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YSI Hydrodata has installed five buoy-mounted automatic water quality monitors at the Tyne Tunnel project to ensure that construction operations do not significantly affect river water quality.

The contractors have not taken ownership of the instrumentation, preferring instead to purchase the data and alarms.

In July 2005, approval was given to the Tyne and Wear Passenger Transport Authority (now the Tyne and Wear Integrated Transport Authority) for a new tunnel and construction began in spring 2008.

The tunnel has been constructed utilising a hybrid method – immersed tubes form the river section and a cut-and-cover method has been deployed on the landside.

The river section involved the construction of four 90m-long, 15m-wide and 8.5m-high concrete units in the nearby Walker dry dock that were subsequently floated downriver and dropped into position.

The riverbed location for the concrete cylinders that would form the tunnel was formed by a ‘cutter suction dredger’ capable of pumping up to 7,000m3 of sediment/water slurry per hour.

In total, the dredger removed 400,000m3 of sediment over a five-week period during December 2009.

YSI Hydrodata was contracted to provide continuous monitoring data from five locations in the river.

It did this by setting up a website accessible to the key stakeholders – Newcastle University, Newcastle City Council, the contractor Bouygues Travaux Publics, the dredging company Jan De Nul, Port of Tyne and the Environment Agency – to which the data were transmitted in real time.

Originally, the plan was to employ a grab dredger to create the trench, with sediment removal by barge and disposal by landfill.

However, the possibility arose of using the dredged material to infill Port of Tyne’s redundant Tyne Dock, thereby reclaiming 13 acres of land for use by Port of Tyne.

The cutter suction dredger removed material with a solids content of approximately 20 per cent so a sheet pile wall was erected in the dock to optimise settlement of the solids.

Furthermore, silt curtains were employed to limit the level of solids in the overflow to the river.

One of the automatic water quality monitors was sited close to the overflow point to ensure that this did not adversely affect water quality.

The Environment Agency reached a legal agreement with Tyne and Wear Integrated Transport Authority that included water quality standards to ensure the protection of the environment and particularly migrating salmon on their way to breed in the upper Tyne catchment.

The agreement also required automatic water quality monitors to demonstrate that these standards were met and to trigger corrective action if they were breached.

The automatic water quality monitors were installed around 12 months prior to the commencement of dredging operations so that ‘normal’ water quality conditions could be established.

The standards were based on differences between measurements of water quality upstream and downstream (depending on the tide) of the dredging operations.

This allowed the impact of the dredging operation to be distinguished from background variations.

Roger Inverarity, a water quality planner at the Environment Agency, said: ‘Having access to continuous live water quality data via a website gave all stakeholders information about what was happening in the river, day or night.

‘It enabled the dredger to respond to any deterioration and us, the regulator, to check that it was doing so,’ he added.

Each of the YSI automatic water quality monitors is fitted with highly accurate sensors that are designed to operate in challenging environments.

These continuously measure dissolved oxygen, turbidity and temperature in addition to velocity.

The turbidity measurement is particularly important in this case because it is a surrogate for both suspended sediments and any contaminants that are associated with them.

The automatic water quality monitors raised alarms on a small number of occasions, largely as a result of high turbidity in the overflow from the dock.

When alarm conditions arose, an e-mail was automatically generated with a copy going to a mobile phone on the dredger, whereupon dredging ceased until water quality had returned to acceptable levels.

This project was unusual in a number of respects, according to YSI Hydrodata.

First, the environmental monitoring took place for a considerable length of time – both during the project and for 12 months beforehand.

Second, the environmental conditions were occasionally severe, and third, the contractor decided that data would be purchased instead of instruments.

This placed the responsibility for installation, ongoing calibration and maintenance with YSI Hydrodata.

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