Background

Following repeated incidents of urban flooding caused by heavy rainfall and flash storm events, Thames Water identified the need for an underground flood alleviation solution at a busy road junction in Wallington, near Sutton.

The location had a history of foul sewer surcharge during storm conditions, leading to flooding of homes and car parking areas. The scheme formed part of wider works to improve network resilience and protect customers from future flood events.

The challenge

The site presented several significant constraints:

  • Extremely limited space at a busy urban junction
  • Requirement for a structure that could be built over and trafficked
  • Multiple underground services clashing within the excavation area
  • A need to minimise disruption to residents and road users
  • Flood risk during construction due to live network conditions

The original design solution proposed a double skin engineering brick chamber, but this proved impractical once the restricted working space and service constraints were fully understood.

The solution

Marshalls proposed a bespoke modular precast chamber, specifically designed to meet the spatial, structural and functional requirements of the site.

The chamber measured 1.7m wide x 3.06m long x 2.96m high and was manufactured off site under controlled factory conditions. It was installed to a depth of 3.26m over a three day period by the KCD major projects team.

The bespoke unit was designed to:

  • House a FLIP pump within one chamber
  • Provide emergency foul storage in a second chamber in the event of pump failure

The complete solution comprised:

  • A one piece precast base
  • Four interconnecting external wall units
  • An internal weir wall
  • Pre fitted watertight seals
  • A cover slab with access openings for inspection and maintenance

Installation works were carefully managed within Tharp Road and the rear access road behind residential properties to reduce the risk of sewer surcharge and protect eight homes that had previously experienced flooding during heavy rainfall

The outcome

The bespoke flat pack chamber delivered a highly effective flood alleviation solution within a severely constrained urban environment.

  • Significant reduction in foul sewer surcharge risk
  • Protection for residential properties previously subject to flooding
  • Rapid installation minimising disruption to residents and traffic
  • A robust, traffic rated below ground structure suited to long term operation

Abi El Mohri, Project Manager at KCD, commented:

“We changed the design mainly to accommodate the space, or lack of it. It was very limited due to the services underground clashing with the space needed for the chamber.”

She added:

“Marshalls were outstanding from my point of view. They delivered as planned and provided us with an expert manager on site which made a complex assembly look really simple.”

The project stands as a strong example of how bespoke precast design can unlock practical flood mitigation solutions where traditional construction methods are not viable.