Thames Tideway Tunnel: a super sewer for London

Creating a tunnel 25 kilometres in length, and running up to 65 metres beneath the River Thames, generated immense volumes of spoil, right in the heart of London. To minimise the impact of transporting this on the capital’s roads, around 4 million tonnes of this material were transported by barge, increasing river traffic by 30 per cent. 

To meet the challenge of increased river use, we developed a four-day course with the Tideway team, designed to ensure boat masters were given the opportunity to demonstrate that they were fully prepared and  aware of the health and safety requirements to work on the Tideway Project. The course used our virtual reality simulation of the tidal River Thames, developed using HR Wallingford’s UK Ship Simulation Centre.

“Our simulation covers the tidal River Thames from Putney Bridge to Margaretness, complete with accurate visual scene, tides, flows, other river traffic and changeable weather conditions,” said Dr James Clarke, Technical Director at HR Wallingford. The Thames simulation is the most detailed visualisation we have developed to date. It includes important visual clues, such as lions heads in the river embankments, marker posts, and even details on the undersides of the bridges.

As well as being assessed during normal operations, the vessel masters had to demonstrate how they would respond in unexpected scenarios, examining credible emergency situations in a safe, risk free environment. 

Many of the temporary works needed during construction and the permanent final structures will be located on the Thames’ foreshore, potentially changing the way water and sediment moves in the river. We’ve looked at these changes as input to understanding the impact they might have on nearby structures, on river navigability, ecology, as well as being able to inform the design of the structures themselves to minimise these effects.

Future proofing the design

The design life of Thames Tideway Tunnel is 120 years. With the predicted effects of climate change, average sea level and river flows are expected to change over this time, so we‘ve used the validated whole estuary numerical model to simulate the potential extreme currents which might occur over the design life of the tunnel, redefining the extreme or ‘most onerous’ hydrodynamic conditions which the designs will need to be ready for. 

Monitoring the effects

The construction of the Thames Tideway Tunnel was split in to three work packages; West, Central and East. Each package includes a number of CSO interception and shaft construction sites. 

For the Central area, we are supported the contractor team – a Ferrovial Agroman and Laing O’Rourke joint venture - to develop and implement a Scour and Accretion Monitoring and Mitigation Plan (SAMMP) for each site of temporary river works. The SAMMP collates the predicted effects of the works on scour and accretion in the river, the understanding of bed level variability and outlines how the river bed level has been monitored over the project construction phase, up to and including removal of the temporary works. The SAMMP then describes the action that will be taken to mitigate the effects where necessary. The SAMMP is an important document for the construction process and needs to be approved by the relevant regulatory bodies.

The SAMMPs for the Thames Tideway Tunnel are based on an exceptional baseline dataset covering quarterly bathymetric surveys since the end of 2013. The baseline data is key in understanding the natural variability in bed levels which would be expected in a dynamic area such as the tidal Thames and defining appropriate thresholds for any mitigation actions to be triggered. The data collected has proven to be a gold mine of information for scour studies enabling scour predictions to be compared to real world data. Our work was presented at the 11th International Conference on Scour and Erosion.