River Severn Sewage Outfall

River Severn

1.7km seweage outfall

1.7km Sewage Outfall in Tidal Estuary

- Challenging marine conditions

The Project

The project was to fabricate and install a 1.7km HDPE gravity-fed sewage outfall within the tidal River Severn estuary. The pipeline consisted of several diameters up to 500mm, and included a 60m horizontal directional drilled outfall. 700m of the pipeline was installed above the high-water mark along the south-eastern banks of the river with the rest of the line being installed within the inter-tidal zone, including the HDD work. This section of project also took the pipeline over a rock outcrop known locally as the Aust or Black Rock.

Risk Assessment

Due to the particular challenges of working within an area well-known for a great tidal range and fast currents SDL carried out extensive risk assessments before work commenced. The risks were not restricted to the inter-tidal zone and working practices were introduced to ensure safe-working throughout the project. Measures taken included the provision of safety wardens to ensure all personnel were logged in and out of the inter-tidal zone and to monitor the condition of the cliff face throughout the works. Specialist PPE equipment was also provided. Before every project SDL will complete a series of assessments covering risk, health and safety and environment. With experience of working in challenging conditions, both on-shore and marine environments and are very familiar with the many environmental constrains present along most pipeline routes. A commitment to minimising any works done is inherent in the SDL approach.


The pipeline route is located within an internationally important site for wintering birds, protected as a SSI/RAMSAR/SPA site, and is known as an area of archaeological interest. In addition, the available project window was restricted to a period of June to October to safeguard the life-cycle of the 'Shad' that spawns in the area, coupled with planning constraints linked to protecting the foreshore salt marsh and the Aust itself this significantly impacted operations - especially plant movements and refueling.

Project Summary

  • 1.7km HDPE pipeline installation.
  • 100m HDD.
  • River Severn estuary.
  • Large tidal range and fast currents.
  • HDD drill and pipeline installation within the inter-tidal region.
  • Major environmental constraints.
  • Tight window of opportunity for project execution.

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Designed at B&W studio