Project outline

Part of a £100m investment over five years, Harrowside outfall is the first element of United Utilities’ Blackpool south strategy to further improve the bathing water quality along the Fylde Coast.

A 1,000 metre-long outfall is used as a storm overflow from two upstream pumping stations; Lennox Gate and Harrowside.  This project was to replace the Harrowside outfall to ensure a discharge flow of 4m3/s at a tide level of 4m AOD before any excess flow is discharged via the beach overflow. 

Key challenges

The low lying nature of the Fylde coast and restricted access to the beach for heavy plant and machinery.

Project delivery

Murphy’s work included the modelling, design, build and commissioning of a replacement for the dilapidated existing cast iron outfall pipe, which was originally constructed in 1938. Work included the design and construction of a new 2.1m diameter solid wall polyethylene gravity outflow pipe to discharge the consented flows 1km offshore and the decommissioning and removal of the existing outfall. Work also involved the reconfiguration of the outfall chambers.

The final stage of the project was to install concrete kennels and mattresses over the pipe, extract the 1,500 steel sheet piles, remove the existing outfall and return the sea bed to its natural state.

The new outfall pipe connects to the Lennox Gate pumping station and will deal with any excess stormwater in the region, as well as increasing the capacity of the existing sewer network.

Collaborative working and the experience of our in-house design team enabled us to complete the project on schedule.

Innovation and value

Conceptual hydraulic design was initially developed by United Utilities and the detailed design was carried out by Murphy’s in-house design team. 

Moving the pipeline about 30m to the north avoided the risk of undermining the existing cast iron pipeline. 

The design brief specified duck-bill valves, however this created the potential for drained sections of the pipe to become buoyant.  To counter this effect, pre-cast concrete ‘kennels’ were installed as additional weigh along the entire length of the beach section.

Using Murphy’s in-house Float-Flood programme we confirmed that the pipeline could be safety flooded into the trench during installation.  The design team developed a concrete weight collar design, which simplified the installation and attachment operation and shortened the programme. 

A shift pattern was adopted to maximise the time available to access the beach, which only had a single ramp.

Key facts
  • New 2.1 metre diameter polyethylene pipe extruded in Norway as two 500-metre sections before being towed by barge to Land & Marine’s Holyhead site and then to Blackpool
  • A 400-metre-long sheet piled cofferdam with guide piles to ensure the new pipe remained in place during all tidal conditions across the beach during installation was built
  • Offshore end of the pipe terminated with a concrete diffuser that housed eight duck bill non-return valves
  • Coordination of offshore activities including pipe delivery, assembly and towing using personnel and vessels at several locations were key factors in executing the construction operations effectively

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