Environmental impacts of
maintenance dredging and disposal
Range of potential
Factors influencing the potential
effects of maintenance dredging and disposal
Range of potential
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
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
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:
the potential effects of maintenance dredging and
- 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
- 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
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
Table - Timespace 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
- 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
- alterations to water currents and wave climates,
which might effect navigation and conservation
- 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.
Removal of benthic animals
and disposal: Suspended sediments and turbidity
and disposal: Organic matter and nutrients
and disposal: Contaminated sediments
Dredging and disposal: Settlement
of suspended sediments
and disposal: Changes to hydrodynamic regime and
Discharge of dredged material at the disposal site