Flood Risk Assessment and SuDS Strategy — Flood Zone 2 Residential Development, Twickenham
We provided a combined flood risk assessment and SuDS strategy for a lower ground floor extension at 4 Riverdale Road, Twickenham — a Flood Zone 2 site near the Thames with high groundwater risk, achieving 50% betterment in peak discharge.
Flood Risk Assessment and SuDS Strategy — Lower Ground Floor Rear Extension in Flood Zone 2, Twickenham
Location: 4 Riverdale Road, Twickenham, TW1 2BS, London Borough of Richmond upon Thames | Client: Daniel James Construction Services Limited | Services: Flood Risk Assessment, SuDS Strategy
Flood Zone 2 Near the Thames — With a SuDS Challenge
Riverdale Road sits within one of Twickenham's more flood-sensitive corridors. At number 4, the Flood Zone 2 designation reflects medium fluvial risk from the Thames, but the real technical complexity lay in two areas: the high groundwater risk that needed addressing for a lower ground floor extension, and a SuDS strategy that had to deliver measurable betterment on a constrained residential plot.
We were appointed by Daniel James Construction Services Limited to prepare a combined FRA and SuDS Strategy for a single-storey lower ground floor rear extension and associated garden works. The existing flat at lower ground floor level already contains bedroom accommodation, with escape available via the rear of the proposed extension.
Flood Risk Assessment
Fluvial risk was classified as medium — Flood Zone 2. Surface water risk was low. Groundwater risk was high, driven by the local geology and the Richmond SFRA's groundwater susceptibility mapping. Sewer flooding risk was low, and artificial source risk was medium. The site draws on the Richmond SFRA (2021), the adopted Local Plan (2025), and the Local Flood Risk Management Strategy (2023).
SuDS Strategy — 50% Betterment
The proposed extension created a net increase in impermeable area of approximately 30m². The SuDS strategy centred on a permeable paving courtyard with a sub-surface granular attenuation layer. The courtyard has a surface area of 11.2m² and incorporates a 300mm depth granular drainage layer with 25% void ratio, providing approximately 0.84m³ of attenuation storage. Designed to the 1-in-100-year storm with 40% climate change allowance, the scheme achieved a 50% betterment in peak discharge rate.
Outcome
The combined FRA and SuDS Strategy demonstrated full NPPF compliance and provided Richmond Council with the technical evidence to support the planning application.