Flood Risk Assessment — Garden Building in Flood Zone 2, Twickenham, Richmond

Flood Risk Assessment — Ancillary Garden Accommodation, TwickenhamLocation: 19 Cole Park Road, Twickenham TW1 1HP, London Borough of Richmond upon Thames | Client: Gillum and Partners | Services: Flood Risk AssessmentContextAncillary garden buildings are easy to underestimate from a flood risk perspective — but a proposed additional living space in Flood Zone 2, within the modelled 1-in-100-year plus climate change flood extent of the River Crane, is anything but straightforward. For this project in a traditional residential neighbourhood in Twickenham, the flood risk picture was genuinely multi-layered: medium fluvial and surface water risk, a medium reservoir breach classification, and a geological profile that introduced nuance even to the groundwater assessment.We were appointed by Gillum and Partners to prepare the FRA, assess all relevant flood sources, and develop a proportionate resilience strategy enabling the development to proceed in compliance with NPPF requirements and Richmond's local planning policies.The Flood Risk ProfileThe site benefits from existing flood defences on the River Crane providing protection to the 1-in-100-year event under current conditions. However, EA climate change mapping placed the site within the design flood extent for both the 1-in-100-year plus 35% and 1-in-100-year plus 70% climate change scenarios — meaning that as the climate changes and flood events intensify over the development's lifetime, the site moves progressively closer to the defended risk envelope. That finding shaped the resilience specification significantly.Surface water flood risk was medium, with EA mapping identifying areas of elevated pluvial risk affecting the rear of the site in higher return period events. The flat topography across the plot — consistent with the surrounding urban landscape — meant no significant localised accumulation points were identified, but surface water management still needed to be addressed in the drainage design.Groundwater risk required careful interpretation. The London Clay bedrock is low permeability and would ordinarily suppress groundwater flood risk. The complicating factor was the Kempton Park Gravel Member superficial deposits — sand and gravel — overlying the clay. These permeable materials are hydraulically active and can transmit groundwater relatively freely, introducing elevated potential for groundwater during prolonged wet periods despite the underlying clay. Borehole data from approximately 190m south-west confirmed gravel and sand over clay, and Soilscapes mapping characterised the soils as freely draining and slightly acid loamy — consistent with reasonable infiltration potential. Groundwater risk was assessed as low overall, but the superficial geology was flagged as a factor warranting consideration in the drainage design.Sewer flooding risk was low, with Thames Water records showing four indoor incidents in the wider area and no outdoor incidents at the site address. Reservoir breach risk was medium, based on EA inundation mapping reviewed under wet-day and dry-day failure scenarios.Flood Resilience StrategyGiven the site's position within the climate change flood extents, the resilience specification was calibrated against the 1-in-100-year plus 35% climate change flood level — with finished floor levels set a minimum of 300mm above this benchmark, pending final mAOD confirmation from the Environment Agency. This provides a meaningful freeboard margin and positions the building above the near-term design flood level whilst the EA level confirmation is obtained.Ground floor construction throughout the garden building was specified in flood-resilient materials: resilient plaster and low water absorption insulation on lower wall sections, moisture-tolerant internal finishes, and minimised service voids to limit pathways for water ingress. All service entry points were sealed, and ground profiling was recommended to eliminate any surface depressions and direct runoff positively away from the building towards drainage infrastructure.Surface Water ManagementWith London Clay at depth limiting deep infiltration potential, the surface water management strategy centred on attenuation rather than full infiltration. Geocellular attenuation systems with Hydro-brake vortex flow control devices were recommended to manage surface runoff from the garden building's roof and associated hardstanding, restricting discharge rates and reducing the burden on the downstream combined drainage network. Where the permeable superficial deposits allowed, soakaway elements were incorporated to maximise on-site infiltration before any controlled discharge.Future occupants were advised to register with the Environment Agency's Floodline warning service, supporting preparedness and informed response during flood events affecting the River Crane corridor.OutcomeThe FRA provided the London Borough of Richmond upon Thames with a comprehensive and clearly evidenced technical assessment across all relevant flood sources. The resilience strategy demonstrated that the proposed garden building could be delivered safely within Flood Zone 2, with appropriate measures addressing both the current defended risk and the longer-term climate change exposure identified by EA mapping. NPPF compliance was confirmed throughout.