Dredging - Plymouth MPA

The river basin and the seabed are highly dynamic environments, with many hundreds of tonnes of sediment constantly on the move by the currents and tides. When currents slow, the sediments carried by the water are deposited on the seabed. If the currents remain slow, then these sediments can build up over time leading to a reduction in water depth and problems for ships. As a result, dredging of the sediment from the seabed is often a necessary part of development in order to secure sufficient depth of water for boats to pass safely.

While vital to the proper function of the port, thereby providing economic benefits, dredging has the potential to cause major impacts through damage to the seabed habitat and its species, changes to the hydrodynamics, and pollution through the resuspension of contaminated material. In order to minimise these impacts, dredging activities should comply with the following measures to reduce the associated impacts.

Measures:

Disturbance of sediments

Impact

Sediments will be disturbed when dredging of the seabed and can result in a large area being impacted. Smothering can be an issue if there are important species not only in close proximity to the dredging site, but can also have lesser impacts further out. The severity of the impact depends on the level of dredging occurring. Impacts of smothering may also arise from dredged material dropped at disposal site.

Further increased levels of suspended sediment during the dredge will impact fisheries, whilst increases in sediment deposition during the dredge will impact on fisheries, shellfisheries and seabed habitats.

Suggested measures to reduce impact(s)

• Choose appropriate dredging plant to minimise levels of suspended sediment.
• Consider beneficial use schemes such as water column recharge to minimise loss of material from the estuarine system and minimise effects of disposal.
• It should be noted that there may be conflicts between water column recharge schemes and the need to reduce mobilisation of contaminated and suspended sediments.

Impact to hydrodynamics

Impact

The location of the dredging can have impacts on the estuarine hydrodynamics through alteration of coastal processes and impacts on sedimentology.

Erosion of the salt marsh habitat could arise if works are located near significant hydrodynamic areas.

Suggested measures to reduce impact(s)

If the dredge is likely to cause loss of intertidal habitat (either directly, or indirectly through alterations to the tidal or wave regime), create compensatory habitat, such as through managed realignment.

Loss of habitat

Impact

Damage to seabed habitat and species within footprint of the dredge will occur along and will be dependent on the scale of dredging work taking place.

Suggested measures to reduce impact(s)

• If the dredge is likely to cause loss of intertidal habitat (either directly, or indirectly through alterations to the tidal or wave regime), create compensatory habitat, such as through managed realignment.
• Undertake dredging works outside salmon migratory season, in consultation with the Environment Agency.

Noise disturbance (birds)

Impact

Potential temporary disturbance to birds during the dredge, due to presence of a dredging plant.

Suggested measures to reduce impact(s)

Consider conducting dredging activities that will have minimal impact on the disturbance of birds, when the least number of birds are present within the area.

Water quality issues

Impact

Issues relating to the re-distribution of contaminated sediments within the estuary, leading to issues of increased levels of suspended sediment, impacting on the quality of the water.

Suggested measures to reduce impact(s)

• Consider beneficial use schemes such as water column recharge to minimise loss of material from the estuarine system and minimise effects of disposal.
• It should be noted that there may be conflicts between water column recharge schemes and the need to reduce mobilisation of contaminated and suspended sediments.

Impacts to migratory fish

Impact

Damage to seabed habitat and species within footprint of the dredge can impact on migratory fish species.

Suggested measures to reduce impact(s)

Undertake dredging works outside salmon migratory season, in consultation with the Environment Agency.