Drains Flooding Heavy Rain Andover | Why It Happens

Heavy rain hits Andover and within twenty minutes some gardens are holding water, some ground-floor loos are gurgling, and at least one road on a low-lying estate is looking more like a ford. It happens often enough that people assume it's just something you put up with. It isn't, not entirely, and understanding why it happens is the first step to knowing what you can and can't fix yourself.

Surface water and foul water are not the same pipe

This is the bit that trips most people up. There are two completely separate drainage systems under your property and under the street.

Foul drainage takes everything that leaves your toilets, sinks, baths, showers, and washing machines. It flows to a sewer, then to a treatment works. It is not supposed to carry rainwater. Ever.

Surface water drainage takes rainwater from roofs, driveways, and paths. In newer developments, like Picket Twenty, East Anton, and much of Saxon Fields, this is a separate system entirely. Water off your roof goes into a surface water drain, treated as relatively clean, and discharged either to a watercourse or a soakaway.

Here is the problem: Andover's older streets, particularly around the town centre, parts of Charlton, Knights Enham, and older sections of Foxcotte, are served by combined sewers. One pipe takes everything. Foul waste and rainwater share the same underground route to the treatment works.

When a storm hits a combined sewer area, the system is doing double duty. It was designed with some overflow capacity, but it was also designed around rainfall patterns from decades ago. Pour several days' worth of rain into it in forty minutes, and something has to give. What gives is usually a garden, a driveway, or in the worst cases, someone's ground floor.

Why low ground makes it worse

Andover sits in the valley of the River Anton, and the surrounding landscape is not flat. Water moves downhill. The estates and roads that sit at the bottom of that slope, or close to the Anton's flood plain, receive not only their own rainfall but everything that runs off from higher ground around them.

Wherwell and Chilbolton, downstream from Andover, know this well. Stockbridge sits at the junction of multiple Test valley flow paths and can flood with surprising speed after rain falls further north. Even within the SP10 postcode, parts of Walworth, areas close to the Anton near Millway, and lower sections around Harroway Road can see surface water accumulate faster than drains can clear it.

This is surface water flooding, and it's largely separate from your drains being blocked. It's the ground being overwhelmed, not your infrastructure failing. Knowing which type you're dealing with matters because the solutions are different.

When it's the sewer surcharging

If rain falls and within an hour your toilet starts to bubble, your shower trap gurgles, or you see water rising from a manhole cover in your garden, that's sewer surcharge. The combined or foul sewer has taken on more than it can handle, and the hydraulic pressure is pushing back up through any available opening.

Ground floor WCs and floor gullies are the most vulnerable. Water finds the lowest point in the system first.

Surcharging on a combined network during storms is sometimes unavoidable, especially on older Andover streets where the pipes were laid in the 1950s and 60s. But it becomes dramatically worse when:

• The pipe itself is part-collapsed or heavily silted, restricting flow before the storm even starts
• Root ingress from trees, common on older Charlton streets and around established gardens in Thruxton and St Mary Bourne, has narrowed the bore
• Fat, grease, and wet wipes have built up a partial blockage that normal flow gets around but storm flow cannot
• A neighbouring property has illegally connected a surface water downpipe to the foul sewer, adding volume to a system that wasn't designed for it

Any one of those reduces the system's spare capacity. All four together means you're flooding before the storm is halfway done.

What you can control on your own property

You can't redesign Southern Water's infrastructure. But there's a reasonable list of things that sit on your side of the boundary that you can actually do something about.

Check your downpipes are connected to the right drain

On a separated system (most post-2000 Andover estates), roof water must go to the surface water drain, never the foul drain. On a combined system it technically doesn't matter, but if your downpipe is connected nowhere and water is just running across the driveway and into the street, you're adding to surface flooding. A quick visual trace from your gutters to the drain connection at ground level takes ten minutes.

Clear your gullies before the rain comes

The square grid covers in your driveway and back yard are gully pots. They sit below a water trap and collect grit, leaves, moss, and whatever else washes off hard surfaces. A blocked gully pot means surface water has nowhere to go. It backs up on the drive, or finds another way in.

Lift the cover. If it's full, it needs clearing. A gloved hand and a bucket gets most of it. For a build-up of compacted silt, high-pressure water jetting clears it properly without damaging the trap.

Know where your manhole covers are

Walk your boundary in dry weather and find every manhole cover. Note which ones are in low spots. In a storm, those are your early warning system. If a cover is lifting under pressure, the sewer behind it is surcharging. That tells you a great deal about where the restriction in the system is.

Don't add to the problem with illegal connections

A new washing machine plumbed into a convenient nearby drain outside, a downpipe connected to the foul drain to save routing it round the house. These are more common than you'd think, particularly on older properties that have had DIY plumbing over the years. They add hydraulic load at exactly the wrong moment. If you've bought a property in Andover and you're not sure where everything connects, a CCTV drain survey will map it out.

Keep your lateral drain clear

The section of drain between your house and the public sewer is your responsibility. If it's carrying a partial blockage when a storm hits, flow backs up into your property before it backs up anywhere else. Preventative jetting, done annually or every couple of years depending on what your drainage is like, removes the build-up before it becomes a crisis. For a house with trees nearby, particularly on older Andover streets with mature limes or horse chestnuts, the root ingress question is real. Roots follow water. If your lateral drain is even slightly cracked, roots will find it.

What you genuinely can't fix

If you're on a combined system and the storm is big enough, your system will surcharge. There's a threshold above which even a perfectly maintained private drain can't prevent the public sewer from pushing back. At that point, your options shift to mitigation rather than prevention.

• Flood gates or door barriers on any openings below the anticipated flood level
• Anti-flood valves fitted to your foul drainage inside the property. These are non-return valves that allow flow out but prevent surcharge pushing back in. A plumber or drainage engineer fits them; they don't stop the sewer flooding, but they stop the water reaching your floor
• Reporting repeat flooding to Southern Water and your local council. Persistent surface water flooding on a public highway is a highways drainage issue. Southern Water's sewerage obligations have a formal complaints and improvement process, not a fast one, but it does exist

Landlords and commercial premises

If you're letting residential property in Andover or running a commercial premises on one of the trading estates off the A303 corridor, heavy-rain drain failures are your liability problem, not your tenant's. A blocked lateral drain that floods a ground-floor flat in Picket Piece is your repair bill, and the clock starts the moment your tenant reports it.

The economics of a reactive call-out versus a scheduled preventative visit are not close. High-pressure jetting and an inspection visit costs a fraction of what emergency work costs when a drain has completely let go during a weekend storm. For commercial kitchens in particular, grease accumulation in drains compounds fast. One heavy rainstorm pushing back through a part-blocked kitchen drain is the kind of event that closes a business for a day.

When to call rather than wait

If water is actively backing up into your property, or a manhole cover is moving, don't wait to see if it settles. Call it in. A drainage engineer can clear the blockage, jet the lateral, and tell you whether the surcharge is coming from a private issue (your pipe) or a public one (the sewer network). Those two have different answers and different people responsible for fixing them.

If it turns out the pipe itself is cracked, root-damaged, or has collapsed, drain relining or repair will carry a 20-year guarantee on the remedial work. Jetting and blockage clearance doesn't come with that kind of guarantee, but it doesn't need to. The work is different.

Drainsco covers Andover and the SP10 and SP11 postcodes, with engineers available 24 hours a day. There's no call-out fee, and you'll get a clear price before anything starts. If you're not sure whether what you're seeing is a blocked private drain or a public sewer issue, give us a ring and we'll help you work out which it is before you commit to anything.