Sediment is an essential, integral and dynamic part of the ecosystem. Over 99% of sediment dumped at sea is locally-generated and results from dredging of harbours and their approaches to ensure they are navigable. Most dredged material is dumped at established sites Figure 9.11. It is also used for purposes such as beach nourishment or land reclamation. Fish wastes and inert material of natural origin, for example rock and mining wastes, may also be dumped at sea. Fish waste is only dumped in small amounts and at a few sites (fewer than 1000 tonnes per year). The phasing out of several types of waste disposal has reduced pressure on the marine environment. Dumping of sewage sludge and of vessels or aircraft has been banned by OSPAR since 1998 and 2004, respectively. Dumping of radioactive wastes has been prohibited since 1999.
Dredging and dumping operations and techniques have changed little over the past ten years. About 90% of all sediments dumped each year are dredged and dumped in the southern North Sea. This is largely from maintaining navigation channels to major seaports such as Hull, Antwerp, Rotterdam, Hamburg and Esbjerg. In 2005, there were around 350 dumpsites in the OSPAR area Figure 9.11. Between 1990 and 2007 the total annual amounts dumped at sea varied from 80 to 130 million tonnes (dry weight) with much of the variation due to capital dredging associated with port expansion and deepening of navigation channels. The level of dumping and dredging activities has been relatively stable over the past decade and is unlikely to fall. The need for dredging may be increased in coming years by a growth in ship size, requiring deeper and wider navigation channels, or a greater frequency and intensity of storm events, and thus sediment movement by waves and currents.
One of the main concerns over dumping and dredging is the release of contaminants to the water column (such as heavy metals and TBT), which is associated with temporary increases in turbidity. This can lead to increased availability of contaminants to the food chain. Contaminants in dredged material are monitored and assessed against action levels to help reduce pollution at dumpsites. There was a clear fall in contaminant concentrations in dredged material from the southern North Sea throughout the 1990s. This trend has since stabilised. In the Netherlands, TBT concentrations in dredged material have fallen since monitoring began in 1998. A further decrease in TBT concentrations is likely following the global ban on TBT-based anti-foulants. Nutrients released from dumped dredge spoil may contribute to eutrophication, but this will generally be of minor significance.
Knowledge about the effects of dredged material disposal on the wider environment is mainly from studies at individual dumpsites and from EIAs. Sediments are part of the marine environment and relocation of non-contaminated sediments to the sea supports the natural processes of the sediment balance. Increased turbidity may also lead to short-lived effects on organisms that are light-dependent, but these are generally considered to be negligible. Dumping sediments on the seabed may smother and crush organisms living on the seafloor and may cause changes in benthic habitats and biological communities. Changes in community structure are restricted to within 5 km of the dumpsite. Continuous maintenance dredging often takes place where navigation channels to ports have high sedimentation rates, such as in estuaries. Areas that are frequently dredged have a permanently changing benthic environment. Dredging in estuaries to create a new harbour, berth or waterway, or to deepen existing facilities, can affect tidal characteristics which may affect sensitive habitats. Dredging and dumping activities also contribute to underwater noise.
Dredging and the dumping of waste and other matter have been well-regulated since the Oslo Convention came into force in 1974. OSPAR guidelines specify best environmental practice (BEP) for managing dredged material. National authorities use these guidelines to manage dredging and dumping and to minimise effects on the marine environment. The main management tools are licence and control systems. These require assessments of the environmental impact of planned disposal activities in relation to a specific dumpsite, sediment characteristics and contamination load. Since the QSR 2000, assessment and licensing procedures for dredged materials in most OSPAR countries have included action levels for contaminant loads based on the OSPAR guidelines. Since 1998, OSPAR has also had guidelines for the dumping of fish wastes.
Management of dredged material should respect the natural processes of the sediment balance. Selecting the appropriate location for a dumpsite is essential to minimise environmental impact. Several dumpsites have been relocated by applying the OSPAR guidelines. A planned site in the Weser estuary was relocated after a site survey detected a mussel bank. Dumpsites have also been relocated or closed to avoid impacts on MPAs, fisheries and shipping. The ban on dumping vessels or aircraft has been implemented successfully.
Existing regulations, including EU legislation, need to be fully implemented and their effectiveness evaluated before additional OSPAR measures are developed. Improved understanding of the effects of dredging and dumping activities on marine ecosystems, including in combination with other pressures, is needed. OSPAR should promote the development of local or regional sediment management plans focusing on maintaining sediment balance, particularly in relation to sensitive marine areas such as OSPAR MPAs and Natura 2000 sites. Greater use should be made of dredged material for beneficial purposes, such as for protecting the stability of coastal and shelf systems.