Sewer daylighting: bringing blue-green infrastructure to Ealing’s Glade Lane

London Borough of Ealing secured approximately £225,000 of GLA Green and Resilient Spaces funding to deliver a sewer daylighting scheme at Glade Lane Canalside Park in Southall. Metis supported Ealing from feasibility and funding stages, before being appointed to develop the outline and detailed design – and provide construction-stage support.

The park is a Grade 1 Site of Importance for Nature Conservation, bordered by the Grand Union Canal, railway land and residential development. The sewer daylighting scheme involved diverting an existing Thames Water surface water sewer into new open channels, wetlands and pond features before reconnecting to the sewer network. Its aims were to reduce downstream flood risk, improve water quality, create new habitats and improve access to green and blue space.

The Challenge

The project required a naturalised drainage feature to be created within an operational public park. This task had to work around significant buried infrastructure. Early Metis reviews identified several constraints, including three large Thames Water trunk water mains, a foul sewer through the site, shallow gradients in the existing sewer network and deeper excavation requirements along the original proposed route.

Metis also identified that the initial alignment had a very shallow gradient, which could have increased the risk of low flow velocities and long-term silt build-up if converted into a meandering open channel.

Key challenges included:

• validating the feasibility of the scheme using sewer, utility, LiDAR, flood risk and environmental data
• supporting Ealing in submitting a successful funding application to Thames Water
• revising the layout to avoid critical Thames Water assets and reduce excavation risk
• agreeing ownership, adoption and maintenance responsibilities with Thames Water and Ealing
• designing a buildable open channel, pond, overflow and reconnection system that could operate safely in normal and high-flow conditions

Our Sewer Daylighting Approach

Outline design

Following appointment, Metis confirmed the feasibility of the project and developed the outline design with Ealing. This was supported by a site walkover, survey scoping and design risk management using the ERIC approach: Eliminate, Reduce, Inform and Control. We recorded risks around trunk mains, deep excavation, the foul sewer and future maintenance, and addressed these through the design.

Metis liaised with Thames Water to agree the approvals route. The scheme moved from a Section 185 sewer diversion approach to a Section 104 adoption agreement. Thames Water adopted the diversion manhole and new pipes, while Ealing retained the open channels, ponds and headwalls.

Detailed design

For the detailed design, Metis used InfoDrainage to model the open channel system and Civil 3D to develop the earthworks design. The design included:

• a main channel of approximately 152m
• an overflow channel of approximately 66m
• a basin with 347m³ of storage
• 1:3 side slopes,
• stone check dams
• a precast diversion manhole with an integrated weir wall

The earthworks strategy sought to reduce material disposal by reusing excavated material on site. The design drawings identified approximately 1500m³ of channel and basin cut, 1200m³ of earth bund fill and a net cut of 300m³.

Metis prepared a tender-ready package including general arrangement drawings, drainage details, earthworks plans, specifications, a Bill of Quantities and an updated design risk register. The package was developed in line with the Manual of Contract Documents for Highway Works and relevant sewer adoption guidance.

Construction

During construction, Metis supported Ealing with contractor queries, Thames Water requirements and buildability issues. When an unrecorded manhole was discovered on site, Metis advised stopping works locally, coordinated with Thames Water and updated the design. This was in order to move the channel 3m away while maintaining future asset access.

Outcomes and Value Delivered

Metis helped Ealing turn an ambitious sewer daylighting concept into a technically robust, buildable blue-green infrastructure scheme. The final Glade Lane Canalside Park design supports:

• improved surface water management
• downstream flood risk reduction
• water quality benefits
• habitat creation
• better public access to green and blue space

The scheme demonstrates how early technical input, stakeholder coordination and practical design development can unlock nature-based drainage solutions in constrained urban parks. It also shows how to balance biodiversity, amenity, asset protection, maintainability and construction risk.

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