56 Cherwell Dr, Marston, Oxford
The Brief
RIDA was appointed to undertake a detailed flood risk assessment for a proposed development at 56 Cherwell Drive, Marston, Oxford. The project involves the alteration and extension of an existing two-storey residential dwelling, encompassing internal reconfiguration and rear external works while retaining its residential use. The primary objective was to evaluate the site’s vulnerability and demonstrate that the expanded property would remain safe from future flooding without increasing flood risk elsewhere.
Why the Client Chose RIDA
The client engaged RIDA due to our established expertise in navigating complex local planning requirements and delivering comprehensive flood risk compliance strategies. Our ability to provide detailed, site-specific mitigation measures ensures that proposed residential developments meet the stringent safety criteria set out by the Environment Agency and local planning authorities.
The Challenge
The site presented several significant hydrological constraints. While situated in fluvial Flood Zone 2 according to the Planning Flood Risk Map, the primary challenges identified were a high risk of surface water flooding (with potential depths of 300mm) and a high risk of groundwater flooding. Furthermore, the property falls within a Critical Drainage Area (CDA). This designation dictates that the local authority expects any new development to actively reduce downstream flood risks rather than having a merely neutral impact.
The RIDA Difference
Our team conducted a detailed flood risk assessment to address these multifaceted challenges in strict accordance with the National Planning Policy Framework (NPPF). We established a Design Flood Level of 59.4m AOD, driven primarily by the high surface water flood risk.
To mitigate this, we recommended retaining the existing Finished Floor Level of 59.25m AOD while incorporating specific flood resilient and resistant design strategies based on CLG 2007 guidance for water exclusion.
We addressed the CDA constraints by recommending the integration of Sustainable Drainage Systems (SuDS) to effectively manage surface water runoff. Finally, a safe egress route was clearly identified via Cherwell Drive toward a nearby primary school situated beyond the flood extent.
The Outcome
The assessment successfully demonstrated that the proposed extension can be developed in full compliance with the NPPF and the requirements of the Environment Agency. By providing a clear, actionable flood mitigation and sustainable drainage strategy, the report effectively supported the planning application, ensuring that the development will be safe over its lifetime and will manage flood risk effectively without adverse off-site impacts.
