A sudden downpour in Birmingham on 27 August overwhelmed urban drainage systems, causing flash flooding in streets and homes. The event exposed the growing challenge UK cities face in managing extreme rainfall on impermeable surfaces.
Surface water reaches sewers in minutes
Hard urban surfaces such as paved streets and car parks prevent rainwater from soaking naturally into the ground. Instead, water runs off rapidly, reaching combined sewers originally designed for much smaller volumes. This influx can cause sewers to flood and discharge untreated flows onto roads and into watercourses.
Recent dry spells and heatwaves have baked the soil harder than usual. When heavy rain arrives, ground infiltration is further reduced, increasing runoff and flood risk.
Beyond upsizing pipes
The instinct to widen pipes and enlarge underground drains often causes costly disruption. Excavating roads and verges to accommodate bigger sewer pipes is expensive and delivers limited improvement because it does not reduce the amount of surface water entering the system.
Instead, a combination of hard and soft sustainable drainage systems (SuDS) offers a better solution. Measures such as rain gardens, permeable paving and attenuation tanks capture rainfall at source, store it temporarily, and release it slowly. When retrofitted into urban catchments, these can drastically cut pressure on sewers during storms.
Small interventions add up
Installing dozens of these surface-level controls across a city can deliver meaningful reductions in flash flood risk. They intercept rainwater before it becomes runoff and overloads drainage infrastructure.
Sam Hawkins, Head of Specification and Design at ACO Water Management, explains: "We keep trying to solve the urban flooding issue by upsizing pipes, but that is the most expensive and least effective solution. The real answer revolves around using a range of SuDS, such as rain gardens and attenuation systems, to reduce the volume of water that makes it to the sewer system. The challenge here is that SuDS uptake remains inconsistent."
Policy gaps slow progress
SuDS uptake in England is hindered by the absence of Schedule 3 in the Flood and Water Management Act. Since this regulation has not been enacted, sustainable drainage remains a matter of policy rather than mandatory law. Adoption and enforcement vary widely between authorities and developers.
Hawkins adds: "Importantly, the same systems that protect us in a downpour also recharge groundwater during drought. As we swing harder between the two extremes, designing for both has become essential."
Tackling climate extremes
With the UK climate presenting hotter dry spells punctuated by more intense rain events, managing surface water has become critical. Sustainable drainage systems can reduce flood risk, protect urban infrastructure, and support groundwater replenishment.
ACO Water Management specialises in surface drainage solutions designed for UK conditions. For more information, visit aco.co.uk.
