Summary of possible effects of maintenance dredging and disposal in marine SACs and suggestions for means of avoiding, minimising and addressing them

(Ben = Beneficial, Min = Minimal, Adv = Adverse)

Port and Harbour Operations

Potential issues, key processes & potential impacts


and comments



Potential impacts on marine SACs




Possible means of avoiding, minimising and addressing impacts

Issue: Removal of marine species at dredge site

Key process: Physical damage (extraction)

Potential impact: Dredging causes the removal of benthic animals at the dredge site.

The removal of benthic animals is unavoidable, however the communities within regularly dredged channels are likely to be degraded and there is relatively rapid recovery.



Consider timing of dredge to avoid sensitive periods for benthic communities in the maintained channels.

Issue: Impacts of reduced water water quality at the dredge site

Key process: Physical damage (siltation)

Non-toxic contamination (suspended sediments, turbidity & organic/nutrient enrichment)

Toxic contamination

Potential impact: Dredging and disposal causes temporary increases in the level of suspended sediments in the water column which can give rise to increased turbidity, and the possible release of oxygen depleting substances (organic or anaerobic sediments), nutrients and contaminants.

The potential effects of these changes on marine life are:

  • Temporary reduction of algal/plant growth due to increased turbidity.
  • Disturbance to sensitive benthic animals and fish due suspended sediments, which may cause temporary disruption of migration of fish.
  • Temporary disturbance of marine animals from the depletion of oxygen due to release of organic-rich material.
  • Nutrient enrichment possibly causing increased food supplies/algal blooms.
  • Uptake of contaminants by marine life possibly causing direct toxic effects or effects further up the food chain.
  • Smothering of benthic animals and plants due to resettlement of suspended sediments.

The suspension of sediments is inevitable, the extent depends on magnitude and frequency of dredging, background water quality, type of material, methods used, channel size and depth, hydrodynamics and the proximity of marine features and sensitive communities. The effects tend to be short term (<1 week after dredge activity) and near-field (<1km from activity). Dredging often generates no greater suspended sediments than natural events or other human activities.


Depends on background water quality, and proximity of algae/plants.

Depends on background water quality, proximity of sensitive species, sediment type and timing.

Only when dredging sediments with high organic content or very anaerobic sediments.

Depends on sediment and water quality.

Depends on sediment quality (most dredged material not polluted, generally low level contaminants).

Some smothering inevitable, depends on hydrodynamic conditions and proximity of sensitive species.






































Select appropriate dredger to minimise resuspension of sediments.

Consider timing to dredge and disposal at most favourable points in the tidal cycle to limit extent of effects.

Use silt curtains where practicable.

Consider timing of dredging to avoid sensitive periods for marine animals.

Issue: Impacts at the disposal site

Key process: : Physical damage (siltation & smothering)

Toxic contamination

Non-toxic contamination (suspended sediments, turbidity & organic/nutrient enrichment)

Potential impact: Disposal of dredged material at sea causes smothering of benthic communities at the disposal site and may cause disturbance and damage to adjacent subtidal and intertidal communities from increased suspended sediments (possibly containing contaminants, organic matter and nutrients) and smothering (see above). Disposal of dredged material may lead to the creation of new subtidal or intertidal habitat, either inadvertently (see below) or through planned sediment recharge schemes.

Smothering is inevitable at disposal site. The communities within regularly used sites are often degraded. Extent of impacts depends on the magnitude and frequency of disposal, background water quality, type and quality of material, size and depth of receiving area, hydrodynamics and the proximity of marine features and sensitive communities. The finer the material and greater the energy at the disposal site, the higher possibility of increased suspended sediments and far-field effects. Potential impacts are minimised under the FEPA licensing process.















Consider alternative beneficial use options to reduce amounts of material disposed at sea.

Select BPEO disposal sites.

The potential effects at the disposal site are minimised under the FEPA licensing process.

Issue: Changes in hydrodynamics and geomorphology at dredge and disposal sites

Issue: Changes in hydrodynamics and geomorphology at dredge & disposal sites.

Key process: Changes to physical regime (bathymetry, tidal flows, currents, waves & sediment transport)

Erosion & accretion

Potential impact: Alteration of bathymetry, tidal currents and sediment-transport processes in the dredge and disposal areas, may cause the alteration of erosion and sedimentation patterns in adjacent areas, which may result in erosion, or creation of intertidal and subtidal habitat.

Impacts are site specific and very difficult to isolate from other natural or man-induced causes (for example sea level rise or reclamation). Effects depend on the scale and frequency of dredge and disposal, and the local conditions at the dredge and disposal site (overall system size, hydrodynamics and sediment-transport processes). Adverse effects are more commonly associated with capital dredging. Generally impacts on geomorphology are little understood and need studying.








Consider site capacity for sediment containment or dispersal when selecting a disposal site.

Consider the disposal of sediments within the system where it is the best practical environmental option.

Consider and undertake where possible beneficial use schemes for habitat creation/restoration.

Useful technical and environmental guidance for maintenance dredging and disposal

  • Beneficial Uses of Dredged Material - A practical guide (PIANC 1992).
  • Dredged material management guide (PIANC 1997).
  • Dredging a handbook for engineers (Bray, Bates & Land 1997).
  • Environmental aspects of dredging (IADC/CEDA 1997/1998)
  • Consists of a series of seven reports of which four are currently available:

No. 1 Players, Processes and Perspectives.

No. 2a Conventions, Codes and Conditions: Marine Disposal.

No. 2b Conventions, Codes and Conditions: Land Disposal.

No. 3 Investigation, Interpretation and Impact.

No. 4 Machines, Methods and Mitigation.

  • Guidance on disposal of dredged material to land (CIRIA 1996).
  • Guidelines for the beneficial use of dredged material (Burt 1996).
  • Handling and treatment of contaminated dredged material from ports and inland waterways (PIANC 1996).
  • ICE design and practice guides (Institute of Civil Engineers 1995).
  • Inland Dredging - Guidance on Good Practice (CIRIA 1997).
  • Management of Aquatic disposal of dredged material (PIANC 1998).


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