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Drains Flooding In Heavy Rain
Gullies, chambers or the garden flood every time there is a downpour, then clear once the rain stops.
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What's happening?
Flooding that only happens in heavy rain is a capacity problem, not a blockage — or at least, not only a blockage. UK drainage is meant to keep foul and surface water apart wherever it can: rainwater to a soakaway, a surface water sewer or a watercourse, and foul to the foul sewer. Where the two share a combined sewer, which is common in older towns, a downpour fills the sewer and it surcharges, pushing water back up the connections into gardens and low-lying gullies. The other common causes are local: a soakaway that has silted up and no longer takes water, downpipes wrongly connected into the foul system, or paving laid over what used to be soft ground so there is more runoff and nowhere for it to go.
What usually causes it?
- A combined sewer surcharging during heavy rainfall, backing water up private connections
- A soakaway that has silted, filled or was never sized for the roof area
- Rainwater downpipes connected into the foul drain, overwhelming it in a storm
- Driveways and patios paved without permeable surfacing or drainage, increasing runoff
- Gullies and channel drains blocked with leaves and silt at exactly the wrong moment
- A partially blocked or collapsed surface water run that copes in dry weather and not in wet
- Ground water rising through a drain in an area with a high water table
Is it dangerous?
High, because of where the water ends up. Surcharge from a combined sewer is diluted foul water, so garden flooding is contaminated, and water rising to threshold level puts the house at risk. Repeated flooding also saturates the ground next to foundations and finds its way into airbricks, cellars and under floors. Standing water over an unseen chamber or a washed-out void is a physical hazard too.
Can I fix it myself?
Maintenance and observation. Clear gullies, channel drains and gutters before autumn and again before winter — a leaf-blocked gully turns a manageable downpour into a flood. Check where each downpipe goes: a pipe entering a foul gully rather than a rainwater one is a common and correctable fault. Photograph the flooding while it is happening and note the date and how heavy the rain was, because that record is what your sewerage company and your insurer will ask for. Check the property's flood risk on the GOV.UK service so you know what you are dealing with.
When should you call a drainage engineer?
Call a drainage engineer to survey the surface water system if flooding recurs, to find whether the soakaway has failed or a run is defective, and to check for wrong connections. Report surcharge flooding to your sewerage company every time it happens: repeated reports are how properties get onto flood mitigation registers, and where the public sewer is the cause the remedy is theirs. If the flooding involves a ditch or a watercourse crossing your land, riparian responsibilities apply and are worth reading before you commission work.
What will the drainage engineer actually do?
- Survey the surface water drainage and establish where each downpipe and gully actually discharges
- CCTV the surface water runs to find silting, root ingress or collapse
- Excavate, clean or rebuild a failed soakaway, sized for the roof area it serves
- Separate wrongly connected rainwater from the foul system
- Fit non-return or anti-flood devices where the property is subject to sewer surcharge
- Install additional channel drainage or a pumped solution where levels leave no gravity option
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