73 Popes Grove
The project involves a new dwelling on a brownfield site in Flood Zone 1 (low flood risk). The FRA justified that the council’s request to raise the finished floor level by 300 mm was unnecessary, as the site is unaffected by fluvial/tidal flooding. A challenge arose from increasing impermeable area (153 m² to 220 m²), risking planning delays without SuDS. To address this, green roofs (6 m²), rainwater planters (600L), a shallow dry pond (0.4 m), and permeable paving were implemented, ensuring compliance with NPPF and local policies, managing runoff, and enhancing biodiversity without increasing costs or delays.
Project Overview
The project at 73 Pope's Grove, Twickenham, TW1 4JT, involves constructing a new residential dwelling on a brownfield site in the London Borough of Richmond upon Thames. Prepared by Air & Flood Consultants for McLaren.Excell, the Flood Risk Assessment (FRA) and Sustainable Drainage Systems (SuDS) Strategy ensures compliance with the National Planning Policy Framework (NPPF), Local Plan Policy LP 21, and London Plan Policy SI 13.
Site Description
Location: Residential area with detached housing, central grid reference E: 5135888, N: 172780.
Characteristics: Flat topography, underlain by London Clay with freely draining loamy soils, supporting infiltration.
Flood Risk Assessment
Flood Zone: The site is in Flood Zone 1 (low probability of fluvial/tidal flooding, <0.1% annual probability), as per the Environment Agency’s Flood Map.
Council Request: The council requested raising the finished floor level (FFL) by 300 mm above the maximum flood level to mitigate flood risk.
Justification Against FFL Increase: The FRA demonstrated that raising the FFL was unnecessary, as the site is not in Flood Zones 2 or 3 and is unaffected by fluvial or tidal flooding. Surface water flood risk is medium (0.2 m depth in localized areas), managed effectively by existing drainage and proposed SuDS, ensuring no impact on the site.
Other Flood Risks:
Groundwater: Medium risk, mitigated by drainage measures.
Sewer: Negligible risk, no significant flooding history.
Artificial Sources: Low risk, unaffected by reservoir breach scenarios.
Identified Issue and Solution
Challenge: The development increased the impermeable area from 153 m² to 220 m², potentially exacerbating surface water runoff without initial SuDS proposals, which could have delayed planning approval and increased costs due to council requirements for sustainable drainage.
Solution: To address this, the following SuDS features were agreed upon:
Green Roof: 6 m², reduces runoff and enhances biodiversity.
Rainwater Planters: 600 liters storage, promotes source control.
Shallow Dry Pond: 0.4 m deep in the rear garden, acts as a detention basin for infiltration during intense rainfall.
Permeable Paving: Proposed in the rear garden, retains existing front paving.
Outcomes
Flood Risk Mitigation: SuDS ensure post-development runoff rates and volumes match or improve pre-development levels, addressing the impermeable area increase.
Compliance: Aligns with NPPF, Local Plan Policy LP 21, and London Plan Policy SI 13, avoiding delays by meeting sustainable drainage requirements.
Benefits: Enhanced water quality, biodiversity, and amenity through vegetated SuDS, with maintenance managed by the homeowner per the CIRIA SuDS Manual (C753).
Conclusion
The FRA justified that raising the FFL by 300 mm was unnecessary due to the site’s Flood Zone 1 status and low flood risk. By proactively addressing the increased impermeable area with green roofs, rainwater planters, a dry pond, and permeable paving, the project mitigated potential planning delays and budget increases, ensuring a sustainable and compliant development.
Planning portal:
https://planning.richmond.gov.uk/richmond/application-details/235587