Physical Effects

River bed sediments would be disturbed during excavation of the trench across the river, bedding the immersed tube sections and backfilling the excavation. Some sediments would be suspended in the flowing water above the river bed, and could be carried away from the construction area. In turn, some of this sediment would be redeposited away from the trench. Computer modelling has demonstrated that these effects would be both reversible and very localised (mainly within a radius of about 50 metres). The Environment Agency has specified maximum allowable values for the concentration of suspended sediment disturbed during construction. Through a modelling study, it has been confirmed that this target would be achieved under almost all foreseeable construction and river flow scenarios.

Fisheries and River Ecology - Chemical Effects

Although the water quality of the Tyne is improving, the sediments that form the river bed contain contaminants from historical industrial and sewage discharges. Some of these contaminants can be toxic to fish and other aquatic life if they are present at elevated concentrations and are in a form that can be absorbed into their bodies.

Additionally, organic material contained within disturbed sediments can potentially reduce the oxygen level of the water in the river. In turn, this can exert physiological stresses on fish and other aquatic animals.

The Environment Agency has therefore proposed a temporary water quality standard for dissolved oxygen (DO) during the immersed tunnel works, to protect aquatic life. Detailed modelling of the water quality effects of the scheme has been undertaken. This study concluded that there would be a very small decrease in DO levels (less than 0.1% compared to normal levels of 5% - 8%) during dredging. This would be well within the Environment Agency targets.

The water quality study also considered the potential toxic effects of sediments disturbed during construction. There would be a short-term, reversible deterioration in water quality in the immediate vicinity of the trench during dredging and construction works. The suspension and transportation of sediments away from the construction area would give rise at worst to a minor adverse impact in areas of the river where the disturbed sediment was finally deposited. This effect is likely to extend no further than 800 metres from the area where dredging takes place. An analysis of potentially toxic substances in the river sediments concluded that these are generally not in a form that poses a risk to aquatic life.

Although no significant water quality effects are expected, careful construction programming and management would aim to ensure that any risks were further reduced.