Key information
Request reference number: MGLA231024-3558
Date of response:
Summary of request
Your request
London Plan Figure 9.4 - Spatial illustration of wastewater drainage capacity across London.
This 2017 London Plan key Climate Change risk information from Thames Water is inaccurate as it fails to map London's much larger spatially, Victorian combined sewers system. For one example, Brent South Kilburn Tall Building Zone (the rivers 'V' shape cutting into Maida Vale Conservation Area and Maida Hill Neighbourhood Plan Area) on figure 9.4, has Victorian combined sewers and a 5 rivers system including the troublesome 'lost' River Westbourne London Plan map Figure 9.4 excluded. The London Flooding Review Stages 3/4 Maida Vale Flood Area offers more accurate and useful information for decision-makers.
What information does GLA policy use in 2024 as to the actual full spatial extent of London's ailing and at over flow capacity Victorian combined sewers system?
Our response
Regarding Figure 9.4 - Spatial illustration of wastewater drainage capacity across London,
which is featured in the current London Plan (2021); Let me provide some clarity and additional context around this figure and the accompanying information in the London Plan:
The purpose of Figure 9.4 is to provide a high-level, strategic-scale illustration of the
wastewater drainage capacity across London. Though the map was produced by the GLA, it used data provided by Thames Water. The Thames Water data included mappable data
(Shapefiles), models and other information concerning London’s sewer system, including the Victorian-era infrastructure.
It is important to note that Figure 9.4 should be understood in conjunction with Paragraph
9.5.14 in the London Plan. This paragraph explains the key details of the underlying Thames Water model, including:
- The model compares the theoretical capacity of the drainage and sewer pipes against the current wastewater flows during a one-in-two-year rainfall event.
- The model's outputs are visualized as a "heat map," where green areas indicate relatively higher capacity and red areas indicate lower capacity to receive additional flows.
- However, it's crucial to understand that even the "green" areas may still require additional drainage infrastructure, as the model does not account for how wastewater is routed through the network.
- The hatched areas on the map represent combined sewer systems, which handle both wastewater and surface water flows.
We acknowledge that some of the data used to produce this figure may now be out of date. To address this, the London Plan team is currently undertaking an evidence-base review, which includes updating the 2018 London Regional Flood Risk Assessment. This revised assessment will seek to provide a more comprehensive and up-to-date technical report, outlining a clear roadmap for enhancing flood resilience in London, with a deeper understanding of the city's spatial wastewater drainage capacities.