When dredging and disposing of non-contaminated fine materials in estuaries and coastal waters, the main environmental effects are associated with suspended sediments and increases in turbidity. All methods of dredging release suspended sediments into the water column, during the excavation itself and during the flow of sediments from hoppers and barges. In many cases, the locally increased suspended sediments and turbidity associated with dredging and disposal is obvious from the turbidity ‘plumes’ which may be seen trailing behind dredgers or disposal sites.

Increases in suspended sediments and turbidity levels from dredging and disposal operations may under certain conditions have adverse effects on marine animals and plants by reducing light penetration into the water column and by physical disturbance (see below). For maintenance dredging, the extent of these environmental affects is near-field and temporary generally only lasting as long as dredging operations are taking place (ABP Research R707 1997; IADC/CEDA 1998).

Increased suspended sediments can effect filter feeding organisms, such as shellfish, through clogging and damaging feeding and breathing equipment (Brehmer 1965; Parr et al 1998). Similarly, young fish can be damaged if suspended sediments become trapped in their gills and increased fatalities of young fish have been observed in heavily turbid water (Wilbur 1971). Adult fish are likely to move away from or avoid areas of high suspended solids, such as dredging sites, unless food supplies are increased as a result of increases in organic material (ABP Research R701 1997).

Increases in turbidity results in a decrease in the depth that light is able to penetrate the water column which may affect submerged seaweeds and plants, such as eelgrass Zostera species, by temporarily reducing productivity and growth rates (Parr et al 1998). The tolerance of eelgrass to high turbidity is indicated by the survival of a very sparse bed near the turbidity maximum of the Severn Estuary, which is one of the most turbid estuaries in the UK (Dee Davison Associates 1998). Although this demonstrates that eelgrass can survive in estuaries with high levels of suspended sediments, this bed declined greatly during the construction of the Second Severn Crossing which was associated with the adverse effects of smothering by the same sediments.

Background suspended solid and turbidity levels in marine SACs are highly variable. In many estuaries and bays background turbidity levels are high, such as the Wash, the Severn, the Dee and the Mersey (Parr et al 1998). Organisms in these environments are able to tolerate continuous exposure to high suspended sediment concentrations, for much longer than would occur in most dredging operations (IADC/CEDA 1998; Peddicord & McFarland 1978; Stern & Stickle 1978). Marine plants and animals living in areas where the waters are normally clear may be especially vulnerable to the effects of increased suspended sediments. For example, fjordic sea lochs in Scotland tend to have very low turbidity levels as do the rocky coasts and rias along the west coast of England.

The degree of resuspension of sediments and turbidity from maintenance dredging and disposal depends on four main variables (Pennekamp & Quaak 1990):

• the sediments being dredged (size, density and quality of the material),
• method of dredging (and disposal),
• hydrodynamic regime in the dredging and disposal area (current direction and speed, mixing rate, tidal state), and
• the existing water quality and characteristics (background suspended sediment and turbidity levels).

Dredging activities often generate no more increased suspended sediments than commercial shipping operations, bottom fishing or generated during severe storms (Parr et al 1998). Furthermore, natural events such as storms, floods and large tides can increase suspended sediments over much larger areas, for longer periods than dredging operations (Environment Canada 1994). It is therefore often very difficult to distinguish the environmental effects of dredging from those resulting from natural processes or normal navigation activities (Pennekamp et al 1996).

In most cases, sediment resuspension is only likely to present a potential problem if it is moved out of the immediate dredging location by tidal processes (Bray, Bates & Land 1997). Therefore when dredging in enclosed areas, such as within locks or dock basins, there is little likelihood that material will be to transported to the wider environment and effect the marine features of the SAC. In general, the effects of suspended sediments and turbidity are generally short term (<1 week after activity) and near-field (<1km from activity). There generally only needs to be concern if sensitive species are located in the vicinity of the maintained channel.

Next section