Information Notes 3 - Environmental Issues

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Conservation Sites
Mudflats and Intertidal Habitats
Sediment and Water Quality
Coastal Processess
Fisheries
Shellfish

Overall, the PLA has a wide remit of environmental considerations. The PLA has therefore translated the issues into the topics set out in Table 1. This section describes the key environmental issues that berth holders should be aware of when planning their dredging campaigns. Each of these issues will need to be considered during the PLA’s application assessment process. However, the PLA will also give consideration to the cost-effectiveness and commercial affordability of any environmental requirements, although where statutorily designated sites are concerned, strict legislation applies. The Authority will continue to aim to strike a balance between environmental protection and a thriving port, in accordance with the theme of sustainable development.

Table 1 : Environmental topics considered during the dredging licence application process

Conservation Sites

As indicated in Figure 1 , the majority of the designated conservation sites in the Thames Estuary are located from approximately Gravesend downstream into the outer Estuary. There are two intertidal SSSIs upstream of Gravesend, one at Thurrock and the other at Rainham. The designations are primarily for overwintering birds and include the intertidal mudflat and sand habitats. These sites are at their most sensitive in the winter months and are intensively used by birds as feeding grounds. The Thames Estuary is also an integral part of the Eastern Atlantic Flyway, and is, therefore, important for birds on their spring and autumn migrations. The designated sites are, therefore, also sensitive to disturbance in the spring and autumn months. Further information on the sites can be obtained from English Nature’s website www.english-nature.org.uk

Keys issues in respect of these designated sites include the following:

• Birds are particularly sensitive to noise and other disturbance, particularly by humans, during the winter months , as well as the spring and autumn months when the sites are used by passage birds, and at low water when large areas of mudflat are exposed.
• This sensitivity increases during periods of very cold weather. Dredging operations in close proximity to SPAs should generally be avoided when the ground has been frozen (or snow covered) for a number of days.
• Smothering of the mudflats by disturbed sediment from dredging operations may affect the food sources for the birds.
• Sediment quality of the dredged material must be acceptable so as to avoid causing contamination of the water column and mudflats, and thus, via the food chain, the protected bird species.
• Any removal or other loss of these designated sites is likely to be unacceptable

Impacts such as those listed above will require consideration under the conservation legislation described in Section 2.1.

In addition to the designated sites described above, the Thames is also designated as a Site of Importance for Nature Conservation. This is a national designation designed to ensure that the feature of the Thames as a wildlife corridor is considered during all applications for development. Licence applicants are encouraged to refer to the Tidal Thames Habitat Action Plan for information on sensitive habitats and species.

Mudflats and Intertidal Habitats

Mudflats are a priority species under the Government’s Biodiversity Action Plan and there is an objective of no net loss of mudflats. In the Thames Estuary, extensive mudflats can be found in the inner estuary between Southend and Mucking, while further upstream there are less extensive mudflats through much of the Thames. Mudflats provide two important environmental functions: the first as a feeding ground for birds and fish, and the second as a wildlife corridor for migratory and juvenile fish. All berth operators should avoid, where possible, any loss or disturbance to mudflats during dredging operations.

Any loss of such intertidal habitat would need to be mitigated, or compensated for by the provision of additional habitat. In practice, this would more usually be relevant to new capital dredging operations or developments than to maintenance dredging.

Wherever dredging or associated works might have an impact on the sites, habitats and species described, early liaison with the PLA will help to determine whether there is an issue to address and, if so, the likely requirements in terms of data and/or time.

Sediment & Water Quality

An assessment of sediment quality is a key part of the dredging licence assessment process. Licence applicants are required to collect and analyse samples prior to submitting their application. A detailed explanation of sampling and analysis requirements and how the PLA interprets this information is provided in Information Note 2.

In addition to the potential contamination issues, the PLA will also consider the effects of dredging operations on dissolved oxygen (DO) and suspended sediment levels. Marine organisms, for example fish, are known to be sensitive to changes in DO and reduced levels can cause fish kills. Research to date does not suggest a link between dredging and DO levels, but the potential for effect will be dependant on the amount of organic material in the dredged sediment. Applicants for dredging licences should consider if their dredge area is located close to a storm water overflow or sewage outfall.

Dredging causes disturbance to the material on the seabed and increases the amount of suspended sediments in suspension in the water column. The increase is dependent on the method of dredging employed; for example, a backhoe dredger will release less suspended sediment that a trailer suction hopper dredger. The Thames has naturally high levels of suspended sediment and, in most cases, increases caused by dredging will be within natural variations. However, some species, e.g. shellfish, are particularly vulnerable to increases in suspended sediments. If an applicant intends to dredge in proximity to shellfish or other sensitive species, then consideration should be given to selecting a dredging method that reduces losses of suspended sediments.

Coastal Processes

An assessment of the effect of the dredging on coastal process is undertaken by the PLA’s River Engineer. However, there may be some instances where additional information (data, modelling, etc.) will be required to fully assess any effects of dredging on erosion, accretion, sediment transport or other aspects of the river regime. On such occasions, the PLA would request that the applicant supply the additional information. In practice, however, for many regular maintenance dredging operations, the effects on coastal process are generally understood and considered to be negligible.

The PLA will be able to advise whether or not such information is required.

Fisheries

The tidal Thames is very important for fisheries and contains 121 species of fish (TEP November 2003; EA, 2003). In particular, the Thames provides spawning and nursery grounds for a variety of fish including sole and bass. Species of conservation interest (e.g. twaite shad) have been observed in the Thames and an extensive salmon restocking programme has attempted since the 1970s.

With the exception of salmon and the conservation species, there is no specific legislation to protect fish resources in England and Wales, but the PLA must consider effects on fish under the reference to conservation of fauna in part (a) of Section 48a of the Harbours Act 1964. The PLA is working with the Environment Agency (EA) and Kent and Essex Sea Fisheries Committee (KESFC) to identify any general measures that can be applied to minimise disturbance to fish at sensitive times. To date, the following principle has been applied to dredging projects on the Thames:

1. The area upstream of Tilbury provides nursery habitat for juvenile fish during the summer months. These fish are particularly sensitive to the combined effects of dredging, outfalls and development during the hot summer months of June to August when dissolved oxygen levels are at their lowest. Where possible and economically viable, dredging operations should be planned to avoid this period.

Further consideration is being given to the sensitivity of sole to dredging operations during the sole spawning period of March to May and nursery period of July to September/October.

While there is no scientific evidence to demonstrate the direct effects of dredging on fish at these sensitive times, the PLA is taking a precautionary approach in recognising the increased sensitivity during nursery and spawning periods. Adult fish are generally considered able to avoid dredgers and sediment plumes from dredging vessels, but juvenile fish and spawning adult fish may be less able to move away from a source of disturbance.

Shellfish

The outer Thames Estuary is a very important and productive shellfishery, with the cockle beds being the most extensive. The shellfish are generally found on the drying banks rather than in the subtidal channels. The PLA is responsible for undertaking maintenance dredging in the navigation channels and will continue to consider any effects from a dredging plume on adjacent shellfish beds, both in terms of smothering and redistribution of contaminants for potential uptake by the shellfish.

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