Selected examples of the effects of the disposal of dredged material

When the maintenance dredgings are disposed of at sea they will have a blanketing and smothering effect on benthic organisms in the immediate disposal site. The continual disposal of maintenance dredging at disposal sites may prevent the development of stable benthic communities, and the partial or complete loss of benthic production is an adverse effect which has to be accepted within regularly used disposal sites (Murray 1994b).

With the exception of the initial smothering of benthic communities at the disposal site which is inevitable, the potential for other effects to possibly occur as a result from disposal operations will be site specific, depending on the characteristics of the dredged material and the hydrodynamic conditions at the disposal site. These potential effects at the disposal site are minimised under the FEPA licensing process, irrespective of whether it is in or adjacent to a marine SAC, which is regulated by MAFF, SOAEFD and DOE(NI).

The finer the material and the greater the energy at the disposal site, the greater the possibility of increased suspended sediments and of far-field effects. However, as mentioned previously, these far-field effects of turbidity and smothering are generally only of high concern in areas of low background levels of suspended solids. Adverse effects may occur if these dredged materials settle out over communities adapted to and dependant upon clear conditions, such as clean swept gravels supporting rich sponge communities.

Disposal sites located in shallow and low energy areas may accept small amounts of fine dredged material occasionally, which is dispersed by tides and waves, ensuring that material does not build up at the site with no effects on adjacent communities. However, if the disposal site is overloaded with large quantities of maintenance dredgings over a short period, shallowing of the disposal site can occur and smothering can adversely effect areas of adjacent subtidal habitat. In contrast, disposal of dredged material may have beneficial effects through the creation of new subtidal or intertidal habitat, depending on the location of the disposal site. Examples of these effects are given below.

• A disposal site near Ramsgate (Thanet Coast SAC) is regularly used for the disposal of small amounts maintenance dredgings without apparent adverse effects on marine ecology. However, disposal of larger quantities of material at the site resulted in the blanketing of adjacent areas and disturbance to local lobster fisheries (Murray 1994b).
• The disposal of capital dredgings offshore of Harwich is reported to have incidentally created a lobster habitat (Murray 1994a).

Just as dredging within highly turbid environments has little effect on the tolerant benthic communities, disposal of maintenance sediments in suitable locations within such estuary systems can also have minimal effects. For example, the disposal of between 5-10 million tonnes each year of fine dredged sediments in the highly turbid environment of the Humber Estuary is reported to have little physical or biological effect as the sediments are redeposited within the estuary (Whitehead ABP Research personal communication 1998; Murray 1994b).

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