Flood Risk Assessment and SuDS Strategy — Residential Extension with Groundwater Risk, Popes Grove, Twickenham
We prepared a combined Flood Risk Assessment and SuDS Strategy for a residential extension at 96 Popes Grove, Twickenham — a Flood Zone 1 site in the Thames corridor at Richmond upon Thames, on behalf of Kevin Eley. Fluvial, surface water, sewer and artificial-source risks were all low, but groundwater was rated medium on the Thames floodplain deposits beneath the site, which removed full infiltration as a reliable disposal route. The scheme reduces impermeable area from approximately 300m² to 270m² and adds two SuDS features: a 280 litre water butt for source control and 30m² of permeable paving over a 300mm drainage layer at 25% porosity, giving 2.25m³ of sub-surface attenuation. The network is designed to the 1-in-100-year event with a 40% climate change allowance, and the assessment confirmed that the extension does not increase flood risk to surrounding properties.
Flood Risk Assessment and SuDS Strategy for a Residential Extension on a Groundwater-Sensitive Thames-Side Plot
Location: 96 Popes Grove, Twickenham, TW1 4JX | Client: Kevin Eley | Services: Flood Risk Assessment; SuDS Strategy
A Low-Risk Flood Zone with a Groundwater Caveat
Popes Grove sits within the Thames corridor at Twickenham, close enough to the river for groundwater behaviour to matter even where fluvial risk is formally low. That distinction is the point of a site-specific assessment. A Flood Zone 1 designation removes the Sequential and Exception Tests from the equation, but it says nothing about how a shallow water table will interact with a new foundation, a lowered threshold or an infiltration feature.
We were appointed by Kevin Eley to prepare a combined Flood Risk Assessment and SuDS Strategy for a residential extension at 96 Popes Grove, Twickenham, TW1 4JX, in the London Borough of Richmond upon Thames, in support of the planning application. At this site the medium groundwater rating meant that soakaway-led drainage could not be assumed to work, and that below-ground construction had to be specified with water ingress in mind.
Evidence Base
We drew on the Environment Agency Flood Map for Planning and the Risk of Flooding from Surface Water mapping, BGS 1:50,000 geological mapping and the borehole records held for the surrounding area, LiDAR topographic data for the plot and its immediate catchment, the Thames Water sewer flooding history for the postcode, and the Richmond upon Thames Strategic Flood Risk Assessment, which remains the policy authority for groundwater and sewer flooding in the borough. Each of the six flood sources was rated against that evidence rather than against the flood zone designation alone.
Site Characteristics and Flood Risk Profile
The site occupies an established residential street of two-storey dwellings with private rear gardens. There are no ordinary watercourses within the immediate vicinity, and topography across the plot is effectively level, consistent with the surrounding urban fabric.
Fluvial and tidal risk are low. The site sits outside the 1-in-100-year plus climate change extent on the Flood Map for Planning, and no historic flood events are recorded at the address. Surface water risk is low, with no significant modelled ponding across the plot and no overland flow route crossing the site. Sewer flooding risk is low, with no incidents recorded at the development address in the Thames Water flooding history data. Reservoir and other artificial-source risk is low.
Groundwater is the exception. We placed the site in a medium groundwater risk band, reflecting the local hydrogeology of the Thames floodplain deposits and the recorded groundwater levels in the wider area. That rating governs both the position the scheme takes in the drainage hierarchy and the resilience specification.
Surface Water Management Strategy
Because the development reduces impermeable area from approximately 300m² to 270m², the drainage strategy is one of betterment rather than mitigation of an increase. Two SuDS features were specified. Rainwater harvesting provides 280 litres of storage in a water butt, giving source control at the head of the system and a reuse benefit for the garden. Permeable paving covers 30m² over a 300mm drainage layer at 25% porosity, providing 2.25m³ of sub-surface attenuation storage.
Together these reduce peak surface water discharge and sit correctly within the drainage hierarchy for a site where full infiltration cannot be relied upon. The surface water network is designed to the 1-in-100-year storm event with a 40% allowance for climate change, which is the allowance applicable to this Thames management catchment for the lifetime of a residential development.
Construction and Resilience
Below-ground elements are specified in waterproof concrete with sealed service penetrations, and moisture-resistant internal finishes are recommended at ground floor level in response to the groundwater rating. Electrical services within the extension are raised, and sealants are specified at construction joints. These are modest measures in cost terms, and they are the proportionate answer to a medium rather than a high groundwater rating.
Outcome
The assessment demonstrated that the extension is at an acceptable level of flood risk for its lifetime and that flood risk to the site and to surrounding properties is not increased. The reduction in impermeable area, combined with source control and sub-base attenuation, gives Richmond Council a proportionate and evidenced drainage position for a minor householder scheme on a groundwater-sensitive Thames-side plot.