Flood Wall Feasibility Review: Assessing a Listed Riverside Home for Future Flood Risk

The owner of a Grade II listed home in a low-lying river valley in eastern England commissioned a Stage 1 flood wall feasibility review. The property had never flooded internally, but the rear garden was inundated regularly during river flood events, and the owner wanted to know whether a low wall around the rear patio would be a viable way to keep floodwater away from the house.

This type of commission is common among homeowners in or close to a floodplain. The trigger is often a near-miss or a worsening flood history, but the underlying need is the same: an accurate, site-specific flood risk assessment, and a realistic view of whether a defence is feasible.

The Approach

The work was carried out as a desk-based Stage 1 flood risk review, with a clearly defined Stage 2 scope recommended to follow once the concept was confirmed as viable.

The review collated the Environment Agency Flood Map for Planning and interrogated a recent EA hydraulic modelling study of the local river reach and its mill sluice structures to establish a design flood level. Because national flood maps are indicative and not suitable for property-level assessment, the modelled in-channel levels were adopted as the primary reference. Fluvial flooding was the main source of risk; surface water and the site’s field drains and ditches were also reviewed, as these connect to the main river and can surcharge during flood events.

EA LiDAR was used to establish ground levels around the property, including the rear patio at approximately 13.2 mAOD and an estimated threshold at approximately 13.35 mAOD, which were compared against the modelled flood levels across a range of return periods. Cross-referencing the largest flood on record, which reached an estimated 12.90 mAOD at the property, gave confidence in the adopted design flood level envelope of approximately 12.80 to 13.30 mAOD. As LiDAR carries a vertical tolerance, a topographic survey was identified as the critical first step of any Stage 2 commission.

mAOD – Levels are given in metres Above Ordnance Datum (mAOD), a national height reference based on mean sea level, so flood and building levels can be compared on the same scale.

Findings

Under present-day conditions, the building footprint sits outside Flood Zones 2 and 3, while the rear garden lies within Flood Zone 3. The largest flood on record, the second largest event in the catchment’s history, reached the rear garden but stopped below patio level, leaving around 450mm of freeboard to the door threshold. This was consistent with the owner’s account and confirmed that the house is not currently at high risk of internal flooding.

Freeboard is the safety margin between the highest-level floodwater is expected to reach (the design flood level) and the top of whatever is protecting the property, such as a wall or a door threshold.

The future picture is more significant. The EA climate change flood extent encroaches towards the house and across the patio, and the limited margin between patio level and the upper design level shows how modest increases in flood severity over the lifetime of the property could erode the current freeboard. On this basis, a low perimeter wall protecting to a crest of around 13.60 mAOD, roughly 400mm above patio level including freeboard, was found to be physically feasible in principle. Any wall would also need to manage rising groundwater and an internal drainage strategy to avoid trapping water within the defended area.

Two constraints shaped the advice. As a listed building, the works would require Listed Building Consent and could not rely on permitted development rights. A localised patio wall was unlikely to need an EA Flood Risk Activity Permit, given the property lies around 170m from the main river and well beyond the 8 metre buffer, but any wall extending into the flood zone would be expected to trigger flood compensation works to offset the lost flood storage. Confirming the permit position is something we cover within our environmental permits advice.

Outcome

The review gave the owner an evidence-based understanding of flood risk from all relevant sources and a clear, proportionate view that a flood wall is feasible and worth progressing. The combination of hydraulic model data, LiDAR ground levels and historic flood records produced a more reliable picture than desk-based mapping alone could provide. Recommendations covered a Stage 2 scope, including a topographic survey and hydraulic level tie-in, a site visit and ground investigation, an outline wall design and options appraisal, and early pre-application engagement with the local planning and heritage officers.

In the meantime, interim property-level resilience measures such as door barriers, water pumps and non-return valves, together with registration for EA flood warnings, were advised to reduce the risk of internal flooding before any permanent wall is in place.

If you are considering a flood wall or other flood defence for your property, our flood risk consultancy service provides site-specific assessments and options appraisals for homeowners and businesses. You can see how we have approached similar projects in our case studies, or get in touch to discuss your site.