial use to the port operator is not a practicable option. However, this may depend on what the material is used for. There is a need to find low cost practical ways of using material beneficially.

Next section