Flood Risk Assessment and SuDS Strategy — Dwelling Replacement with High Surface Water Risk, East Sheen
We prepared a combined Flood Risk Assessment and SuDS Strategy for the demolition and replacement of a dwelling at 2B Vicarage Drive, East Sheen — a Flood Zone 1 site that the Richmond Surface Water Flood Risk mapping (2021) nonetheless places in a high-risk band, on behalf of Richard Simm. Surface water is the governing source and was assessed as high; groundwater was rated medium, which limits reliance on infiltration as the sole disposal route; fluvial, sewer and artificial-source risks are low. The drainage strategy reduces impermeable area from approximately 290m² to 200m² and layers four SuDS components: 11m² of extensive green roof, 450 litres of rainwater harvesting, 115m² of permeable paving and a 4.1m³ soakaway. The network is designed to the 1-in-100-year event with a 40% climate change allowance, delivering a measurable reduction in peak discharge.
Flood Risk Assessment and SuDS Strategy for a Replacement Dwelling on a High Surface Water Risk Plot in East Sheen
Location: 2B Vicarage Drive, East Sheen, London, SW14 8RX | Client: Richard Simm | Services: Flood Risk Assessment; SuDS Strategy
Flood Zone 1 Is Not the Whole Story
A replacement dwelling is a more demanding proposition than an extension. The whole building footprint, the whole drainage arrangement and the whole external surface treatment are in play, which means the Local Planning Authority is entitled to expect a genuine improvement rather than a like-for-like continuation.
At 2B Vicarage Drive that expectation met a site where the Richmond Surface Water Flood Risk mapping (2021) shows a high-risk band across part of the plot, notwithstanding the Flood Zone 1 designation for rivers and the sea. This is the situation the SuDS hierarchy was written for. The governing risk is pluvial, the site is being comprehensively rebuilt, and there is enough external area to deliver a layered drainage train rather than a single end-of-pipe device.
We were appointed by Richard Simm to prepare a combined Flood Risk Assessment and SuDS Strategy for the full demolition of the existing dwelling and its replacement at 2B Vicarage Drive, East Sheen, London, SW14 8RX, in the London Borough of Richmond upon Thames.
Evidence Base
We drew on the Environment Agency Flood Map for Planning and Risk of Flooding from Surface Water mapping, the Richmond upon Thames Surface Water Flood Risk mapping (2021) and the borough Strategic Flood Risk Assessment, BGS geological mapping and local borehole records, LiDAR topographic data for the plot and the surrounding streets, and the Thames Water sewer flooding record for the postcode.
Site Characteristics and Flood Risk Profile
The site sits in a residential street in East Sheen, bounded by dwellings and gardens, with no watercourses in the immediate vicinity. Ground levels are broadly uniform.
Fluvial and tidal risk is low; the site lies outside the 1-in-100-year plus climate change extent. Sewer flooding risk is low, with no recorded incidents at the address. Reservoir and artificial-source risk is low.
Surface water is the governing source and is assessed as high, driven by the borough's own surface water mapping and by the impermeable character of the existing plot. Groundwater risk is medium, reflecting the local geology and recorded water levels, which limits how far infiltration can be relied upon as the sole disposal route.
Surface Water Management Strategy
The drainage strategy delivers a reduction in impermeable area of approximately 90m², from 290m² to 200m², and layers four SuDS components across the plot.
An extensive green roof of 11m² provides interception, biodiversity and water quality benefit at source. Rainwater harvesting provides 450 litres of storage in water butts. Permeable paving covers 115m² across the driveway and hardstanding areas. A soakaway of 4.1m³ provides attenuation, infiltration and filtration before any residual discharge reaches the combined sewer.
The sequence matters as much as the components. Interception at roof level, source control in the water butts, permeable surfacing across the largest external area and a terminal soakaway together give a drainage train that satisfies the hierarchy, rather than a single attenuation tank bolted onto an otherwise conventional scheme. The network is designed to the 1-in-100-year event with a 40% allowance for climate change.
Construction and Resilience
Below-ground elements are specified in waterproof concrete with sealed construction joints and service penetrations, in response to the medium groundwater rating, which also determines that the soakaway performs as an attenuating and filtering feature rather than as the scheme's sole reliance on infiltration. Ground floor construction uses moisture-resistant finishes, and external levels are set so that overland flow is directed away from the building.
Outcome
The assessment demonstrated that a comprehensive rebuild on a high surface water risk plot can be delivered with a material reduction in both impermeable area and peak discharge, and that flood risk to surrounding properties is not increased. The strategy gives Richmond Council a drainage position that answers the borough's surface water mapping directly rather than relying on the Flood Zone 1 designation alone.