Flood Risk Assessment — Flood Zone 3b Residential Extension with Compensatory Floodplain Storage, Oxford

Flood Risk Assessment — Ground and First Floor Extension in Flood Zone 3b, Oxford
Location: 172 Abingdon Road, Oxford, OX1 4RA, Oxford City Council | Client: Arkidraft Ltd | Services: Flood Risk Assessment

Four High-Risk Flood Sources in the Functional Floodplain

Flood Zone 3b — the functional floodplain — is where the planning bar is at its highest. At 172 Abingdon Road in New Hinksey, Oxford, the site carries high risk from four independent flood sources: fluvial, groundwater, sewer, and artificial. The proposed ground and first floor extension had to demonstrate not only that it could proceed safely in this environment, but that it would deliver like-for-like compensatory floodplain storage to offset every cubic metre of flood water displaced by the new footprint.

We were appointed by Arkidraft Ltd to prepare the FRA for Oxford City Council. This was a Revision 02 submission, prepared in direct response to the flood officer's consultation comments received on 20 April 2026 — the original submission required a compensatory storage demonstration, which became the centrepiece of the revised assessment.

Site Context and Ground Conditions

The site is a terraced residential property at grid reference E: 451607; N: 204823, in the predominantly residential area of New Hinksey. The underlying geology comprises Oxford Clay Formation and West Walton Formation (Undifferentiated) mudstone, with Northmoor Sand and Gravel Member (Lower Facet) superficial deposits. A BGS borehole just 45 metres east (reference SP50SW7) confirmed silty clay, clayey sand and medium gravel, with groundwater recorded at just 1.42m depth. Soilscapes mapping characterised the soils as loamy and clayey floodplain soils with naturally high groundwater — ruling out infiltration-based drainage.

The site is in proximity to Eastwyke Ditch, the River Thames and Hinksey Lake, with historic flooding documented in the area by the Oxford City Council SFRA (2026) and EA records.

Flood Risk — Four High-Risk Sources

Fluvial risk was high — Flood Zone 3b, with a 5% annual probability of flooding. EA Product 4 hydraulic modelling confirmed a design flood level of 56.32m AOD for the 1-in-100-year plus 30% climate change event. No formal flood defences protect the site.

Groundwater risk was high. The Oxford City SFRA (2026) confirmed that New Hinksey is underlain by floodplain soils with naturally high groundwater, and borehole data recorded groundwater at just 1.42m below ground level.

Sewer flooding risk was high — 25 sewer flood incidents recorded in the OX1 4 postcode per the SFRA.

Artificial source risk was high — the site falls within the modelled flood extent from reservoir breach on both dry-day and wet-day scenarios.

Surface water risk was the only low-risk source, with EA pluvial mapping confirming the site lies outside elevated surface water flood areas.

Compensatory Floodplain Storage — The Critical Technical Challenge

With a maximum flood level of 56.32m AOD and existing ground level at 55.89m AOD, the proposed extension footprint of 31.68m² would displace approximately 13.62m³ of flood water. To compensate this on a level-for-level, volume-for-volume basis, the extension was designed on stilts with a 430mm void beneath the structure. This ensures no flood storage is lost and the development does not increase flood risk on or off-site.

Finished floor levels were set at 56.62m AOD — providing 300mm freeboard above the design flood level and exceeding the minimum recommended under current planning practice guidance.

The development is a minor householder application, exempt from Sequential and Exception Tests under NPPF note 62.

Flood Mitigation Specification

The resilience strategy addressed all four high-risk sources: FFLs at 56.62m AOD; external ground levels profiled to direct overland flow away from entrances; solid concrete floors with resilient finishes; moisture-resistant materials for skirtings, doorframes and window sills; sealed utility conduits; waterproof concrete and sealants for below-ground structures; sump pumps for groundwater management; and non-return valves on sewer connections.

Outcome

The Revision 02 FRA satisfied the flood officer's requirements, demonstrating that the compensatory storage arrangement — 31.68m² × 430mm void = 13.62m³ — delivers a like-for-like replacement of displaced floodwater. The assessment confirmed compliance with NPPF and Oxford Local Plan Policy RE3, providing Oxford City Council with the technical evidence to support the planning application for this complex multi-risk Flood Zone 3b site.