Ofwat report on future sewer flooding

Last month Ofwat published a report investigating future impacts on sewer systems in England and Wales. A hydraulic modelling exercise was undertaken reviewing the impact of three factors on urban areas.

The report investigated the change in sewer flooding that may result from climate change, urban creep, and new population and housing growth up to about 2040. Unsurprisingly, the study predicts an increase in sewer flooding with the existing network capacity. Each change modelled produces an increase when considered in isolation, apart from for the driest winter storm, with the ‘combined’ effect greater than individual changes. At the 50^th percentile, a 51% increase in sewer flooding is predicted.

Climate change had the largest effect when modelled individually, with 27% median increase in flooding. This confirms the current practice in new design of adding an allowance for climate change, but there is variability for different storm durations. Could we apply better targeted climate change variations in design?

Next largest was urban creep, with a median increase of 12%. Currently standard practice in England and Wales does not include an allowance for this in design guidelines. However, both Sewers for Scotland 2^nd Edition and Sewers for Adoption Northern Ireland 1^st Edition suggest a 10% increase in paved areas to account for future expansion. It will be interesting to see if either Sewers for Adoption 7^th Edition for England and Wales due to be released in September this year or the National Standards for SuDS will follow suit.

Finally, urban growth has a smaller but still significant effect, seeing a 5% median increase in sewer flooding. It is anticipated that enactment of the sustainable drainage section of the Flood and Water Management Act 2010 next year will decrease surface water connections to the sewer. However, foul flows will still need to be accommodated and this is recognised by the Ofwat report in its Suggestions for Adaption Strategies section.

Perhaps there is an opportunity in design to help mitigate some of these effects? Design of new surface water drainage has the potential to minimise discharge to sewers by incorporating sustainable drainage systems (SuDS) where possible. Designing retrofit drainage to reduce the existing sewer discharge will also have a positive effect.

Finally, engineers can use best practice guidance and tools already available to add allowances for climate change and urban creep to ensure we are building adaptable drainage protected from future impacts.

The Ofwat report can be downloaded by clicking here.