Environmental impacts of maintenance dredging and disposal

Range of potential environmental effects

Factors influencing the potential effects of maintenance dredging and disposal

Range of potential environmental effects

The potential environmental effects of maintenance dredging are generally two-fold, firstly as a result of the dredging process itself and secondly as a result of the disposal of the dredged material. During the dredging process effects may arise due to the excavation of sediments at the bed, loss material during transport to the surface, overflow from the dredger whilst loading and loss of material from the dredger and/or pipelines during transport. Depending on where these activities take place, a marine SAC may be affected by either dredging or disposal alone, by both of these activities or by neither.

In considering the environmental effects of maintenance dredging and disposal, the potential benefits of these operations should not be overlooked. These include the removal of contaminated sediments and their relocation to safe, contained areas, and the possible improvement of water quality made by the restoration of water depth and flow. There can be significant beneficial improvements from the use of clean maintenance dredgings to enhance mudflat and saltmarsh habitats, and to mitigate losses of intertidal land through sea level rise and capital dredging operations (Bowles, MAFF personal communication 1999).

The extent to which maintenance dredging and/or disposal might effect marine features in an SAC is highly varied and site specific, depending upon a number of factors shown below:

Factors influencing the potential effects of maintenance dredging and disposal

  • Magnitude and frequency of dredging activity.
  • Method of dredging and disposal.
  • Channel size and depth.
  • The size, density and quality of the material.
  • Intertidal area.
  • Background levels of water and sediment quality, suspended sediment and turbidity.
  • Tidal range.
  • Current direction and speed.
  • Rate of mixing.
  • Seasonal variability and meteorological conditions, affecting wave conditions and freshwater discharges.
  • Proximity of the marine feature to the dredging or disposal activity.
  • Presence and sensitivity of animal and plant communities (including birds, sensitive benthic communities, fish and shellfish).

Prediction of the potential effects that might be caused by maintenance dredging and/or disposal in a marine SAC cannot be made with any degree of confidence if these parameters are not known on a site-by-site basis. Generally, the potential impacts of dredging and disposal can be summarised as follows (IADC/CEDA 1998, ICE 1995, PIANC 1996):

  • Removal of subtidal benthic species and communities.
  • Short-term increases in the level of suspended sediment can give rise to changes in water quality which can effect marine flora and fauna, both favourably and unfavourably, such as increased turbidity and the possible release of organic matter, nutrients and or contaminants depending upon the nature of the material in the dredging area.
  • Settlement of these suspended sediments can result in the smothering or blanketing of subtidal communities and/or adjacent intertidal communities, although this can also be used beneficially to raise the level of selected areas to offset sea level rise or erosion (short-term impact v long-term gain).

The impact of dredged material disposal largely depends on the nature of the material (inorganic, organically enriched, contaminated) and the characteristics of the disposal area (accumulative or dispersive areas) (SOAEFD 1996). The potential impacts of the disposal of maintenance dredgings on the marine environment, such as restricting the disposal of heavily contaminated sediments, is to some extent minimised through the FEPA licensing process by conditions imposed by the licensing authority.

The evaluation of the environmental effects of dredging and disposal must take account of both the short-term and long-term effects that may occur both at the site of dredging or disposal (near field) and the surrounding area (far field). The IADC and CEDA (1998) guide provides a useful table that illustrates the temporal and spatial scales in which various environmental effects of dredging might be realised (see linked table below). Near field effects are simply defined as ‘phenomena occurring within the geographic bounds of the activity, or less than approximately 1 km from the activity’, and far field effects as ‘occurring more than approximately 1 km from the activity'. However, other sources suggest that caution should be used when adopting an arbitrary distance to distinguish between near and far field effects, due to the site-specific nature of the potential effects that arise from dredging.

Table - Time–space matrix of potential effects associated with dredging and dredged material placement

In addition to the environmental effects that may occur as a direct result of dredging and disposal activities, we must also consider the environmental effects that may occur as a result of the physical changes to bathymetry and hydrodynamic processes that dredging makes. Although such changes may occur as a result of maintenance dredging, they are more commonly associated with capital dredging activities. These changes can be summarised as follows (IADC/CEDA 1998):

  • alterations to coastal or estuary morphology, for example alteration of sediment pathways and changes to siltation patterns, which may affect coastal habitats and species in addition to marine ones,
  • alterations to water currents and wave climates, which might effect navigation and conservation interests, and
  • reduction or improvement of water quality.

Each of the potential effects from dredging and disposal are discussed in the following sections. It should be stressed that there will be few maintenance dredging and disposal operations in marine SACs where all of these potential effects will be realised.

Dredging: Removal of benthic animals

Dredging and disposal: Suspended sediments and turbidity

Dredging and disposal: Organic matter and nutrients

Dredging and disposal: Contaminated sediments

Dredging and disposal: Settlement of suspended sediments

Dredging and disposal: Changes to hydrodynamic regime and geomorphology

Disposal: Discharge of dredged material at the disposal site

Disposal: Intertidal recharge

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