SuDS Strategy for Residential Development in Critical Drainage Area, Richmond
We supported the client in discharging a SuDS Strategy condition for a site within a Critical Drainage Area in Richmond. The initial council feedback requested justification for not using infiltration measures and proposed a complex solution involving permeable paving with sub-base attenuation and pumping.
Our revised SuDS Strategy demonstrated that infiltration was unfeasible due to site constraints and groundwater conditions. Instead, we proposed rainwater planters providing 370 litres of above-ground attenuation, improving biodiversity, water quality, and amenity whilst saving the client cost and construction complexity. The condition was discharged with no further comments from the LLFA.
SuDS Strategy for Residential Development in a Critical Drainage Area, Richmond
Location: Richmond, London Borough of Richmond upon Thames | Services: SuDS Strategy, Planning Condition Discharge
Project Background
This project involved a small residential development within a Critical Drainage Area (CDA) in Richmond — a designation that triggers enhanced scrutiny of surface water management regardless of site size. The site also carried increased susceptibility to groundwater flooding, which significantly constrained the drainage options available from the outset.
Following submission of the initial SuDS Strategy, the Lead Local Flood Authority (LLFA) issued a holding objection raising several specific concerns: insufficient justification for not adopting infiltration drainage, no provision for rainwater harvesting, and the absence of hydraulic discharge control or flood exceedance routing. These are common sticking points on constrained urban sites, and addressing them required a methodical, evidence-led response rather than simply revising the drawings.
Technical Assessment
The LLFA's preferred solution — permeable paving with sub-base attenuation and a pumped outlet — was considered in detail. On assessment, this approach proved impractical for the site. Available ground levels were insufficient to accommodate the required sub-base depth, groundwater conditions posed a direct risk to any below-ground attenuation structure, and the construction complexity of a pumped system was disproportionate to the scale and nature of the development.
We documented this feasibility assessment clearly within the updated report, providing the technical justification the LLFA required under Policy LP 21 for departing from the drainage hierarchy's preferred options. Critically, this was not a case of avoiding a technically sound solution — infiltration simply was not viable given the ground conditions, and the evidence supported that conclusion.
The Drainage Solution
With below-ground attenuation ruled out, we designed a surface-level alternative that met policy requirements through a different route. Rainwater planters were integrated into the landscaping scheme, providing 370 litres of above-ground attenuation volume. The planters intercept roof runoff at source, temporarily storing water and releasing it slowly — functioning as a source control measure consistent with the SuDS management train hierarchy.
Beyond the hydraulic function, the planters deliver measurable co-benefits: improved biodiversity through planting, passive water quality treatment as runoff filters through growing media, and visual amenity for future residents. From a cost and programme perspective, above-ground planters also avoided the groundworks complexity and mechanical plant associated with the pumped sub-base system the LLFA had initially proposed.
Flood exceedance routing was defined to demonstrate safe conveyance of flows exceeding the design event, and the report confirmed compliance with both Richmond Local Plan Policy LP 21 and London Plan Policy SI 13.
Outcome
The planning condition was discharged in full with no further comment from the LLFA. The case demonstrates that on constrained urban sites in Critical Drainage Areas, pragmatic above-ground SuDS solutions can satisfy policy requirements where conventional below-ground approaches are technically unfeasible — provided the feasibility assessment is thorough and clearly evidenced.