Jon Fawcett, Head of Technical at Keyline Civils Specialist, discusses how attenuation systems can relieve pressure on existing drainage infrastructure.
chaos, business closures, and communities rocked by flooding.
Extreme rainfall events are becoming commonplace due to climate change, and a growing population brings its own demands on drainage capacity – understandably, a system never designed for such stressors is starting to falter under the pressure.
Victorian drainage systems run through most of our towns and cities – with some estimates putting combined sewers at one fifth of the total drainage network – and many have done an exceptional job at managing wastewater for over 150 years. They remain a key cornerstone of our infrastructure, but we must alleviate the pressure placed on them to protect communities – attenuation systems offer a cost-effective, sustainable way to do so.
A network under pressure
Engineers in the 19th and early 20th centuries did, to an extent, futureproof systems to cope with population growth, but could never have anticipated the unique challenges faced by a modern drainage system.
Not only has the population grown – London’s population, for example, has tripled since the city’s network was largely completed in 1868 – the amount of rainfall itself has also increased. According to a Royal Meteorological Society report, using data from 2015-2024, winter is now 16% wetter than 1961-1990 for the UK.
Add a lack of permeable surfaces in urban environments and you have a mix that inevitably overwhelms drainage systems, making the UK more vulnerable to flooding.
It wreaks havoc on communities, and costs the UK an estimated £2.4 billion annually, according to a Public First report. Prevention, as usual, is much cheaper than cure. DEFRA suggests that for every £1 spent on flood protection, £8 is saved in damages. Investing in flood prevention offers excellent ROI – but how can we maximise that value?
Relieving the burden with attenuation systems
Attenuation systems offer a cost-effective method to take the pressure off. Older sewer systems often rely on combined sewer overflows (CSOs) to discharge excess water into rivers and waterways during heavy rainfall.
This is increasingly unacceptable from both an environmental and regulatory perspective. Water companies are under significant pressure to reduce CSO spills, with AMP8 investment frameworks allocating substantial capital toward drainage improvements and overflow reduction. Attenuation systems represent a cost-effective intervention that helps water companies meet regulatory obligations while protecting Victorian infrastructure – making them an attractive option within constrained capital programs.
Attenuation systems typically use either pre-cast concrete blocks or modular geocellular systems to store water and feature a flow control device to slow the release back into waterways.
These devices vary in sophistication; an orifice plate is the simplest, while vortex controls or penstocks are better suited to more demanding applications, and require little maintenance. They all help reduce the risk of flooding and can be specified according to respective budgets and conditions.
At any rate, attenuation systems are more cost-effective than a like-for-like replacement – where Victorian drainage systems are made of expensive cast iron, geocellular, modular units are made of plastic.
It’s cheaper, easier to install, and often made from 100% recycled material; with a 100+ year design life, they offer a long-term solution to a critical infrastructure problem.
Working alongside existing infrastructure
Ripping out and replacing kilometres of drainage is unfeasible – and unnecessary. Attenuation systems provide a non-disruptive, complementary solution.
Victorian systems are the foundation of urban drainage – we need to support these networks to ensure they work as effectively for the next century as they did their first. Strategically adding sustainable drainage systems like attenuation to areas most vulnerable upstream of CSOs has a two-fold benefit: effectively extending the functional life of the network, and offering better protection for communities from flooding.
And with flooding only worsening year on year, it’s critical to act now.
