Mistys Field, Walton-on-thame
The Brief
RIDA was appointed to undertake a detailed flood risk assessment for a proposed residential extension at 14 Mistys Field, Walton-on-Thames. The development involves extending the existing property across a total area of approximately 225 square metres, converting current water-compatible garden space into a ‘More vulnerable’ residential use with an estimated design life of 50 to 100 years.
Why the Client Chose RIDA
Due to the site’s location within a high-risk flood area, the client required specialist technical guidance to ensure full compliance with National Planning Policy Framework (NPPF) guidelines. They chose RIDA for our proven expertise in evaluating complex, multi-source flood risks and our ability to develop highly specific, pragmatic drainage and mitigation strategies required to satisfy local planning authorities and the Environment Agency.
The Challenge
The proposed extension faced significant environmental constraints, primarily driven by its location within Environment Agency Flood Zone 3, indicating a high probability of fluvial flooding. Beyond river flooding, the assessment identified a high risk of both surface water and groundwater flooding. Furthermore, the property is situated in an area with a high frequency of recorded sewer flooding incidents and falls within a potential reservoir dam breach flooding path, making safe egress and property resilience critical concerns.
The RIDA Difference
RIDA delivered a detailed flood risk assessment that systematically addressed each of the site’s identified constraints. To combat the high fluvial and groundwater risks, RIDA established a design water level and recommended raising the finished floor level to 12.31m AOD, implementing a strict water exclusion strategy. This included specifying suspended concrete floor slabs with a 150mm ventilated void, damp-proof membranes, and closed-cell insulation.
To address the documented sewer flooding risk, the specification of non-return valves on all new network connections was mandated. For surface water management, RIDA integrated Sustainable Drainage Systems (SuDS), recommending permeable surfaces and a storage tank with a flow control device to ensure runoff is sustainably managed. Finally, to protect future occupants from residual risks, RIDA mapped a safe evacuation route directing residents via Mistys Field up to Cottimore Lane and onto Stuart Avenue, safely beyond the flood extent.
The Outcome
The detailed flood risk assessment successfully demonstrated that the residential extension can be developed safely without the need for additional flood compensation volumes, due to the specific scale and location of the project. By detailing robust flood resistance measures, SuDS integration, and a clear emergency evacuation strategy, the report confirmed full compliance with NPPF guidelines. The assessment provided the essential technical evidence to confirm the development will remain safe for its design life and will not increase flood risk elsewhere, effectively supporting the planning application.