Surface water flooding, and the drainage you can prove

August is the month the drainage gets tested. Not by weeks of rain, but by an hour of it landing on ground that cannot take it fast enough.

That is surface water flooding, and it is now the largest single flood exposure in the country. The Environment Agency's latest National Flood Risk Assessment puts 4.6 million properties in England and Wales inside areas at risk from surface water, a 43% increase on the previous national picture, with roughly three times as many properties at high risk from surface water as from rivers and the sea combined (GOV.UK, "Advancing national capability in surface water flood forecasting", April 2026).

The increase is worth reading carefully. The Environment Agency has been clear that the jump comes from better data and better modelling rather than from risk appearing overnight. A large number of sites did not become more exposed. They were always this exposed, and the map has caught up.

Across all sources, 6.3 million properties in England sit at risk of flooding today, rising to around 8 million by the middle of the century, alongside roughly half the rail network and a third of the road network (Environment Agency, July 2026).

Why this one lands on the site, not the building

River and coastal flooding is largely a question of where a building is. Surface water is different, because a meaningful part of it is a question of how the site is maintained.

Water arrives faster than the ground and the drainage can move it. Where it goes next depends on things that sit squarely inside a property manager's control: whether the gullies are clear, whether the channel drain across the car park entrance is silted, whether the attenuation basin still holds its design volume or has slowly filled with sediment and self-seeded willow, whether the outfall to the watercourse is blocked by a fallen branch nobody reported.

The failure points are usually mundane and usually known to somebody. The threshold into a bin store that sits two courses lower than everything around it. The ramp down to a basement car park with a single blocked ACO drain at the bottom. A substation or a lift pit at the lowest point on the site. A neighbouring development that changed the levels three years ago and moved where the water collects.

None of that is exotic engineering. It is a maintenance regime and a record of it.

The question that gets asked afterwards

After a surface water event, the sequence is fairly predictable. The insurer instructs a loss adjuster. The adjuster asks when the drainage was last cleared and inspected, by whom, and what was found.

There are three possible answers. The good one is a dated record with a contractor named and photographs attached. The poor one is a date without evidence. The expensive one is some version of "the grounds contractor does that, I think", which is where a claim starts becoming an argument about whether the damage was caused by rainfall or by a maintenance failure.

The same applies to SuDS features. A permeable surface, a swale, a basin or a below-ground crate system carries an ongoing maintenance obligation, usually written into the planning permission or a section 106 agreement, and often specified in a maintenance and management plan that was submitted years ago by somebody who has since left. Where drainage has not been formally adopted, that obligation stays with the site indefinitely. The plan exists. Whether anyone has read it recently is the real question.

What to hold on the record for each site

Six things, and each one is either written down or it is not.

The drainage layout, showing runs, gullies, chambers, any below-ground storage and the point of discharge. Where the as-built differs from the approved drawing, the as-built is the one that matters.

Who is responsible for each part of it. Adopted by the water company, adopted by the highway authority, or private and therefore yours. A great deal of confusion after a flood comes from nobody having established this before one.

The maintenance schedule, with frequencies that match the design documents rather than the ones that fitted the budget, and a named owner for each task.

The inspection history, dated, with what was found and what was done about it. Gully clearance, silt removal, jetting, vegetation management on basins and swales.

The site's flood risk position from the national mapping, checked rather than assumed, together with any flood risk assessment prepared for planning and any resilience measures actually installed.

The insurance position, including what the policy expects you to be doing by way of maintenance, because that expectation is the standard your records will be measured against.

Where the record lives

This is a land and site problem, so it belongs to the site rather than to any one building.

In SAMRISK the drainage network, the SuDS features and their maintenance obligations sit in drainage and SuDS against the site, with the buried runs and chambers held in subsurface services so that the layout is one record rather than a folder of PDFs. Flooding sits in site risks alongside the rest of the land and site picture, which is where the exposure is scored and reviewed rather than remembered. Gully clearance and basin management are recurring maintenance jobs with owners and completion dates, chained off the compliance calendar so the frequency does not quietly slip a year. The drainage strategy, the maintenance and management plan and the flood risk assessment are versioned files in documents, and anything that does happen is logged in incidents with the photographs attached to it.

A wet hour in August is not preventable. Arriving at the loss adjuster with a dated inspection history, a drainage layout that matches the ground, and a clear answer on who owns which pipe, largely is.

Sources: GOV.UK, "Advancing national capability in surface water flood forecasting", April 2026; Environment Agency, National Flood Risk Assessment (NaFRA2); Environment Agency, "How climate change is impacting the way flooding is managed", July 2026.