Maida Vale And The Tale Of The Throttle Pipe - Passive-Control To Eliminate Foul Pumping

A client outcome-based approach delivered an innovative, sustainable solution to flooding, while embracing safety in design at the outset.

Maida Vale, a suburb of Central London, is served by three of the original ‘Bazalgette’ brick sewers approximately 2m in diameter and built in the late 19th Century. Whilst these sewers highlight the excellent quality of Victorian design and construction, there are a number of areas where they no longer have the capacity to transfer flows. In Maida Vale this resulted in around 200 basement properties suffering from combined sewer flooding during a 1 in 10 year ARI storm event, with some properties being flooded in more frequent events. Consequently, Optimise, a design & construction joint venture between MWH (now Stantec), and Murphy, Barhale, and Clancy Docwra, received instruction from Thames Water to undertake the £21m Maida Vale Flood Alleviation Scheme. This outcome-based project looked to resolve the flooding problem for up to the 1 in 30 year ARI storm event with a demanding contractual deadline of less than three years from initial issue investigation, through full design, with construction completion by March 2015.

Property flooding was found to be in two main clusters, this paper focuses on the Formosa Street sub-catchment where the flooding mechanism was identified as surcharging from Bazalgette’s trunk sewer into the local network.

A traditional approach to the problem would be to isolate the flooding area and pump all flows. In a typical year, this would have resulted in pumping 241,500m3 when the storm flows – the cause of the flooding – was only 21,500m3. As a result of extensive hydraulic modelling to understand the flows within the local and trunk sewers, we developed a means of allowing the base / foul flow to remain connected to the existing sewer network, the source of the flooding problem, whilst managing the risk of storm flows backing up from the trunk sewer and causing flooding.

This was achieved by installing a weir in Formosa Street designed to direct foul flows by gravity to a hydraulically designed throttle pipe into the trunk sewer, and in storm events limiting the reverse flows back from the trunk sewer, over the now-drowned weir into a 20m diameter, 26m deep storage shaft located in nearby Westbourne Green park. Flows are dynamically pumped back into to the receiving sewers as level conditions allow.

This solution is essentially maintenance free, and eliminates the need for foul pumping with the associated savings of capital and maintenance costs. It also minimises the health and safety risks during construction and maintenance of the assets in very deep infrastructure. In addition, the solution minimised cost and disruption to Thames Water’s customers.

This approach was made possible by Thames Water allowing their trusted delivery partners to develop an effective solution to a problem, not just an instruction to construct expensive infrastructure.