Evidencing a history of sewer flooding on your site

Sewer flooding evidence is the weakest part of most flood risk assessments, and officers know it. The maps cover rivers, the sea and surface water run-off. They do not show the night the combined sewer surcharged and came up through the gullies in the car park. National guidance asks for flood risk from all sources to be assessed, so an assessment that deals with fluvial and surface water risk and then says there is no known history of sewer flooding, without saying who was asked, is incomplete on its face.

The records exist. They sit with the sewerage undertaker and the lead local flood authority, and both will release something if the request is framed properly.

Why historic sewer flooding changes a decision

Three consequences follow from a known history, and they are the reason the question gets asked.

First, finished floor levels. Where a sewer has surcharged to cover level, a basement or a ground floor at grade is exposed, and the drainage design needs levels, non-return protection or a pumped arrangement rather than a gravity connection.

Second, the connection itself. An undertaker that has recorded hydraulic overload in the catchment may refuse a surface water connection, cap the discharge rate, or require off-site reinforcement before accepting flows. That changes the drainage strategy and sometimes the programme.

Third, the planning balance. Where neighbours report flooding, the authority has to deal with it, and an assessment that engaged with the history and designed around it is far easier to defend at committee than one that denied it.

What the sewerage undertaker holds

Sewerage undertakers record flooding incidents on their networks, distinguishing internal flooding of a building from external flooding of a garden, yard or highway, and distinguishing hydraulic overload from other causes such as blockage, collapse or equipment failure. The distinction matters: a blockage is an operational event, while repeated hydraulic overload points to a capacity problem that is relevant to your site.

Undertakers also maintain registers of properties at risk of internal sewer flooding, a dataset long known in the industry by its old regulatory reporting code. Properties move on and off that register as the network is improved, so the current position and the history are different questions, and you want both.

Alongside the incident data, the developer services team holds network models, asset plans and capacity assessments for the catchment. That is the material that tells you whether your proposed connection point has headroom.

Making the request and what comes back

Write to the undertaker's developer services or pre-development enquiry function rather than the customer contact centre. Set out the site address, a plan with the red line, the proposed connection points if known, and the catchment or sewer reference if you have the asset plan already.

Ask for five things.

1. Recorded flooding incidents within a defined radius or within the drainage catchment serving the site, for the longest period held, split between internal and external and by cause.
2. Whether any property in that area is currently recorded as at risk of internal flooding due to overloaded public sewers.
3. Modelled capacity at the proposed connection point, and any known hydraulic deficiency downstream.
4. Any planned capital scheme in the catchment and its programme.
5. The asset plan showing public sewers, their size, material and approximate depth.

Expect the response to be aggregated. Incident data is released by postcode sector or catchment rather than by address, because naming affected properties would disclose personal data. That aggregation is usually enough for a flood risk assessment, and the response letter itself is the evidence that the enquiry was made.

A drainage and water search of the kind used in conveyancing also reports whether a property is at risk of internal flooding from overloaded public sewers. It is quicker than a developer enquiry and far narrower, so use it as a first check rather than as the answer.

Lead local flood authority records

The lead local flood authority has a duty under section 19 of the Flood and Water Management Act 2010 to investigate flooding in its area where it considers it necessary or appropriate, and to publish the results. Those section 19 reports are the single most useful local document when they exist, because they usually identify the mechanism, which is often an interaction between surface water run-off, highway drainage and a surcharging sewer rather than any one of them alone.

Also ask for the authority's flood incident records, its local flood risk management strategy, and whether the site sits in a critical drainage area. Many authorities hold a register of historic flooding reported by residents that never reached a formal investigation.

Highway drainage records sit with the highway authority, which may be the same body or may not, and the division of responsibility between a highway drain, a surface water sewer and a combined sewer is often the crux of the matter. That boundary is worth understanding before the meeting, because who takes the flow from surface water and highway drainage determines who has to fix anything.

Resident accounts and how much weight they carry

Neighbour evidence is admissible and sometimes decisive, and consultants are too quick to dismiss it.

An officer reading an objection that describes sewage backing up into a downstairs lavatory twice in five years, with dates and photographs, will give it weight even where the undertaker's records show nothing. Records are incomplete: incidents go unreported, or are logged against a neighbouring address, or were attributed to a blockage when the blockage was itself caused by low flows in an overloaded length.

Deal with it directly. Set out what was reported, by whom in general terms, when, and what the recorded data shows. Where the accounts and the records conflict, say so and design for the worse case. Where the accounts describe a mechanism the design has already addressed, point at the drawing. Silence reads as evasion.

Do not name individuals or reproduce addresses in a public document.

Designing for surcharge

The design response is ordinary engineering once the history is known.

Set the hydraulic design to assume the receiving sewer surcharges to cover level in the design event, and check that no part of the proposed system floods internally as a result. Where a gravity connection cannot be protected, use an anti-flooding device or a pumped connection with storage above the surcharge level, sized for the duration the undertaker's model indicates. Keep foul and surface water separate, and resist any proposal to connect new surface water to a combined sewer, which the undertaker will refuse in most catchments anyway.

Set finished floor levels with freeboard above the surcharge level rather than above ground level. Route exceedance flows deliberately, away from doorways and basement entrances, and show the route on the drawing. Where a basement is proposed, the protection has to be designed rather than assumed, and the ventilation and escape arrangements interact with it.

All of this belongs in the drainage strategy, with the surcharge assumption stated as an input so a reviewer can test it. Where the undertaker's own model has been relied on, the same discipline applies as in reviewing a hydraulic modelling report line by line.

Writing the history into the assessment

In the flood risk assessment, give sewer flooding its own subsection under sources of flood risk, not a single line in a table. Say who was asked, on what date, what reference the response carries, and what it said. Attach the response as an appendix.

An illustrative example of the wording that works: the sewerage undertaker's response, dated and referenced, confirmed a number of recorded external flooding incidents attributed to hydraulic overload within the catchment over the period held, with no property currently recorded as at risk of internal flooding, and the lead local flood authority's published section 19 report identified surcharge of the combined sewer acting with highway run-off at the junction downstream. Then state what the design does about it.

Where residual risk remains, the authority may secure a flood response plan by condition, and it is better to offer one than to have it imposed in terms you have not read. The practicalities of flood response plans secured by planning condition are worth checking before drafting the heads of terms.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.