Flood Risk Assessment — Flood Zone 3a Householder Extension, Riverside, Twickenham

Flood Risk Assessment for a Householder Extension on the Tidal Thames at Riverside, Twickenham
Location: Dial House, Riverside, Twickenham, TW1 3DT | Client: Annie Shephard | Services: Flood Risk Assessment

Building on the Riverside in Flood Zone 3a

Riverside at Twickenham is one of the more demanding flood risk settings in Greater London: a row of historic properties fronting directly onto the tidal Thames, within the highest-probability flood zone, in a borough that scrutinises riverside development closely. For a householder extension the planning question is not whether flood risk exists — it plainly does — but whether the proposal is proportionate, whether it worsens anything, and whether the occupants can be kept safe.

We were appointed by Annie Shephard to prepare a site-specific Flood Risk Assessment for a householder extension at Dial House, Riverside, Twickenham, TW1 3DT, in the London Borough of Richmond upon Thames, in support of the planning application.

Under note 62 of the National Planning Practice Guidance, minor householder extensions are exempt from the Sequential and Exception Tests. That exemption does not remove the requirement for a site-specific FRA. It shifts the assessment onto three things: the design flood level and the freeboard achieved against it, the resilience of the construction, and the arrangements for warning and evacuation.

Evidence Base

We drew on the Environment Agency Flood Map for Planning, the tidal Thames modelled levels and defence information held for this reach, the Risk of Flooding from Surface Water and reservoir inundation mapping, BGS geological mapping, LiDAR topographic data for the plot and the riverside frontage, the Thames Water sewer flooding record for the postcode, and the Richmond upon Thames Strategic Flood Risk Assessment.

Site Characteristics and Flood Risk Profile

The site occupies a riverside plot with ground levels falling gently toward the Thames. The tidal reach here is defended, but the residual risk in a defence overtopping or breach scenario is what places the property in Flood Zone 3a rather than a lower zone.

Fluvial and tidal flooding is the governing source and is assessed as high. Flooding from artificial sources, principally the residual risk associated with upstream impounded water, is assessed as medium. Surface water risk is low, with no significant modelled ponding across the plot. Groundwater risk is low. Sewer flooding risk is low, with no incidents recorded at the address.

Flood Mitigation and Resilience

The mitigation package was built around three principles.

First, finished floor levels within the extension are set to provide freeboard above the modelled 1-in-100-year plus climate change flood level, in line with Environment Agency guidance for the tidal Thames.

Second, the construction is specified to be flood resilient rather than merely flood resistant: solid floors, waterproof concrete to below-ground elements, moisture-resistant plaster and insulation, raised electrical services and sealed service penetrations, so that any water that does enter can be cleaned down and the property returned to use quickly. On a defended tidal reach, where the flooding that reaches the property is residual and short-lived rather than routine, resilience delivers more for the occupants than an attempt to exclude water entirely from a historic riverside building.

Third, the extension is designed so that it does not reduce floodplain storage or impede conveyance across the site, which is the test that protects neighbouring properties from any transfer of risk.

Warning and Evacuation

Occupant safety is addressed through registration with the Environment Agency Floodline warning service and a defined route to safe refuge at first floor level. That is the appropriate response for a defended tidal reach where warning lead times are measurable in hours rather than minutes, and where evacuation across a flooded riverside frontage would expose occupants to greater hazard than remaining above the flood level in a substantial masonry building.

Outcome

The FRA demonstrated that a minor householder extension can proceed at a Flood Zone 3a riverside address without increasing flood risk on or off the site, and that occupants can be kept safe for the lifetime of the development. Richmond Council was provided with a proportionate assessment matched to the scale of the proposals and to the tidal Thames setting.