Sewer flooding records and what they tell you

Sewerage undertakers hold a register of properties they know are at risk of sewer flooding, and in England that history is obtainable. For a site where surface water or foul drainage will connect to a public sewer, those sewer flooding records in planning terms are often the strongest evidence you can put in front of a lead local flood authority, because they describe what the network has actually done rather than what a model predicts.

The records an undertaker holds

Three sets of information are worth asking for, and they are not the same thing.

The flooding risk register, long known in the industry by its regulatory reporting code, lists properties assessed as at risk of internal or external sewer flooding at a stated return period. Entries distinguish internal flooding, where effluent enters a building, from external flooding, where it surfaces in a garden, yard or road.

The incident history is separate. It records reported flooding events with a date, a cause and an outcome, and the cause matters more than the count. Hydraulic overload means the sewer lacked capacity in the rainfall event. Blockage, collapse, equipment failure and third-party action mean something else entirely, and a run of blockage incidents on a flat shared drain says nothing about network capacity.

The sewer records themselves, which undertakers must keep available for inspection under section 199 of the Water Industry Act 1991, give you the pipe layout, diameters, materials and whether the system is combined, foul or separate. Those three documents together let you say whether a connection is sensible.

How to request the history

Start with developer services. Every undertaker runs a pre-development or pre-planning enquiry service that returns sewer records, capacity commentary and often an indication of flooding history for the catchment, and it is the quickest route because the output is formatted for planning use.

Where the response is thin, there is a second route. Sewerage undertakers are treated as public authorities for the purposes of the Environmental Information Regulations 2004, so a request for environmental information about sewer flooding in a defined area can be made under those Regulations, with the usual 20 working day period for a response.

Expect the data to come back aggregated. Addresses are usually withheld or reduced to a postcode sector, because individual property flooding history is personal data. That is normally enough for a planning argument, which is about the catchment rather than one house.

Reading internal and external entries

An internal flooding entry is serious evidence. It means foul water has entered a habitable building from the public sewer, and a cluster of them downstream of your site will shape what the undertaker will accept.

External entries are more common and more ambiguous. Surcharging from a manhole in a road during an extreme event is not the same as a garden filling with sewage twice a year, and the register entry alone will not tell you which. Ask for the return period attached to the entry and the date range of the incidents behind it.

Watch the geography. Records are catchment-based, so an entry 400 m away on the same trunk sewer is more relevant to your connection than one 50 m away on an unconnected branch. Plot the entries against the sewer record rather than against a street map, and the pattern usually resolves itself.

Using the records in a drainage assessment

The records do two jobs in a Drainage Strategy. They justify the discharge hierarchy decision, and they set the design constraint for any connection you do propose.

On hierarchy, national policy and the non-statutory technical standards for sustainable drainage expect surface water to go to the ground first, then to a surface water body, then to a surface water sewer, with a combined sewer last. Documented hydraulic flooding downstream is the clearest possible support for infiltration or an attenuated outfall to a watercourse instead. Our note on which rainfall dataset to cite in a drainage calculation covers the rainfall inputs that sit alongside it.

On design, a history of surcharge means the receiving sewer is under pressure, so a restricted discharge rate, attenuation storage sized for a surcharged outfall, and a non-return valve or pumped connection become sensible rather than cautious. Where foul capacity is the issue, the evidence belongs in a Foul Sewage Assessment and will frame what the undertaker says about reinforcement.

Where the site is also in a mapped flood zone, keep the two strands separate in the Flood Risk Assessment. Sewer flooding is a source of flood risk in its own right and is often omitted from the sources section entirely.

What the records mean for your connection

Section 106 of the Water Industry Act 1991 gives a right to connect to a public sewer, and section 94 places a general duty on the undertaker to maintain and improve the system. In practice the undertaker can impose conditions on the connection and can require the discharge to be restricted or phased while reinforcement is programmed.

That is where a Grampian condition appears, tying occupation to the completion of off-site sewer works. For a developer the question is the programme, because reinforcement schemes run on the undertaker's investment cycle rather than yours. Asking early whether any reinforcement in the catchment is already funded is worth more than arguing about capacity later.

If the answer is that no works are planned, the options narrow to reducing the discharge, storing on site, or a private treatment solution. Each has a cost that belongs in the appraisal before the land is bought.

Where the records understate the problem

Be honest about the limits of this evidence, because a consultee will be.

• The register records known risk. Properties never reported as flooded do not appear, so absence of entries is weak evidence of safety.
• Residents in rented or managed stock report less often, so some catchments are systematically under-recorded.
• Entries can be removed after mitigation works, which means a historic problem may have vanished from the current register.
• Causes are assigned at the time by a field operative, and hydraulic overload is sometimes logged as a blockage.
• Climate change allowances mean a sewer that coped historically may not cope across the development's design life.

So present the records as one line of evidence rather than the whole case. The strongest submissions put the flooding history next to the undertaker's own capacity commentary, the topographic survey and the modelled surface water flow paths, and show that all four point the same way. Our note on managing runoff on a steeply sloping site deals with the case where they do not.

PlanScope drafts drainage and flood documents around the records and calculations you provide, and marks any missing site data rather than filling it in. For specialist technical reports, qualified professional input may be required. Reports should be reviewed before submission, and PlanScope does not guarantee planning approval.